Gas Exchange In Flowering Plants
Photosynthesis and Respiration
All organisms use cellular respiration to release the chemical energy in food molecules, such as
glucose.
- Cellular respiration must continue all the time as organisms need energy for other life
processes
Plants photosynthesis as well as respire.
- For photosynthesis, plant cells need to take in c02 and release 02. However,
photosynthesis can only take place when there is light
At night, plants do not photosynthesize, but they do continue respiration.
- So plants give out c0s and take in 02.
At daybreak, as light intensity increases, the rate of photosynthesis increases
At a particular light intensity, the amount of oxygen produced by photosynthesis will balance out
the amount used by the plant in respiration
- The net production of oxygen will be zero
The point at which photosynthesis and respiration use and produce equal amounts of oxygen and
carbon dioxide is called the compensation point.
As daylight increases, oxygen production from photosynthesis exceeds its use in respiration, and
the opposite for carbon dioxide.
- This continues until light intensity decreases, when the sun sets and a second
compensation point is reached.
Questions - page 107
1. Within a period of 24 hours, when do plant cells respire? Explain your answer
2. Within a period of 24 hours, when do plant cells photosynthesis? Explain your answer
3. What is a compensation point?
4. During which time period is the plant producing more sugars in photosynthesis than it is
using in respiration? Explain your answer
Plant Structure and Gas Exchange
Many adaptations of the leaf for photosynthesis are also adaptations for the rapid exchange of
gases between photosynthesizing cells and the environment.
Adaptations of leaves to maximise gas exchange:
● They are flat
○ Large surface area for diffusion provided by the internal surfaces of the spongy
mesophyll
● They are thin
○ Gives them a short diffusion distance from the photosynthesizing cells to the air
spaces in the leaf
Photosynthesis and Respiration
All organisms use cellular respiration to release the chemical energy in food molecules, such as
glucose.
- Cellular respiration must continue all the time as organisms need energy for other life
processes
Plants photosynthesis as well as respire.
- For photosynthesis, plant cells need to take in c02 and release 02. However,
photosynthesis can only take place when there is light
At night, plants do not photosynthesize, but they do continue respiration.
- So plants give out c0s and take in 02.
At daybreak, as light intensity increases, the rate of photosynthesis increases
At a particular light intensity, the amount of oxygen produced by photosynthesis will balance out
the amount used by the plant in respiration
- The net production of oxygen will be zero
The point at which photosynthesis and respiration use and produce equal amounts of oxygen and
carbon dioxide is called the compensation point.
As daylight increases, oxygen production from photosynthesis exceeds its use in respiration, and
the opposite for carbon dioxide.
- This continues until light intensity decreases, when the sun sets and a second
compensation point is reached.
Questions - page 107
1. Within a period of 24 hours, when do plant cells respire? Explain your answer
2. Within a period of 24 hours, when do plant cells photosynthesis? Explain your answer
3. What is a compensation point?
4. During which time period is the plant producing more sugars in photosynthesis than it is
using in respiration? Explain your answer
Plant Structure and Gas Exchange
Many adaptations of the leaf for photosynthesis are also adaptations for the rapid exchange of
gases between photosynthesizing cells and the environment.
Adaptations of leaves to maximise gas exchange:
● They are flat
○ Large surface area for diffusion provided by the internal surfaces of the spongy
mesophyll
● They are thin
○ Gives them a short diffusion distance from the photosynthesizing cells to the air
spaces in the leaf
