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(a) Light intensity

Photosynthesis is affected by the environment. Three factors with the most influence are light intensity, CO2 level, and temperature.

Light Intensity

As shown in Figure 6-12a, the rate of photosynthesis increases as light intensity increases. Higher light intensity excites more electrons in both photosystems. Thus, the light reactions occur more rapidly. However, at some point all of the available electrons are excited, and the maximum rate of photosynthesis is reached. The rate stays level regardless of further increases in light intensity.

Carbon Dioxide Levels

Increasing levels of CO2 also stimulate photosynthesis until the rate of photosynthesis levels off. Thus, a graph of the rate of photosynthesis versus CO2 concentration would resemble Figure 6-12a.

Temperature

Increasing temperature accelerates the chemical reactions involved in photosynthesis. As a result, the rate of photosynthesis increases as temperature increases, over a certain range. This effect is illustrated by the left half of the curve in Figure 6-12b. The rate peaks at a certain temperature, at which many of the enzymes that catalyze the reactions become ineffective. Also, the stomata begin to close, limiting water loss and C02 entry into the leaves. These conditions cause the rate of photosynthesis to decrease when the temperature is further increased, as shown by the right half of the curve in Figure 6-12b.

(b) Temperature

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