4.1 - Photosynthesis

The photosynthesis reaction

• Photosynthesis is the process by which green plants make their own food.
• Carbon dioxide and water are converted into glucose (also called sugar), and oxygen is produced as well.
• The reaction needs energy, which comes from light, so photosynthesis is described as an endothermic reaction - energy is transferred from the environment into the plant during the reaction.

Where photosynthesis happens

• The reaction happens inside chloroplasts - the small structures found in the cells of leaves and other green parts of a plant.
• Chloroplasts contain a green pigment called chlorophyll, which absorbs light and transfers its energy into the chemical energy stored in the glucose that is made.

The word equation for photosynthesis

Light, absorbed by chlorophyll, provides the energy for the reaction, and is usually written above the arrow with chlorophyll below it.

Uses of the glucose made in photosynthesis

• Some is used straight away in respiration, which releases the energy the plant needs for its life processes and lets it convert the rest of the glucose into other useful substances.
• The remaining glucose is built up into a range of materials.

What the glucose is converted into

• Cellulose - The tough material that makes up plant cell walls.
• Amino acids and proteins - Glucose is combined with nitrate ions taken up from the soil to make amino acids, which are then joined together into proteins.
• Lipids (fats and oils) - Stored, especially in seeds.
• Starch - An insoluble carbohydrate stored in the roots, stems, and leaves as an energy store for when photosynthesis is not happening, such as in winter.
  • Starch is better than glucose for storage because it is insoluble, so it does not dissolve and draw water into the cells by osmosis.
• Sucrose - The soluble sugar in which carbohydrate is transported around the plant in the phloem.