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 CO2 uptake by the plant gives us a more accurate way to measure photosynthetic rate. The only reason the plant takes in CO2 is for the production of carbohydrate via photosynthesis…so more CO2 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 CO2 output by cellular respiration equal the CO2 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 CO2 intake as well.

Temperature

[CO2] & [O2]• 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 CO2 levels – the better it is for the plant.

• C4 plants have developed a “CO2 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.

FIN