3.6.1.1 Plant responses

Question 1

Phototropism

Answer 1

• shoots are positively phototrophic & grow toward sunlight

- roots are negatively phototrophic & grow away from light

Question 2

Geotropism

Answer 2

• shoots are negatively geotropic & grow upwards

- roots are positively geotropic & grown downwards

Question 3

How do auxins stimulate growth of shoots?

Answer 3

• by cell elongation
• auxins stimulate proton pumps in the cell membrane
• protons are pumped into the cell wall from the cytoplasm
• the protons activate expansins (proteins / enzymes)
• expansins break some of the hydrogen bonds between chains of cellulose
• cell walls become ‘ loose and stretchy’ so the cells become longer

Question 4

How do auxins affect root growth?

Answer 4

inhibit root growth

Question 5

How do plants detect the direction of gravity?

Answer 5

• cells near root tips contain amyloplasts (dense organelles / starch grains)
• amyloplasts sink to the bottom of the cell they are in
• this enables the creels to detect the direction of gravity
• IAA is actively transported to the location of the amyloplasts