The maintenance of a stable internal environment within narrow limits.

What is meant by dynamic equilibrium?

Conditions fluctuate around a set point because of continual adjustments.

What are the three parts of a control system?

Receptor, coordinator and effector.

Why do high temperatures stop enzymes working?

Hydrogen bonds break, so the tertiary structure and active site change and the enzyme denatures.

Why do low temperatures slow enzyme reactions?

Less kinetic energy, so fewer collisions and fewer enzyme-substrate complexes.

Why must blood pH be controlled?

A change in pH alters ionic and hydrogen bonds, changing the active site shape so enzymes denature.

What happens to animal cells if blood water potential is too low?

Water leaves by osmosis and the cells shrink.

What happens to animal cells if blood water potential is too high?

Water enters by osmosis and they may burst because they have no cell wall.

What is negative feedback?

A response that reverses a change, returning the condition towards the set point.

What is positive feedback?

A response that amplifies the original change, moving the condition further from the starting level.

Give an example of positive feedback.

Oxytocin release during labour: stretching causes oxytocin, which causes contractions, which cause more stretching.

Which part detects a change in temperature in the body?

Thermoreceptors, for example in the hypothalamus.

How does vasodilation reduce temperature?

More blood flows through skin capillaries, so more heat is lost by radiation.

Homeostasis and feedbackEdexcel A-Level Biology B: Flashcards

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What is homeostasis?

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What is homeostasis?: The maintenance of a stable internal environment within narrow limits.
What is meant by dynamic equilibrium?: Conditions fluctuate around a set point because of continual adjustments.
What are the three parts of a control system?: Receptor, coordinator and effector.
Why do high temperatures stop enzymes working?: Hydrogen bonds break, so the tertiary structure and active site change and the enzyme denatures.
Why do low temperatures slow enzyme reactions?: Less kinetic energy, so fewer collisions and fewer enzyme-substrate complexes.
Why must blood pH be controlled?: A change in pH alters ionic and hydrogen bonds, changing the active site shape so enzymes denature.
What happens to animal cells if blood water potential is too low?: Water leaves by osmosis and the cells shrink.
What happens to animal cells if blood water potential is too high?: Water enters by osmosis and they may burst because they have no cell wall.
What is negative feedback?: A response that reverses a change, returning the condition towards the set point.
What is positive feedback?: A response that amplifies the original change, moving the condition further from the starting level.
Give an example of positive feedback.: Oxytocin release during labour: stretching causes oxytocin, which causes contractions, which cause more stretching.
Which part detects a change in temperature in the body?: Thermoreceptors, for example in the hypothalamus.
How does vasodilation reduce temperature?: More blood flows through skin capillaries, so more heat is lost by radiation.

Exam questions on Homeostasis and feedback

A student records the core body temperature of a healthy volunteer every hour for 24 hours. The temperature fluctuates between 36.4 °C and 37.2 °C, with an average of about 37 °C, even though the volunteer is active during the day and rests at night.
Explain why a fall in core temperature well below 37 °C would slow the rate of enzyme-controlled reactions in cells.2 marks
On a hot day, thermoreceptors in the hypothalamus detect a rise in core body temperature. Nerve impulses lead to the sweat glands secreting more sweat and to the arterioles supplying the skin capillaries dilating. Core body temperature falls back towards 37 °C and the sweating then decreases.
Explain how the dilation of the arterioles in the skin helps to reduce core body temperature.2 marks
During labour, the stretching of the cervix by the baby's head stimulates the release of the hormone oxytocin from the pituitary gland. Oxytocin causes stronger contractions of the uterus muscle. The stronger contractions push the baby's head harder against the cervix, causing more stretching and more oxytocin release. This continues until the baby is born.
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).