Cracking the AP Biology Exam 2018, Premium Edition by Princeton Review

Author:Princeton Review
Language: eng
Format: epub
Publisher: Random House Children's Books
Published: 2017-09-12T04:00:00+00:00

Homeostasis

All living things have a “steady-state” that they try to maintain. This is helpful because the structures and processes within living things are sensitive to things like temperature, pH, pressure, salinity, osmotic pressure, and many other things. There is a set range of conditions that living things can live in successfully, called homeostasis. The body is constantly working to maintain this state by taking measurements and then responding appropriately. That might mean shivering if it is cold or releasing the hormone insulin if blood glucose is high. Depending on the condition, the body will respond accordingly.

Many of these responses are controlled by negative or positive feedback pathways. A negative feedback pathway (also called feedback inhibition) works by turning itself off using the end product of the pathway. The end-product inhibits the process beginning again, shutting down the pathway. This is a common strategy to conserve energy. If plenty of X is made, then the pathway to make X can be turned off. Sometimes, a very distant end product will turn off a process occuring several steps back. For example, ATP turns off the beginning stages of glycolysis, although ATP is not made until much later in cellular respiration.

A positive feedback pathway also involves an end-product playing a role, but instead of inhibiting the pathway, it further stimulates it. For example, once X is made, it tells the pathway to make even more X. This is less common, but it occurs during fruit ripening and labor and delivery, which ramps up and up as it proceeds.

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