Plant Responses and Hormones

Question 1

Why do plants have phototropisms?

Answer 1

They sense the direction of light and grow towards it to maximize light absorption for photosynthesis

Question 2

Why do plants have geotropisms?

Answer 2

They can sense gravity, so their roots and shoots grow in the right direction

Question 3

Why do plants have thigmotropisms?

Answer 3

Climbing plants have a sense of touich, so htey can find things to climb and reach the sunlight.

Question 4

What is herbivory?

Answer 4

Being eaten by animals

Question 5

What are alkaloids and how do they work?

Answer 5

Alkaloids are chemicals with bitter tastes, noxious smells or poisonous characteristics that deter or kill herbivores.

Question 6

What are tannins and how do they work?

Answer 6

Tannins taste bitter, and in some herbivores they can bind to proteins in the gut, making the plant harder to digest. Both of these things deter animals from eating the plant.

Question 7

What are alarm pheromones and what do they do?

Answer 7

Some plants release alarm pheromones into the air in response to herbivore grazing. This can cause nearby plants that detect these chemicals to start making chemical defenses such as tannins.

Question 8

What do some plants do to protect against herbivory when touched?

Answer 8

They fold up in response to being touched. It may help to knock off any small insects feeding on the plant, it may also scare animals trying to eat it.

Question 9

How do some plants respond to abiotic stress e.g extreme cold?

Answer 9

Some plants respond to extreme cold by producing their own form of antifreeze. The antifreeze proteins bind to ice crystals and lower the temperature that water freezes at, stopping more ice crystals from growing.

Question 10

What is a tropism?

Answer 10

A tropism is the response of a plant to a directional stimulus. Plants respond to stimuli by regulating their growth

Question 11

What is a positive tropism?

Answer 11

A positive tropism is growth towards the stimulus

Question 12

What is a negative tropism?

Answer 12

A negative tropism is growth away from the stimulus

Question 13

What parts of the plant are phototrophic?

Answer 13

shoots are positively phototrophic and roots are negatively phototrophic

Question 14

What parts of the plant are geotrophic?

Answer 14

shoots are negatively geotrophic and roots are positively geotrophic.

Question 15

What is a hydrotropism?

Answer 15

Plant growth in response to water. Roots are positively hydrotrophic

Question 16

What is a thermotropism?

Answer 16

Plant growth in response to temperature.

Question 17

What is a thigmotropism?

Answer 17

Plant growth in response to contact with an object.

Question 18

What are plant growth hormones?

Answer 18

They are chemicals that speed up or slow down plant growth.

Question 19

Where are plant growth hormones produced and used?

Answer 19

Growth hormones are produced in the growing regions of the plant and they move to where they are needed in the other parts of the plant.

Question 20

What does the hormone gibberellin do?

Answer 20

It stimulates seed germination, stem elongation, side shoot formation and flowering.

Question 21

What does the hormone auxin do?

Answer 21

They stimulate the growth of shoots by cell elongation. High concentrations of auxins inhibit growth in roots though.

Question 22

Where are auxins produced and give an example of an auxin?

Answer 22

Auxins are produced in the tips of shoots in flowering plants. One important example of an auxin is indoleacetic acid which, like other auxins, works by stimulating cell elongation.

Question 23

How does IAA work?

Answer 23

IAA is moved around the plant to control tropisms- it moved by diffusion and active transport over short distances, and via the phloem over long distances. The uneven distribution of IAA means there is uneven groth of the plant

Question 24

What does IAA do in shoots and roots?

Answer 24

In shoots, it elongates cells while in roots it inhibits growth.

Question 25

How would you carry out a practical investigation into phototropisms?

Answer 25

1. take nine wheat shoots in separate pots
2. cover the tips of 3 shoots with foil caps. leave 3 shoots without caps. wrap the base of the final 3 shoots with foil
3. set all the pots in front of a light source
4. record the amount of growth and direction of growth.

Question 26

How would you carry out practical investigations into geotropisms?

Answer 26

1. line three Petri dishes with damp cotton wool and add 10 cress seeds to each
2. tape a lead and wrap each one in foil
3. leave for a bit in a nice temperature
4. put one at a 90 angle and another at 45 and the last flat, make sure to label them
5. leave the seeds for 4 days and take a look at their shoot and root growth.
6. measure the amount of growth of the shoots and roots and the angle of growth.

Question 27

What is the apical bud?

Answer 27

The shoot tip at the top of a flowering plant

Question 28

What is the benefit of apical dominance?

Answer 28

Apical dominance prevents side shoots from growing, this saves energy and prevents side shots from the same plant competing with the shoot tip for light.

Question 29

What is apical dominance?

Answer 29

Auxins stimulate the growth of the apical bud and inhibit the growth of side shoots from lateral buds.

Question 30

What happens when you remove the apical bud?

Answer 30

The plant won’t produce auxins, so the side shoots will start growing by cell division and cell elongation.

Question 31

Why when the plant is very tall will side shoots start to grow nearer the bottom?

Answer 31

Auxins become less concentrated as they move away from the apical bud to the rest of the plant. If a plant grows very tall, the bottom of the plant will have a low auxin concentration so side shoots will start to grow near the bottom.

Question 32

How would you investigate the role of auxins in apical dominance?

Answer 32

• plant 30 plants in pots
• record number of side shoots
• for 10 remove the top and apply an auxin paste
• for 10 remove the top and apply a normal paste
• leave the last 10
• let grow for 6 days
• count the number of side shoots for each plant
• then analyse the data.

Question 33

Where are gibberellins produced?

Answer 33

They are produced in young leaves and in seeds. They stimulate seed germination, stem elongation, side shoot formation and flowering.

Question 34

How do gibberellins work?

Answer 34

They stimulate the stems of plants to grow by stem elongation, this helps plants to grow very tall. They don’t inhibit the plant growth in any way.

Question 35

How do gibberellins and seeds work?

Answer 35

They stimulate seed germination by triggering the breakdown of starch into glucose in the seed. The plant embryo in the seed can then use the glucose to begin respiring and release the energy it needs to grow. Gibberellins are inhibited by the hormone abscisic acid.

Question 36

How can the role of gibberellins be investigated?

Answer 36

• plant 40 plants (dwarf)
• leave 20 as they are
• leave 20 with a gibberellin solution
• calculate the rate of growth/difference in heights

Question 37

How are auxins and gibberellins synergistic?

Answer 37

This means that they work together to have a really big effect. Auxins and gibberellins work together to help plants grow very tall.

Question 38

How are auxins and gibberellins antagonistic?

Answer 38

This means they oppose each other’s actions. Gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots.

Question 39

What are deciduous plants?

Answer 39

They are plants that lose their leaves in winter.

Question 40

What is the benefit that deciduous plants have?

Answer 40

Losing their leaves helps plants conserve water during the cold part of the year, when it might be difficult to absorb water from the soil, and when there’s less light for photosynthesis.

Question 41

What triggers leaf loss?

Answer 41

Leaf loss is triggered by the shortening day length in the autumn

Question 42

How are auxins involved in leaf loss?

Answer 42

Auxins inhibit leaf loss- auxins are produced by young leaves. As the leaf gets older, less auxin is produced, leading to leaf loss.

Question 43

How is ethylene involved in leaf loss?

Answer 43

Ethene stimulates leaf loss- it is produced by ageing leaves. As the leaves get older, more ethene is produced. A layer of cells develops at the bottom of the leaf stalk. The abscission layer separates the leaf from the rest of the plant. Ethene stimulates the cells in the abscission layer to expand, breaking the cell walls and causing the leaf to fall off.

Question 44

How do guard cells open and close?

Answer 44

When the guard cells are full of water, they are plump and turgid and the pore is open. When the guard cells lose water, they become flaccid, making the pore close.

Question 45

How does ABA trigger stomatal closure?

Answer 45

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

Question 46

How does the fruit industry use hormones to control fruit ripening?

Answer 46

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 47

How do commercial farmers use auxin?

Answer 47

• they are used in selective weedkillers to make weeds grow too fast, so they cannot get enough nutrients or water so they die.
• Auxins are used as rooting hormones which make a cutting of the plant grow roots. The cutting can then be planted nad grown into a new plant. This means lots of the same plant can be grown quickly and cheaply from just one plant.