Photosynthesis » Photosynthesis and the Uses of Glucose

Plants make their own food

Animals have to eat to get energy. Plants do not. Green plants and algae make their own food using a reaction called photosynthesis. It happens inside the chloroplasts, which contain the green pigment chlorophyll.

In photosynthesis, a plant takes in carbon dioxide and water and uses light to turn them into glucose. Oxygen is made as well and leaves the plant as a waste product.

Key terms:

• Photosynthesis: the reaction in which plants and algae use light to make glucose from carbon dioxide and water, releasing oxygen.
• Glucose: a sugar made in photosynthesis. It is the plant's food and its starting material for making other substances.

The equation for photosynthesis

You must be able to write the reaction as a word equation:

Light is not a reactant. It is the energy that makes the reaction happen, so it is written above the arrow.

You also need to recognise the chemical symbols for each substance:

Put together, the symbol equation is:

Notice that the number of atoms of each element is the same on both sides. Atoms are rearranged, not made or lost.

An endothermic reaction

Photosynthesis is an endothermic reaction. This means it takes in energy from the surroundings. In photosynthesis, the energy comes from light. The light energy is transferred from the environment to the chloroplasts.

The energy does not disappear. It is stored in the glucose that is made. The plant can release that energy later.

What happens to the glucose?

The glucose made in photosynthesis does not stay as glucose. A plant can use it in five ways.

Why store glucose as starch?

Starch is insoluble, which means it does not dissolve in water. This makes it a good store. A store of dissolved glucose would draw water into the cells by osmosis. Insoluble starch does not do this, so it can be kept in the cell without causing problems.

Fats and oils are also kept as stores. They are made from the glucose the plant does not need straight away.

Making proteins: nitrate ions

Glucose contains carbon, hydrogen and oxygen only. Proteins also need nitrogen. To make amino acids, the plant combines glucose with nitrate ions. Plants absorb nitrate ions from the soil through their roots.

Without enough nitrate ions, a plant cannot make enough protein.

Putting it together

You can think of photosynthesis as the start of a chain. Light energy is captured and stored in glucose. After that, the plant uses the glucose for energy, for storage and for building new parts.

• Energy now: respiration.
• Energy for later: starch, fat or oil.
• Building materials: cellulose for cell walls, and amino acids for proteins (with nitrate ions).

Why does this matter?

Almost every food chain starts with photosynthesis. The glucose a plant makes, and the starch, fats, oils and proteins built from it, are what other living things eat. When you eat a potato, you are eating a store of starch that a plant made from glucose. When you eat sunflower seeds, you are eating stored oil.

Photosynthesis also adds oxygen to the air. Each time a plant makes a molecule of glucose, it releases six molecules of oxygen, as the balanced equation shows.

Exam-style question

(a) Write the word equation for photosynthesis. [2 marks]

(b) Photosynthesis is an endothermic reaction. Explain what this means. [2 marks]

(c) Give two ways a plant can use glucose made in photosynthesis. [2 marks]

(d) Name the ions that plants take in from the soil to make proteins. [1 mark]