The process of photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy. It mainly occurs in the chloroplasts of plant cells. During photosynthesis, plants take in carbon dioxide from the air and water from the soil. Using sunlight, they transform these into glucose (a sugar) and oxygen. This process is essential for producing food and oxygen, supporting life on Earth.

The overall reaction can be summarised as:

Use this page to revise the following concepts of photosynthesis:

• The chloroplast
• The light-dependent stage of photosynthesis
• The light-independent stage of photosynthesis
• How the two stages of photosynthesis are connected

The chloroplast

The chloroplast is the key organelle responsible for photosynthesis.

Chloroplasts are double-membraned organelles found in plant cells and some algae, responsible for photosynthesis.

This unique structure allows chloroplasts to efficiently convert light energy into chemical energy through photosynthesis.

Click on the key parts of the chloroplast to find out more about its function.

The light-dependent stage of photosynthesis

The first stage of photosynthesis is the light dependent stage.

It involves the capture and conversion of sunlight into chemical energy. This process occurs in the grana of the chloroplast, or more specifically in the thylakoid membranes.

Light is absorbed by the chlorophyll in the thylakoid membranes and the light energy is used to split a water molecule. The released electrons and H⁺ ions from the water molecule are absorbed by the energy carrier NADP⁺_, which becomes a loaded electron acceptor known as NADPH.

The energy carrier ADP absorbs the released energy and is converted to ATP.

The oxygen from the split water molecule is released into the atmosphere, or it may be used as an input for cellular respiration in the plant. It is a waste product of this reaction.

The light-independent stage of photosynthesis

The second stage of photosynthesis is the light-independent stage or Calvin cycle.

Despite its name, the light independent reaction can occur when it is light, however, this stage does not directly rely on sunlight.

The Calvin cycle involves building carbon molecules into energy-rich organic molecules, such as glucose. This process occurs in the stroma of the chloroplast.

Carbon dioxide is absorbed into the Calvin cycle. In the first step of the cycle, the enzyme RuBisCO catalyses a reaction between CO₂ and a molecule called RuBP. RuBP is combined with carbon dioxide to ultimately produce two 3-carbon molecules. Numerous reactions occur during the Calvin cycle until the eventual formation of glucose.

The energy carriers, NADPH and ATP, provide energy for this process to occur. They also assist to regenerate RuBP within the cycle, making it ready to react with the next CO₂ molecule and keep the cycle going. In the Calvin cycle, ATP and NADPH are converted back to ADP +P_i and NADP⁺, respectively, which can be returned to the light-dependent reactions.

How the two stages of photosynthesis are connected

The light dependent stage and light independent stage of photosynthesis are connected through the energy carriers.

The light dependent stage provides the loaded energy carriers (NADPH and ATP), while the light independent stage uses the energy from the loaded energy carriers to assist in the production of glucose.

The overall input and output equation of photosynthesis is summarised by:

6CO₂ + 6H₂O → C₆H₁₂O + 6O₂.

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