4.5.3.7 - Negative feedback

Higher Tier boundary and key idea

This specification point is Higher Tier only. It sits inside hormonal coordination, so the key idea is that hormones are chemical messengers released by glands into the blood. The blood carries a hormone to target organs or tissues, where it produces an effect.

In this lesson, the two named hormones are adrenaline and thyroxine. Adrenaline is linked to fear or stress and rapid preparation for action. Thyroxine is linked to basal metabolic rate, growth, development and negative feedback.

Negative feedback is a control mechanism. A change away from the normal level is detected, a response reverses that change, and the condition moves back towards the normal level. For AQA 4.5.3.7, the required negative-feedback example is thyroxine control.

Do not turn this lesson into every example of homeostasis. For AQA 4.5.3.7, the named hormones are adrenaline and thyroxine, and the required feedback example is the control of thyroxine levels.

Adrenaline and flight or fight

Adrenaline is produced by the adrenal glands in times of fear or stress. In this specification point, exam questions usually care more about the hormone's effects than the exact position of the glands.

Adrenaline increases heart rate. This means the heart pumps blood around the body faster, so oxygen and glucose are delivered more quickly to the brain and muscles. Muscles can then respire more rapidly, releasing energy needed for rapid action.

AQA wording often uses flight or fight. Fight or flight means the same thing, but keep the biological explanation precise: adrenaline prepares the body for rapid action by increasing delivery of oxygen and glucose, especially to the brain and muscles.

In exam answers, avoid vague phrases such as "adrenaline gives energy". Hormones do not make energy. A stronger answer says that adrenaline increases oxygen and glucose delivery, so cells can respire more rapidly and release energy.

Thyroxine and metabolic rate

Thyroxine is produced by the thyroid gland. The thyroid gland is in the neck, but again the main exam focus is the hormone's role.

The basal metabolic rate is the rate of chemical reactions in the body when it is at rest. Thyroxine stimulates this rate, so it affects how quickly cells carry out metabolic reactions. One important example is respiration, where cells release energy from glucose.

Thyroxine also plays an important role in growth and development. That does not mean you need detailed clinical disorders or childhood medicine for this lesson. At GCSE, keep the role clear: thyroid gland produces thyroxine; thyroxine stimulates basal metabolic rate; thyroxine is important in growth and development.