F215:04:02 How plant respond to the environment

Question 1

Why are plants cell often limited in ability to divide and expand (unlike animal cells)?

Answer 1

As they have cells walls

It means they can only expand where there are meristems

Question 2

What is are meristems?

Answer 2

immature cells that are still capable of dividing

Question 3

What are the 4 types of meristem cells?

Answer 3

Apical
Lateral Bud
Lateral Intercalary

Question 4

Where are apical meristems found?

Answer 4

at the tips or spices of roots and shoots and are responsible for the roots and shoots getting longer

Question 5

What are apical meristems responsible for?

Answer 5

the roots and shoots getting longer

Question 6

Where are lateral bud meristems found?

Answer 6

found in the buds

allow side buds to grow

Question 7

What are lateral bud meristems responsible for?

Answer 7

The growth of side buds

Question 8

Where are lateral meristems found?

Answer 8

in a cylinder near the outside of roots and shoots

make them wider

Question 9

What are lateral meristems responsible for?

Answer 9

making roots and shoots longer

Question 10

Where are intercalary meristems found?

Answer 10

between the nodes (where the leaves and buds branch off the stem)
Growth between the nodes is responsible for the shoots getting longer

Question 11

What are intercalary meritstems responsible for?

Answer 11

the shoots getting longer

Question 12

In a shoot, where does cell division and cell elongation occur?

Answer 12

division: closest to the apex
elongation: just behind the apex

Question 13

Where are auxins produced?

Answer 13

At the tip of the apex

Question 14

What is the role of auxins?

Answer 14

They cause cells to elongate, and therefore the shoots to grow

Question 15

How do auxins travel?

Answer 15

Either by diffusion or active transport

Question 16

What is the extent of which the cells elongate proportional to?

Answer 16

The concentration of auxins

Question 17

How do auxins encourage cell elongation?

Answer 17

They increase the stretchiness of the cell wall by promoting the active transport of hydrogen ions, by an ATPase enzyme on the plasma membrane into the cell wall
The resulting low pH provides optimum conditions for expansins to work

Question 18

What are expansins?

Answer 18

wall loosening enzymes

Question 19

What do expansins do?

Answer 19

Break bonds within the cellulose so the walls become less rigid and can expand as the cell takes in water

Question 20

What happens in a phototropic response?

Answer 20

a shoot bends towards the light source

Question 21

How do shoots bend towards a light source?

Answer 21

the cells on the shaded side elongate faster than those on the illuminated side
As light causes auxins to be transported to the shaded side where they promote elongation

Question 22

What is thought to cause a redistribution of auxin?

Answer 22

Althought there is still some uncertainty, two enzymes have been identified (phototropin 1 and 2) whos activity is promoted by blue light, so there is a lot of phototropin 1 activity on a light side and progressively less heading to the dark side
This gradient is thought to be the cause

Question 23

What do cytokinins do?

Answer 23

they stop the leaves of deciduous trees from senescing

Question 24

What does senescing mean?

Answer 24

ageing

turning brown and dying

Question 25

How do cytokinins stop the leaves of deciduous trees from senescing?

Answer 25

by making sure the leaf acts as a sink for phloem transport

Question 26

Why is it important that cytokinins make sure leaves act as a sink for phloem transport?

Answer 26

ensures the plant gains a good supply of nutrients

Question 27

What is abscission?

Answer 27

When leaves start to shred

Question 28

What happens if cytokinin levels drop?

Answer 28

the supply of nutrients dwindle and senecence begins

This is usually followed by leaves beginning to shed

Question 29

Despite the fact auxin inhibits abscission by acting on cells in the abscission zone, why is it not entirely effective?

Answer 29

• Leaf senescence causes auxin production at the tip of the leaf to drop
• This causes cells in the abscission zone more sensitive to another growth substance called ethene
• If auxin concentration drops, causes an increase in ethene production as well
• These increases cause an increase in production of the enzyme cellulase which digests the walls of the cells in the abscission zone and eventually separates the petiole from the stem

Question 30

Tropic responses are described as growth responses

Do they actually involve growth?

Answer 30

‘growth’ is limited to getting larger, and is irreversible as it is an increase in cell division whereas tropic responses are not limited to just getting larger

Question 31

Roots are negatively phototropic. However, auxins still control their response and are still thought to travel to the more shaded side of the root.
Suggest how auxin in roots may be responsible for negative phototropisms

Answer 31

Roots grow in response to low levels of auxins, but high levels of auxins inhibit growth
Auxin migrates away from the light side of the root to the dark side of the root but promotes growth in the light side
So the root bends away from the light and into the soil where it is dark

Question 32

What effect do auxins have on cells?

Answer 32

They cause cell elongation by increasing the stretchiness of the cell wall by promoting H+ active transport by the ATPase enzyme on the plasma membrane into the cell wall
This lowered pH provides optimum conditions for wall loosening enzymes to work which break the bonds with cellulose (and at the same time the increased H+ ions disrupt the hydrogen bonds within cellulose)
So the walls become less rigid and can expand as the cell takes in water

Question 33

Explain how auxin is involved in shoot growth

Answer 33

Auxins stimulate shoot growth by causing cell elongation

The extent to which cells elongate and the shoot elongates is proportional to the concentration of auxins

Question 34

Explain how auxin is involved in leaf abscission

Answer 34

Usually auxin inhibits abscission by acting on cells in the abscission zone
However:
- Leaf senescence causes auxin production at the tip of the leaf to drop
- This causes cells in the abscission zone more sensitive to another growth substance called ethene
- If auxin concentration drops, causes an increase in ethene production as well
- These increases cause an increase in production of the enzyme cellulase which digests the walls of the cells in the abscission zone and eventually separates the petiole from the stem

Question 35

Explain how positive phototropisms in plant shoots and negative phototropism in plant roots would be selected for

Answer 35

Plants whose shoots grow towards the light would have the advantage over others as they can photosynthesise more
Plants whose roots are negatively phototropic have the advantage as they would grow down and gain more water and stabilise the plant more
Allows them to survive longer and more likely to reproduce
After many generations, the whole populations of plants would respond to light through phototropisms