2.1. Cell physiology[[Student version, December 8, 2002]] 35
Figure 2.5: (Electron micrograph.) A prokaryotic cell,E. coli. Cw, cell wall; N, nucleoid; R, ribosomes. The
plasma membrane lies just under the cell wall. [From (Wolfe, 1985).] [Copyrighted figure; permission pending.]
Membrane-bounded structures The simplest and most ancient type of cells are theprokary-
otes,including the familiarbacteria(Figures 2.3b and 2.5).^2 Typically about one micrometer in
length, the gross anatomy of a bacterium consists mainly of a thick, rigidcell wall,creating a
single interior compartment. The wall may be studded with a variety of structures, such as one or
severalflagella,long appendages used for swimming (Chapter 5). Just inside the wall lies a thin
plasma membrane.The material inside the plasma membrane sometimes shows a poorly defined
division into an interior “nucleoid” and the rest, but this region has no sharp edge and indeed has
no bounding membrane.
Plants, fungi, and animals are collectively calledeukaryotes;they consist of “eukaryotic cells.”
Bakers’ yeast, orSaccharomyces cerevisiæ,is a simple example of a eukaryotic cell (Figure 2.6).
Eukaryotic cells are bigger than prokaryotes, typically 10μmor more in diameter. They too are
bounded by a plasma membrane, though the cell wall may either be absent (in animal cells) or
present (in plants and fungi). They contain various well-defined internal compartments (examples
oforganelles), each bounded by one or more membranes roughly similar to the plasma membrane.^3
In particular, eukaryotic cells are defined by the presence of a nucleus. The nucleus contains the
material that condenses into visible chromosomes during cell division (Section 3.3.2); the rest of
the cell’s contents is collectively called thecytoplasm. During this process the nucleus itself may
lose its definition, then re-form after the division is complete.
In addition to a nucleus, eukaryotic cells containmitochondria,sausage-shaped organelles about
1 μmwide (Figure 2.7). The mitochondria carry out the final stages of the metabolism of food and
the conversion of its chemical energy into molecules of ATP, the internal energy currency of the cell
(^2) Since prokaryotes were originally defined only by the absence of a well-defined nucleus, it took some time to
realize that they actually consist of two distinct kingdoms, the “bacteria” (including the familiar human pathogens)
and the “archæa” (including many of those found in environments with extreme acidity, salt, or high temperature).
(^3) One definition of organelle is a discrete structure or subcompartment of a cell specialized to carry out a particular
function.