B1: Hormones and Plant Hormones

Question 1

What are hormones?

Answer 1

Chemical messengers that are released directly into the blood to activate target cells.

Question 2

Where are hormones produced?

Answer 2

In various glands but the main ones are the pituitary gland and the ovaries.

Question 3

What are the differences between nerves and hormones?

Answer 3

1. Nerves send a very fast message compared to hormones
2. Nerve messages act for a very short time unlike hormones
3. Hormones act in a very general way whereas nerves act on a very precise area.

Question 4

What is auxin?

Answer 4

A plant hormone that controls growth near the tips and roots and shoots.

Question 5

How does auxin control growth?

Answer 5

It controls growth of a plant in response to phototropism, geotropism and moisture.

Question 6

What is phototropism?

Answer 6

The plant’s response to light.

Question 7

What is geotropism?

Answer 7

The plant’s response to gravity.

Question 8

What happens when auxin is produced in the tips?

Answer 8

• It moves backwards to stimulate the cell elongation process
• This process occurs in the cells just behind the tips

Question 9

How does auxin promote growth in the shoots?

Answer 9

1. When a shoot is exposed to light, auxin accumulates on the underside of the shoot (the shaded area)
   - This makes the cell grow faster on the shaded side so the shoot bends towards the light.
2. When a shoot grows sideways, gravity creates an unequal distribution of auxin in the tip, with more auxin on the lower side
   - This causes the lower side to grow faster, bending the shoot upwards.

Question 10

When does auxin inhibit growth?

Answer 10

When it is present in the roots.

Question 11

How does auxin control growth in the roots?

Answer 11

1. A root growing sideways will have more auxin on its lower side.
   - The extra auxin inhibits growth, which means the cells on top will elongate and the root will bend downwards
2. An uneven amount of moisture causes more auxin to be produced on the side with more moisture.
   - This inhibits growth on the side with more moisture, causing the root to bend in that direction because the cells on the other side grow faster.

Question 12

What uses do plant hormones have in nature?

Answer 12

1. Most weeds are broad leaved (crops are narrow-leaved) so selective weed-killers are made of plant growth hormones that only affect broad-leaved plants. Therefore, whilst the weeds are killed, the crops are not harmed.
2. Rooting powder contains auxin so plant cuttings will produce roots rapidly and start growing. This produces lots of clones of a desired plant very quickly.