16 Plant responses

Question 1

What does auxin do?

Answer 1

• control cell elongation
• prevent leaf fall (abscission)
• involved in tropisms
• maintain apical dominance
• prevent lateral bud growth

Question 2

What does gibberellin (growth hormones) do?

Answer 2

• Gibberellins stimulate stem elongation and seed germination
• Taller plants have more Le gene which codes for enzymes that forms gibberellins
• stimulate pollen tube growth in fertilisation
• Seed absorbs water, water reaches endodermis layer and stimulates release of gibberellins. Gibberellins stimulate seed germination by triggering the breakdown of starch into glucose. Glucose used as substrate for respiration by the plant embryo and can release energy needed for growth.
• They also stimulate side shoot formation
• gibberellins switch on genes which code for amylases and proteases- the digestive enzymes required for germination.

Question 3

What does ethene do?

Answer 3

Causes fruit ripening, promotes abscission

Question 4

What does ABA (Abscisic acid) do?

Answer 4

Maintains dormancy of seeds and buds, stimulates stomatal closing

Question 5

What are examples of auxins?

Answer 5

Indoleacetic acid (IAA)

Question 6

What are tannins?

Answer 6

phenols, toxic to insects: they bind to the digestive enzymes produced in saliva and inactivate them.

Question 7

What are alkaloids?

Answer 7

• Alkaloids are a large group of very bitter tasting, nitrogenous compounds found in many plants.

Question 8

What hormones play a role in seed germination?

Answer 8

Gibberellins and ABA

Question 9

How does seed germination occur?

Answer 9

1) When seeds absorb water, the embryo is activated and begins to produce gibberellins.
2) They in turn stimulate the production of enzymes that break down the food stores found in the seed.
3) The embryo plant uses these food stores to produce ATP for building materials so it can grow and break through the seed coat.
4) Evidence suggests that gibberellins switch on genes which code for amylases and proteases- the digestive enzymes required for germination.

Question 10

Where is indoleacetic acid produced?

Answer 10

Indoleacetic acid is produced in the shoots and roots in flowering plants.

Question 11

What is the role of auxins such as indoleacetic acid?

Answer 11

• They stimulate the growth of the main, apical shoot.
• Evidence suggests that auxins affect the plasticity of the cell wall; the presence of auxins means the cell wall stretches more easily.

Question 12

What is apical dominance?

Answer 12

• When high concentrations of auxins suppress the growth of lateral shoots.
• The apical bud (the shoot at the top) is dominant over the lateral buds.
• Prevents side shoots from growing

Question 13

What does synergism mean?

Answer 13

When different hormones work together and complement each other, giving a greater response than on their own.

Question 14

Give an example of when hormones are synergistic?

Answer 14

Auxins and gibberellins work together to help plants grow very tall.

Question 15

What does antagonism mean?

Answer 15

When hormones have opposite effects

Question 16

Give an example of when hormones are antagonistic.

Answer 16

Gibberellins stimulate growth of side shoots but auxins inhibit the growth of side shoots.

Question 17

What do low concentrations of auxins promote?

Answer 17

Root growth

Question 18

What is abiotic stress? Give examples.

Answer 18

Abiotic stress is anything that’s potentially harmful to a plant e.g changes in day length, cold and heat, lack of water, excess water, high winds

Question 19

What is photoperiodism?

Answer 19

When plants are sensitive to a lack of light in their environment

Question 20

How do leaves respond to the falling auxin concentration?

Answer 20

They produce the gaseous plant hormone ethene

Question 21

What is the abscission zone?

Answer 21

• base of the leaf stalk is a region called the abscission zone
• made up of two layers of cells sensitive to ethene.

Question 22

What does ethene do?

Answer 22

Ethene seems to initiate gene switching in cells resulting in the production of new enzymes.
- These digest and weaken the cell walls in the outer layer of the abscission zone (separation zone)

Question 23

How do leaves fall off?

Answer 23

• phytochromes detect low light levels, higher levels of ethene and low levels of auxins, ethene triggers production of digestive enzymes like cellulase in the abscission zone. Cellulase digests the cell wall in the separation zone
• The vascular bundles which carry materials into and out of the leaf are sealed off. At the same time fatty material is deposited in the cells on the stem side to form a protective scar when the leaf falls, preventing entry of pathogens.
• Further abiotic factors such as low temperatures, strong winds puts further strain and leaf separates from the plant.
• A neat waterproof scar is left behind

Question 24

How do leaves prevent freezing?

Answer 24

• The cytoplasm of the plant cells and the sap in the vacuoles contain solutes which lower the freezing point.
• Some plants produce sugars, amino acids and even proteins which act as antifreeze to prevent cytoplasm from freezing
• Most species produce chemicals which make them hardy and frost resistant during winter months

Question 25

How does the hormone abscission acid (ABA)trigger stomatal closure?

Answer 25

1) ABA binds to the receptors on the guard cell membranes. This causes specific ion channels to open, which allows calcium ions to enter the cytosol from the vacuole.
2) The increased concentration of calcium ions in the cytosol causes other ion channels to open. These ion channels allow ions (potassium ions) to leave the guard cells, raising the water potential of the cells.
3) Water then leaves the guard cells by osmosis.
4) The guard cells become flaccid and the stomata close.

Question 26

How are auxins antagonistic to ethene?

Answer 26

Auxins inhibit leaf loss (as the leaf gets older, less auxin is produced leading to leaf loss) whereas ethene stimulates leaf loss (as leaves get older, more ethene is produced)

Question 27

What is a pheromone?

Answer 27

A pheromone is a chemical made by an organism which affects the social behaviour of other members of the same species.

Question 28

Give an example of pheromones.

Answer 28

If a maple tree is attacked by insects, it releases a pheromone which is absorbed by leaves on other branches. These leaves then make chemicals such as callose to help protect them if they are attacked.

Question 29

What are volatile organic compounds?

Answer 29

Chemicals. that act like pheromones between themselves and other organisms e.g insects

Question 30

Give an example of when volatile organic compounds are used.

Answer 30

When corn plants are being eaten by caterpillars, they can produce pheromones which attract parasitic wasps. These wasps then lay their eggs in the caterpillars, which eventually kill them.

Question 31

Give an example of a plant that folds up in response to being touched.

Answer 31

Mimosa pudica, it contains a toxic alkaloid and has sharp prickles
- It frightens off larger herbivores and dislodges small insects

Question 32

What is tropism?

Answer 32

A tropism is a response of a plant to a directional stimulus

Question 33

What is phototropism?

Answer 33

The growth of plants in response to light

Question 34

What is geotropism?

Answer 34

The response to gravity

Question 35

What is chemotropism?

Answer 35

The growth of plants in response to chemicals

Question 36

What is thigmotropism?

Answer 36

The plant growth in response to touch

Question 37

Shoots are ______ phototropic

Answer 37

Shoots are positively phototropic

Question 38

Roots are __________ phototropic

Answer 38

negatively

Question 39

Shoots are _______ geotropic

Answer 39

negatively

Question 40

Roots are _________ geotropic

Answer 40

positively

Question 41

Why is ethene commercially used in climacteric fruits?

Answer 41

• Ethene stimulates enzymes that break down cell walls, break down chlorophyll and convert starch into sugars.
• This makes the fruit soft, ripe and ready to eat

Question 42

How is ethene used to ripen bananas?

Answer 42

Bananas are harvested and transported before they’re ripe so they’re less likely to be damaged this way. They’re then exposed to ethene on arrival so they all ripen at the same time on the shelves and in peoples homes.

Question 43

How is auxin used in selective weed killers?

Answer 43

They make weeds produce long stems instead of lots of leaves. This makes the weeds grow too fast, so they can’t get enough water or nutrients, so they die.

Question 44

How are auxins used as rooting hormones? (e.g in rooting powder)

Answer 44

They make a cutting grow roots.

• The cutting can then be planted and grown into a new plant.
• Many cuttings can be taken from just one original plant and treated with rooting hormones, so lots of the same plant can be grown quickly and cheaply from just one plant.

Question 45

what do cytokinins do ?

Answer 45

overrides apical dominance and promotes lateral bud growth

Question 46

why do plants need to be able to respond to their environment ?

Answer 46

• maximise photosynthesis
• avoid herbivory
• avoid abiotcic stress
• cope with changing conditions
• ensure germination in suitable conditions