Stimuli And Responses

Question 1

What is a tropism?

Answer 1

A tropism is the growth of part of a plant in response to a directional stimulus.

Question 2

What is a positive tropism?

Answer 2

Growth towards stimulus

Question 3

What is a negative tropism?

Answer 3

Growth away from stimulus

Question 4

Example of tropisms in the shoots

Answer 4

SHOOTS - so that their leaves are in the most favourable position to capture light for photosynthesis.
● Positive phototropism - towards light
● Negative gravitropism - away from gravity

Question 5

Examples of tropisms in the roots

Answer 5

ROOTS - increases the probability that roots will grow into the soil, where they are belier able to absorb water and mineral
ions
● Negative phototropism - away from light
● Positive gravitropism - towards gravity

Question 6

What is Phototropism?

Answer 6

Shoots grow towards light (positively phototropic) because light is needed for
photosynthesis.

Question 7

What is Gravitropism?

Answer 7

Plants need to be firmly anchored in the soil. Roots are sensitive to gravity and grow in the direction of its pull (positively gravitropic).

Question 8

What is Hydrotropism?

Answer 8

Almost all plant roots grow towards water (positively hydrotropic) in order to absorb it for use in photosynthesis and other metabolic processes, as well as for support.

Question 9

Plant growth factors

Answer 9

➔ They exert their influence by affecting growth
➔ Can be made by cells located throughout the plant rather than in particular organs.
➔ Unlike animal hormones, some plant growth factors affect the tissues that release them rather than acting on a
distant target organ.
➔ Plant growth factors are produced in small quantities.

Question 10

What are IAA?

Answer 10

Indoleacetic acid (IAA) is a type of auxin

Question 11

What is the role of IAA in elongation growth?

Answer 11

● Controls cell elongation in shoots
● Inhibits the growth of cells in the roots.

Question 12

Where is IAA made?

Answer 12

It is made in the tip of the roots and shoots but moved by diffusion and active transport over short distances and by the phloem over long distances to other cells.

Question 13

What direction is IAA transported?

Answer 13

The transport of IAA is in one direction - away from the tip of shoots and roots where it is produced.

Question 14

What is the effect of IAA on plant cells?

Answer 14

increasing the plasticity (ability to stretch) of their cell walls.

Question 15

Acid growth hypothesis

Answer 15

It involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall causing the cell wall to
become more plastic allowing the cell to elongate by expansion.

Question 16

Control of phototropism by IAA in Shoots

Answer 16

Grow towards unilateral light as it is needed for the LDR. Positive phototropism:
1. Cells in the tip of the shoot produce IAA, which is then transported by diffusion down the shoot.
2. The IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
3. Light causes the movement of lAA from the light side to the shaded side of the shoot where it accumulates.
4. A greater concentration of IAA builds up on the shaded side of the shoot than on the light side.
5. As IAA causes elongation of shoot cells, the cells on the shaded side elongate more.
6. The shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light.

Question 17

Control of phototropisms by IAA in Roots

Answer 17

Do not require light as they do not photosynthesis, and are more able to anchor the plant if they are deep in the soil away
from light. They grow away from light which is negative phototropism:
1. Cells in the tip of the root produce IAA, which is then transported by diffusion down the root.
2. The IAA is initially transported evenly throughout all regions as it begins to move down the root.
3. Light causes the movement of lAA from the light side to the shaded side of the shoot where it accumulates.
4. A greater concentration of IAA builds up on the shaded side of the shoot than on the light side.
5. In roots, a high concentration of IAA inhibits cell elongation.
6. Causing roots cells to elongate more on the lighter side and so the root bends away from light

Question 18

Control of gravitropisms by IAA in Root

Answer 18

Positive gravitropism:
1. Cells in the tip of the root produce IAA, which is then transported by diffusion along the root.
2. The IAA is initially transported to all sides of the root.
3. Gravity influences the movement of IAA from the upper side to the lower side of the root.
4. A greater concentration of IAA builds up on the lower side of the root than on the upper side.
5. In root cells, IAA inhibits the elongation so the cells on the lower side elongate less than those on the upper side.
6. The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend
downwards towards the force of gravity.

Question 19

Control of gravitropisms by IAA in shoots

Answer 19

Negative gravitropism:
1. Cells in the tip of the shoot produce IAA, which is then transported by diffusion along the shoot.
2. The IAA is initially transported to all sides of the shoot.
3. Gravity influences the movement of IAA from the upper side to the lower side of the shoot.
4. A greater concentration of IAA builds up on the lower side of the shoot than on the upper side.
5. In shoots, the greater concentration of IAA on the lower side increases cell elongation and causes this side to elongate more than the upper side.
6. As a result, the shoot grows upwards away from the force of gravity