Principles of homeostasis and negative feedbackAQA A-Level Biology: Revision notes
Section 1
What homeostasis is
Homeostasis is the maintenance of a stable internal environment within restricted limits, despite changes outside the body or in activity. In mammals it uses physiological control systems, each with:
- Receptors that detect a departure from the set point
- A coordinator (e.g. the hypothalamus) that processes the information
- Effectors (e.g. muscles, glands) that bring about the response
Section 2
Why core temperature and pH must be stable
Core temperature: enzymes have an optimum temperature. If it is too low, there is less kinetic energy, so fewer enzyme–substrate complexes form and reactions slow. If it is too high, hydrogen bonds break, the tertiary structure and active site change shape, and the enzyme is denatured.
Blood pH: hydrogen ions interfere with the ionic and hydrogen bonds holding the tertiary structure, so the active site changes and enzymes denature. Rates of metabolic reactions fall.
Section 3
Why blood glucose must be stable
Glucose is the main respiratory substrate. If blood glucose is too low, cells (especially neurones) cannot make enough ATP.
If it is too high, the water potential of the blood falls, so water leaves cells by osmosis and they become dehydrated.
A stable concentration gives a constant supply of substrate and a steady water potential.
Section 4
Negative and positive feedback
Negative feedback reverses a change, restoring the system to its original level. When the level returns to normal the response is reduced. This keeps the variable fluctuating around the set point.
Separate mechanisms control departures in each direction, e.g. sweating and vasodilation to cool, shivering and vasoconstriction to warm. Active correction in both directions gives greater control than a single mechanism.
Positive feedback amplifies a change, moving the system further from its starting point, e.g. oxytocin in childbirth: stretch causes oxytocin release, which causes stronger contractions and more stretch, until the baby is born.
Positive feedback is not 'good' feedback: it amplifies the change. In the exam, state clearly whether the response reverses or increases the change.
Section 5
Hormones as chemical messengers
Hormones are secreted by endocrine glands into the blood and carried to all parts of the body. They affect only target cells, which have receptors with a tertiary structure complementary to the hormone.
Compared with nervous coordination, hormone responses are usually slow, long-lasting and widespread, because the hormone is carried in the blood, persists until broken down, and reaches many target cells.
When asked why only some cells respond to a hormone, link receptor tertiary structure to the shape of the hormone.
That's the notes covered.
Carry on to the next subtopic.
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
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).