14b Hormones, homeostasis and tropic responses | IGCSE Biology 0610

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🎯What you need to be able to do

Describe a hormone; identify the adrenal glands, pancreas, testes and ovaries and their hormones; describe the effects of adrenaline.
Compare nervous and hormonal control; describe homeostasis; state that insulin decreases blood glucose.
Explain negative feedback, blood glucose control by insulin and glucagon, type 1 diabetes, the skin and temperature control EXTENDED.
Describe and investigate gravitropism and phototropism; explain the role of auxin EXTENDED.

📚The biology

Hormones

A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

An outline of a human body showing endocrine glands: the adrenal glands on top of the kidneys secrete adrenaline; the pancreas secretes insulin and glucagon; the ovaries in a female secrete oestrogen; the testes in a male secrete testosterone. — The four glands on the syllabus and their hormones.

Adrenaline is secreted in “fight or flight” situations. Its effects: increased breathing rate, increased heart rate, and increased pupil diameter. EXTENDED It also increases the blood glucose concentration, so more glucose is available to muscles for respiration.

nervous control: very fast; short-lasting effect

hormonal control: slower; longer-lasting effect

Homeostasis

Homeostasis is the maintenance of a constant internal environment. Insulin decreases blood glucose concentration.

EXTENDED Homeostatic control works by negative feedback around a set point: a change away from the set point is detected, and the response reverses the change.

A negative feedback loop. When blood glucose rises after a meal, the pancreas secretes insulin; liver and muscles take up glucose and the liver stores it as glycogen, returning glucose to normal. When blood glucose falls, the pancreas secretes glucagon; the liver converts glycogen to glucose and releases it, returning glucose to normal. — EXTENDED Two hormones with opposite effects, both from the pancreas, both acting mainly on the liver.

EXTENDED In type 1 diabetes the pancreas does not produce enough insulin, so blood glucose can rise dangerously high. It is treated by injecting insulin, monitoring blood glucose regularly, and managing diet and exercise.

Controlling body temperature EXTENDED

A section through the skin. A hair in a follicle with a hair erector muscle; a sweat gland deep in the skin with a duct to a pore at the surface; a receptor with a sensory neurone; an arteriole with capillary loops reaching up near the surface; a layer of fatty tissue below. — EXTENDED The structures you must identify in the skin.

EXTENDED The brain monitors blood temperature and sends impulses to the skin and muscles. Insulation by fatty tissue (and by hairs raised to trap air, in many mammals) reduces heat loss. When too hot, sweat is secreted, and heat is lost as it evaporates. When too cold, muscles shiver, releasing heat from respiration.

Two diagrams of skin. Too hot: vasodilation; the arterioles supplying surface capillaries widen, so more blood flows near the surface and heat is lost by radiation. Too cold: vasoconstriction; the arterioles narrow, so less blood flows through surface capillaries and less heat is lost. — EXTENDED It is the *arterioles* that widen or narrow; capillaries do not move.

Tropic responses

Gravitropism is a response in which parts of a plant grow towards or away from gravity. Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source. Shoots show positive phototropism and negative gravitropism; roots show positive gravitropism.

Three shoots. Lit from above, a shoot grows straight up. Lit from one side, it bends towards the light, and auxin collects on the shaded side. With the tip covered by foil, it does not bend. — The foil experiment shows that the tip detects the light.

A germinating seed laid on its side; a few days later its shoot has curved upwards and its root has curved downwards. — The shoot grows away from gravity; the root grows towards it.

EXTENDED These are examples of the chemical control of plant growth by auxin:

EXTENDED auxin is made in the shoot tip;
auxin diffuses through the plant from the shoot tip;
auxin is unequally distributed in response to light and gravity (in a shoot lit from one side, it collects on the shaded side);
auxin stimulates cell elongation, so the shaded side grows longer and the shoot bends towards the light.

✏️Worked example

(a) A student is startled by a loud noise. Name the hormone released and give two of its effects. [3] (b) Give one difference between how this response is coordinated and how the pupil reflex is coordinated. [1] (c) EXTENDED After a long run, a student’s blood glucose falls. Explain how it is returned to normal. [3] (d) EXTENDED Explain why a shoot lit from one side bends towards the light. [3]

(a) Adrenaline. Any two: faster breathing rate; faster heart rate; wider pupils (increased blood glucose, Extended).

(b) The adrenaline response is hormonal, slower and longer-lasting; the pupil reflex is nervous, faster and short-lived.

(c) The fall is detected; the pancreas secretes glucagon; glucagon causes the liver to convert glycogen to glucose and release it into the blood, raising the concentration back to the set point (negative feedback).

(d) Auxin, made in the tip, collects on the shaded side; it stimulates cell elongation there, so the shaded side grows more and the shoot bends towards the light.

Check it. Insulin lowers blood glucose; glucagon raises it. If an answer uses the same hormone for both directions, it is wrong.

Glucagon and glycogen. Glucagon is the hormone; glycogen is the stored carbohydrate in the liver. They are spelled differently and are different substances.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which gland secretes testosterone? A: adrenal gland. B: pancreas. C: testes. D: ovaries.

2. (Theory.) Define a hormone. [2]

A chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

3. (Theory.) Define homeostasis, and state the effect of insulin on blood glucose. [2]

The maintenance of a constant internal environment. Insulin decreases blood glucose concentration.

4. (Practical.) Describe an experiment to show that roots grow towards gravity. [3]

Place germinating seeds with their roots pointing in different directions (e.g. pinned to damp cotton wool in a dish, or laid on their side) in the dark; after a few days, observe that all the roots have curved to grow downwards.

5. (Theory.) EXTENDED Explain how sweating cools the body. [2]

Sweat is secreted on to the skin surface; as the water evaporates it takes heat energy from the skin, cooling it.

6. (Theory.) EXTENDED A person’s skin looks pale on a very cold day. Explain why. [3]

Vasoconstriction: arterioles supplying the surface capillaries narrow, so less blood flows near the surface; less heat is lost by radiation, and the skin looks pale.

7. (Theory.) EXTENDED Explain the term negative feedback, using body temperature as an example. [2]

A change from the set point is detected and triggers responses that reverse it: if body temperature rises above 37 °C, sweating and vasodilation increase heat loss, bringing it back down.

8. (Theory.) EXTENDED The tip is cut off a shoot, which is then lit from one side. Predict and explain the result. [2]

It does not bend (and grows little): auxin is made in the shoot tip, so no auxin reaches the region of elongation.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Diabetes UK — type 1 diabetes explained
Science and Plants for Schools (SAPS) — phototropism investigations with seedlings