Photosynthesis

The Equation for Photosynthesis: 6 molecules of carbon dioxide and 6 water molecules are converted by plants in the prescence of light to 1 molecule of glucose (sugar) and 6 molecules of oxygen.

Reactants: CO2 + H2O

Products: Sugar + oxygen

The rate of photosynthesis may be measured by the quantity of carbon dioxide consumed per unit time, or by measuring the quantity of oxygen produced per unit time. In addition, the increase in biomass of a pre-determined area of plants may be measured per unit time. Biomass is the total dry mass of organic matter in organisms or ecosystems.

1. Measure uptake of carbon dioxide

2. Measure production of oxygen

3. Measure increase of plant biomass (Biomass is the total dry mass of organic matter in organisms or ecosystems.)

The rate of photosynthesis may be measured by the quantity of carbon dioxide consumed per unit time, or by measuring the quantity of oxygen consumed per unit time. Since photosynthesis is a very complex process, the rate at which photosynthesis occurs is determined by the step that is occuring the most slowly. This step is then known as the rate-limiting step. Factors which affect the rate of photosynthesis include, light intensity, temperature and carbon dioxide concentration. Each factor affects a different rate-limiting step.

Light Intensity

Carbon Dioxide Concentration

Temperature

As light intensity increases, the photosynthetic rate increases until a point is reached where the rate begins to level off. At low light intensity, photosynthesis occurs slowly because only a small quantity of ATP and NADPH is created by the light dependent reactions. As light intensity increases, more ATP and NADPH are created, thus increasing the photosynthetic rate. At high light intensity, photosynthetic rate levels out, not due to light intensity but due to other limiting factors, including competition between oxygen and carbon dioxide for the active site on RUBP carboxylase.

How Carbon Dioxide Concentration Affects Photosynthetic Rate

As carbon dioxide concentration increases, the rate of photosynthesis increases. At high concentrations, the rate of photosynthesis begins to level out due to factors not related to carbon dioxide concentration. One reason might be that some of the enzymes of photosynthesis are working at their maximum rate.

In general, carbon dioxide is found in low concentration in the atmosphere, and so atmospheric carbon dioxide levels may be a major limiting factor on photosynthesis when at low levels.

How Temperature Affects Photosynthetic Rate

As temperature increases above freezing, the rate of photosynthesis increases. This ocurs because molecules are moving more quickly and there is a greater chance of a collision resulting in a chemical reaction. At some point, a temperature is reached that is an optimum temperature. The photosynthetic reaction rate is at its quickest rate at this point. Above that temperature, the enzymes begin to denature (as in RUBP carboxylase), slowing the rate of photosynthesis until a temperature is reached where photosynthesis does not occur at all.