Tropism

Question 1

Responses for survival and stimulus?

Answer 1

• a stimulus is a detectable change in the environment
• these changes can be detected by receptors
• organisms increase chance of survival by responding to stimuli via different responses mechanisms

Question 2

Tropism?

Answer 2

• when plants respond, via growth, to stimuli
• can be positive or negative, growing towards or away from stimulus
• controlled by specific growth factors (e.g. indoleacetic acid, IAA)

Question 3

IAA?

Answer 3

• type of auxin
• controls cell elongation in shoots and inhibits growth of cells in roots
• made in tip or roots and shoots but can diffuse to other cells

Question 4

Positive phototropism?

Answer 4

Shoots:
- light needed for LDR in photosynthesis, so plants grow and bend towards light (positive phototropism)
1. Shoot tip cells produced IAA, causing cell elongation
2. IAA diffuses to other cells
3. If there is unilateral light, IAA diffuses towards shaded side of shoot - results in higher concentration of IAA there
4. Cells on shaded side elongate more and results in plant bending towards light source

Question 5

Negative phototropism?

Answer 5

Roots:
- don’t photosynthesise so don’t require light
- must anchor plant deep in the soil
- high concentration of IAA inhibits cell elongation - causes root cells to elongate more on lighter side and so root bends away from light

Question 6

Negative gravitropism?

Answer 6

Shoots:
- IAA diffuses from upper side to lower side of shoot
- if plant is vertical, causes plant cells to elongate and plant grows upwards
- if plant on its side, causes shoot to bend upwards

Question 7

Positive gravitropism?

Answer 7

Roots:
- IAA moves to lower side of roots - upper side elongates and root bends down towards gravity and anchors plant