Lesson 10 - Stimulus Response

Question 1

How do plants change their response to their environment? Name 3 ways.

Answer 1

1. By opening and closing their STOMATA
2. Turning their leaves towards a light source
3. sprouting or shedding leaves

Question 2

What is a plant’s response to an external stimulus called?

Answer 2

Tropism

Question 3

What is it called when a plant grows towards a stimulus?

Answer 3

Positive Tropism

Question 4

What is it called when a plant grows AWAY from a stimulus?

Answer 4

Negative Tropism

Question 5

When the shoot system of a plant grows TOWARDS a LIGHT source it is called?

Answer 5

Positive Phototropism

Question 6

Experiment involving Charles and Francis Darwin attempting to determine where the mechanism for phototropism is located within the shoot.

Answer 6

Tip of Shoot vs Body of Shoot

Question 7

2nd Experiment involving Charles and Francis Darwin attempting to determine whether the mechanism in the TIP was on the LIT or UNLIT side of the shoot.

Answer 7

Lit Side vs Unlit Side

Question 8

3rd Experiment involving Peter Boysen-Jensen involved determining whether or not the “signal” for phototropism was CHEMICAL or PHYSICAL.

Answer 8

Type of Mechanism Responsible for Phototropism (Chemical vs Physical)

Question 9

4th/Final Experiment conducted by Frits Went was to confirm that the mechanism for phototropism was chemical. (auxin from young stem tips and let this chemical diffuse into small blocks of agar (3 trials)

Answer 9

Chemical Responsible for Phototropism

Question 10

Biologists now know that this chemical is responsible for causing plant cells to ELONGATE which causes plant growth.

Answer 10

AUXIN

Question 11

Do shaded areas of the plant or lit areas of the plant create more auxin?

Answer 11

Shaded

Question 12

Since the shaded areas of a plant create more auxin, does the shaded side of the plant experience more growth?

Answer 12

Yes! This pushed the plant towards the light.

Question 13

What are the three types of TROPISM?

Answer 13

1. Phototropism
2. Gravitropism
3. Hydrotropism

Question 14

When does GRAVITROPISM occur in a plant?

Answer 14

When the plant has to maintain its access to SOLAR energy - shoot system.
When the plant has to maintain its access to WATER - root system.
Heavy starch particles within vacuoles sink to the bottom of the cytoplasm allowing the plant to sense which way is “up and down”.

Question 15

When does HYDROTROPISM occur in a plant?

Answer 15

Shoot system - weak negative hydrotropism as it opens its stomata under water and this does not allow for gas exchange
Root system - strong positive hydrotropism growing towards water to allow the plant to obtain this necessary ingredient for photosynthesis