The role of diffusion in gas exchange is simple:
Gases move from an area dense in gas to an area less dense in gas. This is prevalent in gas exchange in the lungs. The oxygen in the alveoli diffuse into the bloodstream as it is less dense with oxygen. On the other hand, the carbon dioxide in the bloodstream diffuses into the alveoli as they are less dense with carbon dioxide.
Respiration is the conversion of oxygen and glucose into carbon dioxide and water. Oxygen has been exchanged with carbon dioxide.
On the other hand, photosynthesis is the conversion of carbon dioxide and light into oxygen and glucose.
Equation for photosynthesis:
Carbon dioxide and light = glucose and oxygen
The rate of photosynthesis is affected in three ways:
1. Light intensity
2. Temperature
3. Concentration of carbon dioxide in the air.
Temperature would affect photosynthesis as the enzymes involved in the chemical reactions during photosynthesis would not work efficiently in cool temperatures while becoming denatured in hot temperatures.
A high concentration of carbon dioxide would maximise the rate of photosynthesis.
A high light intensity would increase the rate of photosynthesis.
Image courtesy of BBC Bitesize.
| Photosynthesis v respiration | Overall result | |
|---|---|---|
| Dark |
Respiration
No photosynthesis |
Oxygen taken in
Carbon dioxide given out |
| Dim light | Photosynthesis rate equals respiration rate | Neither gas is taken in or given out, as each cancels the other out |
| Bright light | Photosynthesis rate greater than respiration rate |
Carbon dioxide taken in
Oxygen given out |
Plants and how they are adapted for gas exchange
Plants are thin, enabling a short diffusion distance for carbon dioxide to be absorbed. There are stomata situated at the bottom of the leaf to allow the gases to diffuse into the leaves. When the guard cell (of the stomata) is turgid, it bends and opens, allowing the gas to diffuse into the leaf.
Stomata in gas exchange
Photosynthesis produces oxygen as a byproduct. However, this oxygen is used in respiration. But, photosynthesis can only work in light therefore gas exchange only occurs during the day. As photosynthesis only occurs in the day, guard cells only absorb water during light hours. When they absorb water, they become turgid and the stomata opens. [stoma for singular]. During the night, the guard cells lose water through respiration, as only respiration continues during the night. Therefore, the guard cells become flaccid and close the stomata, preventing any gas from diffusing into the leaves.
An experiment to investigate the effect of light on the net release of carbon dioxide from a plant using hydrogen-carbonate indicator
| Concentration | Indicator turns |
|---|---|
| Highest | Yellow |
| Higher | Orange |
| Atmospheric level | Red |
| Low | Magenta |
| Lowest | Purple |
A leaf is placed in a stoppered boiling tube containing somehydrogen carbonate indicator solution. The effect of light intensity can then be investigated.
The table shows some typical results.
| Tube 1 | Tube 2 | Tube 3 | Tube 4 | |
|---|---|---|---|---|
| Light turned on | ✓ | ✓ | ✓ | ✓ |
| Paper on tube | Black paper | Tissue paper | None | None |
| Leaf | Living | Living | Living | Dead (boiled) |
| Indicator colour at the end | Yellow | Magenta | Purple | Red |
| Carbon dioxide concentration | Highest | Low | Lowest | Atmospheric level |
| Respiration | ✓ | ✓ | ✓ | ✗ |
| Photosynthesis | ✗ | ✓ | ✓✓ | ✗ |
- Tube 4 was a control. The results in tubes 3 and 4 show that the leaf has to be alive for the carbon dioxide concentration to change.
- Tubes 1, 2 and 3 show the effect of increasing the light intensity. The black paper stopped light reaching the leaf in tube 1, so only respiration could happen.
- The tissue paper stopped some of the light reaching the leaf in tube 2, and the leaf in tube 3 received the most light.
- Photosynthesis happened as well as respiration in tubes 2 and 3, so there was a net absorption of carbon dioxide.
- The rate of photosynthesis was greatest in the leaf in tube 3, and it had the greatest net absorption of carbon dioxide.
From this experiment we can see that the concentration of carbon dioxide produced fluctuated due to the light intensity varying. Gas exchange occurs at the greatest rate when respiration and photosynthesis occur at the same time.
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