14.3 Metabolism

177

with soluble enzymes had already become established. Most proteins are attached to

membranes, and the cytosol is a highly crowded, viscous hydrogel. ⁸

Lipid membranes occupy a very important place in the cell. Most of the organelles

are membrane-bounded, and their surfaces are the sites of most enzyme activity.

Chloroplasts are virtually filled with internal membranes. Curiously, the most promi-

nent membrane of all, the one which surrounds the cell, has, even today, a rather

obscure function; for example, it is often asserted that it is needed to control ion fluxes

into and out of the cell but, experimentally, potassium flux is much less affected by

the removal of the membrane than one might suppose. ⁹

Although prokaryotes (which are typically much smaller than eukaryotes) lack

most of the internal membrane-based structure seen in eukaryotes, they are still

highly heterogeneous in terms of the nonuniform distributions of components, from

macromolecules down to small ions.

If they are tagged (by synthesizing them with unusual isotopes, or attaching a flu-

orescent label or a nanoparticle), individual molecules, or small groups of molecules,

can be localized in the cell, by spatially resolved secondary ion mass spectrometry

(SIMS), fluorescence microscopy, and so forth. These measurements can usually

be carried out with fair time resolution (milliseconds to seconds); hence, both local

concentrations and fluxes of the tagged molecules can be determined.

Spontaneous assembly. Take the isolated constituents (e.g., head, neck, and legs)

of a phage virus and mix them together, and a functional virus will result. ¹⁰ This

exercise cannot be repeated successfully with larger, more complex structures closer

to that state we call “living”. Nor does it work if we break down the phage constituents

into individual molecules.

14.3

Metabolism

The fundamental purpose of metabolism is to provide energy for survival (think-

ing, mobility, and repair) and components for growth (including the production of

offspring). It may be defined as the set of chemical reactions needed to maintain life—

to grow, reproduce, repair, and respond (adapt). Traditionally, it is subdivided into

catabolism, concerned with breaking large, usually polymeric, molecules imported

as food from the external world into the cell down into monomers and submonomeric

components in order to provide energy, and anabolism, concerned with building up

large molecules and supramolecular structures. Metabolism is largely carried on by

enzymes and coenzymes, the latter being molecules auxiliary to enzyme action that

transfer chemical functional groups (e.g., NADSuperscript plus⁺/NADH).

Digestion is typically carried out extracellularly (in the mouth, stomach, and

gut—the gastro-intestinal tract) and breaks macromolecular food (proteins, polysac-

8 See Ellis (2001).

9 Solomon (1960).

10 See Kellenberger (1972) for a review.