- Photosynthesis taking place - although some CO2 comes from respiration of cells, most has to be obtained from external air - some oxygen from photosynthesis is used in respiration but most of it diffuses out of the plant.
- Photosynthesis not occuring - oxygen diffuses into the leaf because it is constantly used by cells during respiration - carbon dioxide produced diffuses out.
Structure of a leaf and gas exchange
- No living cell is far away from external air (source of oxygen and CO2)
- Diffusion takes place in the gas phase (air) - so…