Learning outcomes

2.39
understand gas exchange (of carbon dioxide and oxygen) in relation to respiration and photosynthesis
2.40
understand that respiration continues during the day and night, but that the net exchange of carbon dioxide and oxygen depends on the intensity of light
2.41
explain how the structure of the leaf is adapted for gas exchange
2.42
describe the role of stomata in gas exchange
2.43
describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator
Remind yourself of the structure of the leaf (Section 2e - Nutrition in Flowering Plants).

Remember that CO2 and O2 diffuse in and out of leaves through stomata. Remember that CO2 is used in photosynthesis and produced by respiration, whereas O2 is used in respiration and produced in photosynthesis!
Both processes run all the time. So the net amount of glucose the plant produces (i.e. the amount it gets to use for growth etc) is governed by the formula;

Net Glucose = Total production – Amount used in respiration

The amount the plant uses in respiration in nearly constant. However, the total production is not. It is dependent on the rate limiting factors (i.e. light intensity, CO2 level, water availability, temperature etc). In winter the net glucose production is virtually zero, whereas in summer the net glucose production is large. Therefore, plants grow a lot during the summer and not much during winter!


Leaf Structure and its adaptation for gas exchange


Air Spaces - Increase the surface area inside the leaf to maximise gas exchange across the surface of the Spongy Mesophyll cells

Stoma - Allow exchange of CO2 and O2

Mesophyll cells - Have a large surface area and moist surfaces, which speeds gas exchange

Leaf shape - Leaves are thin, which increases diffusion speeds and leaves also have a very large surface area, which also increases diffusion speed.

Stomata distribution - Stomata are spread out over leaves, which means waste gases produced by the leaf can diffuse away quickly, this stops the build-up of excreted products, which would slow gas exchange.


You need to know an experiment which will show the effect of light intensity on the rate of gas exchange. The best example is to seal two leaves (still attached to the plant) in separate plastic bags with some bicarbonate indicator solution. One of the bags is black and the other is translucent. The leaf in the black bag produces CO2 via respiration and the colour of the bicarbonate indicator changes quickly to yellow. The leaf in the translucent bag produces O2 via photosynthesis and the bicarbonate indicator solution changes to red slowly.

Bicarbonate Indicator colours:

Red in the presence of O2
Yellow in the presence of CO2

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