2. Chemistry for A&P

Atoms, Molecules, Water, pH, and the Organic Molecules of Life

Learning Objectives

2.1 Matter, Elements, and the Atoms That Matter in the Body

Matter is anything that occupies space and has mass. That is the whole definition, and it covers everything in this book: the calcium in Amara's bones, the air in her alveoli, the plasma her troponin is dissolved in. Energy — the capacity to do work — is not matter. It has no mass and takes up no space, and the entire discipline of physiology is a description of matter being rearranged by energy.

Note the distinction between mass and weight, because clinical measurements depend on it. Mass is the amount of matter in an object and does not change. Weight is the force gravity exerts on that mass. A 79.8 kg patient has the same mass in an ambulance, on the moon, and in free fall; her weight differs in all three. Laboratory chemistry uses mass exclusively.

Elements

An element is a substance that cannot be broken into simpler substances by ordinary chemical means. Each is defined by a single number — how many protons its atoms carry — and is represented by a one- or two-letter symbol drawn from its Latin or English name: Na from natrium for sodium, K from kalium for potassium, Fe from ferrum for iron.

An atom is the smallest unit of an element that retains its properties. A molecule is two or more atoms bonded together; if the atoms are of different elements, the substance is a compound. So O₂ is a molecule but not a compound; H₂O is both; and a lone iron atom sitting in a heme group is neither.

Of the 118 known elements, roughly 24 are present in the human body in measurable amounts, and about 20 have an established function. Four of them account for 96.1% of body mass.

The trace elements

A trace element constitutes less than 0.01% of body mass. The total mass of all of them together in Amara's 79.8 kg body is under 15 grams. Their significance is entirely out of proportion to that mass, because almost every one sits at the catalytic centre of an enzyme or a carrier protein — positions where a single atom does the work of the whole molecule.

Read that table for the pattern rather than the list. In nearly every row, one atom of a metal sits in the active site of a protein and performs an electron transaction the protein's amino acids cannot perform on their own. Proteins are built from atoms that are poor at handling single electrons — carbon, hydrogen, oxygen, nitrogen, sulfur. Transition metals excel at it, because they hold several accessible oxidation states. So biology recruits them, one atom at a time, and inserts them where the chemistry is hard. Iron sits at the centre of heme because iron can hold O₂ reversibly and carbon cannot.

The four in Amara's panel

Look back at her basic metabolic panel. Sodium, potassium, chloride, and calcium are four of the lesser elements from the table above, and the lab is not reporting them as elements at all. It is reporting them as ions — charged atoms dissolved in her plasma. Sodium metal reacts violently with water and chlorine gas was used as a chemical weapon; Na⁺ and Cl⁻ are the two most abundant solutes in her extracellular fluid and are entirely benign. Gaining or losing a single electron changes an element's behaviour completely, and §2.3 explains why.

2.2 Atomic Structure, Isotopes, and Why Medicine Uses Radioactive Atoms

The three particles

An atom has a dense central nucleus containing protons (charge +1) and neutrons (charge 0), surrounded by a diffuse cloud of electrons (charge −1). Protons and neutrons each have a mass of approximately 1 atomic mass unit (amu, or dalton). An electron has 1/1836 of that, which is why the mass of an atom is effectively the mass of its nucleus and why electrons — nearly massless — nonetheless determine every chemical property the atom has.

The scale is worth pausing on. If a hydrogen nucleus were a marble on the centre spot of a football stadium, its single electron would be somewhere out in the stands, and everything in between would be empty. Atoms are, dimensionally, almost entirely nothing. The reason your hand does not pass through a table is not that atoms are solid; it is that the electron clouds of the two surfaces repel each other electrically.

   THE ATOM — THREE PARTICLES, TWO REGIONS, ONE RULE THAT MATTERS
   ══════════════════════════════════════════════════════════════════════

     NUCLEUS   diameter ~10^-14 m   ·   >99.9% of the atom's MASS
     ┌────────────────────────────────────────────────────────────────┐
     │  PROTON   p+   charge +1   mass 1 amu                          │
     │           number of protons = ATOMIC NUMBER (Z) = the ELEMENT  │
     │  NEUTRON  n0   charge  0   mass 1 amu                          │
     │           varying the neutrons = ISOTOPES of the same element  │
     └────────────────────────────────────────────────────────────────┘
     ELECTRON CLOUD   diameter ~10^-10 m   ·   <0.1% of the MASS
          ELECTRON  e-  charge -1  mass 1/1836 amu
          → determines ALL chemical behaviour. Chemistry is electrons.

     MASS NUMBER = protons + neutrons        ATOMIC NUMBER = protons

   ══════════════════════════════════════════════════════════════════════
   SHELL FILLING          shell 1 holds 2      shell 2 holds 8
                          shell 3 holds 8 (18 when deeper shells fill)

   The OCTET RULE: an atom is stable when its VALENCE (outermost) shell
   holds 8 electrons — or 2, if shell 1 is the only shell it has.
   Everything an atom does chemically is an attempt to reach that state.

   ══════════════════════════════════════════════════════════════════════
   VALENCE IS DESTINY

    Element  Z   shell1 shell2 shell3   valence e-   to reach octet →
    ──────────────────────────────────────────────────────────────────
      H      1     1      -      -          1        share 1 → 1 BOND
      C      6     2      4      -          4        share 4 → 4 BONDS
      N      7     2      5      -          5        share 3 → 3 BONDS
      O      8     2      6      -          6        share 2 → 2 BONDS
      Na    11     2      8      1          1        LOSE  1 → Na+
      Mg    12     2      8      2          2        LOSE  2 → Mg2+
      P     15     2      8      5          5        share 5 (or 3)
      S     16     2      8      6          6        share 2 (or 6)
      Cl    17     2      8      7          7        GAIN  1 → Cl-
      K     19     2      8      8 +1       1        LOSE  1 → K+
      Ca    20     2      8      8 +2       2        LOSE  2 → Ca2+
    ──────────────────────────────────────────────────────────────────

        ╭───────────╮        ╭───────────╮        ╭───────────╮
        │   ● ●     │        │   ● ●     │        │   ● ●     │
        │  ●  6p+   │        │  ●  8p+  ●│        │ ●  11p+   │
        │  ●  6n0  ●│        │ ● ● 8n0 ● │        │ ●  12n0  ●│
        │   ● ●     │        │   ● ● ● ● │        │ ●  ● ● ● ●│  ● ← lone
        ╰───────────╯        ╰───────────╯        ╰───────────╯    outer e-
          CARBON               OXYGEN               SODIUM
        4 of 8 — must         6 of 8 — needs       1 spare — easier
        share four            two more             to LOSE than gain 7
        → 4 bonds; the        → 2 bonds; H2O       → becomes Na+ and
          backbone of           and every            behaves like a
          organic life          C=O group            different substance

Isotopes

All atoms of one element have the same number of protons — that is what makes them that element. They need not have the same number of neutrons. Atoms of the same element with different neutron numbers are isotopes, and they are written with the mass number after the name or as a superscript: carbon-12, carbon-13, carbon-14, or ¹²C, ¹³C, ¹⁴C.

Here is the fact that makes nuclear medicine possible, and it is worth stating twice.

Isotopes of an element are chemically identical. Chemistry is a property of electrons, and isotopes differ only in the nucleus. Every enzyme, every transporter, every receptor in the body treats ¹³¹I exactly as it treats ¹²⁷I, because from the outside they are indistinguishable.

Naturally occurring elements are mixtures of isotopes, which is why the atomic weight on the periodic table is rarely a whole number: it is the mass-weighted average of the naturally occurring mixture. Chlorine's atomic weight is 35.45 because natural chlorine is about 76% ³⁵Cl and 24% ³⁷Cl.

Radioisotopes and half-life

Some isotopes have nuclei that are unstable. They decay — reorganize spontaneously into a more stable configuration — and emit particles or energy in the process. These are radioisotopes, and their emissions come in three principal forms.

Half-life (t½) is the time for half of a sample's radioactive atoms to decay. It is a constant of the isotope and cannot be altered by temperature, pressure, or chemical environment. It is also the single most important practical property of a medical radioisotope, because it sets the compromise between two competing demands: the isotope must last long enough to be delivered and to reach its target, and it must disappear fast enough that the patient is not irradiated for weeks afterwards.

Note technetium-99m's 6-hour half-life against carbon-14's 5,730 years. After 24 hours — four half-lives — only 6% of an injected ⁹⁹ᵐTc dose is still radioactive; after 60 hours, essentially none. That is why it is the most-used medical isotope in the world.

2.3 Chemical Bonds, and the One Idea Behind All of Them

Most textbooks present the bond types as a list to memorize. They are better understood as one continuum, and the variable that generates the continuum is electronegativity.

Electronegativity is a measure of how strongly an atom pulls on shared electrons. On the Pauling scale it runs from about 0.7 to 4.0, and only a handful of values matter for physiology.

Now take any two atoms and subtract. The difference (ΔEN) tells you what kind of bond forms.

   THE BOND CONTINUUM — ONE AXIS, FOUR NAMES
   ══════════════════════════════════════════════════════════════════════

   ΔEN  0.0 ──────── 0.4 ──────────── 1.8 ──────────────────────► 3.3
        │             │                │
   NONPOLAR COVALENT  │  POLAR COVALENT│         IONIC
   electrons shared   │  shared but    │  electron TRANSFERRED
   equally            │  pulled to one │  → two IONS that then attract
                      │  side          │
   ─────────────────────────────────────────────────────────────────────
   C-C   ΔEN 0.00  ───┤                │            Na-Cl  ΔEN 2.23
   C-H   ΔEN 0.35  ───┤                │            K-Cl   ΔEN 2.34
                      │  O-H  ΔEN 1.24 ┤            Ca-O   ΔEN 2.44
                      │  N-H  ΔEN 0.84 ┤
                      │  C-O  ΔEN 0.89 ┤
                      │  C-N  ΔEN 0.49 ┤
   ─────────────────────────────────────────────────────────────────────

   ═════════ THE FOUR BONDS SIDE BY SIDE ═══════════════════════════════

   1 IONIC              Na  ● ─────→  ●●●●●●● Cl
     ΔEN > 1.8          Na loses its lone outer e-; Cl gains it
     ~300-400 kJ/mol        [Na]+  [Cl]-   now attract electrostatically
     in a dry crystal
     BUT only ~5 kJ/mol    In WATER the lattice falls apart:
     once dissolved!            NaCl  →  Na+  +  Cl-
                           These are ELECTROLYTES — they conduct current
                           and they are what a metabolic panel reports.

   2 NONPOLAR COVALENT       H : C : H     electrons sit midway
     ΔEN < 0.4               charge is even; no partial charges
     ~350-420 kJ/mol         → the molecule is HYDROPHOBIC
                             → fats, oils, steroid rings, membrane cores

   3 POLAR COVALENT               δ-                electrons pulled
     ΔEN 0.4-1.8                  O                 toward O
     ~460 kJ/mol (O-H)           / \                → PARTIAL charges
                            δ+  H   H  δ+           → the molecule is a
                                                      permanent DIPOLE
                             → water, sugars, amino acids: HYDROPHILIC

   4 HYDROGEN BOND        δ+H──Oδ- ······· δ-O──Hδ+
     ~5-20 kJ/mol         an attraction, NOT a bond between atoms:
     1/20th of a          a δ+ H already bonded to O, N, or F is
     covalent bond        attracted to a nearby δ- O, N, or F
                          Individually trivial. Collectively decisive:
                          they hold water together, coil the alpha helix,
                          zip the two strands of DNA, and set the shape
                          of every protein you own.

   ═════════ ALSO PRESENT, ALSO WEAK, ALSO ESSENTIAL ═══════════════════
   VAN DER WAALS  ~1-4 kJ/mol   fleeting dipoles between any two atoms
                                that come very close. Vast numbers of
                                them hold a folded protein's core shut.
   HYDROPHOBIC INTERACTION      not an attraction at all — nonpolar
                                groups are pushed together because WATER
                                is more stable when it is not forced to
                                cage them. Drives membrane formation.

Ionic bonds, ions, and why the lab reports milliequivalents

When ΔEN exceeds about 1.8, one atom does not merely pull harder on the shared electrons — it takes them. The donor becomes a positively charged cation; the acceptor becomes a negatively charged anion; and the two attract. In dry solid sodium chloride, that attraction locks billions of ions into a crystal lattice, and the bond is strong.

Put that crystal in water and the situation reverses. Water molecules, being strong dipoles, crowd around each ion and screen its charge so effectively that the lattice falls apart. The result is a solution of free electrolytes — ions that conduct electricity and that are the substrate of every electrical event in the body.

This is the answer to Amara's first case question. Sodium, potassium, chloride, and bicarbonate are reported in milliequivalents per litre because their physiological job is electrical and osmotic, not chemical mass. An equivalent is one mole of charge. For an ion with a single charge, mEq/L and mmol/L are numerically identical — her sodium of 138 mEq/L is 138 mmol/L. For a divalent ion they differ by a factor of two: her calcium of 9.2 mg/dL is 2.30 mmol/L, which is 4.60 mEq/L, because each calcium ion carries two charges.

Why does it matter? Because the body balances charge, not mass. The sum of the cations in plasma must equal the sum of the anions — plasma is electrically neutral to a fantastic degree of precision. That constraint is what makes the anion gap meaningful:

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) = 138 − (102 + 24) = 12 mEq/L (normal 8–12)

The gap is not a real gap. It is the concentration of anions the panel does not measure — mostly plasma proteins, plus phosphate, sulfate, and organic acids. When an unmeasured acid floods the plasma, its anion joins that pool and the gap widens. Amara's gap is 12: at the top of normal, but normal. Hold that number; §2.5 shows exactly how much it tells you.

Covalent bonds

When two atoms of similar electronegativity meet, neither can strip the other, and they share. A shared pair is a single bond; two shared pairs a double bond; three a triple bond. Covalent bonds are the strongest bonds in the body — 350 to 460 kJ/mol — which is precisely why they are used for structure. A protein's backbone must not come apart because the temperature rose a degree.

The distinction that matters physiologically is not single versus double but polar versus nonpolar:

• Nonpolar covalent (C–C, C–H, ΔEN < 0.4). No partial charges. Water cannot form favourable interactions with these regions, so molecules built mostly from them are hydrophobic. Every fat, oil, steroid, and membrane interior in the body is a mass of C–C and C–H bonds.
• Polar covalent (O–H, N–H, C=O, ΔEN 0.4–1.8). Electrons sit closer to the more electronegative atom, giving it a partial negative charge (δ−) and leaving a partial positive charge (δ+) behind. Water can hydrogen-bond to these regions, so molecules containing many of them are hydrophilic.

That single division — polar versus nonpolar — predicts where a molecule goes in the body, how it is transported, whether it crosses a membrane unaided, how it is excreted, and how a drug must be formulated. It is the most useful single sorting rule in this chapter.

Hydrogen bonds

A hydrogen bond forms when a hydrogen already covalently bonded to O, N, or F — and therefore carrying a substantial δ+ because those atoms have pulled its electron away — is attracted to a δ− oxygen, nitrogen, or fluorine on another molecule or another part of the same molecule.

At 5–20 kJ/mol, a hydrogen bond is roughly one twentieth the strength of a covalent bond. Thermal motion at body temperature breaks and reforms hydrogen bonds on a picosecond timescale. And yet:

• Liquid water exists at body temperature only because of hydrogen bonds (§2.4).
• The α-helix holds its shape because every backbone C=O hydrogen-bonds to the N–H four residues along the chain (§2.9).
• The two strands of DNA are held together entirely by hydrogen bonds — two between A and T, three between G and C (§2.9).
• Every enzyme recognizes its substrate largely through hydrogen bonds.

The design logic is exact. Structures that must be permanent are held by covalent bonds. Structures that must be specific but reversible — a substrate binding an enzyme, a hormone binding a receptor, two DNA strands separating for replication and rejoining afterwards — are held by large numbers of weak bonds. Individually each is trivial to break. Collectively they require a precise complementary shape, and they release on demand. Weakness, deployed in bulk, is the mechanism of biological specificity.

2.4 Water: Four Properties, Four Physiological Consequences

Water is 50–60% of body mass. For Amara at 79.8 kg, that is roughly 40 litres — about 25 L inside her cells and 15 L outside them, of which about 3 L is the plasma her troponin is dissolved in. Every number on her lab report is a concentration in water.

Water's remarkable properties all descend from one structural fact. The oxygen atom pulls the shared electrons of both O–H bonds strongly toward itself (ΔEN 1.24), and the molecule is bent at 104.5° rather than linear. A bent molecule with two polar bonds cannot cancel its dipoles. Water is therefore a permanent dipole: δ− at the oxygen, δ+ at each hydrogen. Everything below follows.

   WATER: ONE SHAPE, FOUR CONSEQUENCES
   ══════════════════════════════════════════════════════════════════════

   THE MOLECULE                        HYDROGEN BONDING
                                       each H2O can make FOUR H-bonds:
          δ-                           2 through its own H's (donor)
          O                            2 through its O lone pairs
         / \   104.5 degrees                    (acceptor)
    δ+  H   H  δ+                          H
                                           |
   bent + polar = permanent          H--O······H--O
   DIPOLE. Not cancelled.               |         |
   THIS IS THE WHOLE STORY.             H    ·    H
                                             ·
                                        H--O······H--O
                                           |         |
                                           H         H
                                    ~3.4 partners at any instant,
                                    each lasting ~1 picosecond

   ══ CONSEQUENCE 1 · UNIVERSAL SOLVENT ════════════════════════════════

     NaCl crystal            water pulls it apart          HYDRATION
     ┌─────────┐            δ-O                            SHELLS
     │Na+Cl-Na+│         H       H     ┌────────────┐
     │Cl-Na+Cl-│   →      \     /      │  H2O H2O   │
     │Na+Cl-Na+│           ●───●       │ H2O Na+ H2O│  Na+ never
     └─────────┘        (Na+ pulled    │  H2O H2O   │  meets Cl-
     lattice held by     out by the    └────────────┘  again
     ionic attraction    O's δ- pole)   O poles face IN toward Na+
                                        H poles face IN toward Cl-
     PHYSIOLOGY: plasma carries 138 mEq/L Na+, glucose, amino acids,
     urea, hormones, drugs, and wastes — all in solution. Anything
     NOT water-soluble (cholesterol, O2, steroid hormones, fats)
     requires a CARRIER. Lipoproteins exist because of this line.

   ══ CONSEQUENCE 2 · HIGH HEAT CAPACITY (4.184 J/g/degC) ══════════════

     Heat must first break H-bonds before it can speed molecules up,
     so temperature rises slowly.  40 L of water = a 40 kg thermal
     flywheel inside the body.

     Hard exercise makes ~15 kcal/min of heat.
     Body heat capacity ~0.83 kcal/kg/degC x 80 kg = 66 kcal/degC
     → without ANY heat loss: 1 degC every 4.4 min → 41 degC in 25 min
     PHYSIOLOGY: core temperature holds within 1-2 degC for hours.

   ══ CONSEQUENCE 3 · HIGH HEAT OF VAPORIZATION (~580 kcal/L) ══════════

     To evaporate, a water molecule must break ALL its H-bonds at once.
     Evaporating 1 L of sweat removes ~580 kcal from the skin.

     PHYSIOLOGY: above ~35 degC ambient, evaporation is the ONLY route
     of heat loss — radiation and convection ADD heat. Sweat that
     DRIPS removes almost nothing; only sweat that EVAPORATES cools.

   ══ CONSEQUENCE 4 · COHESION + SURFACE TENSION (72 mN/m) ═════════════

     H-bonds pull surface molecules inward and sideways.

     ┌───────── parietal pleura ─────────┐   Two wet surfaces SLIDE
     ═══════ 10 um of serous fluid ══════    freely but will not
     └───────── visceral pleura ─────────┘   PULL APART. The lung
                                             follows the chest wall
     PHYSIOLOGY: serous membranes, the tear film, mucus.
     AND THE PROBLEM: surface tension in 480 million alveoli would
     collapse every one of them. SURFACTANT is the countermeasure.

Property 1 · Polarity makes water the universal solvent

A solution is a homogeneous mixture of a solvent (the dissolving medium, water in every biological case) and one or more solutes. Because water is a dipole, it surrounds and stabilizes any solute that carries charge or partial charge: ions, sugars, amino acids, and the polar surfaces of proteins. Each dissolved particle acquires a hydration shell of oriented water molecules that screens its charge and prevents it re-associating.

The physiological payoff is that plasma can be a single fluid carrying everything at once. The physiological cost is that anything nonpolar cannot dissolve in it. Cholesterol, triglyceride, steroid hormones, fat-soluble vitamins, and oxygen itself are all poorly soluble in water, and every one requires a workaround — a carrier protein, a lipoprotein particle, or hemoglobin. The existence of Amara's lipid panel is a direct consequence of this paragraph.

Concentration can be expressed several ways, and it matters which:

Osmolality deserves a moment, because it counts particles rather than molecules. One mole of glucose dissolves into one mole of particles; one mole of NaCl dissolves into two. Normal plasma osmolality is 275–295 mOsm/kg and can be estimated:

Osmolality ≈ 2 × [Na⁺] + glucose/18 + BUN/2.8 = 2(138) + 212/18 + 18/2.8 = 276 + 11.8 + 6.4 = 294 mOsm/kg

Amara's calculated osmolality sits at the very top of the normal range, and the reason is visible in the arithmetic: her glucose is contributing 11.8 mOsm/kg where a normal fasting value would contribute 5. Chapter 3 shows what an osmolality at 294 does to the water distribution between her cells and her plasma.

Property 2 · High heat capacity buffers body temperature

Specific heat capacity is the energy required to raise one gram of a substance by one degree Celsius. Water's is 4.184 J/g·°C (1 cal/g·°C) — among the highest of all common liquids, and roughly four times that of air and nine times that of iron.

The reason is hydrogen bonding. Adding heat to water first goes into breaking hydrogen bonds rather than into increasing molecular motion, and only motion registers as temperature. Water therefore absorbs a great deal of energy for a small temperature change, and releases a great deal on cooling.

The consequence is that a 40-litre water reservoir sits inside every adult acting as a thermal flywheel. Amara's metabolism is producing heat continuously; every ATP hydrolysis is about 60% efficient at best, and the remainder is heat. Without water's heat capacity, ordinary daily metabolism would cook her.

Property 3 · High heat of vaporization makes sweating work

To leave the liquid, a water molecule must break every hydrogen bond holding it, all at once. That costs an enormous amount of energy — about 2,257 kJ per kilogram at 100 °C, and about 2,430 kJ/kg (580 kcal/L) at skin temperature.

That energy comes out of the skin. Evaporating one litre of sweat removes roughly 580 kcal, enough to cool 40 kg of body water by about 14 °C if nothing replaced it. This is why evaporation is the dominant mechanism of heat loss during exercise and the only effective one once ambient temperature exceeds skin temperature — above about 35 °C, radiation and convection deliver heat to the body rather than removing it.

Two clinical corollaries follow immediately. Sweat that drips off the skin has removed almost nothing: only the fraction that evaporates carries heat away, which is why high humidity is so much more dangerous than high temperature alone. And Amara's diaphoresis in triage is not thermoregulatory sweat at all — her core temperature is 36.8 °C. It is sympathetic sweating, which is why her skin is cool and wet rather than hot and wet (Chapter 1, §1.3).

Property 4 · Cohesion, surface tension, and the films that hold you together

Hydrogen bonds make water molecules cling to one another (cohesion) and to polar surfaces (adhesion). At an air–water interface, molecules are pulled inward and sideways but not outward, producing surface tension — 72 mN/m for pure water at 25 °C, among the highest of any liquid.

Three physiological consequences, running from useful to dangerous:

1. Serous membranes work because of cohesion. Chapter 1 described the pleura as two layers with 5–15 mL of fluid between them. That thin film lets the layers slide with almost no friction while making them nearly impossible to pull apart — the same reason two wet glass slides slide freely but resist separation. This is what couples the lung mechanically to the chest wall so that expanding the thorax expands the lung.
2. Lubricating and protective films. The tear film over the cornea, the mucus blanket of the airway, and synovial fluid in a joint all depend on water's ability to form a continuous coherent layer.
3. Alveolar collapse — the problem surfactant solves. In the lung, surface tension acts to shrink each of the 480 million alveoli, and by Laplace's law the collapsing pressure is inversely proportional to radius, so small alveoli collapse into larger ones. Untreated, this would make breathing impossible. Pulmonary surfactant — a phospholipid-rich secretion, §2.8 — inserts between water molecules at the air–liquid interface and interrupts their hydrogen bonding, dropping surface tension by roughly tenfold. Chapter 22 develops this fully; the point here is that a phenomenon caused by hydrogen bonds is neutralized by a molecule designed to prevent them.

Water as a reactant, and as padding

Two smaller roles, both important. Water is a reactant in every hydrolysis reaction in the body — digestion is literally the addition of water across bonds (§2.6) — and it is a product of every dehydration synthesis. And water cushions: cerebrospinal fluid floats the 1.4 kg brain to an effective weight of about 50 g, amniotic fluid protects a fetus, and synovial fluid absorbs joint loads.

2.5 Acids, Bases, pH, and Buffers

This section is the foundation of Chapter 31, and it is the most consequential chemistry in the book. Read it slowly.

Acids and bases

An acid is a proton (H⁺) donor. A base is a proton acceptor. Because a hydrogen atom consists of one proton and one electron, a hydrogen ion stripped of its electron is a bare proton — the smallest, most reactive chemical entity in biology.

Strong acids dissociate essentially completely: hydrochloric acid in gastric juice releases virtually every proton it has. Weak acids dissociate only partly and reversibly:

H₂CO₃ ⇌ H⁺ + HCO₃⁻

That double arrow is the whole of buffering. A weak acid holds a reservoir of protons it can release or reabsorb depending on which way the surrounding chemistry pushes it. Strong acids cannot buffer, because they have nothing left to give back.

pH

Hydrogen ion concentration in body fluids is inconveniently small, so it is expressed logarithmically:

pH = −log₁₀[H⁺], with [H⁺] in moles per litre

Consequences of the log scale, all of which students get wrong at least once:

• pH falls as acidity rises. The sign is negative.
• Each whole pH unit is a tenfold change. pH 6 is ten times more acidic than pH 7 and one hundred times more acidic than pH 8.
• pH 7.0 is neutral at 25 °C. At 37 °C, neutrality is about 6.8, so blood at 7.40 is slightly more alkaline relative to true neutrality than the numbers suggest.

Now do the arithmetic on Amara's blood. At pH 7.40, [H⁺] = 10⁻⁷·⁴⁰ = 4.0 × 10⁻⁸ M = 40 nanomoles per litre. Her measured pH of 7.38 corresponds to about 42 nmol/L.

Set that beside her sodium: 138 millimoles per litre. Sodium is present at roughly 3.3 million times the concentration of hydrogen ion. And yet sodium is permitted to range over 135–145 — a 7% band — while hydrogen ion is defended between about 35 and 45 nmol/L, and a doubling of it to 80 nmol/L (pH 7.10) is a medical emergency.

That asymmetry is the point of this section. The body regulates its least abundant ion most tightly, and it does so because H⁺ is the ion that binds to and alters every protein it touches.

   THE pH SCALE, WITH REAL BODY FLUIDS PLACED ON IT
   ══════════════════════════════════════════════════════════════════════

  pH   0    1    2    3    4    5    6    7    8    9   10   11   12   13
       │    │    │    │    │    │    │    │    │    │    │    │    │    │
  [H+] 1   .1  .01  1e-3 1e-4 1e-5 1e-6 1e-7 1e-8 1e-9 ...        (mol/L)
       ◄────────── MORE ACIDIC ─────┼───── MORE ALKALINE ──────────►
                                 NEUTRAL
                                (7.0 at 25 degC; ~6.8 at 37 degC)

    ▼1.5-3.5                                  ▼7.35-7.45
    GASTRIC JUICE                             ARTERIAL BLOOD
    (parietal cells;                          THE DEFENDED VARIABLE
     pumps H+ against a
     3-million-fold gradient)         ▼7.31 CEREBROSPINAL FLUID
                                      ▼7.35 VENOUS BLOOD (more CO2)
         ▼3.8-4.5 VAGINAL             ▼7.0-7.2 CYTOSOL (inside cells)
         ▼4.5-8.0 URINE ──────────────────────► the widest range in the
         │  (the kidney's disposal route         body: it is the DUMP
         │   for fixed acid)                     for whatever plasma
         ▼4.7-5.8 SKIN SURFACE                   must not keep
         │  ("acid mantle": inhibits
         │   bacterial colonization)             ▼7.6-8.6 BILE
              ▼6.35-6.85 SALIVA                  ▼8.0    PANCREATIC
                                                          JUICE
                                                 (neutralizes gastric
                                                  acid entering the
                                                  duodenum)

   ══ THE SURVIVAL BAND ═════════════════════════════════════════════════

     6.8 ──── 7.35 ══════ 7.40 ══════ 7.45 ──── 7.8
      │         │           │           │        │
    death   ACIDEMIA    SET POINT    ALKALEMIA  death
             begins                   begins

     Total survivable range: about ONE pH UNIT out of fourteen.
     Normal operating band: 0.10 pH units = 35 to 45 nmol/L of H+.

     For comparison: [Na+] = 138,000,000 nmol/L, allowed a 7% swing.
                     [H+]  =         42 nmol/L, allowed a 12% swing
                                                 before it is named
                                                 a disorder.

The acid load the body must handle every day

Two streams of acid are generated continuously.

• Volatile acid. Every cell producing ATP aerobically makes CO₂ — about 15,000 mmol per day in a resting adult, and several times that during exercise. CO₂ is not itself an acid, but in water it becomes one, which is why the lab prints "carbon dioxide" on a metabolic panel and means bicarbonate. The lungs excrete this stream.
• Fixed (non-volatile) acid. Metabolism of sulfur-containing amino acids yields sulfuric acid; phospholipid metabolism yields phosphoric acid; incomplete oxidation of fats and carbohydrates yields organic acids. Total: 50–100 mEq per day. The kidneys excrete this stream.

Fifteen thousand millimoles of potential acid per day into a plasma pool holding 42 nanomoles of free H⁺ per litre. If the buffering systems paused for even a few minutes, blood pH would fall past the survivable limit. They do not pause.

Buffers

A buffer is a solution that resists change in pH when acid or base is added. Every buffer is a conjugate pair: a weak acid (which can donate H⁺ if the solution becomes too alkaline) and its conjugate base (which can absorb H⁺ if it becomes too acidic).

Three buffer systems operate in the body.

Hemoglobin deserves special mention because it is the ideal buffer for the job: it sits in red cells travelling exactly where CO₂ is produced and unloaded, and deoxygenated hemoglobin binds H⁺ better than oxygenated hemoglobin does — so hemoglobin's buffering capacity increases at exactly the moment tissue is offloading oxygen and producing CO₂. Chapter 22 develops this as the Haldane effect.

The bicarbonate buffer system, built one step at a time

Step 1 — CO₂ meets water. Carbon dioxide dissolves in water and a small fraction hydrates to carbonic acid. Uncatalyzed this reaction is slow, taking many seconds — far too slow for physiology.

CO₂ + H₂O ⇌ H₂CO₃

Step 2 — the catalyst. The enzyme carbonic anhydrase, present in red blood cells, renal tubular cells, gastric parietal cells, and elsewhere, accelerates that hydration by a factor of about 10⁴ to 10⁶. It is one of the fastest enzymes known, turning over up to a million molecules per second per enzyme molecule. It contains a single zinc atom at its active site — one of the trace elements from §2.1.

Step 3 — dissociation. Carbonic acid is a weak acid and dissociates readily:

H₂CO₃ ⇌ H⁺ + HCO₃⁻

Step 4 — put it together.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Now apply Le Châtelier's principle: a system at equilibrium responds to a disturbance by shifting in the direction that opposes it. Run the equation in both directions.

• Acid is added (say, lactic acid from sprinting). The added H⁺ pushes the equation leftward: H⁺ combines with HCO₃⁻ to form H₂CO₃, which becomes CO₂ and water. The free H⁺ is consumed, pH barely moves, bicarbonate is spent, and the CO₂ generated is blown off by the lungs.
• Base is added, or acid is lost (vomiting gastric HCl). Free H⁺ falls, so the equation shifts rightward: more H₂CO₃ dissociates to replace the missing protons.

Step 5 — why this buffer beats every other one. On paper, bicarbonate is a mediocre buffer for blood: its pKa is 6.1, and a buffer works best within about one pH unit of its pKa. At pH 7.40 it is 1.3 units away, which should make it feeble.

It is nonetheless the body's dominant extracellular buffer, for a reason that has nothing to do with chemistry and everything to do with anatomy: it is an open system with two independently regulated ends.

• The CO₂ end is controlled by the lungs, which can change alveolar ventilation within seconds and alter PCO₂ over a range of at least 20–80 mm Hg.
• The HCO₃⁻ end is controlled by the kidneys, which reabsorb, regenerate, or excrete bicarbonate over hours to days.

A closed-system buffer is consumed as it works. This one is continuously replenished at one end and vented at the other. That is why it dominates, and it is the single most important idea in acid–base physiology.

   THE BICARBONATE BUFFER SYSTEM — TWO ORGANS, ONE EQUATION
   ══════════════════════════════════════════════════════════════════════

        LUNGS                                            KIDNEYS
     (seconds-minutes)                              (hours-days)
     control the LEFT end                       control the RIGHT end
            │                                            │
            ▼                                            ▼
    ┌───────────────┐                            ┌────────────────┐
    │  EXHALE CO2   │                            │ reabsorb HCO3- │
    │  ~15,000      │                            │ regenerate new │
    │  mmol/day     │                            │ HCO3-          │
    │               │                            │ excrete H+ as  │
    │  raise or     │                            │ NH4+ and       │
    │  lower rate   │                            │ titratable acid│
    │  and depth    │                            │ 50-100 mEq/day │
    └───────┬───────┘                            └────────┬───────┘
            │                                             │
            ▼                                             ▼
   ═══════════════════════════════════════════════════════════════════
                    carbonic
        CO2  +  H2O ══════════►  H2CO3  ◄══════►  H+   +   HCO3-
         ▲              anhydrase    ▲              ▲         ▲
         │             (Zn at the    │              │         │
      VOLATILE         active site;  │          THE VARIABLE  │
      ACID             10^6/sec)     │          BEING         │
      15,000 mmol/d                  │          DEFENDED      │
                                     │          42 nmol/L     │
   ═══════════════════════════════════════════════════════════════════

   ══ RUN IT LEFTWARD: acid is added (sprinting, ketoacidosis) ═════════

       H+ ↑  →  H+ + HCO3-  →  H2CO3  →  CO2 + H2O  →  EXHALED
                    │
                    └──► HCO3- is CONSUMED   → serum HCO3- FALLS
                         pH barely moves     → this is COMPENSATION
                         and breathing rises → PCO2 FALLS

   ══ RUN IT RIGHTWARD: acid is lost (vomiting) or base added ══════════

       H+ ↓  →  H2CO3 dissociates to replace it → HCO3- accumulates
                                                → serum HCO3- RISES
                                                → breathing SLOWS to
                                                  retain CO2

   ══ HENDERSON-HASSELBALCH — THE WHOLE SYSTEM IN ONE LINE ═════════════

              pH  =  6.1  +  log ( [HCO3-] / (0.03 x PCO2) )
                              ▲            ▲
                     KIDNEY sets this   LUNG sets this

     NORMAL:   6.1 + log(24 / (0.03 x 40))  = 6.1 + log(24/1.20)
             = 6.1 + log 20  = 6.1 + 1.30   = 7.40
                       ▲
              THE 20:1 RATIO. pH depends on the RATIO, not on either
              number alone — which is why compensation works at all.

     AMARA:    6.1 + log(24 / (0.03 x 42))  = 6.1 + log(24/1.26)
             = 6.1 + log 19.0 = 6.1 + 1.28  = 7.38   ✓ matches her ABG

2.6 Chemical Reactions, Energy, ATP, and Enzymes

Reaction types

Two reaction patterns dominate this chapter's second half, and they are inverses of each other. Dehydration synthesis joins two monomers by removing an –OH from one and an –H from the other, releasing water and forming a covalent bond. Hydrolysis does the reverse: it inserts a water molecule across a bond and splits it. Every macromolecule in §2.7 to §2.9 is built one way and dismantled the other. Digestion is hydrolysis; growth is dehydration synthesis.

Redox reactions deserve one clarifying line, because their naming is counterintuitive. Oxidation is the loss of electrons; reduction is the gain. In biological molecules electrons travel with hydrogen atoms, so a molecule that loses hydrogens has been oxidized. Fats have more C–H bonds than carbohydrates, which is to say more electrons to give away, which is exactly why they yield more energy per gram (§2.8).

Energy and the two laws

Energy is the capacity to do work. It exists as kinetic energy (motion) and potential energy (stored position or chemical configuration), and it converts among chemical, electrical, mechanical, and radiant forms. Two constraints govern every conversion:

• First law. Energy is neither created nor destroyed. Every calorie Amara consumes is stored, converted to work, or released as heat.
• Second law. Every conversion loses some energy as heat. Muscle is 20–25% efficient at converting chemical energy to mechanical work; the other 75–80% is heat, which is why shivering warms you and why exercise requires the water chemistry of §2.4.

A reaction that releases energy is exergonic and proceeds spontaneously. One that requires energy is endergonic and will not proceed unless coupled to an exergonic one. Cells survive by coupling, and the coupling agent is ATP.

ATP

Adenosine triphosphate is adenine + ribose + three phosphate groups in series. The bonds linking the second and third phosphates are phosphoanhydride bonds, and hydrolysing the terminal one releases about 7.3 kcal/mol (30.5 kJ/mol) under standard conditions and roughly 11–13 kcal/mol under the conditions actually present in a cell.

ATP + H₂O → ADP + Pi + energy

Why so much energy from one bond? Three reasons stack. The three phosphates carry four negative charges packed adjacent to one another and repel electrostatically, so the intact molecule is strained. The released inorganic phosphate is resonance-stabilized in ways the attached form is not. And both products are more favourably hydrated than the reactant. Hydrolysing ATP is therefore less "breaking a high-energy bond" than "releasing a compressed spring."

The turnover numbers are extraordinary. A cell holds only 1–2 seconds' worth of ATP at any moment; total body ATP is about 250 g; and a resting adult synthesizes and consumes roughly 40–75 kg of it per day, recycling each molecule several hundred times. ATP is not a storage molecule. It is a currency in constant circulation, and the entire apparatus of Chapter 24 exists to keep it circulating.

Reaction rates and enzymes

Four variables change reaction rate: temperature (a 10 °C rise roughly doubles or triples rate), concentration of reactants, particle size or surface area, and the presence of a catalyst.

Every reaction must first climb an energy barrier called the activation energy — the energy needed to distort reactant bonds into the transition state — even if the reaction is overwhelmingly favourable overall. At body temperature almost no biological reaction clears that barrier at a useful rate on its own.

An enzyme is a biological catalyst, almost always a protein, that lowers activation energy by binding substrate in a strained, precisely oriented configuration at its active site. Four properties follow, and each is worth committing to memory:

1. Enzymes lower activation energy. They do not change ΔG or the equilibrium position. An enzyme makes a reaction reach equilibrium faster, in both directions equally. It cannot make an unfavourable reaction favourable — only coupling to ATP does that.
2. Enzymes are specific, because catalysis depends on shape complementarity. Binding induces a conformational change in the enzyme that tightens the fit — induced fit.
3. Enzymes are not consumed. Carbonic anhydrase turns over up to 10⁶ substrate molecules per second, indefinitely.
4. Enzymes are regulable, by substrate concentration, by pH and temperature, by inhibitors, by covalent modification (phosphorylation), and by allosteric effectors. This is where nearly all metabolic control lives, and where most drugs act.

Many enzymes require a non-protein helper: a cofactor (usually a metal ion — the zinc in carbonic anhydrase, the magnesium required by every kinase) or a coenzyme (an organic molecule, characteristically vitamin-derived: NAD⁺ from niacin, FAD from riboflavin, coenzyme A from pantothenate). This is the chemical reason vitamin deficiencies produce multi-system disease: remove one coenzyme and dozens of unrelated enzymes stop at once.

Because enzymes are proteins, they are exquisitely sensitive to pH and temperature — both of which alter the weak bonds holding their three-dimensional shape (§2.9). Most human enzymes peak near pH 7.2 and 37 °C. Pepsin, working in gastric juice, peaks at pH 2.0 and is irreversibly destroyed at pH 7. An enzyme's optimum is a statement about where in the body it belongs.

2.7 Carbohydrates

Carbohydrates have the general formula (CH₂O)ₙ — literally "hydrated carbon." They are the body's most accessible fuel and, in a form most students never hear about, its primary identification system.

Monosaccharides

The simple sugars. Three hexoses matter: glucose, fructose, and galactose, all with the identical formula C₆H₁₂O₆ and differing only in the spatial arrangement of their atoms. They are isomers, and the differences are not trivial — the body has separate transporters and separate metabolic routes for each. Two pentoses matter: ribose in RNA and ATP, and deoxyribose in DNA.

Glucose is the universal currency of fuel. Plasma glucose is held at 70–99 mg/dL fasting. That corresponds to roughly 4 grams of glucose dissolved in the entire circulating blood volume — about one teaspoon. The brain alone consumes about 120 g/day. The whole of glucose homeostasis is the problem of keeping a teaspoon's worth in circulation while the brain draws that much every 45 minutes.

Amara's glucose of 212 mg/dL means roughly 10 g in her bloodstream — two and a half teaspoons. The absolute quantity is small; the consequences of it being sustained are the subject of most of the rest of her story.

Disaccharides and polysaccharides

Two monosaccharides joined by dehydration synthesis form a glycosidic bond.

Polysaccharides are long chains. Starch is the plant storage form; glycogen is the animal one; cellulose is the plant structural form. Cellulose differs from starch only in the orientation of its glycosidic bond (β-1,4 rather than α-1,4), and humans possess no enzyme that cleaves the β form — which is the entire chemical basis of dietary fibre.

Glycogen is worth a structural note, because it is a clean structure-determines-function case. It is highly branched, with a branch point roughly every 8–12 glucose residues. Branching creates a large number of free ends, and the enzymes that mobilize glycogen work only from the ends. A branched polymer can therefore be dismantled at hundreds of points simultaneously, which is what allows liver glycogen to defend blood glucose within seconds. A linear polymer of the same mass could be attacked at only two points. Amara's liver holds about 100 g of glycogen; her skeletal muscle holds about 400 g, but muscle glycogen is unavailable to the blood because muscle lacks the enzyme that releases free glucose.

Glycoproteins, glycolipids, and the glycocalyx

Most proteins destined for the cell surface or for secretion are glycosylated — sugar chains are attached enzymatically in the endoplasmic reticulum and Golgi apparatus (Chapter 3). Those sugar chains form the glycocalyx, the fuzzy carbohydrate coat on the outer face of every cell membrane, and they are the body's identification system. ABO blood group antigens are sugar chains. Sperm–egg recognition, leukocyte adhesion to inflamed endothelium, viral attachment, and the immune distinction between self and non-self are all read at this layer.

Glycation versus glycosylation — the HbA1c answer

Here is the distinction that answers Amara's second case question, and it hinges on a single word.

Glycation is what happens when a sugar's reactive aldehyde group collides with a free amino group on a protein and simply sticks. No enzyme is involved, no regulation is possible, and the rate is a pure function of how much glucose is present and for how long.

Hemoglobin A1c is glycated hemoglobin. Glucose attaches to the N-terminal valine of the hemoglobin β-chain, forming first a reversible Schiff base and then, over hours, undergoing an Amadori rearrangement into a stable ketoamine that cannot come off. Because red blood cells live about 120 days and cannot repair or replace their hemoglobin, the fraction of hemoglobin bearing this adduct is a running integral of blood glucose over the red cell population's lifespan — weighted toward the recent past, with roughly 50% of the value contributed by the last 30 days and 25% by the 30–60 day window.

Amara's HbA1c of 7.4% (57 mmol/mol) converts to an estimated average glucose of

eAG = (28.7 × 7.4) − 46.7 = 166 mg/dL

So her single reading of 212 is not an aberration produced by a stressful morning. Her average glucose over the past three months has been about 166 mg/dL, at a time when 100 would be normal. The test looks backwards in time because the molecule it measures is permanent and its carrier cell is not replaced.

2.8 Lipids

Lipids are defined by a physical property rather than a chemical structure: they are insoluble in water and soluble in nonpolar solvents. Structurally they are dominated by C–C and C–H bonds — the nonpolar covalent bonds of §2.3 — which is the whole reason they behave as they do.

Why fat stores nine calories per gram

Fat yields 9 kcal/g; carbohydrate and protein yield 4. Two independent reasons:

1. Chemistry. Fatty acid carbons are highly reduced — surrounded by hydrogens, rich in electrons available to hand to oxygen. Carbohydrate carbons already carry oxygen and are partly oxidized already, so less energy remains to extract.
2. Physics. Glycogen is stored hydrated, carrying 2–3 g of water per gram of glycogen. Fat is stored anhydrous. Per gram of stored mass, the advantage is therefore closer to sixfold.

The consequence is dramatic. Amara at 79.8 kg with a body fat fraction near 38% carries roughly 30 kg of triglyceride ≈ 270,000 kcal — enough for two months of total starvation. Her 500 g of glycogen holds about 2,000 kcal — less than one day. Evolution chose energy density for the long reserve and accessibility for the short one.

Triglycerides and fatty acids

A triglyceride is one glycerol molecule esterified to three fatty acids — long hydrocarbon chains with a carboxyl group at one end. Formed by dehydration synthesis, broken by hydrolysis.

Saturated fatty acids have no C=C double bonds; every carbon carries its full complement of hydrogens; the chains are straight and pack tightly, so they are solid at room temperature. Unsaturated fatty acids contain one or more double bonds. In the naturally occurring cis configuration, each double bond puts a permanent 30° kink in the chain, preventing tight packing — so unsaturated fats are liquid.

That single geometric fact does a great deal of work in this book. It is why olive oil pours and butter does not; it is why cell membranes stay fluid at 37 °C (§2.8, and Chapter 3); and it is why industrially produced trans fats are harmful — hydrogenation converts cis double bonds to trans, which are straight, so a trans fat behaves like a saturated fat while being labelled unsaturated.

Eicosanoids — prostaglandins, thromboxanes, and leukotrienes — are 20-carbon signalling molecules made from arachidonic acid on demand. They mediate inflammation, pain, fever, uterine contraction, and platelet aggregation. They also explain a tablet Amara swallowed in triage: aspirin irreversibly acetylates the cyclooxygenase enzyme in platelets, so those platelets can make no thromboxane A₂ for the remainder of their 7–10 day lifespan and cannot aggregate normally. Platelets have no nucleus and cannot synthesize replacement enzyme. A single small dose therefore disables a cell for its whole life — which is why aspirin is dosed once daily and why it must be stopped a week before surgery.

Phospholipids and self-assembly

Replace one of a triglyceride's three fatty acids with a phosphate-containing group and you have a phospholipid: a molecule with a charged, hydrophilic head and two nonpolar, hydrophobic tails. A molecule with both characters is amphipathic, and amphipathic molecules do something remarkable in water — they organize themselves.

The organizing force is not attraction between the tails. It is entropy in the water. Water forced to surround a nonpolar chain must adopt an ordered cage-like arrangement, which is entropically expensive. Clustering the nonpolar chains together minimizes the total surface water must cage. The tails are not pulled together; they are pushed. This is the hydrophobic effect, and it builds every membrane in your body without a single enzyme.

Geometry determines what forms. A single-tailed molecule is cone-shaped and packs into a spherical micelle. A two-tailed phospholipid is roughly cylindrical and packs into a flat bilayer. Bile salts make micelles that ferry dietary fat to the intestinal wall; phospholipids make the bilayer that is the boundary of every cell you own.

   AMPHIPATHIC MOLECULES IN WATER — FOUR STRUCTURES, ONE FORCE
   ══════════════════════════════════════════════════════════════════════

   THE MONOMER                       WHY IT ASSEMBLES
   ┌──────┐  ← PHOSPHATE + choline   Water must build an ordered CAGE
   │ HEAD │    polar, charged,       around any nonpolar chain. That
   └──┬───┘    HYDROPHILIC           costs entropy. Hiding the chains
      │                              from water is the cheapest option.
   ═══╪═══  glycerol backbone
      │                              → THE HYDROPHOBIC EFFECT
    ║   ║   ← two FATTY ACID tails     Tails are not PULLED together.
    ║   ╲     nonpolar, HYDROPHOBIC    They are PUSHED. No enzyme,
    ║    ╲    (one kinked = cis        no ATP, no template required.
    ║     ║    double bond)

   ══════════════════════════════════════════════════════════════════════
   1 MICELLE (single-tail, cone-shaped)   2 BILAYER (two-tail, cylinder)

            ○○○○○○                       ○○○○○○○○○○○○○○○○○○○○○  heads
          ○ ║║║║║║ ○                     ║║║║║║║║║║║║║║║║║║║║║  tails
         ○ ║║ ●● ║║ ○   ← fat or         ║║║║║║║║║║║║║║║║║║║║║   ~5 nm
         ○ ║║ ●● ║║ ○     vitamin        ○○○○○○○○○○○○○○○○○○○○○  heads
          ○ ║║║║║║ ○      cargo
            ○○○○○○                       WATER above and below; the
      Bile salts do this in the          interior is an oil film 3 nm
      gut, carrying fat to the           thick. THE CELL BOUNDARY.
      absorptive surface.                (Chapter 3 builds on this.)

   ══════════════════════════════════════════════════════════════════════
   3 LIPOPROTEIN — a micelle with an ADDRESS

        ╭──────────────────────────────╮
        │  ○○ PHOSPHOLIPID MONOLAYER ○○│ ← polar face out: the particle
        │ ○  ╔══════════════════════╗ ○│   dissolves in plasma
        │○   ║ TRIGLYCERIDE +       ║  ○│
        │○   ║ CHOLESTERYL ESTER    ║  ○│ ← nonpolar cargo, hidden
        │ ○  ║   (the cargo)        ║ ○ │
        │  ○ ╚══════════════════════╝○  │
        │   ▓▓▓ APOLIPOPROTEIN ▓▓▓      │ ← THE ADDRESS LABEL.
        ╰──────────────────────────────╯   Determines which receptor
          + free cholesterol wedged        on which organ accepts it.
            among the phospholipids

   ══════════════════════════════════════════════════════════════════════
   4 CHOLESTEROL IN A MEMBRANE — the fluidity buffer

     warm (37 degC): rings sit BETWEEN tails, restrain their motion
                     → membrane LESS fluid than it would be
     cold:           rings BLOCK tails from packing into a solid
                     → membrane MORE fluid than it would be
     One molecule, opposite effects, because it interferes with
     whatever the tails were about to do.

        ○○○○○○○○○○○○○○○○○○○○○
        ║║║▓║║║║║▓║║║║║║▓║║║║   ▓ = cholesterol's rigid four-ring
        ║║║▓║║║║║▓║║║║║║▓║║║║       core wedged among the tails
        ○○○○○○○○○○○○○○○○○○○○○

Steroids and cholesterol

Steroids are built on a characteristic skeleton of four fused carbon rings. Cholesterol is the parent compound and it has three distinct jobs:

1. Membrane component. Roughly one cholesterol molecule per two phospholipids in a typical plasma membrane, acting as the fluidity buffer described in Figure 2.6.
2. Precursor. Every steroid hormone in the body — cortisol, aldosterone, testosterone, estradiol, progesterone — is made from cholesterol, as are the bile salts that emulsify dietary fat and vitamin D₃ made in skin.
3. Structural raw material for growth and repair, especially of nervous tissue.

Cholesterol is therefore essential, and the body does not rely on diet for it: the liver and other tissues synthesize 700–1,000 mg/day, against 300–500 mg in a typical diet. The rate-limiting enzyme of that synthetic pathway is HMG-CoA reductase, and it is one of the most consequential drug targets in medicine.

Lipoproteins — the answer to Amara's third case question

Cholesterol is nonpolar and cannot dissolve in plasma. It travels inside lipoproteins: spherical particles with a nonpolar cargo core, a phospholipid monolayer, and one or more surface apolipoproteins that act as molecular addresses, determining which receptor on which organ will take the particle up. Lipoproteins are classified by density, which is inversely related to lipid content: more lipid, less protein, lower density.

So LDL and HDL are not two kinds of cholesterol. They are the same cholesterol travelling in opposite directions. LDL carries it outward from the liver to peripheral tissue; HDL performs reverse cholesterol transport, scavenging excess cholesterol from cells — including from macrophages inside arterial walls — and returning it to the liver for disposal in bile.

Why LDL is atherogenic and HDL is not is a matter of size and destination. LDL particles are small enough to cross a damaged endothelium into the subendothelial space of an artery, where they are trapped by matrix proteoglycans and chemically oxidized. Oxidized LDL is taken up by macrophage scavenger receptors, which — unlike the ordinary LDL receptor — are not downregulated by intracellular cholesterol. The macrophage keeps eating, becomes a lipid-laden foam cell, and dies there. That accumulation is the atherosclerotic plaque, and the plaque that ruptured in one of Amara's coronary arteries is the reason she is in an emergency department.

Now the arithmetic that answers her third question. Her LDL of 168 was never measured. It was calculated by the Friedewald equation:

LDL = Total cholesterol − HDL − (Triglycerides ÷ 5) = 255 − 38 − (244 ÷ 5) = 255 − 38 − 48.8 = 168 mg/dL

The triglyceride-over-five term is an estimate of the cholesterol carried in VLDL. It is a useful approximation that becomes unreliable when triglycerides exceed about 400 mg/dL, which is why non-HDL cholesterol — total minus HDL, or 217 mg/dL for Amara — is increasingly preferred: it requires no assumption and captures every atherogenic apoB-bearing particle at once.

One more number. Her triglyceride-to-HDL ratio is 244/38 = 6.4, against a desirable value below 2. A high ratio is a chemical signature of insulin resistance and is associated with a shift toward small, dense LDL particles — which penetrate the arterial wall more readily than large buoyant ones. Her LDL number is bad; the particles behind it are probably worse than the number suggests.

2.9 Proteins and Nucleic Acids

Amino acids and the peptide bond

Proteins are polymers of amino acids. Every amino acid has the same four groups attached to a central carbon: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group or side chain. There are twenty in human proteins, and the R group is the only difference between them. Nine are essential — the body cannot synthesize them and they must be eaten.

R groups sort into four classes, and the classification predicts where in a folded protein each residue will end up:

Three side chains are special enough to name individually. Glycine has a single hydrogen as its R group, making it tiny and unusually flexible — which is why it appears at every third position in collagen's triple helix, where no larger residue would fit. Proline's side chain loops back to its own backbone nitrogen, putting a rigid kink in the chain and terminating α-helices. Cysteine carries a thiol (–SH) group that can oxidize with a second cysteine to form a covalent disulfide bridge — the only covalent cross-link inside a folded protein, and the reason keratin in hair and nails is so tough.

Amino acids join by dehydration synthesis: the carboxyl of one condenses with the amino group of the next, releasing water and forming a peptide bond. The resulting chain has direction — a free amino end (N-terminus) and a free carboxyl end (C-terminus) — and is always written and synthesized N to C.

The four levels of structure

   PROTEIN STRUCTURE — FOUR LEVELS, AND THE BOND THAT HOLDS EACH
   ══════════════════════════════════════════════════════════════════════

   PRIMARY (1°)   the SEQUENCE          held by: PEPTIDE BONDS (covalent)
   ─────────────────────────────────────────────────────────────────────
     N-Val-His-Leu-Thr-Pro-Glu-Glu-Lys...-C     (human beta-globin)
                              ▲
                          position 6
     SICKLE:   Glu → Val   ONE substitution out of 146.
     Glu is acidic and charged; Val is nonpolar. A hydrophobic patch
     now sits on the molecule's SURFACE, where water cages it — so
     deoxygenated molecules stick to one another and POLYMERIZE into
     rigid fibres that deform the whole cell. One atom's worth of
     chemistry, one disease.
     → 1° DETERMINES EVERYTHING BELOW. It is the only level encoded
       by DNA; the rest is self-assembly.

   SECONDARY (2°)  local repeating shape    held by: HYDROGEN BONDS
   ─────────────────────────────────────    between BACKBONE atoms only
                                            (C=O ····· H-N), NOT R groups
     ALPHA HELIX                  BETA PLEATED SHEET
       ╭─╮ ╭─╮ ╭─╮                 ═══════════►
      ╱   V   V   ╲                 ┊ ┊ ┊ ┊ ┊   ← H-bonds between
     ╰─╯ ╰─╯ ╰─╯ ╰─╯               ◄═══════════    adjacent strands
     3.6 residues per turn          ┊ ┊ ┊ ┊ ┊
     C=O of residue i bonds        ═══════════►
     to N-H of residue i+4
     (keratin, myosin tails)       (silk, antibody domains, amyloid)

   TERTIARY (3°)  the whole 3-D fold of ONE chain
   ─────────────────────────────────────────────────────────────────────
     held by R-GROUP interactions, listed by strength:
        DISULFIDE BRIDGE  Cys-S-S-Cys        ~250 kJ/mol  COVALENT
        IONIC / salt bridge  Asp- ···· +Lys   ~20 kJ/mol
        HYDROGEN BOND     Ser-OH ···· O=C     ~10 kJ/mol
        HYDROPHOBIC CORE  Leu/Val/Phe packed    entropic — and it is
                          away from water       the DOMINANT force
        VAN DER WAALS     everywhere close     ~2 kJ/mol each

     ┌────────────────────────┐   POLAR residues face OUT (water)
     │  ○ ○ ▓▓▓▓▓▓▓ ○ ○       │   NONPOLAR residues face IN
     │ ○  ▓ hydro- ▓  ○       │   → the protein turns itself inside
     │ ○  ▓ phobic ▓  ○       │     out relative to a membrane, where
     │  ○ ○ ▓▓▓▓▓▓▓ ○ ○       │     the arrangement is REVERSED
     └────────────────────────┘

   QUATERNARY (4°)  two or more folded chains assembled
   ─────────────────────────────────────────────────────────────────────
     HEMOGLOBIN = 2 alpha + 2 beta chains + 4 heme groups + 4 Fe atoms
        ╭───╮╭───╮        Not all proteins have a 4° level.
        │ a ││ b │        Those that do can show COOPERATIVITY:
        ╰───╯╰───╯        one subunit binding O2 changes the shape
        ╭───╮╭───╮        of the others. That is where the sigmoid
        │ b ││ a │        oxygen dissociation curve comes from.
        ╰───╯╰───╯

   ══ DENATURATION ═════════════════════════════════════════════════════
     Heat >41 degC · extreme pH · heavy metals · alcohol · radiation
        break 2°, 3°, and 4° — the WEAK bonds
        leave 1° intact — the peptide bonds survive
     → shape lost → FUNCTION LOST. Usually irreversible in vivo.
        Egg white. Fever. Burns. Fixation of a histology specimen.
        And, deliberately: gastric acid denaturing dietary protein so
        that pepsin can reach the peptide bonds.

Denaturation

Denaturation is the loss of a protein's three-dimensional shape without breaking its peptide bonds. Because function depends entirely on shape — an active site is a shape, a receptor is a shape, a channel is a shape — denaturation abolishes function.

The agents are exactly the things that disrupt weak bonds: heat (which supplies enough kinetic energy to break hydrogen bonds), extreme pH (which adds or removes protons from acidic and basic side chains, destroying ionic bonds), heavy metals (which bind thiol groups), alcohols and detergents (which disrupt the hydrophobic core), and radiation.

The body exploits denaturation deliberately in one place: gastric acid at pH 1.5–3.5 unfolds dietary protein so that pepsin can reach peptide bonds that were buried inside the folded structure. Elsewhere, denaturation is damage.

What proteins do

Proteins are the most functionally diverse class of molecule in the body, and roughly 20% of body mass. Fibrous proteins — collagen (25–30% of all protein in the body), elastin, keratin — are structural, insoluble, and built from repetitive sequences. Globular proteins — enzymes, antibodies, hemoglobin, transporters — are compact, soluble, and functional.

Troponin — a protein that answers Chapter 3's question

Since Amara's troponin is the reason she is being admitted, it is worth meeting the molecule here, in chemical terms.

Troponin is not one protein but a complex of three, sitting at regular intervals along the thin filament of striated muscle: troponin T (which binds tropomyosin), troponin I (which inhibits the actin–myosin interaction at rest), and troponin C (which binds Ca²⁺). When calcium arrives, troponin C binds it, the complex changes shape, tropomyosin shifts, and myosin can engage actin. Contraction follows. Chapter 9 develops this in full.

Two chemical facts make the blood test possible.

First, troponin is intracellular and structural. It is bound to the contractile apparatus inside the cell. There is no secretory pathway for it and no reason for it ever to appear in plasma. Its presence in blood is therefore not a change in regulation — it is evidence that cells have physically broken open. Chapter 3 pursues that.

Second, the cardiac isoforms differ in primary sequence from the skeletal muscle isoforms. Cardiac troponin I carries an extra 31 amino acids at its N-terminus that no skeletal isoform has. That sequence difference produces a shape difference, and a shape difference is something an antibody can recognize (§2.6: specificity is shape complementarity). The assay uses two antibodies raised against cardiac-specific epitopes, so a value of 2.4 ng/mL means cardiac muscle, not the muscle she strained lifting a patient last week.

A difference in primary structure, translated into a difference in shape, translated into a diagnosis. That is the chain of reasoning this whole chapter has been building.

Nucleic acids

A nucleotide has three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. Polymerize them through phosphodiester bonds between the sugar of one and the phosphate of the next, and you have a nucleic acid.

Bases come in two shapes: purines (adenine, guanine) are double-ringed; pyrimidines (cytosine, thymine, uracil) are single-ringed. Pairing is always purine-to-pyrimidine, which keeps the double helix a constant width: A pairs with T through two hydrogen bonds, G pairs with C through three. That difference in bond count is why GC-rich DNA requires more energy to separate, and it is measurable as a higher melting temperature.

The packing problem is worth a number. A human diploid nucleus contains about 6 billion base pairs, roughly 2 metres of DNA, inside a nucleus about 6 µm across. The compaction ratio exceeds 100,000-fold, and achieving it without tangling — while keeping every gene accessible on demand — is the job of the histone proteins and chromatin structure that Chapter 3 introduces and Chapter 29 completes.

Finally, note that ATP is a nucleotide — adenine, ribose, and three phosphates rather than one. So are the electron carriers NAD⁺ and FAD, the second messenger cyclic AMP, and coenzyme A. The same chemical building block serves as the medium of information, the currency of energy, and the vocabulary of signalling. Evolution reuses what works.

   THE FOUR CLASSES OF BIOLOGICAL MACROMOLECULE, COMPARED
   ══════════════════════════════════════════════════════════════════════
                CARBOHYDRATE      LIPID          PROTEIN       NUCLEIC ACID
   ─────────────────────────────────────────────────────────────────────
   ELEMENTS     C H O            C H O          C H O N S     C H O N P
                (H:O = 2:1)      (little O)     (+ N and S!)  (+ P!)

   MONOMER      monosaccharide   (no true       amino acid    nucleotide
                glucose,          monomer)      20 kinds      = sugar +
                fructose,        glycerol +                     phosphate
                galactose        fatty acids                    + base

   BOND         glycosidic       ester          peptide       phospho-
                                 linkage                       diester

   BUILT BY     dehydration      dehydration    dehydration   dehydration
   BROKEN BY    hydrolysis       hydrolysis     hydrolysis    hydrolysis
                ── the SAME two reactions build and break ALL four ──

   ENERGY       4 kcal/g         9 kcal/g       4 kcal/g      not a fuel
   YIELD        fast access      dense, slow    last resort

   WATER        soluble          INSOLUBLE      varies by     soluble
   BEHAVIOUR    (polar OH        (nonpolar      surface R      (charged
                 groups)          C-H bonds)    groups         backbone)
                                  → needs a
                                    CARRIER

   MAIN JOBS    immediate fuel   long-term      EVERYTHING:   information
                glycogen store   fuel store     structure,    storage and
                cell-surface     membranes      catalysis,    transfer
                ID (glycocalyx)  hormones       transport,
                                 insulation     signalling,
                                 protection     immunity

   IN AMARA'S   glucose 212      LDL 168        troponin I    (the gene
   LAB REPORT   HbA1c 7.4%       HDL 38         2.4 ng/mL     variants
                                 TG 244         albumin       behind all
                                                (anion gap)   of it —
                                                              Ch. 29)

   FAILURE      hyperglycemia    atherosclerosis denaturation  mutation
   MODE         glycation/AGEs   membrane damage misfolding    (Ch. 29)
   ─────────────────────────────────────────────────────────────────────

Chapter Summary

§2.1 Matter is anything with mass occupying space; energy is the capacity to rearrange it. Four elements — oxygen, carbon, hydrogen, nitrogen — constitute 96.1% of body mass, though hydrogen is the most abundant atom. Trace elements matter out of all proportion to their mass because each sits at the active site of a protein that cannot function without it.

§2.2 Protons set the element, neutrons set the isotope, electrons set all chemical behaviour. Valence — the number of outer-shell electrons — predicts bonding: carbon four bonds, nitrogen three, oxygen two, hydrogen one. Radioisotopes are chemically indistinguishable from their stable forms, so the body distributes them normally while a detector tracks them; half-life sets the clinical compromise between delivery and dose.

§2.3 Electronegativity difference alone generates the whole bond continuum: below 0.4 nonpolar covalent, 0.4–1.8 polar covalent, above 1.8 ionic. Polar/nonpolar predicts solubility, transport, and excretion. Hydrogen bonds at one twentieth of covalent strength are the mechanism of biological specificity, because weak bonds in bulk are both selective and reversible. Electrolytes are reported in milliequivalents because the body balances charge, not mass.

§2.4 Water's bent, polar shape produces four properties with four consequences: it dissolves anything charged (so nonpolar molecules need carriers); its high heat capacity buffers body temperature; its high heat of vaporization makes evaporative sweating the only cooling route above 35 °C ambient; and its cohesion holds serous membranes together while creating the alveolar surface tension that surfactant must defeat.

§2.5 pH is the negative log of [H⁺]. Arterial blood is held at 7.35–7.45, or 35–45 nmol/L — the least abundant ion, most tightly defended. The bicarbonate system, CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, dominates extracellular buffering not because its pKa is well matched but because it is an open system with the lungs controlling one end in seconds and the kidneys the other in days. Henderson–Hasselbalch shows that pH depends on the 20:1 ratio, which is why compensation works.

§2.6 Reactions are synthesis, decomposition, exchange, or redox; dehydration synthesis and hydrolysis build and break every macromolecule. ATP couples exergonic to endergonic reactions and is a currency in constant circulation, not a store. Enzymes lower activation energy without altering equilibrium, are specific by shape, are regulable, and often require metal cofactors or vitamin-derived coenzymes.

§2.7 Carbohydrates are fuel and identification. Glycogen's branching exists to permit rapid mobilization. Enzymatic glycosylation is functional; non-enzymatic glycation is damage whose rate depends only on glucose concentration and time — which is what HbA1c measures, integrated over the 120-day red cell lifespan.

§2.8 Lipids are defined by insolubility. Fat stores 9 kcal/g because its carbons are reduced and it is stored anhydrous. Cis double bonds kink chains and keep membranes fluid. Amphipathic molecules self-assemble into micelles and bilayers driven by water's entropy, not by attraction. Lipoproteins solve the transport problem: LDL carries cholesterol outward and is atherogenic, HDL returns it to the liver, and LDL is usually calculated rather than measured.

§2.9 Proteins are amino acid polymers whose primary sequence dictates all higher structure. Secondary structure comes from backbone hydrogen bonds, tertiary from side-chain interactions dominated by hydrophobic packing, quaternary from subunit assembly. Denaturation destroys shape and function while sparing sequence. Nucleic acids store and transfer information through complementary base pairing, and the same nucleotide chemistry supplies ATP, NAD⁺, FAD, and cyclic AMP.

Case File 2 · Resolution

Question 1 — Why are electrolytes reported in mEq/L while glucose and cholesterol are in mg/dL, and why does the panel print "carbon dioxide" for a number about acid?

Because the units answer different questions (§2.3, §2.4).

Milliequivalents count charge. An equivalent is one mole of charge, so mEq/L = mmol/L × valence. Sodium, potassium, chloride, and bicarbonate are reported this way because their physiological jobs — generating membrane potentials, driving osmotic water movement, and maintaining electroneutrality — depend on how much charge is present, not how much mass. For monovalent ions the two numbers coincide: Amara's sodium is 138 mEq/L and 138 mmol/L. For calcium they do not: 9.2 mg/dL is 2.30 mmol/L but 4.60 mEq/L, because each ion carries two charges.

Milligrams per decilitre count mass, which is the right unit for glucose and cholesterol because those molecules are uncharged and act as substrates and cargo rather than as charge carriers. What matters is how many grams of fuel are in circulation.

The charge convention also produces the anion gap. Plasma must be electrically neutral, so Na⁺ − (Cl⁻ + HCO₃⁻) = 138 − (102 + 24) = 12 mEq/L is not a gap at all: it is the concentration of anions the panel does not measure, chiefly albumin and phosphate. When an unmeasured acid accumulates, its anion joins that pool and the gap widens. Amara's gap of 12 is normal, and that normality is a positive finding (§2.5).

As for "carbon dioxide": the analyte is not dissolved CO₂ gas but total CO₂ content, which is over 95% bicarbonate. The name is a historical artefact of how the measurement was originally made. Chemically it is the base member of the body's dominant buffer pair, and it belongs on the panel because CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ makes bicarbonate and acid two views of the same equilibrium. Reading her HCO₃⁻ of 24 alongside her pH of 7.38 and PCO₂ of 42 confirms the Henderson–Hasselbalch arithmetic exactly: 6.1 + log(24/1.26) = 7.38.

Question 2 — What does HbA1c actually measure, and why does it look backwards in time?

It measures glycated hemoglobin — hemoglobin molecules with a glucose permanently attached to the N-terminal valine of the β-chain (§2.7).

The reaction is non-enzymatic glycation, not enzymatic glycosylation. Glucose's reactive aldehyde group collides with a free amino group and forms a reversible Schiff base, which over hours undergoes an Amadori rearrangement into a stable ketoamine that cannot detach. No enzyme catalyzes it, nothing regulates it, and nothing reverses it. The rate depends on exactly two variables: glucose concentration and exposure time.

It looks backwards because of the carrier. A red blood cell lives about 120 days and has no nucleus, no ribosomes, and no capacity to replace its hemoglobin. Every glucose adduct acquired stays until the cell is destroyed. Measuring the percentage of hemoglobin bearing the adduct across the whole red cell population therefore integrates glucose exposure over the preceding two to three months, weighted toward the recent past — about half the value comes from the last 30 days.

Amara's 7.4% converts to an estimated average glucose of (28.7 × 7.4) − 46.7 = 166 mg/dL. So her reading of 212 this morning is not a stress response to a heart attack. It is roughly consistent with where she has been living for months, and it tells you her hyperglycemia predates today by a long way. That single inference is why the test exists.

And note what the same reaction does elsewhere: glycation of long-lived collagen, elastin, and lens crystallin produces advanced glycation end products that stiffen arteries and cloud lenses. The A1c is not just a number about sugar. It is a rate meter for a damage process running in every long-lived protein she has.

Question 3 — Cholesterol is one molecule. Why two numbers, and why does the LDL value exist without having been measured?

Because LDL and HDL are not kinds of cholesterol. They are transport particles carrying the same cholesterol in opposite directions (§2.8).

Cholesterol is nonpolar (§2.3) and cannot dissolve in plasma, so it travels inside lipoprotein particles: a nonpolar core, a phospholipid monolayer, and a surface apolipoprotein that acts as an address. LDL carries apoB-100 and delivers cholesterol from the liver to peripheral tissues. HDL carries apoA-I and performs reverse transport, collecting excess cholesterol from cells — including foam-cell macrophages in artery walls — and returning it to the liver, which is the only organ that can dispose of it, since no human enzyme degrades the steroid ring.

The opposite clinical meanings follow from the direction of travel. LDL particles are small enough to enter the subendothelial space of an artery, where they are retained, oxidized, and consumed by macrophage scavenger receptors that are not downregulated by cholesterol load. The macrophage keeps eating and dies as a foam cell. That deposit is the plaque. HDL is running the same traffic in reverse.

Her LDL of 168 was calculated, using the Friedewald equation:

LDL = TC − HDL − TG/5 = 255 − 38 − (244/5) = 255 − 38 − 48.8 = 168 mg/dL

The TG/5 term estimates the cholesterol carried by VLDL. It is a serviceable approximation below about 400 mg/dL of triglyceride and unreliable above it, which is why non-HDL cholesterol — 255 − 38 = 217 mg/dL — is often the better number: it assumes nothing and counts every atherogenic particle.

Finally, her TG/HDL ratio of 6.4 (desirable below 2) is a chemical signature of insulin resistance and predicts a shift toward small dense LDL particles, which enter arterial walls more readily than large buoyant ones. Read together with her glucose of 212 and HbA1c of 7.4%, the lipid panel is not an independent finding. It is the same metabolic disturbance viewed through a different molecule.

Systems Integration Case File · Entry 2

Review

Level 1 · Recall

2.1 Which four elements make up approximately 96% of body mass?

a) carbon, hydrogen, oxygen, calcium    b) oxygen, carbon, hydrogen, nitrogen    c) oxygen, carbon, calcium, phosphorus    d) hydrogen, oxygen, sodium, chlorine

2.2 An atom of carbon-14 differs from an atom of carbon-12 in its number of:

a) protons    b) electrons    c) neutrons    d) valence electrons

2.3 A bond in which electrons are shared but pulled toward one atom is:

a) ionic    b) nonpolar covalent    c) polar covalent    d) hydrogen

2.4 Which property of water is most directly responsible for the effectiveness of sweating?

a) high specific heat capacity    b) polarity    c) high heat of vaporization    d) cohesion

2.5 A patient's arterial pH is 7.20. Compared with a pH of 7.50, the hydrogen ion concentration is approximately:

a) 0.3 times as great    b) 2 times as great    c) 10 times as great    d) 100 times as great

2.6 Enzymes accelerate reactions by:

a) increasing the free energy released    b) lowering activation energy    c) shifting the equilibrium toward products    d) raising the temperature locally

2.7 HbA1c reflects average blood glucose over approximately the preceding:

a) 24 hours    b) 7 days    c) 2–3 months    d) 12 months

2.8 Which lipoprotein returns cholesterol from peripheral tissues to the liver?

a) chylomicron    b) VLDL    c) LDL    d) HDL

Level 2 · Comprehension

2.9 Explain why hydrogen bonds — twenty times weaker than covalent bonds — are responsible for the specificity of nearly every recognition event in the body.

2.10 The bicarbonate buffer has a pKa of 6.1, which is poorly matched to blood pH of 7.4. Explain why it is nevertheless the body's most important extracellular buffer.

2.11 Distinguish glycosylation from glycation, and explain why the distinction matters clinically.

2.12 Amara's calculated plasma osmolality is 294 mOsm/kg. Explain how it was calculated, which solute is responsible for it being at the top of the normal range, and one consequence.

Level 3 · Clinical Application

2.13 A patient with severe vomiting for three days has arterial pH 7.52, PCO₂ 48 mm Hg, and HCO₃⁻ 38 mEq/L. Explain each number chemically, identify the disorder, and state whether compensation is occurring.

2.14 A 62-year-old man on a statin develops muscle aching and a creatine kinase of 900 U/L. Explain the biochemistry of statins, one proposed mechanism for the muscle symptoms, and why a raised creatine kinase is analogous to Amara's raised troponin.

2.15 A marathon runner collapses at mile 22. She is confused, her core temperature is 40.8 °C, her skin is wet, and her serum sodium is 126 mEq/L. Analyse this using water chemistry.

Level 4 · Integration and Synthesis

2.16 Amara's HbA1c is 7.4%, her LDL is 168, her HDL is 38, and her triglycerides are 244. Construct the causal chain linking a single upstream defect to all four numbers and to the coronary event that brought her in. Identify at least one point where the chain feeds back on itself.

2.17 Argue for or against: "Water is the most important molecule in physiology, more so than DNA or ATP." Use at least four specific properties and at least two clinical examples.

Concept Map to Complete

Copy this onto blank paper and fill every bracket from memory before checking the chapter.

                    ELECTRONEGATIVITY DIFFERENCE
                               │
             ┌─────────────────┼─────────────────┐
        ΔEN < [ ___ ]     ΔEN [ ___ ]      ΔEN > [ ___ ]
        [ __________ ]    [ __________ ]   [ __________ ]
         COVALENT          COVALENT          BOND
             │                 │                 │
        molecule is       molecule is      dissociates in
        [ __________ ]    [ __________ ]   water into
             │                 │           [ __________ ]
        examples:         examples:        reported by the
        [ ____ ][ ____ ]  [ ____ ][ ____ ] lab in [ ______ ]
             │                 │
             └────────┬────────┘
                      │
              WATER is [polar/nonpolar]
                      │
       ┌──────────┬───┴──────┬──────────────┐
   property1   property2   property3    property4
   [ ______ ]  [ ______ ]  [ ______ ]   [ ______ ]
       │           │           │            │
   consequence consequence consequence  consequence
   [ ______ ]  [ ______ ]  [ ______ ]   [ ______ ]

                 THE BICARBONATE BUFFER
        [ ___ ] + H2O ⇌ [ _____ ] ⇌ [ __ ] + [ _____ ]
              ▲                              ▲
        set by the [ _____ ]          set by the [ _____ ]
        in [ seconds / days ]         in [ seconds / days ]

        pH = [ ___ ] + log ( [ _____ ] / 0.03 x [ _____ ] )
        normal ratio = [ ___ ] : 1

Lab / Self-Exploration

1. Watch the hydrophobic effect. Put a tablespoon of oil into a glass of water and stir vigorously. Time how long the droplets take to coalesce. Now add a drop of dish detergent — an amphipathic molecule — and stir again. Explain, using §2.8, why the emulsion now persists, and name the molecule in your own small intestine that does the same job.
2. Measure evaporative cooling. Wet the back of one hand and leave the other dry. Wave both in the air for thirty seconds. The temperature difference you feel is water's heat of vaporization, and it is roughly 580 kcal per litre evaporated. Predict what would happen to the effect in a steam-filled bathroom, then test it.
3. Build the pH scale from your kitchen. Using pH strips (inexpensive online), test lemon juice, vinegar, tap water, saliva, baking soda solution, and dish soap. Place each on the scale in Figure 2.4. Then test your saliva before and five minutes after eating something sweet, and explain what changed.
4. Denature a protein three ways. Take three portions of egg white. Heat one, add vinegar to the second, and add rubbing alcohol to the third. All three should turn opaque and solid. Explain what each agent disrupted, and state what was not disrupted in any of them.
5. Read a real label as chemistry. Take any packaged food and locate total fat, saturated fat, trans fat, total carbohydrate, sugars, fibre, and protein. Calculate the calories from each using 9 kcal/g for fat and 4 for carbohydrate and protein, and check your total against the printed one. Then explain why fibre is listed under carbohydrate but contributes almost no calories.
6. Reproduce Amara's arithmetic. Using her values, calculate (a) her anion gap, (b) her calculated osmolality, (c) her LDL by the Friedewald equation, (d) her estimated average glucose from HbA1c, and (e) her pH from Henderson–Hasselbalch. Check each against the chapter. Doing this once by hand is worth more than reading it three times.

Key Terms

Next: Chapter 3 · Cells — where the troponin in Amara's plasma is traced back to the inside of the cardiac muscle cells it escaped from, and where the membrane that was supposed to keep it there is built molecule by molecule.