What Is Life? The Chemistry of Living Things

Built From Four Families

Life is assembled from four classes of macromolecule, each a polymer of small repeating units.

Every one of these bonds is made by condensation, releasing one water molecule per bond, and broken by hydrolysis, consuming one. So joining $n$ monomers into a single chain forms $n-1$ bonds and releases $n-1$ waters, a small piece of arithmetic that recurs throughout biochemistry.

Proteins are the workhorses, and their function follows their shape through four levels: the primary sequence of amino acids, secondary helices and sheets held by hydrogen bonds, the tertiary three-dimensional fold, and the quaternary assembly of several chains. Denature a protein and the sequence survives while the function does not, which is exactly why the higher levels matter.

Enzymes are catalytic proteins that lower a reaction's activation energy without being consumed. They do not change the equilibrium position, only the speed of arrival, the same rule that governs any catalyst.

Membranes are lipid bilayers, and they form spontaneously because phospholipids are amphipathic: a hydrophilic phosphate head and two hydrophobic fatty-acid tails. No machinery assembles them; water does, by excluding the tails.

Finally, a constraint that shapes all of cell biology. A sphere's surface area goes as $r^2$ while its volume goes as $r^3$, so

$$\frac{SA}{V} = \frac{3}{r}$$

Doubling the radius halves the surface available per unit of contents. Since everything a cell needs must cross that surface, cells stay small, and large organisms are built from many small cells rather than from few large ones.

Common pitfall: thinking a big cell simply needs a bigger membrane. It has one, but volume outgrows area, so the supply per unit of demand keeps falling. This is why the answer to being large is more cells, not bigger ones.