Flashcards in Homeostasis in Plants (Chapter 14) Deck (29)
Why is homeostasis in plants important?
Because mesophyll cells in leaves require a constant supply of CO2 to make the best use of light energy for photosynthesis
What controls the entry of CO2 into the leaves?
What is a stoma?
Technically, the hole between guard cells but the term is used to reefer to the 2 guard cells and the hole between them
What are guard cells?
Highly specialised cells that respond to a wide range of environmental stimuli and ∴ control the internal atmosphere of the leaves
Describe the daily rhythms of opening and closing shown by stomata (even when kept in constant light or dark)?
- 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
What factors to stomata open in response to?
1) increasing light intensity
2) low [CO2] in the air spaces within the leaf
What happen when stomata are open?
Leaves gain CO2 for photosynthesis, but tend to lost much water in transpiration
What do stomata respond to?
Changes in environmental conditions
What factors to stomata close in response to?
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
What is the disadvantage of closing stomata?
During daylight, the supply of CO2 decreases ∴ the rate of photosynthesis decreases
What is the advantage of closing stomata?
Water is retained inside the leaf, which is important during times of water stress
What is each stomatal pore surrounded by?
2 guard cells
When do guard cells open?
When they gain water to become turgid
When do guard cells close?
When they lose water to become flaccid
How do guard cells gain and lose water and what does this mean?
Osmosis ∴ a decrease in water potential is needed before water can enter the cell and open
Explain how a stoma opens
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
Why do K+ move into the guard cells when there is a decrease in [H+]?
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
Why does the stoma open as water moves into the guard cells?
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
When/how to stomata close?
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
How does the release of ABA lead to the closing of stomata?
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
Why does stomatal closure only occur when reducing the loss of water vapour and conserving water is the most important factor?
Because stomatal closure reduces the rate of photosynthesis and transpiration
What is transpiration used for?
Cooling the plant and maintaining the transpiration stream that supplies water and mineral ions to the leaf
What happens in conditions of water stress?
The hormone, abscisic acid (ABA) is produced/secreted in plants to stimulate stomatal closure
Where is ABA found and synthesised?
ABA is found in every part of the plant and synthesised in almost all cells with chloroplasts or amyloplasts
Why is ABA a stress hormone?
Bc it coordinates the responses to (water) stress
Under what difficult environmental conditions would a plant respond by secreting ABA?
Very high temperatures or much reduced water supplies
What does high [ABA] stimulate?
The stomata to close, reducing the loss of water vapour from the leaf
Where are stomata located on a leaf?
In the lower (+upper) epidermis