The maintenance of a stable internal environment within restricted limits.

Name the three parts of a control system.

Receptors, a coordinator and effectors.

Why must core temperature be stable?

Low temperatures slow enzyme reactions (less kinetic energy); high temperatures denature enzymes by breaking bonds and changing the active site.

Why must blood pH be stable?

Hydrogen ion concentration changes affect the bonds in an enzyme's tertiary structure, changing the active site, so enzymes denature.

Why must blood glucose not be too low?

Glucose is the respiratory substrate, so cells would lack the substrate to make enough ATP.

Why must blood glucose not be too high?

It lowers the water potential of the blood, so water leaves cells by osmosis.

Define negative feedback.

A response that reverses a change from the set point, returning the variable to its original level.

Why do separate mechanisms for departures in each direction give greater control?

A change either way can be actively reversed, so the variable is held closer to the set point.

Define positive feedback.

A response that increases the original change, moving the variable further from its starting level.

Give an example of positive feedback.

Oxytocin during childbirth: stretch of the cervix causes oxytocin release, stronger contractions and more stretch.

What is a target cell?

A cell with receptors that have a tertiary structure complementary to a particular hormone.

How do hormone responses compare with nervous responses?

They are usually slower, longer-lasting and more widespread.

Principles of homeostasis and negative feedbackAQA A-Level Biology: Flashcards

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What is homeostasis?

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What is homeostasis?: The maintenance of a stable internal environment within restricted limits.
Name the three parts of a control system.: Receptors, a coordinator and effectors.
Why must core temperature be stable?: Low temperatures slow enzyme reactions (less kinetic energy); high temperatures denature enzymes by breaking bonds and changing the active site.
Why must blood pH be stable?: Hydrogen ion concentration changes affect the bonds in an enzyme's tertiary structure, changing the active site, so enzymes denature.
Why must blood glucose not be too low?: Glucose is the respiratory substrate, so cells would lack the substrate to make enough ATP.
Why must blood glucose not be too high?: It lowers the water potential of the blood, so water leaves cells by osmosis.
Define negative feedback.: A response that reverses a change from the set point, returning the variable to its original level.
Why do separate mechanisms for departures in each direction give greater control?: A change either way can be actively reversed, so the variable is held closer to the set point.
Define positive feedback.: A response that increases the original change, moving the variable further from its starting level.
Give an example of positive feedback.: Oxytocin during childbirth: stretch of the cervix causes oxytocin release, stronger contractions and more stretch.
What is a target cell?: A cell with receptors that have a tertiary structure complementary to a particular hormone.
How do hormone responses compare with nervous responses?: They are usually slower, longer-lasting and more widespread.
What do the thermoreceptors in the hypothalamus detect?: The temperature of the blood.

Exam questions on Principles of homeostasis and negative feedback

In a healthy human, core temperature is held at about 37 °C and blood pH at about 7.4. Both stay within very narrow limits, even though the environment and the person's activity change from hour to hour.
Explain what is meant by negative feedback.2 marks
Hormone X is secreted by an endocrine gland into the blood. It circulates to every organ in the body, but only liver cells show a response to it. The response begins within minutes and lasts for several hours.
Explain why a response to a hormone is usually slower than a response to a nerve impulse.2 marks
During childbirth, the baby's head stretches the cervix. Stretch receptors send impulses to the hypothalamus, which causes oxytocin to be released from the posterior pituitary gland into the blood. Oxytocin stimulates the muscle of the uterus to contract more strongly, which pushes the baby's head harder against the cervix.
Explain why this is an example of positive feedback.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).