The Dimensions of Prokaryotic Cells and Their ConstituentsEscherichia coli cells are about...

The Dimensions of Prokaryotic Cells and Their Constituents

Escherichia coli cells are about 2 µm (microns) long and 0.8 µm 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 mm.)

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b. What is the volume of an E. coli cell? (Assume it is a cylinder, with the volume of a cylinder given by V=πr2h, where π= 3.14.)

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c. What is the surface area of an E. coli cell? What is the surface-to-volume ratio of an E. coli cell?

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d. Glucose, a major energy-yielding nutrient, is present in bacterial cells at a concentration of about 1 mM. What is the concentration of glucose, expressed as mg/ml? How many glucose molecules are contained in a typical E. coli cell? (Recall that Avogadro’s number = 6.023 x 1023.)

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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. coli 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 kD, what is its concentration, expressed as mg/ml?

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f. An E. coli cell contains about 15,000 ribosomes, which carry out protein synthesis. Assuming ribosomes are spherical and have a diameter of 20 nm (nanometers), what fraction of the E. coli cell volume is occupied by ribosomes?

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g. The E. coli chromosome is a single DNA molecule whose mass is about 3.0 x 109 daltons. This macromolecule is actually a circular array of nucleotide pairs. The average molecular weight of a nucleotide pair is 660 and each pair imparts 0.34 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.)