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Photosynthesis and Respiration
When these two reactions occur in the same cytoplasm, the measurement of the products and reactants for each of these processes becomes difficult. You must eliminate one or work with chloroplasts and mitochondria in isolation. It is easy to eliminate photosynthesis: switch off the light.
Switch off the light = no photosynthesis = respiration only.
You can therefore deduce the volume of CO2 produced and O2 consumed by respiration alone.
Switch on the light = Photosynthesis + Respiration
To find the real rate of photosynthesis add on the rate of CO2 produced or the rate of O2 consumed whilst the plant was in the dark.
When the rates of photosynthesis = the rate of respiration this is called the compensation point. This is not the same for all plants.
(This assumes that the rate of respiration is the same in the light and in the dark. The calculations are made more complicated by a process called photorespiration which takes place in the light)
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