Homeostasis in Plants (Chapter 14)

Question 1

Why is homeostasis in plants important?

Answer 1

Because mesophyll cells in leaves require a constant supply of CO2 to make the best use of light energy for photosynthesis

Question 2

What controls the entry of CO2 into the leaves?

Answer 2

Stomata

Question 3

What is a stoma?

Answer 3

Technically, the hole between guard cells but the term is used to reefer to the 2 guard cells and the hole between them

Question 4

What are guard cells?

Answer 4

Highly specialised cells that respond to a wide range of environmental stimuli and ∴ control the internal atmosphere of the leaves

Question 5

Describe the daily rhythms of opening and closing shown by stomata (even when kept in constant light or dark)?

Answer 5

• Opening during the day maintain the inwards diffusion of CO2 and outward diffusion of oxygen and water vapour in transpiration
• The closure of stomata at night when photosynthesis cannot occur reduces rates of transpiration and conserves water

Question 6

What factors to stomata open in response to?

Answer 6

1) increasing light intensity

2) low [CO2] in the air spaces within the leaf

Question 7

What happen when stomata are open?

Answer 7

Leaves gain CO2 for photosynthesis, but tend to lost much water in transpiration

Question 8

What do stomata respond to?

Answer 8

Changes in environmental conditions

Question 9

What factors to stomata close in response to?

Answer 9

1) darkness
2) high [CO2] in the air spaces in the leaf
3) low humidity
4) high temperature
5) water stress - when the supply of water from the root is limited and/or there are high rates of transpiration

Question 10

What is the disadvantage of closing stomata?

Answer 10

During daylight, the supply of CO2 decreases ∴ the rate of photosynthesis decreases

Question 11

What is the advantage of closing stomata?

Answer 11

Water is retained inside the leaf, which is important during times of water stress

Question 12

What is each stomatal pore surrounded by?

Answer 12

2 guard cells

Question 13

When do guard cells open?

Answer 13

When they gain water to become turgid

Question 14

When do guard cells close?

Answer 14

When they lose water to become flaccid

Question 15

How do guard cells gain and lose water and what does this mean?

Answer 15

Osmosis ∴ a decrease in water potential is needed before water can enter the cell and open

Question 16

Explain how a stoma opens

Answer 16

1) ATP-powered proton pumps (transporter proteins) in the CSM actively transport H+ out of the guard cells
2) the decrease in [H+] inside the cells causes channel proteins (voltage-gated channels) in the CSM to open so that K+ move into the cell due to the electrochemical gradient
3) the extra K+ inside the guard cells lowers the water potential ∴ water moves into the cell through aquaporins in the CSM
4) this increases the volume of water and turgor inside the guard cells and the stoma opens

Question 17

Why do K+ move into the guard cells when there is a decrease in [H+]?

Answer 17

1) the removal of H+ ions has left the inside of the cell negatively charged compared with the outside
2) bc K+ have a positive charge, they are drawn down an electrical gradient towards the negatively charged region
3) they also diffuse into the cells down a concentration gradient - this combined gradient = an electrochemical gradient

Question 18

Why does the stoma open as water moves into the guard cells?

Answer 18

1) guard cells have unevenly thickened cell walls - the wall adjacent to the pore is very thick, whereas the wall furthest from the pore is thin
2) bundles of cellulose microfibrils are arranged as hoops around the cells so that, as the cell becomes turgid, these hoops ensure that the cell mostly increase in length and not diameter
3) bc the ends of the guard cells are joined and the thin outer walls bend more readily than the thick inner walls, the guard cells become curved, opening the pore between the cells

Question 19

When/how to stomata close?

Answer 19

1) stomata close when the H+ pump proteins stop and K+ leave the guard cells and enter neighbouring cells
2) now, there is a water potential gradient in the opposite direction
3) ∴ water leaves the guard cells, so that they become flaccid and close the stomata

Question 20

How does the release of ABA lead to the closing of stomata?

Answer 20

1) guard cells have ABA receptors on their CSMs and when ABA binds with these, it inhibits the proton pumps to stop H+ being pumped out
2) ABA also stimulates opening of voltage-gated channels in the CSM and ∴ the movement of Ca2+ into the cytoplasm through the CSM and tonoplast
3) Ca2+ acts a second messenger to activate channel protein to open that allow negatively charged ions to leave the guard cells
4) this stimulates the opening of channel proteins which allow the movement of K+ out of the cell
5) at the same time, Ca2+ also stimulate the closure of the channel proteins that allow K+ to enter
6) the loss of ions raises the water potential of the cells, water passes out by osmosis, the guard cells become flaccid and the stomata close

Question 21

Why does stomatal closure only occur when reducing the loss of water vapour and conserving water is the most important factor?

Answer 21

Because stomatal closure reduces the rate of photosynthesis and transpiration

Question 22

What is transpiration used for?

Answer 22

Cooling the plant and maintaining the transpiration stream that supplies water and mineral ions to the leaf

Question 23

What happens in conditions of water stress?

Answer 23

The hormone, abscisic acid (ABA) is produced/secreted in plants to stimulate stomatal closure

Question 24

Where is ABA found and synthesised?

Answer 24

ABA is found in every part of the plant and synthesised in almost all cells with chloroplasts or amyloplasts

Question 25

Why is ABA a stress hormone?

Answer 25

Bc it coordinates the responses to (water) stress

Question 26

Under what difficult environmental conditions would a plant respond by secreting ABA?

Answer 26

Very high temperatures or much reduced water supplies

Question 27

What does high [ABA] stimulate?

Answer 27

The stomata to close, reducing the loss of water vapour from the leaf

Question 28

Where are stomata located on a leaf?

Answer 28

In the lower (+upper) epidermis

Question 29

Why do K+ ions have to be moved into the guard cells?

Answer 29

Because K+ affect water potential more than H+