Homeostasis in PLANTS Flashcards
plants carry out homeostasis why?
they need to maintain a constant internal environment
For example, mesophyll cells in leaves require a constant supply of carbon dioxide for photosynthesis
What does stomata (specifically guard cells) control?
the diffusion of gases in and out of leaves
this means stomata control the entry of CO2 into leaves
response of guard cells and stomata
What are the environmental stimuli causing stomata to open?
- increasing light intensity
- low CO2 conc in the air spaces within the leaf
What are the environmental stimuli causing stomata to close?
- darkness
- high CO2 conc in the air spaces within the leaf
- low humidity
- high temp
- water stress
what is water stress?
when the supply of water from the roots is limited/ or there are high rates of transpiration
What does the regulation of stomatal aperture balance the need for?
CO2 uptake by diffusion with the need to minimise loss by transpiration
What is the advantage of stomata open during the day?
leaves gain CO2 for photosynthesis
what is the advantage of stomata closed during the day?
water is retained inside the leaf, which is important in times of water stress
what is a disadvantage of stomata open during the day?
leaves lose large amounts of water by transpiration
what is the disadvantage of stomata closed during the day?
Supply of CO2 decreasing so the rate of photosynthesis decreases
Stomata open and close in a
daily rhythm
Even when the plant is kept in constant light or constant darkness, the daily rhythm of opening and closing of the stomata continues
Opening of stomata during the day:
maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen
allows the outward diffusion of water vapour in transpiration
Closing of stomata at night when photosynthesis cannot occur:
reduces the rate of transpiration
conserves water
what is each stoma surrounded by?
2 guard cells
Guard cells have the following features
- Thick cell walls facing the air outside the leaf and the stoma
- Thin cell walls facing adjacent epidermal cells
- Cellulose microfibrils arranged in bands around the cell
- Cell walls have no plasmodesmata
- Cell surface membrane is often folded and contains many channel and carrier proteins
- Cytoplasm has a high density of chloroplasts and mitochondria
- Chloroplasts have thylakoids but with few grana (unlike those in mesophyll cell chloroplasts)
- Mitochondria have many cristae
- Several small vacuoles rather than one large vacuole
the mechanism to open stomata
- ATP- powered proton pump transports H+ ions out of cell
- K+ ions diffuse into cell down electrochemical grad
- this lowers water potential inside the cell
- water enters by osmosis
- the turgid pressure in the cell increases
- the thin outer walls bend more easily than the thick inner walls. the cell becomes curved and the stoma opens
Mechanism to close stomata
When certain environmental stimuli are detected (that lead to the closing of the stomata), the proton pumps in the guard cell surface membranes stop actively transporting hydrogen (H+) ions out of the guard cell
The potassium (K+) ions leave the guard cells
The water potential gradient is now reversed and water leaves the guard cells by osmosis
This causes the guard cells to become flaccid, closing the stoma
during times of water stress what hormone is produced by plants?
abscisic acid (ABA)
what does abscisic acid (ABA) stimulate?
the closing of their stomata
what type of environmental conditions can cause water stress?
very high temperatures or reduced water supplies
What type of receptors do guard cells have?
ABA receptors on CSM
What does ABA bind to?
ABA binds with ABA receptors, inhibiting the proton pumps and therefore stopping the active transport of H+ ions out of the guard cells
what does ABA cause ?
Ca2+ ions to move into the cytoplasm of the guard cells through CSM
what doe Ca2+ ions act as?
secondary messengers
The calcium ions act as second messengers:
- what do they cause to open ?
They cause channel proteins to open that allow negatively charged ions to leave the guard cells
This stimulates the opening of further channel proteins that allow potassium (K+) ions to leave the guard cells
The calcium ions also stimulate the closing of channel proteins that allow potassium (K+) ions to enter the guard cells
This loss of ions increases the water potential of the guard cells
Water leaves the guard cells by osmosis
The guard cells become flaccid, causing the stomata to close
summary of closure of a stoma in response to ABA
- Abscisic acid binds to ABA receptors
- Ca ions enter guard cells
- K+ ions leave guard cells
- water leaves guard cells by osmosis due to increased water potential in guard cells
- guard cells become flaccid
