AP Biology: Notes and PracticeThe Endocrine System and Feedback Mechanisms

Model 1For a cell to regulate its activities, it uses transcription factors to turn genes on or off, creating proteins to respond to its needs. But how does an organism, made up of billions of cells, regulate itself system-wide? If an organism is too hot, or needs to digest a large meal, or dehydrated, how does it coordinate the actions of billions of cells to ultimately benefit the organism as a whole? The answer to this and other biological processes is hormones, molecules secreted into blood that travel to cells to communicate messages. Each hormone targets a specific receptor, only signaling cells that are needed for the desired response.

This process of chemical signaling is a function of the endocrine system, a collection of glands and cells that secrete hormones into the surrounding fluid to communicate with other cells of the body. The endocrine system works closely with the nervous system, another system responsible for communication within an organism.

The major glands and tissues of the endocrine system are displayed below.

Figure 1

AP Biology: Notes and Practice1. Refer to the pathway to the right, demonstrating the

response to acidic conditions in the small intestine:a. In figure 1, explain why “low pH in the duodenum”

is labeled as the “Stimulus.”

b. In figure 1, explain why “bicarbonate release” is labeled as the “Response.”

Figure 1

c. What is the role of secretin in the pathway displayed in Figure 1? Explain your reasoning.

d. On the way to the target cells, the hormone secretin moves into blood vessels that pass by the kidney. Propose an explanation for what happens when secretin comes in contact with kidney cells.

e. Secretin triggers the release of bicarbonate, which helps to raise the pH in the duodenum back to normal levels. Would this occur in an individual with a mutated version of the gene that codes for the hormone secretin? Why or why not?

AP Biology: Notes and PracticeModel 2The heating system of a house works to keep the temperature constant. If the house gets too cold, then the heat automatically turns on to warm the house. The heat stops when the present temperature is reached. This is an example of a feedback mechanism. Organisms use many feedback mechanisms to either maintain or amplify important chemical systems. Many of these mechanisms are regulated by the endocrine system. Figure 1 below displays a general model for positive and negative feedback mechanisms. Figures 2 and 3 display specific feedback mechanisms that occur in mammals.

Figure 1

Thermoregulation in Mammals

Figure 2

Childbirth and Contractions in Mammals

Figure 3

2. According to Figure 1, describe the similarities and differences between negative and positive feedback mechanisms.

3. According to Figure 1 and Figure 2:a. Identify the stimulus, signal, and response in the feedback loop mammals carry out when they are

too warm.

AP Biology: Notes and Practiceb. Identify the stimulus, signal, and response in the feedback loop mammals carry out when they are

too cold.

c. Are the feedback loops that occur to regulate body temperature positive or negative? Explain your reasoning.

4. According to Figure 1 and Figure 3:a. Identify the stimulus, signal, and response in the feedback loop mammals carry out during child

birth.

b. During childbirth, uterine contractions increase the frequency with which the baby pushes against the cervix. Are the feedback loops that occur to regulate contractions during childbirth positive or negative? Explain your reasoning.

5. Many of the systems in the body are delicate. They function only under a specific range of parameters. Enzyme will denature if they get too hot or cold or if the pH of the solution they are in is too high or too low. Cells will not be able to process glucose for energy if the concentrations of oxygen in the blood are not high enough. Feedback mechanisms are used to keep the body in homeostasis. That is, many systems are in place that monitor and regulate important parameters of the body and keep them within normal levels.

Determine which type of feedback mechanism (positive or negative) is responsible for maintaining homeostasis. Justify your response using evidence from Figures 1, 2 and 3 and give a real-world example.

AP Biology: Notes and PracticeModel 3All living things use glucose as a source of energy. In vertebrates, it is critical that the levels of glucose in the blood are consistent. If the glucose concentration in the blood gets too high, a coma could result. If the glucose concentration in the blood gets too low, the person could experience seizures, go into a coma, or die. In humans, hormone levels help regulate the glucose concentration in the blood and keep us in homeostasis. Figure 1 below displays the relative concentrations in the blood of glucose and two hormones, insulin and glucagon, before and after a meal.

When insulin is present, the transfer of glucose into cells increases. This takes the glucose out of the bloodstream and stores it as glycogen. Oppositely, when glucagon is present, glycogen will be converted back into glucose, as glycogen cannot be used by cells directly as an energy source, but glucose can. These feedback mechanisms are displayed in Figure 2.

Figure 1

Figure 2

6. According to Figure 1, which molecule from the graph above is found in the blood at the highest concentrations? Based on your prior knowledge, why do cells need this molecule?

7. According to the graph, what happens to blood glucose levels after a meal has been eaten? Based on your knowledge of what types of polymers contain glucose, explain why this is logical.

8. According to Figure 1, describe the following trends:a. When glucose levels are at baseline (remaining constant), how do the levels of hormones insulin and

glucagon change?

AP Biology: Notes and Practiceb. As glucose levels increase above baseline, which hormone changes in concentration AND how does

it change?

c. As blood glucose levels drop below baseline, which hormone changes in concentration AND how does it change?

9. Return to Figure 1 and add a curve to show the relative concentration of glycogen. Explain your reasoning for the curve you drew.

10. According to Figures 1 and 2:a. Determine the stimulus, signal, and response in Cycle A.

b. Determine the stimulus, signal, and response in Cycle B.

11. Are the feedback loops described in question 9 positive or negative? Explain your reasoning.

12. In Type I diabetes, the Beta cells of the pancreas produce little to no insulin. Draw a graph showing the changes in the levels of glucose and insulin before and after a meal for an individual with Type I diabetes. Explain your reasoning for the changes that you indicate in your graph.

AP Biology: Notes and PracticeAntidiuretic hormone (ADH) is a nine amino acid peptide secreted from the posterior pituitary. ASH binds to receptors on cells in the collecting ducts of the kidney and promotes reabsorption of water back into the circulation. In the absence of ADH, the collecting ducts of the kidneys are virtually impermeable to water.

ADH stimulates water reabsorption by stimulating insertion of "water channels" or aquaporins into the membranes of kidney tubules. These channels transport solute-free water through tubular cells and back into blood, leading to a decrease in blood plasma osmolarity (solute concentration) and an increase in osmolarity of urine. Figure 1 displays the feedback loop when blood plasma osmolarity increases above normal levels. Figure 2 displays the salt concentration in the blood plasma of an individual over time.

Figure 2

Figure 1

a. According to Figure 1 and the passage, determine the signal and response for a feedback loop in which the stimulus is decreased plasma osmolarity.

b. Using Figure 2, draw another curve on the graph that displays the change in concentration of ADH over time. Explain your reasoning for the curve that you draw.

Rela

tive

Conc

entr

ation

Time

Salt in blood plasma

AP Biology: Notes and Practicec. Various factors can affect ADH production. Alcohol consumption inhibits the release of ADH.

How will this affect the amount of water in the blood plasma, compared to normal conditions? Explain your reasoning.

How will this affect the amount of water in the urine, compared to normal conditions? Explain your reasoning.

d. The drug acetaminophen (the active ingredient in painkillers like Tylenol) stimulates the release of ADH.

How will this affect the amount of water in the blood plasma, compared to normal conditions? Explain your reasoning.

How will this affect the amount of water in the urine, compared to normal conditions? Explain your reasoning.