Introduction to Metabolism and Homeostasis Practice Assignment

1. The hypothalamus in the human brain is involved in thermoregulation of the body. This organ has been compared to a thermostat that uses a negative feedback mechanism to control the heating and cooling in a house. Explain whether or not this comparison is accurate. Justify your response.

2. An enzyme was isolated from wild-type bacteria (WT). Three mutant forms of this same enzyme were made using a mutagenesis technique, and then each mutant enzyme (1E8, 5F8, D249G) was isolated. All four samples of purified enzymes were tested for thermal stability. To do this, each enzyme was incubated at the temperatures indicated on the graph for 15 minutes and then tested for the percentage of original catalytic activity that remained after the heat treatment. Describe the effects of the mutations on the thermal stability of the wild-type enzyme. Analyze the data to rank the four in order from least heat-stable to most heat-stable.

3. A company that grows and sells fresh pears is concerned because workers notice that pears placed in storage seem to ripen slowly at first, but then suddenly display extremely rapid ripening. Perplexed, company officials call in two independent science teams to evaluate the problem.

Team A makes the statement: "Your pears are under positive feedback control." Team B makes the statement: "Ethylene amplifies the fruit-ripening process by triggering both its own production in fruit and the production of enzymes catalyzing the breakdown of cell walls."

Both science teams made statements that help explain the observed acceleration of pear ripening in storage. The pears are ripening because ethylene builds up, which signals an increase in cell-wall breakdown, a reaction that is part of the ripening process. Ripening goes slowly at first because there is little ethylene present, but because it is under positive feedback control, a little ethylene induces more and more ethylene production as time goes on. This means that ethylene concentrations amplify very quickly, with the result that ripening is also amplified very quickly.

A representative of the pear company explains that pears are typically stored in small, tightly sealed storage bins just after harvesting. He asks whether the ripening process would be different if the same number of pears were stored in a much larger room with better ventilation. Predict whether these two scenarios can be expected to yield the same results, and explain your reasoning.

4. Bone is a dynamic tissue constantly undergoing change. As shown in the diagram below, two sets of bone cells work in opposition to one another to maintain bone homeostasis. Osteoblasts build bone, while osteoclasts break down bone. Blood calcium levels change in response to these cell activities.

a. Calcitonin is a hormone produced by the thyroid gland. Calcitonin is released whenever blood calcium levels go up. Calcitonin travels through the blood to bone, where it activates one of the two types of bone cells and inhibits the other type. Predict the type of cells that will be activated by calcitonin. Predict the type of cells that will be inhibited by calcitonin. Justify your responses.

b. Refine the diagram to show the regulatory mechanism that triggers the changes in bone-cell activity described in part a of the question.

5. Sawflies are insects that lay eggs inside of plant tissue. The eggs hatch into larvae and feed on the plant before pupating and transforming into adult flies. Sawflies target pine trees in particular, and their larvae can destroy healthy trees in a short period of time. In defense, pine trees produce volatile organic compounds that attract predatory wasps that feed on either eggs or the larvae.

Researchers studied the induction of mRNA transcripts for a gene involved in synthesis of a volatile organic compound by pine trees in response to egg-laying sawflies. The data below show one control group and four experimental groups. The control (C) consisted of pine branches with no eggs. E3 were branches laden with eggs tested 3 days after egg-laying. E14 were branches laden with eggs tested 14 days after egg-laying, just when larvae hatch. E+L were branches laden with eggs that hatched and were fed on by larvae for two days.

a. Analyze the data in the graph to determine how the timing of the pine tree's production of a defensive volatile organic compound relates to the life cycle of the sawfly larvae. Explain the mechanism of evolution of this process.

b. Draw a diagram to describe how information moves among the three organisms discussed in this example. In your diagram, show the internal changes taking place in the pine-tree cells as they receive information from one organism, respond to that information, and then transmit other information to the second organism.

6. The figure below shows the experimental setup used to test female cichlid fish's mating preferences. The barriers separating the central tank from the end tanks are transparent in one direction but opaque in the other, so that the female in the central tank can observe one male in each of the tanks on the ends, but the males cannot observe the other male. In 16 out of 20 repeated trials, females preferentially produced eggs in the spawning nests nearer the larger male.

Describe how information moves between organisms in this example. Explain what the results of this experiment suggest about differential reproductive success in cichlids and how it relates to evolution.