Plant Hormones

Question 1

What is phototropism?

Answer 1

A plant’s growth response to light.

Question 2

What is an auxin?

Answer 2

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

Question 3

What is the difference between auxins in the shoot and the root?

Answer 3

Promote growth in the shoot

Inhibit growth in the roots

Question 4

What happens if the tip of a shoot is removed?

Answer 4

No auxins will be available

So plant stops growing

Question 5

Describe a shoots growth towards light:

Answer 5

• more auxin accumulate on the shaded side than the side exposed to light
• this makes the cells elongate faster on the shaded side
• so the shoot bends towards the light

Question 6

Shoot’s growth in response to gravity:

Answer 6

• when a shoot is growing sideways, gravity produces an unequal distribution of auxins at the tip
• with more auxin on the lower side
• the lower side grows faster
• bending the shoot upwards

Question 7

Root’s growth response to gravity:

Answer 7

• when a root is growing sideways, there is an unequal distribution of auxin at the tip
• so there is more auxin on the lower side
• but in a root, auxins inhibit growth
• this means the cells on the top elongate faster
• and the root bends downwards

Question 8

Required Practical 8: Plant Responses

Investigating the effect of light intensity on the height of seedlings.

Answer 8

Independent variable: light intensity
Dependant variable: height of the seedlings
Control variable: volume of water, type of seed

1. Place cotton wool in three Petri dishes.
2. Soak them with equal volumes of water.
3. Place 10 mustard seeds in each dish.
4. Leave the dishes in a warm place and allow the seeds to germinate.
5. Water the seeds every day with the same volume of water.
6. Seeds germinate after a few days.
7. Make sure there are the same number of seedlings in each dish, if not, remove seedlings so all the dishes are the same.
8. Use a ruler to measure the height of each seedling.
9. Hold the seedlings straight for this.
10. Place the three dishes in different conditions: full sunlight, partial sunlight, darkness.
11. Measure the height of each seedling everyday for 5 consecutive days.
12. Record the results in a table.
13. When experiment is finished, we calculate a mean seedling height for each day.

Question 9

Describe the results for the light intensity investigation.

Answer 9

Full light and partial light: growth was similar as chlorophyll is very efficient at absorbing light energy. And the seedlings grew towards the light source, due to phototropism.
Darkness: seedlings grew the longest. This is because seeds germinate underground, so they grow rapidly to reach the light.
The leaves are small and yellow. this is because, once the seedlings have used all of their energy stores, they cannot carry out photosynthesis in the dark.

Question 10

Investigating the effect of gravity on seedlings.

Answer 10

1. A dish of seedlings is placed on its side in the dark.
2. The shoots have grown upwards, against the direction of gravity.
3. The roots have grown downwards, towards the direction of gravity.
4. Auxin inhibits cell growth in roots.
5. Gravity causes auxin to build up on the lower side of the roots.
6. This grows more slowly than on the upper side.
7. Which may make the root grow in the direction of gravity.