14.2 Mitochondria

175

ically crushing it in a cylinder in which a tightly fitting piston moves); the results

obtained from fractionating such homogenates give a quite misleading impression

of the constitution of a living cell.

The cell membrane (also called “plasma membrane” or “plasmalemma”), often

described as a robust and fairly impermeable coating around the cytoplasm, has a

function that, strictly speaking, remains somewhat mysterious, since modern, and not

so modern, research has shown that cells remain viable even when their membranes

are significantly disrupted. The image of a cell as a toy balloon filled with salt

solution, which would immediately spurt out if the balloon were punctured, is not in

agreement with the experimental facts. ²

The Structure of a Cell

The two great divisions of cell types are the prokaryotes (bacteria and archaea) and

the eukaryotes (protozoa, fungi, plants, and animals) (cf. Table 5.1). As the name

suggests, the eukaryotes possess a definite nucleus containing the genetic material

(DNA), which is separated from the rest of the cell by a lipid-based membrane,

whereas the prokaryotes do not have this internal compartmentation. Moreover, the

eukaryotes possess other internal compartments known as organelles: the mitochon-

dria, sites of oxidative reactions where food is metabolized; chloroplasts (only in

plants), sites of photosynthesis; lysosomes, sacs of digestive enzymes for decom-

posing large molecules; the endoplasmic reticulum, a highly folded and convoluted

lipid membrane structure to which the ribosomes (RNA–protein complexes responsi-

ble for protein synthesis from mRNA templates ³) are attached, and contiguous with

the Golgi body, is responsible for other membrane operations such as packaging

proteins for excretion to outside the cell; and so on.

14.2

Mitochondria

The mitochondria and chloroplasts possess their own DNA, which codes for some,

but not all of their proteins; they are believed to be vestiges of formerly symbi-

otic prokaryotes living within the larger eukaryotes. The present interrelationship

between cell and mitochondrion is highly convoluted. The yeast mitochondrion, for

example, has about 750 proteins, of which only 8 are templated by the mitochondrial

genome, the remainder coming from the principal genome of the cell.

2 See Kellermayer et al. (1986).

3 See Yamano (2011) for a description of the structure, and Bernstein and Allis (2005) for a review

of the interaction between RNA and chromatin.