Plant Responses

Question 1

Outline the stages of germination and the role of plant hormones in this process

Answer 1

1. Water enters the seed
2. Embryo in the seed is activated and begins to produce gibberellins
3. Gibberellins begin to stimulate the production of enzymes like amylases and maltases
4. These enzymes hydrolyse food stores in the seed endosperm
5. The embyro now has sugars to produce ATP for growth

Question 2

How does auxin control growth by elongation?

Answer 2

1. Auxins are synthesised by the meristem cells in the shoot tips and diffuse down the shoot
2. Auxins bind to receptor sites of the cell surface membranes
3. pH reduces
4. The cells absorb water by osmosis, forming vacuoles and increasing internal pressure
5. Cell walls stretch
6. Cells elongate and the plant grows

Question 3

Outline the stages of abscission

Answer 3

1. A reduction in light levels causes auxin concentrations to reduce
2. Leaves produce the gaseous hormone ethene
3. Ethene helps switch on genes in cells in the abscission zone at the base of the leaf stalk
4. The cells produce enzymes that digest cell walls in the separation layer of the abscission zone
5. Vascular bundles supplying the leaf are cut off
6. Fatty material is deposited in the cells in the inner layer of the abscission zone, forming a protective scar when the leaf falls to prevent entry of pathogens
7. Cells in the separation zone retain water and swell, leading to the leaf separating from the plant

Question 4

How is the plant hormone ABA involved in the response to low water availability?

Answer 4

1. Plants produce ABA in response to low water availability
2. ABA is transported to the leaves
3. ABA binds to receptors on the plasma membrane of guard cells
4. ABA activates changes in the ionic concentration of guard cells, reducing water potential and therefore the turgor of the cells
5. This closes the stomata, helping the plant to conserve water

Question 5

Explain what happens to the shoot of a plant when it is exposed to light on one side of the shoot

Answer 5

1. IAA is produced in the tips of plant shoots
2. IAA transported downwards
3. Light stimulates IAA to move towards the shaded side of the shoot
4. IAA becomes more concentrated and stimulates more cell elongation on the shaded side of the shoot
5. Shoot bends towards the light