- Photosynthesis is a series of enzyme controlled reactions that occur in chloroplasts in some cells of green plants.
- Water and carbon dioxide are needed and glucose and oxygen are produced.
- Glucose can be used in respiration to make ATP, be stored as starch for later use, or changed into one of many chemicals such as cellulose, fat and amino acids the plant needs to survive.
- The reactions of photosynthesis are divided into two phases:
1) light-dependent phase - occurs in the thylakoids of chloroplast. The walls of the thylakoids have lipid bilayers that contain the light absorbing pigment chlorophyll.
The chlorophyll traps energy from light which is used to split water. The hydrogen from water is transferred to the carrier molecule NADP+ to produce NADPH and the enzyme ATP synthase to produce ATP. Oxygen from water is released as a waste product.
The membranes in the thylakoids provide a large surface area to hold the enzymes and chlorophyll that are needed.
2) Light-independent-phase. This takes place in the stroma of chloroplasts where the NADPH and ATP from the light phase are used to convert CO2 to glucose.
Chloroplasts are found in their greatest number in palisade cells near the upper surface of the leaf. Here as much light energy as possible can be absorbed by chlorophyll. Chloroplasts are pushed up close to the inside of the cell membrane which reduces the diffusion distance for CO2 and water so photosynthesis can occur at a higher rate.
- Photosynthesis is important as it changes solar energy to chemical energy as organic molecules (eg carbohydrates). Plants are producers of food, they are the starting point of food chains.