The Photosynthesis Equation

• Biology
• Lower Secondary (Key Stage 3)

Starter Task

1. What gas do plants take in?

2. What gas do plants give out?

3. What do plants take in via the roots?

4. What do plants make as a supply of energy?

Challenge: What type of energy is required for photosynthesis?

The Photosynthesis Equation

We now know that plants take IN  carbon dioxide and water and produce (give OUT) oxygen and glucose during photosynthesis.

We also know plants need light energy to carry out photosynthesis and that chlorophyll in the leaves is the site of the reaction.

The Photosynthesis Equation

Photosynthesis is a chemical equation. 

From previous learning, we know that things required for a chemical reaction to occur are reactants, and that things produced are the products. 

The Photosynthesis Equation

Using the information we have just learned, use it to construct a chemical word equation for the photosynthesis reaction.

          light

carbon dioxide + water   --->   oxygen + glucose

         chlorophyll

Where carbon dioxide and water are the reactants (so on the left of the arrow) and oxygen and glucose are the products (so on the right of the arrow).

It is important to add the light energy and the chlorophyll as without these, photosynthesis cannot happen.

           light

carbon dioxide + water   --->   oxygen + glucose

          chlorophyll

As scientists, we can also write this equation as a symbol equation. 

If C6H12O6 is the symbol for glucose, can we now write the reaction as a symbol equation?

          light

carbon dioxide + water   --->   oxygen + glucose

        chlorophyll

light

CO2     +     H2O        --->     O2   +   C6H12O6

chlorophyll

light

CO2     +     H2O        --->     O2   +   C6H12O6

chlorophyll

The last problem with the equation is that it isn't balanced. In all chemical reactions, what goes into the reaction, has to come out. It may look different, but it is still there! 

light

CO2     +     H2O        --->     O2   +   C6H12O6

chlorophyll

If we look at H - hydrogen for example, we can see 2 atoms of hydrogen are in the reactants, but there are 12 in the products! Can we balance this equation do you think?

light

6CO2     +     6H2O        --->     6O2   +   C6H12O6

chlorophyll

The equation is now balanced. All we need to remember about the photosynthesis equation is that we insert 6 in front of everything but glucose.

Exit Task

1. What is the name of the green pigment where photosynthesis takes place?

2. What energy is required for photosynthesis? 

3. What is the chemical symbol for glucose?

Challenge: Is photosynthesis an exothermic or endothermic reaction? Explain your answer.