Escherichia coli cells are about \(2 \mu \mathrm{m}\) (microns) long and \(0.8
\mu \mathrm{m}\) in diameter.
a. How many \(E\). coli cells laid end to end would fit across the diameter of a
pinhead? (Assume a pinhead diameter of \(0.5 \mathrm{mm}\).)
b. What is the volume of an \(E\). coli cell? (Assume it is a cylinder, with the
volume of a cylinder given by \(V=\pi r^{2} h,\) where \(\pi=3.14 .\)
c. What is the surface area of an \(E\). colicell? What is the surface-to volume
ratio of an \(E .\) colicell?
d. Glucose, a major energy-yielding nutrient, is present in bacterial cells at
a concentration of about \(1 \mathrm{m} M\). What is the concentration of
glucose, expressed as \(\mathrm{mg} / \mathrm{mL}\) ? How many glucose molecules
are contained in a typical \(E\) coli cell? (Recall that Avogadro's number
\(=6.023 \times 10^{23}\).)
e. A number of regulatory proteins are present in \(E\). coli at only one or two
molecules per cell. If we assume that an \(E\). colicell contains just one
molecule of a particular protein, what is the molar concentration of this
protein in the cell? If the molecular weight of this protein is \(40
\mathrm{kD},\) what is its concentration, expressed as \(\mathrm{mg} /
\mathrm{mL} ?\)
f. \(\operatorname{An} E .\) coli cell contains about 15,000 ribosomes, which
carry out protein synthesis. Assuming ribosomes are spherical and have a
diameter of \(20 \mathrm{nm}\) (nanometers), what fraction of the \(E .\) colicell
volume is occupied by ribosomes?
g. The \(E\) coli chromosome is a single DNA molecule whose mass is about \(3
\times 10^{9}\) daltons. This macromolecule is actually a linear array of
nucleotide pairs. The average molecular weight of a nucleotide pair is \(660,\)
and each pair imparts \(0.34 \mathrm{nm}\) to the length of the DNA molecule.
What is the total length of the
E. coli chromosome? How does this length compare with the overall dimensions
of an \(E\). coli cell? How many nucleotide pairs does this DNA contain? The
average \(E\). coli protein is a linear chain of 360 amino acids. If three
nucleotide pairs in a gene encode one amino acid in a protein, how many
different proteins can the
E. coli chromosome encode? (The answer to this question is a reasonable
approximation of the maximum number of different kinds of proteins that can be
expected in bacteria.)