C3 and C4 plants have two things in common, they both are found in hot and very dry areas and they both use photorespiration. However, the difference is the way they react toward water loss which will affect photosynthesis. If a C3 plant starts to transpire rapidly because it is photosynthesizing and the stomata are open, then it will close the stomata, ending photosynthesis. What the plant does next is undergo a counterproductive cycle called photorespiration. In the C3 plant photorespiration actually uses up 50% of the CO2 fixed by the Calvin cycle. What happens is the plant closes it's stomata and as a result stops transpiration and photosynthesis. What happens is the O2 level rises and the CO2 drops. Then the plant must use O2 in place of CO2 to input into the Calvin cycle. A two carbon molecule comes out and is broken down by peroxisomes into CO2. This just uses up CO2 and doesn't benefit the plant because no energy or food is made in this process. In a C4 plant it is different. When it needs to close it's stoma to stop transpiration, it undergoes photorespiration as well, however, it's photorespiration is productive. Preceding the Calvin Cycle it incorporates CO2 into an organic compound first, or the CO2 into phosphoenolpyruvate to form oxaloacetate. PEP carboylase has a higher affinity for CO2 then rubisco does so PEP can fix CO2 efficiently and rubisco can't. The O2 starts to rise and the CO2 drops. After the C4 plant fixes CO2, the mesophylls export there 4 carbon products to bundle sheath cells through plasmodesmata. Within the bundle sheath cells, the 4 carbon compounds release CO2, which is reassimilated into an organic material by rubisco and the Calvin cycle. In effect the mesophyll cells pump CO2 into bundlesheath cells keeping CO2 high enough in the bundle sheath cells for rubisco to accept CO2 instead of O2. That is how the C3 and C4 plants react to desiccation.
Plants are closer to humans then you would think.
