B3.1 - What happens during photosynthesis?

What happens in photosynthesis?

Photosynthesis takes place inside the chloroplasts of the cell. Plant leaves contain many chloroplasts so that they can make as much glucose as possible using the sun’s energy. Chloroplasts contain a green chemical called chlorophyll. The cells that carry out most photosynthesis (such as the cells at the top of a leaf) contain lots of chloroplasts.

Plant Cell

The word and symbol equations for photosynthesis are given below: -

Photosynthesis Equation

The equation is a summary of the whole process, in reality the process takes place in lots of small steps each catalysed by an enzyme. For GCSE we split the process of photosynthesis into two main reactions – one that depends on light and one that can take place without light. Photosynthesis is an endothermic process; this means that it takes in energy (in this case light energy) which is stored in the bonds of the glucose molecule formed.

The first stage, the light dependent reaction involves using sunlight energy to split water molecules into hydrogen and oxygen. The hydrogen is used in the second stage of photosynthesis and the oxygen is given off as a waste product. This reaction takes place using the green pigment chlorophyll which is found inside the chloroplasts.

Light dependent equation

The second stage does not involve light (and is known as the light independent reaction) and involves using the hydrogen formed from the first stage of photosynthesis and reacting it with carbon dioxide to form glucose and some more oxygen which is also given off as a waste product.

Light independent equation

These two equations merged together will make the overall equation for photosynthesis.

The first stage reaction: -

Light dependent equation

The second stage reaction: -

Light independent equation

If we take everything from before the arrow and everything after the arrow in each equation and merge them together.

Photosynthesis Equation Merged

If we then cancel out anything that has been repeated on both sides.

Photosynthesis Equation Merged

If we then add up the two lots of oxygen on the products side then we get the overall equation for photosynthesis.

Photosynthesis Equation Merged

 

Knowledge Check

What is the word and symbol equation for photosynthesis? (2)
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Word equation - Carbon Dioxide + Water Glucose + Oxygen (✔).
Symbol equation - 6CO2 + 6H2O C6H12O6 + 6O2(✔)

Why is photosynthesis described as an endothermic process? (1)
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Photosynthesis is described as an endothermic process as it takes in energy(✔) from the sun which it stored in the bonds. (ENdothermic - Energy ENters the reaction.)

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Biomass and Photosynthesis   Testing a leaf for starch