Biology 107 (Dr. Harrington) Problem set 2 answers

Biology 107 B2 - Answers to Problem set 2

Question 1 (exam) Woese divided life into three domains: Archaea, _____ , and _____.

A. Bacteria ... Eukarya

Question 2 (thinking) How could you determine experimentally if the Archaean Methanobrevibacter oralis is a cause of dental root disease and not just a consequence?

The best way to answer this question is to consider Koch's postulates (Robert Koch, 1843-1910). They state that in order for a microorganism to be considered to be the cause of a disease:

(1) the organism must be found in all animals with the disease
(2) the organism must be isolated from a diseased animal and grown in a laboratory culture
(3) the cultured organism must cause the disease when introduced into a healthy animal
(4) the organism must be reisolated from the infected animal

Question 3 (thinking) Recall that the hemoglobin protein is made of two α-Globin and two β-Globin polypeptides. The human β-Globin is 146 amino acids long. The gorilla version is also 146 aa long and there is only a single aa difference. There are 8 differences between the human and rhesus monkey β-Globins, 27 differences between the human and mouse β-Globins, and 67 differences between the human and the frog β-Globins. Make a diagram showing the evolutionary relationships between these five species.

Question 4 (exam) Arabidopsis thaliana, a small well-studied plant, responds to cool temperatures by synthesizing FAD8 proteins. FAD8 is a fatty acid desaturase enzyme which converts certain C-C bonds in free fatty acids into C=C bonds. Why is this enzyme made when the plants are in cold environments?

By changing the pool of free fatty acids from mostly saturated to mostly unsaturated the plant ensures that the phospholipids being made will be better suited for the low temperature e

Question 5 (exam) A human red blood cell would _____ if placed in tap water because this is a/an _____ environment.

C. swell ... hypotonic environment.

Question 6 (exam) Muscle cells consume molecular oxygen (O₂) as they synthesize ATP.

a) How does the oxygen enter the cell? Why is this mechanism used in this situation?

The oxygen enters by diffusion through the phospholipid bilayer. This mechanism works because the oxygen molecules are (i) small enough to diffuse through the phospholipid bilayer and (ii) moving down their concentration gradient.

b) An oxygen molecule has just entered a muscle cell. What, if anything, prevents it from leaving the cell?

Nothing prevents it from leaving. Diffusion of oxygen involves a net movement from a region of high oxygen concentration to low oxygen concentration - it does not refer to what any individual molecule will do. Ultimately the oxygen molecule will be consumed in the muscle cell during the process of cellular respiration.