Page 211 - Color Atlas of Biochemistry

P. 211

202 Organelles

tors, 30 aminoacyl–tRNA synthases, 340
Cell components and cytoplasm tRNA molecules, 2–3 mRNAs (each of which
is 10 times the length of the section shown),
The Gram-negative bacterium Escherichia coli and six molecules of RNA polymerase.
(E. coli) is a usually harmless symbiont in the • About 330 other enzyme molecules, includ-
intestine of mammals. The structure and char- ing 130 glycolytic enzymes and 100 en-
acteristics of this organism have been partic- zymes from the tricarboxylic acid cycle.
ularly well characterized. E. coli is also fre- • 30000 small organic molecules with
quently used in genetic engineering (see masses of 100–1000 Da—e. g., metabolites
p. 258). of the intermediary metabolism and coen-
zymes. These are shown at a magnification
10 times higher in the bottom right corner.
A. Components of a bacterial cell • And finally, 50000 inorganic ions. The rest
Asingle E. coli cell has a volume of about consists of water.
3
0.88 µm . One-sixth of this consists of mem- The illustration shows that the cytoplasm of
branes and one-sixth is DNA (known as the cells is a compartment densely packed with
“nucleoid”). The rest of the internal space of macromolecules and smaller organic mole-
the cell is known as cytoplasm (not “cytosol”; cules. The distances between organic mole-
see p. 198). cules are small. They are only separated by a
The main component of E. coli—as in all few water molecules.
cells—is water (70%). The other components All of the molecules are in motion. Due to
are macromolecules (proteins, nucleic acids, constant collisions, however, they do not ad-
polysaccharides), small organic molecules, vance in a straight path but move in zigzags.
and inorganic ions. The majority of the macro- Dueto their largemass, proteins areparticu-
molecules are proteins, which represent ca. larly slow. However, they do cover an average
55% ofthe dry mass ofthe cell. When anum- of 5 nm in 1 ms—a distance approximately
ber of assumptions are made about the dis- equal to their own length. Statistically, a pro-
tribution and size (average mass 40 kDa) of tein is capable of reaching any point in a
proteins, it can be estimated that there are bacterial cell in less than a second.
approximately 250000 protein molecules in
the cytoplasm of an E. coli cell. In eukaryotic
cells, which are about a thousand times larger, C. Biochemical functions of the cytoplasm
it is estimated that the number of protein In eukaryotes, the cytoplasm, representing
molecules is in the order of several billion. slightlymorethan 50% of thecellvolume, is
the most important cellular compartment. It
is the central reaction space of the cell. This is
B. Looking inside a bacterial cell
where many important pathways of the inter-
The illustration shows a schematic view in- mediary metabolism take place—e. g., glycol-
side the cytoplasm of E. coli, magnified ap- ysis, the pentose phosphate pathway, the ma-
proximately one million times. At this magni- jority of gluconeogenesis, and fatty acid syn-
fication, a single carbon atom would be the thesis. Protein biosynthesis (translation; see
size of a grain of salt, and an ATP molecule p. 250) also takes place in the cytoplasm. By
would be as large as a grain of rice. The detail contrast, fatty acid degradation, the tricarbox-
shown is 100 nm long, corresponding to ylic acid cycle, and oxidative phosphorylation
about 1/600th of the volume of a cell in E. arelocated in themitochondria (see p. 210).
coli. To make the macromolecules clearer,
small molecules such as water, cofactors,
and metabolites have all been omitted from
the illustration. The section of the cytoplasm
shown contains:
• Several hundred macromolecules,which
are needed for protein biosynthesis—i. e.,
30 ribosomes, more than 100 protein fac-

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