What is Photosynthesis


Photosynthesis is the process by which green plants convert light energy from the sun to produce carbohydrates. It can be summarised as follows:

                        6CO2 + 6H2O + light energy = C6H1206 + 6O2

It is an exceedingly important process – it is the route by which virtually all energy enters the biosphere. Life would not exist without this process.

Photosynthesis occurs in two phases, only one of which actually requires light.

The Light Reactions

The first step in the conversion of light energy to chemical energy is the absorption of light. Pigment molecules absorb photons (particles of light energy). The pigments involved are chlorophyll – the molecules that actually use the light energy - and carotenoids and phycobilins, which are accessory pigments that act as an antennae-like network for gathering light.

The light energy is transferred from one pigment molecule to the next until it reaches the reaction centre which is a special form of chlorophyll. As the chlorophyll absorbs energy, electrons are boosted to a higher energy level and transferred to an acceptor molecule to initiate electron flow. As the electrons are removed, they are replaced by electrons from a water molecule, and oxygen is produced. This oxygen is the main source of atmospheric oxygen which is essential for supporting life.

The Dark Reactions

In the second phase the energy products of the first phase are used to reduce carbon from carbon dioxide to a simple sugar – this conversion of CO2 into organic compounds is called carbon fixation and involves the process known as the Calvin Cycle.

Each step in the Calvin Cycle is regulated by a specific enzyme. At each full turn of the cycle a molecule of carbon dioxide enters the cycle and is reduced, and a molecule of RuBP (ribulose, 1,5-bisphosphate) is regenerated. Three turns of the cycle produce a three-carbon molecule, glyceraldehyde 3-phosphate, two molecules of which can combine to form a glucose molecule.

Environmental factors which affect photosynthesis:

1. Light

As a general rule, as light increases so too does photosynthesis. In relation to C3 and C4 plants, C4 plants will continue to photosynthesise in higher light intensities whereas a C3 plant stops its rate of photosynthesis at its saturation point.

2. Carbon dioxide concentration

As CO2 increases, so too does photosynthesis.

3. Temperature

Photosynthesis will increase with temperature until a point of equilibrium is met. At this point photosynthesis stays uniform as temperature increases. Growth is defined as photosynthesis minus respiration. As temperature increases, respiration will continue to increase, therefore at some point respiration will exceed photosynthesis resulting in no net growth.

4. Water balance

Stomatal aperture is controlled by light energy, turgor pressure of the surrounding guard cells, temperature, CO2 concentration in the stomatal cavity, and chemicals such as anti‑transpirants.

5. Leaf age

As the leaf becomes older in age, the photosynthetic capacity of the leaf decreases.

6. Environmental history

Longitude, latitude, elevation all effect photosynthesis.