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Photosynthesis is the process plants, algae, and some bacteria use to convert light energy into chemical energy stored in sugar. It matters because it supplies most of the food energy in ecosystems and releases the oxygen that many organisms need for cellular respiration. In plants, photosynthesis mainly occurs in chloroplasts inside leaf cells, where pigments capture sunlight. A leaf is shaped and structured to collect light, exchange gases, and move water and sugars efficiently.
Photosynthesis has two major stages: the light-dependent reactions and the Calvin cycle. In the light-dependent reactions, chlorophyll absorbs light energy, water is split, oxygen is released, and energy-carrying molecules are made. In the Calvin cycle, carbon dioxide is fixed into organic molecules and eventually used to build glucose and other carbohydrates. The overall process links sunlight, water, and carbon dioxide to the chemical bonds in sugar.
Key Facts
- Overall photosynthesis equation: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
- Photosynthesis occurs mainly in chloroplasts, especially in leaf mesophyll cells.
- Light-dependent reactions occur in the thylakoid membranes and produce ATP, NADPH, and O2.
- The Calvin cycle occurs in the stroma and uses CO2, ATP, and NADPH to build sugar molecules.
- Chlorophyll absorbs red and blue light strongly and reflects much green light, making leaves appear green.
- Stomata allow CO2 to enter and O2 to leave, but they can also cause water loss by transpiration.
Vocabulary
- Photosynthesis
- Photosynthesis is the process that uses light energy to convert carbon dioxide and water into sugars and oxygen.
- Chloroplast
- A chloroplast is a plant cell organelle where photosynthesis takes place.
- 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 occur.
- Calvin Cycle
- The Calvin cycle is the set of reactions that uses carbon dioxide to help build sugar in the chloroplast stroma.
Common Mistakes to Avoid
- Saying plants get their food from soil is wrong because most of the mass of plant sugars comes from carbon dioxide in the air, not minerals in the soil.
- Forgetting to balance the photosynthesis equation is wrong because atoms must be conserved, so the standard balanced equation uses 6 CO2 and 6 H2O to make 1 glucose and 6 O2.
- Thinking oxygen comes from carbon dioxide is wrong because the oxygen gas released during photosynthesis comes mainly from the splitting of water in the light-dependent reactions.
- Mixing up the two stages is wrong because the light-dependent reactions make ATP and NADPH in thylakoids, while the Calvin cycle uses those molecules to fix carbon in the stroma.
Practice Questions
- 1 Using the balanced equation 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2, how many molecules of CO2 are needed to produce 4 molecules of glucose?
- 2 If a plant produces 18 molecules of O2 during photosynthesis, how many molecules of glucose are produced according to the balanced equation?
- 3 A plant is kept in bright light but its stomata close during a hot, dry day. Explain how this would affect carbon dioxide intake, water loss, and the rate of sugar production.