Question

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When the stomata are open, there is a constant exchange of carbon dioxide between a leaf and the surrounding atmosphere. The actual amount of CO2 taken up is the balance between CO2 uptake for photosynthesis and CO2 evolution from photorespiration and cellular respiration. This balance is called the net carbon exchange rate (CER). In the dark or in light levels barely sufficient to drive photosynthesis, the CER is actually negative. This means that the rate of CO2 evolution from respiration is greater than the rate of uptake for photosynthesis. As light levels increase, respiration remains constant but photosynthesis and, consequently, CO2 uptake increases. The point where CER reaches zero is called the light compensation point. At this point, the light level is high enough that CO2 uptake for photosynthesis "compensates" for the CO2 evolved by respiration.

For most plants, the light compensation point is equivalent to a well-lighted office or classroom. With increasing light beyond the compensation point, the CER continues to increase until a light saturation level is reached and further increases in light result in little or no increase in CER. A C3 plant typically reaches light saturation at one-quarter to one-half of full sunlight. At light saturation, photosynthesis in no longer limited by light-it is now limited by the concentration of CO2 in the air. Beyond the point of light saturation, further increases in the rate of photosynthesis can be achieved only by increasing the amount of CO2 in the atmosphere. It appears that CO2 is often the limiting factor in photosynthesis, at least in C3 plants.

Answer 1

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Since plant cells need carbon dioxide for photosynthesis, carbon dioxide concentrations are another key factor. If carbon dioxide concentrations inside the leaf start to fall, the plant will open its stomata so that more CO2 can enter, even under dry conditions when the stomata would ordinarily be closed.