Photosynthesis & Plant Responses

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Photosynthesis

Photosynthesis takes place inside chloroplasts found in plants and algae. The reaction is endothermic, meaning that it requires energy. The source of this energy is sunlight, which is trapped by a chemical called chlorophyll inside chloroplasts in plant cells.

Photosynthesis

The Rate of Photosynthesis

The four key factors affecting the rate of photosynthesis are:

Temperature

Increasing the temperature increases the rate of photosynthesis because more energy is provided.
However, if the temperature is increased to above about 45°C, the enzymes that catalyse (speed-up) the reaction begin to denature (not work anymore).
This causes the rate of the reaction to drop sharply until it stops altogether.

Light intensity

Increasing light intensity increases the rate of photosynthesis because more energy is provided.
However, if the light intensity is increased above a certain threshold, the rate of photosynthesis will not increase because another factor (such as temperature) is limiting the rate of the reaction.

Carbon dioxide concentration

Increasing the carbon dioxide concentration increases the rate of photosynthesis.
However, above a certain threshold, further increases in the carbon dioxide concentration do not increase the rate of photosynthesis because another factor (such as light intensity) is limiting the rate of reaction.

Chlorophyll concentration

High chlorophyll concentration gives a high rate of photosynthesis.

Limiting Factors

A limiting factor restricts the rate of a reaction. The identity of the limiting factor in photosynthesis depends on the conditions faced by the plant.

Winter

In winter, temperature is usually the limiting factor.

Night

At night, light intensity is usually the limiting factor.

Warm and bright

In warm and bright conditions, carbon dioxide concentration is usually the limiting factor.

Mineral-deficient soil

If plants are grown in mineral-deficient (lacking minerals) soil, they may not absorb enough minerals to produce lots of chlorophyll.
In this case, chlorophyll concentration can be the limiting factor.