Chemistry of life

A USEFUL CHAIN FOR STRUCTURE-TO-FUNCTION EXPLANATIONS

Five of the nine problems below are the same argument wearing different nouns. Learn the shape once and most of the unit comes with it:

When a prompt asks for an explanation, naming the two endpoints leaves the mechanism implicit. The arrows make the causal claim complete.

ONE CAUSE, FIVE CONSEQUENCES: HYDROGEN BONDING

Oxygen is more electronegative than hydrogen — it holds the electrons of each shared O–H pair more tightly. No electron has been given away; the charge is only unevenly distributed. That is what polar means, and everything below follows from it.

That dashed line is the whole unit. It breaks and re-forms constantly, and nothing in the list below involves breaking a covalent bond. Every property here is what happens when an enormous number of those weak attractions act at once:

• Cohesion — water sticks to water: unbroken columns, droplets, transpiration streams.
• Adhesion — water sticks to other polar surfaces. Capillary action is the two together.
• Surface tension — surface molecules have no water above them, so their bonds pull sideways and down, and the surface behaves like a skin under tension.
• High specific heat — a thermometer reports the average kinetic energy of the molecules, and energy spent disrupting hydrogen-bond interactions during heating is energy not spent speeding molecules up. During cooling, formation of those interactions releases energy as molecular motion is lost. Water therefore changes temperature slowly in either direction. It buffers climate, and cytoplasm buffers the cell.
• High heat of vaporization — escaping into the gas phase requires enough energy to overcome the attractions holding a surface molecule in the liquid. Higher-energy surface molecules are more likely to escape; they carry energy away, lowering the average kinetic energy of what remains. That is evaporative cooling: sweat, panting, transpiration.

Also: water is the solvent for polar and ionic substances, and ice floats because hydrogen bonds lock into an open lattice.

THE FOUR CLASSES, AND HOW POLYMERS ARE BUILT AND BROKEN

All four classes commonly contain C, H, and O, so elemental composition narrows identity but does not determine it. Sulfur supports a protein identification; phosphorus supports either a phospholipid or nucleic acid; nitrogen supports a nucleic acid and is also common in proteins. Finish the identification with structural evidence — the subunits, linkage, polarity, or function. Lipids themselves are grouped by shared hydrophobic behavior rather than one repeating monomer structure.

Fats store long-term energy and may insulate. Steroid hormones regulate growth, development, metabolism, and homeostasis; cholesterol contributes structural stability to animal cell membranes; amphipathic phospholipids assemble into membrane bilayers.

Building and breaking. The departing H and OH form water, and the reaction runs both ways:

The names are the events. Dehydration, water removed; synthesis, something built. Hydro, water; lysis, splitting. Digestion is hydrolysis, from the amylase in your saliva onward.

The counting rule, and why it is $n-1$. In one connected, acyclic chain or branched tree, the first monomer formed no linkage by being there. Each later monomer joined through one new linkage. So $n$ monomers means $n-1$ linkages — therefore $n-1$ waters released while building it, and $n-1$ waters consumed while hydrolysing those linkages. A ring or an additional crosslink adds another linkage without another monomer, so inspect the topology before applying the shortcut.

What that does to the mass. For the dehydration-built linkages modeled here, each linkage released the equivalent of one water, so the polymer weighs less than the monomers you began with: For $n$ identical monomers in the stated acyclic structure,

For a heteropolymer, replace the first term with the sum of the individual monomer molar masses. Nothing was destroyed — the missing mass is sitting in the beaker as water. Use that as a check: if a polymer mass comes out larger than the sum of its monomers, you added the water instead of subtracting it.

PROTEIN STRUCTURE: FOUR LEVELS, AND WHAT HOLDS EACH ONE

• Primary — the amino acid sequence, held by peptide bonds (covalent). Constrains how the chain can fold.
• Secondary — $\alpha$-helices and $\beta$-pleated sheets, defined by hydrogen bonds between backbone atoms: the C=O of one residue and the N–H of another. R groups are not the bonds that define this level, though side chains can influence which conformations a sequence favors.
• Tertiary — the whole chain folded into one three-dimensional shape, held by interactions between R groups of residues that may sit far apart in the sequence. Four common classes used here depend on R-group chemistry and local conditions (geometry, solvent, pH, ions, and neighboring residues): hydrogen bonds between polar R groups; ionic bonds between an R group carrying a negative charge and one carrying a positive charge; hydrophobic clustering, where nonpolar R groups cluster away from water; and disulfide bridges, genuine covalent S–S bonds between two cysteines. Three weak, one covalent — that split helps predict which interactions are more vulnerable under the stated conditions.
• Quaternary — two or more separate polypeptides assembled through the same types of interactions that stabilize tertiary structure. A single-chain protein has none.

The common amino-acid plan. Each amino acid has a central carbon bonded to a hydrogen, an amine group, a carboxyl group, and a variable R group. A peptide bond joins the carboxyl group of one amino acid to the amine group of the next. The R group supplies chemical variation that helps determine tertiary interactions; classify its relevant property rather than memorizing twenty structures.

Methionine also contains sulfur, but it lacks the thiol pair used in the usual cysteine S–S bridge.

pH can change the protonation of ionizable groups. AP prompts most often foreground acidic carboxylates and basic side chains: adding $H^{+}$ can protonate a carboxylate and remove its negative charge; high pH can remove a proton from a positive group. Cysteine’s thiol is also ionizable, so do not turn the common AP examples into an exhaustive rule.

Denaturation follows directly from that split. Heat adds kinetic energy: the chain flexes and vibrates more strongly, making weak noncovalent arrangements less stable. A change in pH works by a different route to the same place: it adds or removes protons from charged R groups, so the charges that were attracting one another stop existing. Either way, it is the weak interactions that are more readily disrupted. The covalent peptide bonds and, under many ordinary denaturing treatments, disulfide bridges are more likely to persist.

As enough stabilizing interactions are disrupted, secondary, tertiary, and, where present, quaternary organization can change while the primary sequence remains. Because a protein’s job depends on a particular three-dimensional arrangement — for example, an enzyme pocket whose geometry and chemistry favor a substrate or family of substrates — a shape change can reduce function. Active-site residues often sit far apart in the sequence and are brought together by folding, which is why unfolding can disrupt catalysis. Under ordinary denaturing conditions, loss of shape need not mean hydrolysis of the peptide backbone.

NUCLEIC ACIDS: DIRECTIONALITY IS CHEMISTRY, NOT NOTATION

A nucleotide is a phosphate, a five-carbon sugar, and a nitrogenous base. The sugar’s carbons are numbered $1^{\prime }$ through $5^{\prime }$; the phosphate hangs off carbon $5^{\prime }$, and a free hydroxyl sits on carbon $3^{\prime }$. A phosphodiester bond joins nucleotides in the sugar–phosphate backbone. Standard polymerases extend a strand by using its free $3^{\prime }$-OH to react with the activated $5^{\prime }$ phosphate of an incoming nucleotide. Biological information is encoded in the order of the nucleotide bases.

So standard synthesis proceeds $5^{\prime }\to 3^{\prime }$ by extending the $3^{\prime }$ end. That is a property of the polymerase reaction, not a claim that a $5^{\prime }$ end can participate in no chemistry. Antiparallel means the two strands of a duplex run in opposite directions.

Pairing is A–T (2 hydrogen bonds) and G–C (3), with a purine (two rings: A, G) always opposite a pyrimidine (one ring: C, T, U) — so every rung of the ladder is the same width and the helix neither bulges nor pinches. The bond count has a consequence worth carrying: because a G–C pair is held by three hydrogen bonds and an A–T pair by only two, otherwise comparable high-G+C DNA generally requires more energy to separate. Base-stacking interactions also contribute to duplex stability, so hydrogen-bond count is not the only physical term. DNA vs. RNA: deoxyribose vs. ribose, thymine vs. uracil, double- vs. usually single-stranded.

THE QUANTITATIVE KIT — REFERENCE-SHEET BRIDGES

Surface area and volume (given on the reference sheet): cube $\mathrm{SA}=6s^2$, $V=s^3$; sphere $\mathrm{SA}=4\pi r^2$, $V=\frac{4}{3}\pi r^3$; rectangular solid $\mathrm{SA}=2\mathrm{lh}+2\mathrm{lw}+2\mathrm{wh}$, $V=\mathrm{lwh}$. Divide and the variables mostly cancel: a cube gives $\mathrm{SA}:V=6s^2∕s^3=6∕s$ and a sphere gives $4\pi r^2∕\frac{4}{3}\pi r^3=3∕r$. Notice that the units do not fully cancel — $\mu {\mathrm{\text{m}}}^2∕\mu {\mathrm{\text{m}}}^3=\mu {\mathrm{\text{m}}}^{-1}$ — so the ratio carries units of reciprocal length. It is not a pure number, and it is meaningless unless the area and the volume were measured in the same length unit to begin with.

Molarity and dilution (not on the sheet — memorize):

Water potential (given on the reference sheet): $\mathrm{\Psi }={\mathrm{\Psi }}_p+{\mathrm{\Psi }}_s$ and ${\mathrm{\Psi }}_s=-\mathrm{iCRT}$.

What $\mathrm{\Psi }$ actually is, because the formula is unusable without it: water potential compares water’s tendency to move. Water runs downhill in $\mathrm{\Psi }$ the way a ball runs downhill in height, and net movement stops when connected regions reach equal potential. Pure water in an open, unpressurized container is defined as zero. Dissolved solute lowers the solute term below zero, while positive pressure can raise total $\mathrm{\Psi }$ above zero. Negative values are therefore common, not automatic errors.

• $\mathrm{\Psi }$ — water potential, in bars. Pure water in an open, unpressurized container is $\mathrm{\Psi }=0$.
• ${\mathrm{\Psi }}_p$ — pressure potential. Zero in an open container (the sheet says so in print); positive in a turgid walled cell.
• ${\mathrm{\Psi }}_s$ — solute potential. Always $\le 0$: adding solute only ever lowers water potential.
• $i$ — ionization constant: $1$ for sucrose and glucose, $2$ for NaCl, $3$ for $\mathrm{CaC}{l}_2$. Count the ions.
• $C$ — mol/L. $R=0.0831$ L$\cdot$bar/(mol$\cdot$K). $T$ — kelvin ${=}^{\circ }$C $+273$.

Units check: $\left(\mathrm{\text{mol∕L}}\right)\left(L\cdot \mathrm{bar}\cdot \mathrm{mo}{l}^{-1}{K}^{-1}\right)\left(\mathrm{\text{K}}\right)=\mathrm{\text{bar}}$. The direction rule: water moves from higher $\mathrm{\Psi }$ to lower $\mathrm{\Psi }$. Compare signed values, never magnitudes; when pressure potential is positive, the lower value need not be negative.