Gas Exchange (plants)

Question 1

how do plants exchange gases?

Answer 1

BY DIFFUSION

• e.g. when a plant photosynthesises, it uses up lots of CO2, so there’s hardly any in the leaf.
• this makes more CO2 move into the leaf by diffusion (area of high conc to area of lower conc)
• At same time lots of O2 is being made as a waste product. some is used in respiration and the rest diffuses out through the stomata (area of high conc to area of low conc)

Question 2

how are leaves adapted for efficient gas exchange?

Answer 2

1. BROAD = large SA for diffusion
2. THIN = short diffusion distance
3. AIR SPACES = spongey mesophyll layer. lets gases move easily between cells. also increases SA for gas exchange.
4. STOMATA =lower epidermis. let gases diffuses in and out and allow water to escape.
5. GUARD CELLS = control stomata.

Question 3

what are the role of STOMATA in gas exchange?

Answer 3

1. begin to close as it gets dark. photosynthesis can’t happen in the dark so they don’t need to be open to let CO2 in.
2. also close when water levels are low.this stops the plant from photosynthesising and therefore drying out and dying.
3. GUARD CELLS increase in volume to open stomata and decrease in volume to close them.

Question 4

what does the net exchange of gases depend on?

Answer 4

LIGHT INTENSITY.

• photosynthesis only happens during the day i.e when theres light available.
• but plants must respire all the time to get the energy they need to live.
• during the day, when light intensity is high plants make more oxygen by photosynthesis than they use in respiration. so in daylight, they release oxygen. they also use up more carbon dioxide than they produce, so they take in carbon dioxide.
• at night when light intensity is low, plants only respire. they take in oxygen and release carbon dioxide.

Question 5

what does hydrogen-carbonate indicator show?

Answer 5

CHANGES IN CO2 CONCENTRATION.

• a solution of hydrogen-carbonate indicator in air with normal CO2 conc is ORANGE.
• if the CO2 conc of the air increases, more CO2 will dissolve in it and it becomes YELLOW.
• and as the CO2 conc of the air decreases, CO2 will come out of the solution and it becomes purple.

Question 6

PRACTICAL: investigate the effect of light on gas exchange from a leaf using hydrogen-carbonate indicator METHOD

Answer 6

1. add the same volume of hydrogen-carbonate indicator to four boiling tubes
2. put similar sized, healthy looking leaves into 3 of the tubes and seal with a rubber bung. trap the leaf stem with the bung to stop in falling into the solution. keep the fourth tube empty as a control.
3. completely wrap tube in foil, and a second tube in gauze.
4. place all tubes in bright light. this will let plenty of light on to the uncovered leaf, and a little light onto the gauze covered. the leaf covered in foil will get no light.
5. leave for an hour then check results.

Question 7

PRACTICAL: investigate the effect of light on gas exchange from a leaf using hydrogen-carbonate indicator RESULTS

Answer 7

• there shouldn’t be any change in the colour of the control tube.
• you’d expect the indicator in the darkened tube to go yellow. respiration will still take place but there will be no photosynthesis so the CO2 conc in the tube will increase.
• you’d expect the indicator in the shaded tube to stay similar colour. (orange). with a little photosynthesis and some respiration, roughly equal amounts of CO2 will be taken up and produced by the leaf so the conc wont change much.
• you’d expect the indicator in the uncovered tube to go purple. there will be some respiration but lots of photosynthesis, leading to net uptake of CO2 by the leaf. this will lower the CO2 conc in the tube.