Which of the following statements is TRUE? Feedback inhibition is a negative feedback system for controlling substrate activity. Feedback inhibition is difficult to reverse and requires synthesis of new enzymes. Feedback inhibition regulates the flow through all biosynthetic pathways. Feedback inhibition takes place when a substrate used early in a reaction pathway is inhibited by an enzyme used late in the pathway. Feedback inhibition takes place when an enzyme acting early in a reaction pathway is inhibited by a late product of that pathway.

In this type of regulation, the product of the process actively inhibits the previous formation chain to stabilize the compound so that its level doesn't rise more than necessary.

Let's take for example the Hypothalamic-pituitary-adrenal axis.

Upon receiving a stimulus (for example stress), the hypothalamic cells secrete corticotropin-releasing hormone (CRH). This hormone is transported through the portal blood vessels to the anterior pituitary where it stimulates the secretion of stored adrenocorticotropic hormone (ACTH).

The ACTH travels through blood to the adrenal cortex of the adrenal gland where it stimulates the biosynthesis of glucocorticoids (mainly cortisol).

The hormone produced in the adrenal cortex will negatively feedback to inhibit both the hypothalamus and the pituitary gland. This reduces the secretion of CRH and the release of ACTH.

Hypothalamus

CRH ← Inhib. ← ↑

↓ ↑

Anterior Pituitary ↑

ACTH ← Inhib. ← ↑

↓ ↑

Adrenal cortex ↑

Cortisol → → ↑

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