How plants turn light into sugar
HESI A2 topic: Photosynthesis
- Photosynthesis uses light to turn CO₂ and water into sugar and oxygen.
- Light reactions in the thylakoid membrane make ATP, NADPH, and oxygen.
- The oxygen comes from splitting water, not from CO₂.
- The Calvin cycle in the stroma uses ATP and NADPH to build sugar.
- Plants run both photosynthesis and cellular respiration.
Key facts
- Overall equation: 6CO2 + 6H2O → C6H12O6 + 6O2, powered by light energy.
- A chloroplast has an outer and inner membrane, stacked thylakoids (a stack is a granum), and a fluid called the stroma around them.
- Chlorophyll sits in the thylakoid membrane and absorbs mainly blue and red light; it reflects green, which is why leaves look green.
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- The light-dependent reactions happen in the thylakoid membrane and make ATP, NADPH, and oxygen; the oxygen comes from splitting water.
- The Calvin cycle happens in the stroma; the enzyme RuBisCO fixes CO2 onto RuBP, and ATP and NADPH turn the product into the sugar G3P.
- The Calvin cycle is called light-independent, but it depends on light indirectly because it needs ATP and NADPH from the light reactions.
- The products of photosynthesis (sugar and O2) are the reactants of cellular respiration, and the products of respiration (CO2 and water) are the reactants of photosynthesis.
Quick check1 question · 2 more at the end
In which part of the chloroplast do the light-dependent reactions of photosynthesis take place?
Correct answer: A.
The light-dependent reactions happen in the thylakoid membrane, where chlorophyll and the photosystems are embedded. The Calvin cycle happens in the stroma, the fluid around the thylakoids. The mitochondrial matrix is a site of cellular respiration, not photosynthesis.
A Correct: the light-dependent reactions occur in the thylakoid membrane, which holds chlorophyll.
B The outer membrane encloses the chloroplast but does not hold the photosystems.
C The stroma is where the Calvin cycle (light-independent reactions) takes place; this mixes up the two stages.
D The mitochondrial matrix is part of a mitochondrion and hosts the citric acid cycle, not photosynthesis.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Photosynthesis uses light energy to turn carbon dioxide and water into sugar and oxygen. In the thylakoid membranes, the light-dependent reactions split water, release oxygen, and make ATP and NADPH. In the stroma, the Calvin cycle spends that ATP and NADPH to fix CO2 into sugar. Its products, sugar and oxygen, are the reactants of cellular respiration.
1 What does photosynthesis do?Photosynthesis captures light energy and stores it in the chemical bonds of sugar. 1 min
Photosynthesis captures light energy and stores it in the chemical bonds of sugar. Organisms that make their own food this way, such as plants, algae, and cyanobacteria, are photoautotrophs. Heterotrophs such as animals and fungi must get their energy from organic molecules other organisms made.
The overall equation is:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Carbon dioxide and water go in; sugar and oxygen come out. The work happens in two stages, in two parts of the chloroplast.
2 Where does photosynthesis happen?Photosynthesis happens in the chloroplast, and each stage has its own compartment there. 1 min
Photosynthesis happens in the chloroplast, and each stage has its own compartment there. Carbon dioxide enters the leaf, and oxygen leaves, through small pores called stomata, usually on the underside of the leaf. Guard cells open and close them.
| Part of the chloroplast | What it is | Stage that happens there |
|---|---|---|
| Outer and inner membrane | Double envelope around the organelle | None |
| Thylakoids | Flattened discs; their membrane holds chlorophyll | Light-dependent reactions |
| Granum (plural grana) | A stack of thylakoids | Light-dependent reactions |
| Thylakoid lumen | The space inside each thylakoid | H⁺ builds up here |
| Stroma | Fluid around the grana | Calvin cycle |
Chlorophyll absorbs mainly blue and red light and reflects green, which is why leaves look green.
3 What happens in the light-dependent reactions?The light-dependent reactions turn light energy into two energy carriers, ATP and NADPH, and give off oxygen. 1 min
The light-dependent reactions turn light energy into two energy carriers, ATP and NADPH, and give off oxygen. All of this happens in the thylakoid membrane.
- Light energizes electrons in photosystem II.
- To replace those electrons, photosystem II splits water. This releases oxygen, so the O₂ a plant gives off comes from water.
- The electrons pass along an electron transport chain to photosystem I, and their energy pumps H⁺ into the thylakoid lumen.
- H⁺ flows back out through ATP synthase, which makes ATP.
- Photosystem I re-energizes the electrons, which convert NADP⁺ into NADPH.
4 What happens in the Calvin cycle?The Calvin cycle spends ATP and NADPH to build sugar from carbon dioxide in the stroma. 1 min
The Calvin cycle spends ATP and NADPH to build sugar from carbon dioxide in the stroma. It has three steps:
- Fixation: the enzyme RuBisCO attaches CO₂ to a five-carbon molecule called RuBP.
- Reduction: ATP and NADPH convert the product into a three-carbon sugar, G3P.
- Regeneration: most of the G3P is used, with more ATP, to rebuild RuBP so the cycle can continue.
Each turn fixes one CO₂, so building one six-carbon glucose takes six turns.
The cycle is called light-independent because light does not drive it directly. It still depends on light indirectly, since it runs on the ATP and NADPH the light reactions make, and light also switches on several of its enzymes. The old name "dark reactions" is misleading, because it suggests the cycle happens only at night.
5 How do the two stages compare?The two stages split the work, one capturing light energy and the other spending it to build sugar. 1 min
The two stages split the work, one capturing light energy and the other spending it to build sugar.
| Light-dependent reactions | Calvin cycle | |
|---|---|---|
| Where | Thylakoid membrane | Stroma |
| Inputs | Light, water, ADP, NADP⁺ | CO₂, ATP, NADPH |
| Outputs | ATP, NADPH, O₂ | G3P (sugar), ADP, NADP⁺ |
6 How is photosynthesis linked to cellular respiration?Photosynthesis and cellular respiration run in a loop, each using what the other makes. 1 min
Photosynthesis and cellular respiration run in a loop, each using what the other makes. Photosynthesis uses CO₂ and water and makes sugar and O₂. Cellular respiration uses sugar and O₂ and releases CO₂ and water.
Plants do both. Plant cells have chloroplasts for photosynthesis and mitochondria that make ATP through cellular respiration.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: The oxygen released by photosynthesis comes from water, not from carbon dioxide.
Watch out: The light-dependent reactions happen in the thylakoid membrane; the Calvin cycle happens in the stroma, not the other way around.
Watch out: The Calvin cycle is not a night-only 'dark reaction'; it does not use light directly, but it depends on the ATP and NADPH the light reactions make.
Watch out: Plants do not replace respiration with photosynthesis; plant cells have mitochondria and carry out cellular respiration too.
How to remember it
Memory hook: "Light in the Lamp (thylakoid stacks), Calvin in the Cup (the stroma fluid)." And "water gives the oxygen, CO₂ gives the carbon."
Self-check:
- Name the two products of the light reactions that the Calvin cycle needs.
- Say which molecule is split to release O₂, and where that happens.
- Explain why the Calvin cycle is called light-independent even though it depends on light indirectly.