9.3 chemical control in plants

Question 1

where are the main areas of cell division in plants?

Answer 1

meristems that are just behind the tips of the roots or shoots

Question 2

what are the three main plant hormones?

Answer 2

• auxins
• gibberellins
• cytokinins

Question 3

what are auxins?

Answer 3

plant hormones that act as powerful growth stimulants that are involved in apical dominance and tropic responses

Question 4

how do auxins work?

Answer 4

1- auxins synthesised in the meristem cells and diffuse away from tip
2- auxins bind to receptor sites on cell surface membranes activating hydrogen ions to be pumped into the membrane
3- low pH of the cell breaks bonds between cellulose microfibrils meaning the cell wall is very flexible and plastic
4- cells expand as they absorb water
5- enzymes destroy auxins and elongation stops

Question 5

what are giberellins?

Answer 5

plant hormones that act as growth regulators and promote the growth of fruit and are involved in breaking the dormancy in seeds during germination

Question 6

how do giberellins work?

Answer 6

1- seed absorbs water and swells as the embryo becomes active
2- the embryo secretes giberrelin that diffuses to aleurone layer
3- gibberellin stimulates aleurone layer to produce amylase that diffuses into endosperm and breaks down food stores to provide embryo with materials for respiration and growth
4- enzymes produced in response to gibberellin digest the endosperm
5- products released from endosperm are used by emrbyo to make new cells and germinate

Question 7

what are cytokinins?

Answer 7

plant hormones that promote cell division in the atypical meristems and promote lateral bud development

Question 8

what is synergy?

Answer 8

when two hormones work together

Question 9

what is antagonism?

Answer 9

when hormones work oppositely to create a balance

Question 10

what is apical dominance?

Answer 10

• one lead shoot in a young plant grows bigger and faster than all the others and the growth of all other lateral buds is inhibited, as a result of the high auxin levels produced by the first shoot to emerge
• the auxin acts antagonistically with cytokinin, which stimulates the development of the lateral buds

Question 11

what is a phytochrome?

Answer 11

a plant pigment that reacts with different types of light and affects the responses of the plant

Question 12

what are the two forms of phytochrome?

Answer 12

Pr : which absorbs red light
Pfr : which absorbs far-red light

Question 13

which pigement can convert itself back in darkness?

Answer 13

Pfr –>Pr

Question 14

which pigment is biologically active?

Answer 14

Pfr