Coordination in Plants and Hormones in Animals: Quick Revision

A brief note on Coordination in Plants and Hormones in Animals. Read it before a test. Try each question at the end before you open its answer.

Plant movements

• Plants have no nerves and no muscles, but they sense changes and respond.
• A quick movement, such as the folding of a touch-me-not leaf, comes from cells that lose water and change shape. Nothing grows.
• A growth movement comes from one side of a part growing faster than the other. A growth movement towards or away from a stimulus is a tropism.
• Shoots grow towards light (positive phototropism) and away from gravity. Roots grow with gravity (positive geotropism) and towards water (hydrotropism). A pollen tube grows towards the ovule (chemotropism).

Plant hormones

• Auxin is made at the shoot tip. In light from one side, it moves to the shaded side, whose cells grow longer, so the shoot bends towards the light.
• Gibberellins help stems grow. Cytokinins promote cell division.
• Abscisic acid slows growth and makes leaves wilt.

Hormones in animals

• Endocrine glands release hormones straight into the blood. Only target cells respond.
• Hormones are slower than nerve impulses, but they reach every cell and act for longer.

• The hypothalamus tells the pituitary when to release its hormones. Testes and ovaries make the hormones of puberty.
• Too little iodine causes goitre. Too little insulin causes diabetes.
• Feedback keeps the level of each hormone right: high blood sugar brings more insulin, and low blood sugar brings less.

A worked example

A shoot tip makes $90$ units of auxin, and light from the left sends two thirds of it to the shaded side. Which way does the shoot bend, and why?

1. Shaded side. Light comes from the left, so the right side is shaded. It gets $\dfrac{2}{3} \times 90 = 60$ units.
2. Lit side. It gets $90 - 60 = 30$ units.
3. Growth. The right side has more auxin, so its cells grow longer.
4. The bend. The longer right side is on the outside of the curve, so the shoot bends to the left, towards the light.

Practice with new numbers

A program to try

The program prints each gland with its hormone and one job of the hormone.

Mistakes to avoid

• Saying that the lit side of a shoot grows more. The shaded side grows more.
• Calling the touch-me-not's folding a tropism. It is not a growth movement.
• Saying that hormones travel along nerves. In animals, the blood carries them.
• Thinking that every plant hormone promotes growth. Abscisic acid slows it.

Check yourself

1. What makes the cells on one side of a shoot grow longer than on the other side?

2. Name a plant movement made by chemotropism.

3. Why does the body need feedback control of hormones?

4. Which hormone contains iodine, and what happens if the food lacks iodine?

5. A person's blood sugar stays high for hours after every meal. Which hormone is likely lacking?