5.5 Plant And Animal Responses

Question 1

Auxins

Answer 1

A group f hormones produced by the tip of a shoot or root that regulate plat growth

Question 2

Auxins in shoots

Answer 2

Stimulates cell elongation and thus high concentration of auxin promote growth

Question 3

Auxins in roots

Answer 3

Inhibits cell elongation and thus high concentration of auxin limit growth

Question 4

How does auxin cause cell elongation

Answer 4

1. Auxin activates a proton pump in the plasma membrane
2. This causes the secretion of H+ ions into the cell wall
3. The resultant decrease in pH of the cellulose cell wall is the optimum pH for the enzyme expanase
4. Expanase breaks the bonds in the cellulose fibres
5. Cell wall becomes more flexible
6. The cell can now take in more water and become more turgid
7. The elongates the cell

Question 5

Why do plants need to defend themselves

Answer 5

Avoid predation- some plants produce toxic substances
Avoid abiotic stress- carrots produce antifreeze to bind to ice crystals and lower freezing point of water
Survive- to reproduce

Question 6

Why do plants need to respond to their environment

Answer 6

To cope with environmental conditions
Avoid abiotic stress
Maximise photosynthesis
Avoid bherbivory (biotic stress)

Question 7

Tannins

Answer 7

Very bitter taste
Toxic to microorganisms and insects

Question 8

Alkaloids

Answer 8

Chemicals with bitter tastes, noxious smells or poisonous characteristics that deter or kill herbivores
Eg tobacco plants produce the alkaloid nicotine in response to tissue damage. Nicotine is highly poisonous to many insects

Question 9

Pheromones

Answer 9

Chemicals which are released by one individual, which can affect behaviour and physiology of another member of the same species

Question 10

Nastic response

Answer 10

A non-directional response to stimuli

Question 11

Tropism

Answer 11

A directional growth response in which the direction of the response is determined by the direction of the external stimulus

Question 12

Positive tropism

Answer 12

A growth response towards the stimulus eg. Light

Question 13

Negative Tropism

Answer 13

A growth response away from the stimulus eg. Gravity

Question 14

Types of plant tropism

Answer 14

Phototropism-growth response to light
Geotropism- growth response to gravity
Chemotropism- growth response to chemicals
Thigmotropism- growth response to touch
Hydrotropism-growth response to water

Question 15

Ethene

Answer 15

Stimulates leaf loss

Question 16

How does ethene stimulate leaf loss

Answer 16

As leaf gets older, more ethene produced
A layer of cells develops at the bottom of leaf stalk and the abscission layer separates leaf from the plant. Ethene stimulates cells in abscission layer to expand, breaking the cell walls and causing the leaf to fall off

Question 17

How does abscisic acid (ABA) trigger stomata closure

Answer 17

1. Abscisic acid (ABA) binds to specific receptors on the guard cell membranes
2. ABA causes calcium ions to move into the cytoplasm of the guard cells
3. The increased concentration of calcium ions causes other ion channels to open
4. Ions leave the guard cell increasing the water potential inside the guard cell
5. Water leaves the guard cells by osmosis
6. The guard cells become flaccid causing the stoma to close

Question 18

Role of gibberellin in the germination of a seed

Answer 18

1. Water absorbed
2. Gibberelins synthesised by embryo
3. gibberellin stimulates aleurone layer cells to synthesise amylase
4. Amylase hydrolyses starch to form maltose
5. Maltose converted to glucose
6. Glucose transported to embryo providing energy for growth

Question 19

Auxins negative geotropism in shoot

Answer 19

Auxin moves to lower side
The cells of the shoot grow more on the side with most auxin, so it stimulates cells to grow more here
This makes the shoot bend and grow away from the ground

Question 20

Apical dominance

Answer 20

The central stem of the plant is dominant and grows more strongly than the side branches

Inhibition of lateral bud growth due to hormones produced by the apical buds

Question 21

Cytokinins

Answer 21

Promote bud growth