chapter 16 pt 1

Question 1

tropisms

Answer 1

plants showing directional growth in response to environmental cues such as light and gravity

Question 2

The key limitations on plants are that

Answer 2

they are rooted - they are not mobile, and they do not have a rapidly responding nervous system.
They are, however, coordinated organisms that show clear responses to their environment, communication between cells, and even communication between different plants.

Question 3

The timescales of most plant responses are

Answer 3

slower than animal responses, but they still respond as a result of complex chemical interactions.
Plants have evolved a system of hormones - chemicals that are produced in one region of the plant and transported both through the transport tissues and from cell to cell and have an effect in another part of the plant.

Question 4

Important plant hormones include

Answer 4

auxins, gibberellins, abscisic acid (ABA), and ethene.

Question 5

Plants produce chemicals which

Answer 5

which signal to other species - for example, to protect themselves from attack by insect pests - and may communicate with other plants. They also produce chemical defences against herbivores.
In plant responses, chemicals are essential.

Question 6

Plant hormones work at very

Answer 6

low concentrations, so isolating them and measuring changes in concentrations is not easy.
The multiple interactions between the different chemical control systems also make it very difficult for researchers to isolate the role of a single chemical in a specific response.

Question 7

key role of plant hormones highlighted.

Answer 7

Question 8

Plant hormones and seed germination:
For a plant to start growing, the seed must germinate.
part 1

Answer 8

When the seed absorbs water, the embryo is activated and begins to produce gibberellins.
They in turn stimulate the production of enzymes that break down the food stores found in the seed.
The food store is in the cotyledons in dicot seeds and the endosperm in monocot seeds.
The embryo plant uses these food stores to produce ATP for building materials so it can grow and break out through the seed coat.

Question 9

Plant hormones and seed germination:
For a plant to start growing, the seed must germinate.
part 2

Answer 9

Evidence suggests that gibberellins switch on genes which code for amylases and proteases - the digestive enzymes required for germination.
There is also evidence suggesting that another plant hormone, ABA, acts as an antagonist to gibberellins (interferes with the action of gibberellin), and that it is the relative levels of both hormones which determine when a seed will germinate.

Question 10

Experimental evidence supporting the role of gibberellins in the germination of seeds includes:

Answer 10

Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins.
These seeds do not germinate.
If gibberellins are applied to the seeds externally, they then germinate normally.

If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot make the gibberellins needed for them to break dormancy.
If the inhibition is removed, or gibberellins are applied, the seeds germinate.

Question 11

diagram of the role of gibberellins in the germination of seeds

Answer 11

Question 12

Auxins:

Answer 12

• Auxins such as indoleacetic acid (IAA) are growth stimulants produced in plants.
• Small quantities can have powerful effects.
• They are made in cells at the tip of the roots and shoots, and in the meristems.
• Auxins can move down the stem and up the root both in the transport tissue and from cell to cell.
• The effect of the auxin depends on its concentration and any interactions it has with other hormones.
• Auxins have a number of major effects on plant growth.

Question 13

Function of auxin

Answer 13

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.
Auxin molecules bind to specific receptor sites in the plant cell membrane, causing a fall in the pH to about 5.
This is the optimum pH for the enzymes needed to keep the walls very flexible and plastic.
As the cells mature, auxin is destroyed.
As the hormone levels fall, the pH rises so the enzymes maintaining plasticity become inactive.
As a result, the wall becomes rigid and more fixed in shape and size and the cells can no longer expand and grow.

Question 14

effect of auxin on apical shoot growth diagram

Answer 14

Question 15

High concentrations of auxins suppress the growth of lateral shoots…

Answer 15

• This results in apical dominance.
• Growth in the main shoot is stimulated by the auxin produced at the tip so it grows quickly.
• The lateral shoots are inhibited by the hormone that moves back down the stem, so they do not grow very well.
• Further down the stem, the auxin concentration is lower and so the lateral shoots grow more strongly.

Question 16

experimental evidence for the role of auxins in apical dominance

Answer 16

if the apical shoot is removed, the auxin-producing cells are removed and so there is no auxin.
- As a result, the lateral shoots, freed from the dominance of the apical shoot, grow faster.
- If auxin is applied artificially to the cut apical shoot, apical dominance is reasserted and lateral shoot growth is suppressed.

Question 17

Low concentrations of auxins promote…

Answer 17

root growth
Up to a given concentration, the more auxin that reaches the roots, the more they grow.
Auxin is produced by the root tips and auxin also reaches the roots in low concentrations from the growing shoots.
If the apical shoot is removed, then the amount of auxin reaching the roots is greatly reduced and root growth slows and stops.
Replacing the auxin artificially at the cut apical shoot restores the growth of the roots. High auxin concentrations inhibit root growth.

Question 18

diagram of apical dominance

Answer 18