Click image to open full size
Photosynthesis
How Plants Turn Sunlight Into Food
Related Tools
Related Labs
Related Worksheets
Photosynthesis is the process plants use to turn light energy into chemical energy stored in sugar. It matters because it supplies most of the food energy in ecosystems and releases the oxygen many living things need to survive. In a leaf, chloroplasts act like tiny solar factories that capture sunlight and build glucose from carbon dioxide and water. This process connects sunlight, plants, animals, and the atmosphere in one energy cycle.
Photosynthesis happens in two main stages inside chloroplasts. The light-dependent reactions capture sunlight in chlorophyll, split water, and release oxygen while making energy carriers. The Calvin cycle uses carbon dioxide and those energy carriers to assemble glucose. Factors such as light intensity, carbon dioxide concentration, temperature, and water supply can change the rate of photosynthesis.
Key Facts
- Overall equation: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
- Photosynthesis takes place mainly in chloroplasts inside leaf cells.
- Chlorophyll absorbs light most strongly in the blue and red parts of the visible spectrum.
- Light-dependent reactions occur in the thylakoid membranes and produce O2, ATP, and NADPH.
- The Calvin cycle occurs in the stroma and uses CO2, ATP, and NADPH to build sugar.
- Glucose stores chemical energy that plants can use for growth, repair, respiration, or storage as starch.
Vocabulary
- Photosynthesis
- Photosynthesis is the process by which plants, algae, and some bacteria use light energy to make sugars from carbon dioxide and water.
- Chloroplast
- A chloroplast is an organelle in plant and algal cells where photosynthesis occurs.
- Chlorophyll
- Chlorophyll is a green pigment that absorbs light energy for photosynthesis.
- Thylakoid
- A thylakoid is a flattened membrane sac inside a chloroplast where the light-dependent reactions take place.
- Stoma
- A stoma is a tiny pore in a leaf that allows carbon dioxide to enter and oxygen and water vapor to exit.
Common Mistakes to Avoid
- Thinking plants get their food from soil, which is wrong because most plant biomass comes from carbon dioxide converted into sugars during photosynthesis.
- Forgetting water is a reactant, which is wrong because water provides electrons and hydrogen and is split during the light-dependent reactions.
- Saying oxygen comes from carbon dioxide, which is wrong because the oxygen released by photosynthesis comes mainly from split water molecules.
- Mixing up photosynthesis and cellular respiration, which is wrong because photosynthesis stores energy in glucose while respiration releases energy from glucose.
Practice Questions
- 1 Using the equation 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2, how many molecules of oxygen are produced when 18 molecules of carbon dioxide are used?
- 2 A plant produces 4 molecules of glucose during photosynthesis. According to the balanced equation, how many molecules of water are needed?
- 3 A plant is placed in bright light but its stomata close because of dry conditions. Explain how this would affect photosynthesis and why.