Negative & Positive Feedback - Online Tutor, Practice Problems & Exam Prep

Negative feedback in metabolic pathways inhibits earlier steps, acting like a red light to slow down production. For example, the final product can inhibit an enzyme, reducing the conversion of molecule A to B, thus limiting the final product F. In contrast, positive feedback stimulates earlier steps, functioning like a green light to accelerate production. The final product can enhance enzyme activity, leading to increased production of the final product. Understanding these feedback mechanisms is crucial for grasping metabolic regulation and homeostasis in biological systems.

Negative Feedback

In this video, we're going to begin our lesson on negative and positive feedback by starting off with negative feedback. Negative feedback is when the final product of a metabolic pathway inhibits an earlier step of the same exact pathway. Because negative feedback inhibits an earlier step in the same exact pathway, negative feedback is kind of acting like the red light to inhibit the metabolic pathway. Just like at a red light when you're in a car, you need to stop or slow down, with negative feedback, it's going to stop or slow down the metabolic pathway. Notice over here on the right-hand side we're showing you an example of negative feedback occurring within a metabolic pathway.

Notice that this metabolic pathway converts molecule A into molecule B using enzyme 1, then molecule B into C using enzyme 2, C into D using enzyme 3, D into E using enzyme 4, and then E into the final product F using enzyme 5. Perhaps the cell is using this metabolic pathway to create product F, the final product. However, maybe the cell does not want to create too much of product F and it wants to limit the production of product F, and so this can be when negative feedback can be very, very useful.

With negative feedback, notice that the final product of this metabolic pathway is able to come back to an earlier step within the same metabolic pathway and inhibit an enzyme in the same metabolic pathway. The little negative sign that you see here is going to be used to represent inhibition, essentially blocking or slowing down the enzyme to prevent the conversion of molecule A into molecule B. By inhibiting an earlier step in the same pathway, ultimately that is going to lead to the decreased production of the final product. Negative feedback is acting like the red light to this metabolic pathway to stop the metabolic pathway or inhibit the metabolic pathway and limit the production of the final product.

This here concludes our introduction to negative feedback, and we'll be able to compare this to positive feedback in our next video. So, I'll see you all there.

Positive Feedback

So now that we've introduced negative feedback in our last lesson video, in this video we're going to introduce positive feedback, which is pretty much the exact opposite of negative feedback, and that's because positive feedback is when the final product of a metabolic pathway stimulates an earlier step in the same metabolic pathway. And so because positive feedback stimulates an earlier step in the same pathway, positive feedback pretty much acts like the green light to further stimulate metabolic pathways. And so notice down below, we're showing you an image of a green light. And so just like when you're driving, the green light represents stimulation of your car where your car is going to accelerate and speed up. Positive feedback here is going to act like the green light for metabolic pathways. And so notice here in this metabolic pathway the final product is product f and product f notice is going backwards to stimulate an earlier step of the same metabolic pathway, essentially stimulating enzyme 1 here so that enzyme 1 is able to operate faster, even faster than it normally does, stimulating this enzyme. And so ultimately when the enzyme 1 is stimulated it's going to create more of product b, or molecule b here. And then ultimately, this is going to lead to even more of the final product f. And so if a cell is in a scenario where they want to create even more of the final product of a metabolic pathway, then that is when positive feedback can be very very useful to the cell because positive feedback acts like the green light to the entire metabolic pathway allowing even more of the final product F to be made. And so the positive sign that you see here, the plus, represents the stimulation of the enzyme here. And so this here concludes our introduction to positive feedback and we'll be able to get some practice applying the concepts that we've learned in our next few videos. So I'll see you all there.

________ is when the product of a biochemical pathway activates the production of itself.

a) Negative feedback inhibition.

b) Positive feedback.

c) Substrate feedback inhibition.

d) Product feedback inhibition.

• A. Negative feedback inhibition.
• B. Positive feedback.
• C. Substrate feedback inhibition.
• D. Product feedback inhibition.

Which of the following is TRUE about feedback inhibition?

a) Feedback inhibition has no physiological importance.

b) Multiple products are required for feedback inhibition.

c) Feedback inhibition of a pathway can only be accomplished by the products of that pathway.

d) Feedback inhibition involves products binding to the active site to prevent enzyme activity.

• A. Feedback inhibition has no physiological importance.
• B. Multiple products are required for feedback inhibition.
• C. Feedback inhibition of a pathway can only be accomplished by the products of that pathway.
• D. Feedback inhibition involves products binding to the active site to prevent enzyme activity.

Here’s what students ask on this topic:

What is negative feedback in metabolic pathways?

Negative feedback in metabolic pathways occurs when the final product of a pathway inhibits an earlier step in the same pathway. This inhibition acts like a red light, slowing down or stopping the pathway to prevent the overproduction of the final product. For example, if a metabolic pathway converts molecule A to molecule F through a series of enzymes, the final product F can inhibit an enzyme that converts A to B. This reduces the production of F, maintaining balance and preventing excess accumulation. Negative feedback is crucial for maintaining homeostasis in biological systems.

How does positive feedback differ from negative feedback in metabolic pathways?

Positive feedback is the opposite of negative feedback. In positive feedback, the final product of a metabolic pathway stimulates an earlier step in the same pathway, acting like a green light to accelerate production. For instance, if a pathway converts molecule A to F, the final product F can enhance the activity of an enzyme that converts A to B, leading to increased production of F. This mechanism is useful when a cell needs to rapidly produce more of a specific product. In contrast, negative feedback inhibits earlier steps to slow down production, maintaining balance and preventing overproduction.

Why is negative feedback important for homeostasis?

Negative feedback is essential for homeostasis because it helps maintain stable internal conditions by regulating metabolic pathways. When the final product of a pathway accumulates, it inhibits an earlier step, reducing its own production. This prevents the overaccumulation of substances, which could disrupt cellular functions and overall homeostasis. For example, in the regulation of blood glucose levels, insulin acts through negative feedback to lower blood sugar when it is too high. By inhibiting further glucose production and promoting its uptake and storage, negative feedback ensures that physiological parameters remain within optimal ranges.

Can you provide an example of positive feedback in biological systems?

An example of positive feedback in biological systems is the process of blood clotting. When a blood vessel is injured, platelets adhere to the site and release chemicals that attract more platelets. This accumulation of platelets continues to stimulate further platelet aggregation, rapidly forming a clot to seal the wound. The initial signal is amplified through positive feedback, ensuring a swift and effective response to injury. This mechanism is crucial for preventing excessive blood loss and promoting healing.

How do negative and positive feedback mechanisms contribute to metabolic regulation?

Negative and positive feedback mechanisms are vital for metabolic regulation. Negative feedback inhibits earlier steps in a pathway to prevent overproduction of the final product, maintaining balance and preventing resource wastage. For example, the end product of a pathway can inhibit an enzyme that catalyzes an early step, reducing its own synthesis. Positive feedback, on the other hand, stimulates earlier steps to accelerate production when more of the final product is needed. This amplification ensures rapid response and adaptation to changing cellular demands. Together, these feedback mechanisms ensure efficient and balanced metabolic processes, crucial for cellular function and homeostasis.