Environmental Factors & the Rate of Photosynthesis

Photosynthetic Rate? To measure the rate of photosynthesis we need to consider what is made by photosynthesis…Glucose. But, we can’t use the levels of glucose made to measure photosynthetic rate. Glucose is: –Made into ATP via Cellular Respiration. –Stored as starch –Used for growth as cellulose. Carbon dioxide is used directly to make glucose so the rate of CO 2 uptake by the plant gives us a more accurate way to measure photosynthetic rate. The only reason the plant takes in CO 2 is for the production of carbohydrate via photosynthesis…so more CO 2 in means more photosynthesis occurring.

Light Intensity Irradiance is a measure of light intensity per unit of leaf. As irradiance increases, the levels of photosynthesis increase up to a point known as the light-saturation point. At this point the enzymes are saturated with substrate and just can’t work any faster – they are maxed out!! Also note the light-compensation point. This is a low level of light that sees the levels of CO 2 output by cellular respiration equal the CO 2 intake by photosynthesis.

Light Intensity

Temperature The Calvin cycle is affected by temperature more so than the light reactions. As temperature increases, so does the rate of photosynthesis, up to a temperature of about 25°C. After 30°C, the rate of photosynthesis begins to lower. Why? –Proteins become unhappy and threaten to denature due to the elevated temperature. –The stoma close to limit water loss through evaporation. This also limits CO 2 intake as well.

Temperature

[CO 2 ] & [O 2 ] The atmosphere is about 21% oxygen while less then 1% is carbon dioxide. This presents a problem as oxygen is favoured by the rubisco (enzyme) that is required to start of the Calvin cycle and start the production of carbohydrate (glucose). The greater the CO 2 levels – the better it is for the plant. C4 plants have developed a “CO 2 only” pathway for the start of the Calvin cycle.

Water Water is required to replenish the electrons lost in the light reactions by the chlorophyll molecules. If there is a water shortage, the chlorophyll degrades and this results in an obvious decrease in photosynthetic rate. Plants do this naturally when in the fall as they prepare for the oncoming winter.

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