Chloroplasts are the organelles in plants and algae that make photosynthesis possible. They capture light energy from the Sun and use it to help build sugar from carbon dioxide and water. This process supports nearly all food webs because it stores energy in chemical bonds.
Understanding the chloroplast helps explain how leaves grow, why plants need light, and how oxygen enters the atmosphere.
A chloroplast has a double outer membrane, a fluid-filled stroma, and stacks of membrane sacs called thylakoids. Light-dependent reactions occur in the thylakoid membranes, where chlorophyll absorbs light and helps produce ATP and NADPH. The Calvin cycle occurs in the stroma, where carbon dioxide is fixed into sugars using energy from ATP and NADPH.
Chloroplasts also contain their own circular DNA and ribosomes, which supports the idea that they evolved from photosynthetic bacteria through endosymbiosis.
Key Facts
- Overall photosynthesis equation: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
- Chlorophyll absorbs mainly red and blue light and reflects green light.
- Thylakoid membranes contain photosystems, electron transport chains, and ATP synthase.
- Grana are stacks of thylakoids that increase surface area for light-dependent reactions.
- The Calvin cycle occurs in the stroma and uses CO2, ATP, and NADPH to build sugar molecules.
- Chloroplasts have circular DNA and divide by a process similar to bacterial binary fission.
Vocabulary
- Chloroplast
- A membrane-bound organelle in plants and algae that carries out photosynthesis.
- Thylakoid
- A flattened membrane sac inside a chloroplast where the light-dependent reactions take place.
- Granum
- A stack of thylakoids that helps organize photosynthetic membranes and increase light-capturing surface area.
- Stroma
- The fluid-filled space inside a chloroplast where the Calvin cycle and sugar-building reactions occur.
- Endosymbiosis
- The evolutionary process in which one cell lived inside another cell and became a permanent organelle.
Common Mistakes to Avoid
- Saying chloroplasts make energy from nothing is wrong because they transform light energy into chemical energy stored in molecules such as ATP, NADPH, and sugar.
- Placing the Calvin cycle in the thylakoid membrane is wrong because the Calvin cycle occurs in the stroma, while the light-dependent reactions occur in thylakoid membranes.
- Thinking oxygen gas comes from carbon dioxide is wrong because the O2 released during photosynthesis comes from the splitting of water molecules.
- Calling grana the same thing as chloroplasts is wrong because grana are only stacks of thylakoids inside the larger chloroplast organelle.
Practice Questions
- 1 A leaf cell contains 40 chloroplasts, and each chloroplast contains 25 grana. How many grana are in the cell?
- 2 During photosynthesis, a plant uses 12 molecules of CO2. Using 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2, how many glucose molecules and oxygen molecules can be produced?
- 3 Explain why the presence of circular DNA and ribosomes inside chloroplasts supports the endosymbiotic theory.