Hormones, Homeostasis and Tropisms

Hormones

A hormone is a chemical messenger. It is made in a gland, carried around the body in the blood, and changes the activity of its target organs. Glands that release hormones into the blood are called endocrine glands.

Each hormone affects only its own target organs, even though it is carried to every part of the body in the blood.

ExtendedA second pancreatic hormone

The pancreas also secretes glucagon. Insulin and glucagon work together to control blood glucose (page 6).

Worked example (gland and hormone): A hormone is secreted by the adrenal glands. Name the hormone and state how it reaches its target organs. Step 1. The adrenal glands secrete adrenaline. Step 2. Hormones are carried by the blood to their target organs. Answer: adrenaline; in the blood.

Adrenaline and Control

Adrenaline is secreted in ‘fight or flight’ situations, such as when you are frightened. It is carried in the blood and has three Core effects you must know.

Nervous and hormonal control

The nervous system and hormones both coordinate the body. They differ in speed of action and duration of effect.

ExtendedAdrenaline and metabolic activity

Adrenaline also controls metabolic activity. It increases blood glucose concentration and increases heart rate. The extra glucose and the faster blood flow supply the muscles with more of what they need for respiration.

Worked example (comparing control): A person jumps back from a snake in less than a second. State whether this is likely to be nervous or hormonal control, and why. Step 1. The response is very fast. Step 2. Nervous control has a fast speed of action; hormonal control is slower. Answer: nervous; fast action.

Homeostasis

Homeostasis is the maintenance of a constant internal environment. Blood glucose concentration is one thing that the body keeps steady.

Insulin

Insulin is a hormone secreted by the pancreas. It decreases blood glucose concentration. After a meal, blood glucose rises and insulin brings it back down.

Blood glucose concentration is one part of the internal environment. Keeping it steady is a good example of homeostasis.

Worked example (insulin): State what happens to blood glucose concentration when insulin is secreted. Step 1. Insulin is secreted by the pancreas. Step 2. Insulin decreases blood glucose concentration. Answer: it decreases.

ExtendedNegative feedback and the set point

Homeostasis works by negative feedback. The body has a set point. If a value moves away from it, the change is detected and a response brings it back. The value then wavers around the set point.

Controlling Blood Glucose

ExtendedThe liver, insulin and glucagon

The liver and the pancreas control blood glucose concentration. Insulin and glucagon have opposite effects.

ExtendedType 1 diabetes

In Type 1 diabetes the pancreas does not make enough insulin, so blood glucose concentration can rise too high. It is treated by injections of insulin, usually with careful control of the diet.

Worked example (blood glucose control): Blood glucose concentration falls too low between meals. Describe how the body brings it back to normal. Step 1. The pancreas secretes glucagon. Step 2. The liver converts glycogen to glucose. Step 3. Blood glucose concentration rises. Answer: glucagon; glycogen to glucose.

The Skin

ExtendedStructure of the skin

You must be able to identify these structures in diagrams and images of the skin.

Worked example (identifying structures): Which structure in the skin carries impulses from a temperature receptor to the brain? Step 1. Receptors detect the change. Step 2. Impulses are carried by a sensory neurone. Answer: sensory neurone.

Worked example (linking structure to role): State the role of the fatty tissue in the skin. Step 1. Fatty tissue lies under the skin. Step 2. It acts as insulation, reducing heat loss from the body. Answer: insulation.

Temperature Control

ExtendedKeeping body temperature constant

Mammals maintain a constant internal body temperature. The brain receives impulses from temperature receptors and coordinates the responses.

ExtendedVasodilation and vasoconstriction

Widening of the arterioles is vasodilation; narrowing is vasoconstriction.

Worked example (temperature control): Describe how the body responds when it becomes too cold. Step 1. Hairs are raised, trapping air as insulation. Step 2. Muscles contract rapidly (shivering), releasing heat. Step 3. Arterioles narrow (vasoconstriction), so less heat is lost from the skin. Answer: insulation; shivering; vasoconstriction.

Tropic Responses

Plants cannot move from place to place, but parts of them grow in response to stimuli. These growth responses are called tropic responses.

Investigating tropisms

To investigate phototropism, grow identical seedlings in a box with light coming from one side only, and compare them with seedlings that receive light from all sides. To investigate gravitropism, place seedlings on their side in the dark, so that light cannot affect them. Record the direction of growth of the shoots and roots after a few days.

Worked example (investigation): A seedling is placed on its side in the dark. Suggest why it is kept in the dark. Step 1. The aim is to test the effect of gravity. Step 2. Darkness removes light as a second stimulus, so any bending is due to gravity. Answer: so only gravity acts.

Auxin and Shoot Growth

ExtendedThe chemical control of shoot growth

Phototropism and gravitropism of a shoot are examples of the chemical control of plant growth. The chemical is auxin.

1. Auxin is made in the shoot tip.

2. Auxin diffuses through the shoot from the tip.

3. Auxin is unequally distributed in response to light and gravity.

4. Auxin stimulates cell elongation.

ExtendedExplaining the bending

With light from one side, auxin collects on the shaded side. Cells there elongate more than cells on the lit side, so the shoot bends towards the light. In a shoot on its side, auxin collects on the lower side, so the lower cells elongate more and the shoot bends upwards, away from gravity.

Worked example (explaining phototropism): Explain why a shoot grows towards a light source. Step 1. Auxin is made in the shoot tip and diffuses down the shoot. Step 2. With light from one side, auxin is unequally distributed and collects on the shaded side. Step 3. Auxin stimulates cell elongation, so the shaded side grows faster and the shoot bends towards the light. Answer: more auxin on shaded side; more elongation.