Investigate and explain how feedback mechanisms maintain homeostasis in living organisms (North Carolina Standard Course of Study, Biology, LS.Bio.3).

What this topic is asking

North Carolina LS.Bio.3 asks you to explain how feedback mechanisms maintain homeostasis, the stable internal environment that living things need. For the Biology EOC you need to define homeostasis, describe how a negative feedback loop detects and reverses a change, work through examples (body temperature and blood glucose), and recognize that positive feedback amplifies a change instead. Items often give a scenario and ask you to identify the feedback type or fill in a loop.

What homeostasis is

Homeostasis matters because the cell's chemistry is sensitive. Enzymes work best at a particular temperature and pH and denature outside it, so the body must hold those conditions steady. The internal "target" value is called the set point, and the body works to keep each condition close to its set point. This ties the topic back to enzymes: homeostasis exists largely to protect enzyme function.

Negative feedback: reversing a change

Two examples appear again and again on the EOC.

• Body temperature. If you get too hot, receptors detect the rise, and the body responds by sweating (evaporation cools you) and widening blood vessels near the skin (releasing heat). If you get too cold, you shiver (releasing heat) and narrow blood vessels (conserving heat). Each response opposes the change, returning temperature toward the set point.
• Blood glucose. After a meal, blood glucose rises; the pancreas releases insulin, which helps cells take up glucose and the liver store it, lowering blood glucose. When blood glucose falls (for example between meals), the pancreas releases glucagon, which makes the liver release stored glucose, raising it. The two hormones push glucose back toward the set point from opposite directions.

Positive feedback: amplifying a change

Positive feedback does the opposite of negative feedback: instead of reversing a change, it amplifies it, driving the condition further from the starting point until a specific event is completed. It is much less common in the body, and the EOC usually just wants you to distinguish it from negative feedback. Examples include childbirth (contractions trigger hormone release that causes stronger contractions, until birth) and blood clotting (the start of a clot triggers more clotting until the wound is sealed). Positive feedback is used for processes that should run to completion, not for keeping a value steady.

Try this

Q1. Define homeostasis and give one example of a condition it controls. [2]

• Cue. The maintenance of a stable internal environment; for example body temperature, blood glucose, water balance, or pH.

Q2. Explain the difference between negative and positive feedback. [2]

• Cue. Negative feedback reverses a change to restore the set point; positive feedback amplifies a change, driving it further until an event completes.