Homeostasis in PLANTS

Question 1

plants carry out homeostasis why?

Answer 1

they need to maintain a constant internal environment

For example, mesophyll cells in leaves require a constant supply of carbon dioxide for photosynthesis

Question 2

What does stomata (specifically guard cells) control?

Answer 2

the diffusion of gases in and out of leaves

this means stomata control the entry of CO2 into leaves

Question 3

response of guard cells and stomata

What are the environmental stimuli causing stomata to open?

Answer 3

• increasing light intensity
• low CO2 conc in the air spaces within the leaf

Question 4

What are the environmental stimuli causing stomata to close?

Answer 4

• darkness
• high CO2 conc in the air spaces within the leaf
• low humidity
• high temp
• water stress

Question 5

what is water stress?

Answer 5

when the supply of water from the roots is limited/ or there are high rates of transpiration

Question 6

What does the regulation of stomatal aperture balance the need for?

Answer 6

CO2 uptake by diffusion with the need to minimise loss by transpiration

Question 7

What is the advantage of stomata open during the day?

Answer 7

leaves gain CO2 for photosynthesis

Question 8

what is the advantage of stomata closed during the day?

Answer 8

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

Question 9

what is a disadvantage of stomata open during the day?

Answer 9

leaves lose large amounts of water by transpiration

Question 10

what is the disadvantage of stomata closed during the day?

Answer 10

Supply of CO2 decreasing so the rate of photosynthesis decreases

Question 11

Stomata open and close in a

Answer 11

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

Question 12

Opening of stomata during the day:

Answer 12

maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen

allows the outward diffusion of water vapour in transpiration

Question 13

Closing of stomata at night when photosynthesis cannot occur:

Answer 13

reduces the rate of transpiration
conserves water

Question 14

what is each stoma surrounded by?

Answer 14

2 guard cells

Question 15

Guard cells have the following features

Answer 15

• 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

Question 16

the mechanism to open stomata

Answer 16

1. ATP- powered proton pump transports H+ ions out of cell
2. K+ ions diffuse into cell down electrochemical grad
3. this lowers water potential inside the cell
4. water enters by osmosis
5. the turgid pressure in the cell increases
6. the thin outer walls bend more easily than the thick inner walls. the cell becomes curved and the stoma opens

Question 17

Mechanism to close stomata

Answer 17

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

Question 18

during times of water stress what hormone is produced by plants?

Answer 18

abscisic acid (ABA)

Question 19

what does abscisic acid (ABA) stimulate?

Answer 19

the closing of their stomata

Question 20

what type of environmental conditions can cause water stress?

Answer 20

very high temperatures or reduced water supplies

Question 21

What type of receptors do guard cells have?

Answer 21

ABA receptors on CSM

Question 22

What does ABA bind to?

Answer 22

ABA binds with ABA receptors, inhibiting the proton pumps and therefore stopping the active transport of H+ ions out of the guard cells

Question 23

what does ABA cause ?

Answer 23

Ca2+ ions to move into the cytoplasm of the guard cells through CSM

Question 24

what doe Ca2+ ions act as?

Answer 24

secondary messengers

Question 25

The calcium ions act as second messengers:
- what do they cause to open ?

Answer 25

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

Question 26

summary of closure of a stoma in response to ABA

Answer 26

1. Abscisic acid binds to ABA receptors
2. Ca ions enter guard cells
3. K+ ions leave guard cells
4. water leaves guard cells by osmosis due to increased water potential in guard cells
5. guard cells become flaccid