Biochemistry : Negative Feedback

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Negative Feedback

The following is a hypothetical metabolic pathway:

Which of the following is an example of negative feedback?

Possible Answers:

Product B activates the formation of product D

Product B inhibits the formation of product D

Product D inhibits the formation of product B

Product D activates the formation of product B

Correct answer:

Product D inhibits the formation of product B

Explanation:

Remember that negative feedback means the production of a metabolite will prevent the formation of either itself or any other metabolite preceding it. Product D inhibiting the formation of product B is the only choice that satisfies the definition of negative feedback. 

Example Question #2 : Negative Feedback

Which of the following physiological processes is not an example of negative feedback?

Possible Answers:

The formation of a blood clot at a site of injury in response to tissue damage

The secretion of renin by the kidneys in response to low blood pressure

The secretion of atrial natriuretic peptide by the atria of the heart in response to high blood pressure

The secretion of insulin by the pancreas in response to high blood sugar

The down-regulation of dopamine receptors on the membrane of a post-synaptic neuron in response to elevated levels of dopamine

Correct answer:

The formation of a blood clot at a site of injury in response to tissue damage

Explanation:

To answer this question, we'll need to take a look at all of the answer choices and see which one of them does not display feedback inhibition.

Generally speaking, negative feedback mechanisms help ensure that a homeostatic set point will be maintained. Thus, negative feedback can be viewed as a process that helps to maintain an equilibrium level. In other words, when things begin to deviate away from a set point, negative feedback helps shift things back to that set point.

When blood pressure is low, one of the consequences of this is that the kidneys begin to decrease the rate at which they filter the blood. Because the vasculature of the kidneys is very sensitive to even tiny changes in its filtration rate, it is well poised to detect and alter blood pressure. Upon sensing a reduced filtration rate, the kidneys increase their production of renin. This enzyme goes on to initiate a chain of events that ultimately result in increased blood pressure. This heightened blood pressure, in turn, reduces the kidneys' release of renin. Thus, this is an example of negative feedback.

Conversely, when blood pressure is too high, this can put increased stress on the walls of the heart. When the walls become slightly more distended and stretched due to increased pressure, the atria can release a compound known as atrial natriuretic peptide (ANP). This peptide then goes on to affect several other physiological events that finally culminate in a decrease in blood pressure. The reduced pressure, in turn, causes the heart to turn down its secretion of ANP. Consequently, this is also an example of negative feedback.

Levels of sugar in the blood also need to be regulated, as having too low or too high of a concentration can have many adverse consequences. As expected after eating a meal, blood sugar levels rise. In order to ensure homeostasis, the pancreas increases its production of insulin, a hormone that enables many cells to take up glucose across their plasma membrane, which cause a reduction in blood sugar levels. Therefore, the secretion of insulin by the pancreas is yet another example of negative feedback.

We can also see negative feedback at the molecular level when considering the interaction of neurotransmitters with their receptors. In this hypothetical example, we're told that elevated levels of dopamine have caused down-regulation in dopamine receptors. What this essentially means is that because there has been so much dopamine hanging out near the post-synaptic membrane, the cell has been constantly affected by the presence of all that dopamine. In response, the cell has decided to turn down its production of dopamine receptors. This is a process that happens inside the cell, where the action of certain genes results in the synthesis of dopamine receptors that are eventually deposited in the cell membrane. By manufacturing less dopamine receptors, the cell will have a reduced capacity for responding to all that dopamine that's been outside of the cell. Thus, this is certainly an example of negative feedback.

Lastly, let's take a look at blood clotting. The blood contains a large variety of clotting factors. However, in the absence of tissue damage, there are regulatory mechanisms that keep these clotting factors inactivated. This is important, because if these clotting factors were able to from clots, unimpeded by any kind of regulation, the blood would end up clotting too much and thus the function of the circulatory system to carry nutrients and wastes throughout the body would be compromised. Under normal physiological conditions (meaning in the absence of any pathology or disease), blood clotting will only happen at sites of cell or tissue damage. Once activated by chemicals and other factors that are associated with injury, clotting factors begin to initiate an extraordinarily complex cascade that culminates in the formation of a clot. One such compound, known as fibrinogen, becomes converted into its active form, fibrin. Multiple fibrin molecules are then able to cross-link to help form the clot. As more and more clotting factors and fibrin molecules are assembled and assimilated, the result is that more and more clotting factors are recruited to the site of injury to help ensure that a clot forms. Although there aren't too many examples of positive feedback in biochemistry, this is one that stands out.

Example Question #3 : Negative Feedback

Which biological process is not regulated by negative feedback?

Possible Answers:

Thermoregulation

Blood pH regulation

Blood sugar regulation

Wound healing

Allosteric control of enzymes

Correct answer:

Wound healing

Explanation:

The answer is "wound healing." In negative feedback systems, the product of the process will inhibit the process when it reaches a certain level. This is true whether that product is the actual product of an enzymatic reaction, or just a change in body temperature, blood sugar levels, or blood pH. This is because all of those processes are working to stay at a fairly precise level to maintain homeostasis. Wound healing, however, involves positive feedback. Platelets attract more platelets and induce other factors in the healing response.

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