Photosynthesis

The photosynthetic reaction

• Photosynthesis happens in the chloroplasts of plant and algal cells. Chlorophyll absorbs light

$$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}$$ $$6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

• It is an endothermic reaction: energy is transferred from the environment to the chloroplasts by light
• Carbon dioxide enters the leaf through the stomata; water comes up the xylem from the roots; oxygen leaves through the stomata; glucose is used or transported away in the phloem

Uses of glucose

• The glucose made in photosynthesis may be:
  • Used in respiration to release energy
  • Converted into insoluble starch for storage (insoluble, so it does not affect osmosis)
  • Used to make fat or oil for storage, especially in seeds
  • Used to make cellulose, which strengthens the cell wall
  • Used to make amino acids for protein synthesis. This also needs nitrate ions absorbed from the soil
• The starch test: boil a leaf in water to kill it, then in ethanol to remove the chlorophyll, rinse, and add iodine. Blue-black shows starch, so photosynthesis has happened there
  • A variegated leaf goes blue-black only in the green parts: no chlorophyll, no photosynthesis, no starch
  • A leaf kept in the dark stays orange-brown: no light, no photosynthesis