Gaseous Exchange in Humans and Plants
Senior 2 · Biology
Gaseous Exchange in Humans and Plants
Lesson 5 of 5
20 min
Let's begin
Plants need oxygen for respiration all the time, day and night, just as you do. In the light they also take in carbon dioxide for photosynthesis. So which gas does a leaf take in, and which does it give out? The answer changes with the light, and you can see it change colour in a test tube.
Hydrogencarbonate indicator is orange-red when in balance with ordinary air. It turns yellow when carbon dioxide is added to it and purple when carbon dioxide is removed.
Worked example
Four sealed tubes of orange-red indicator are set up, each with a leaf held above the liquid on a wire gauze (except D). After two hours:
| Tube | Conditions | Colour | Explanation |
|---|---|---|---|
| A | leaf in bright light | purple | photosynthesis is faster than respiration, so the leaf removes carbon dioxide |
| B | leaf, tube wrapped in foil (dark) | yellow | only respiration happens, so the leaf adds carbon dioxide |
| C | leaf in dim light | orange-red | photosynthesis and respiration are equal, so there is no overall change |
| D | no leaf, bright light | orange-red | the control: shows the light alone does not change the indicator |
The light intensity at which photosynthesis exactly balances respiration (tube C) is called the compensation point. Plants pass through it around dawn and dusk.
| Condition | Processes | Overall gas exchange |
|---|---|---|
| Bright light | photosynthesis faster than respiration | takes in carbon dioxide, gives out oxygen |
| Compensation point | photosynthesis equals respiration | no overall exchange |
| Dark (night) | respiration only | takes in oxygen, gives out carbon dioxide |
Each stoma is a pore between two bean-shaped guard cells. The inner wall of each guard cell, next to the pore, is thicker and less stretchy than the outer wall.
This is a trade-off: open stomata let carbon dioxide in for photosynthesis but also let water vapour out.
Worked example
A student peels the lower epidermis from a leaf and counts 12 stomata in a microscope field of view with an area of 0.2 mm². Stomatal density = 12 ÷ 0.2 = 60 stomata per mm².
Tip
Light: photosynthesis beats respiration, so carbon dioxide goes in and oxygen comes out. Dark: respiration only, so oxygen goes in and carbon dioxide comes out.
Hydrogencarbonate indicator: Yellow = more CO₂, Purple = less CO₂. Turgid guard cells = open stoma.
A leaf in a sealed tube of hydrogencarbonate indicator is kept in the dark for two hours. What colour will the indicator become, and why?
Solution: Yellow. In the dark the leaf only respires, releasing carbon dioxide into the tube.
What is the compensation point?
Solution: The light intensity at which the rate of photosynthesis equals the rate of respiration, so there is no overall exchange of carbon dioxide or oxygen.
Explain why stomata close when a plant is short of water, and state one disadvantage.
Solution: The guard cells lose water, become flaccid and close the pore, which reduces water loss. The disadvantage is that carbon dioxide can no longer enter, so photosynthesis slows.
A field of view of area 0.25 mm² contains 20 stomata. Calculate the stomatal density.
Solution: 20 ÷ 0.25 = 80 stomata per mm².
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
