DAT Review · Biology
Photosynthesis
On this page 7 sections
In 30 seconds
- Light reactions (thylakoid membrane): split H₂O, release O₂, produce ATP and NADPH. Photosystem II comes before Photosystem I (confusingly — they were named in order of discovery).
- Calvin cycle (stroma): fixes CO₂ using RuBisCO, consumes ATP and NADPH, produces G3P → glucose.
- Photosynthesis is essentially the reverse of cellular respiration — know the comparison table cold.
The college version
Core Review
Chloroplast Structure
Chloroplasts are double-membraned organelles found in plant cells and photosynthetic algae. The internal membrane system consists of flattened sacs called thylakoids, which are stacked into columns called grana (singular: granum). The thylakoid membrane houses the photosystems, electron transport chains, and ATP synthase. The fluid-filled space surrounding the thylakoids is the stroma, which contains the enzymes for the Calvin cycle, chloroplast DNA, and ribosomes.
Light Reactions (Thylakoid Membrane)
The light reactions convert solar energy into chemical energy in the form of ATP and NADPH.
Photosystem II (PSII): Light energy excites electrons in the reaction center chlorophyll (P680). These high-energy electrons are passed to the primary electron acceptor and then down an electron transport chain (similar to the mitochondrial ETC). To replace the lost electrons, PSII splits water molecules: 2 H₂O → 4 H⁺ + 4 e⁻ + O₂. This is the source of all oxygen released by photosynthesis. As electrons travel through the ETC, the energy released pumps protons (H⁺) from the stroma into the thylakoid lumen, creating a proton gradient.
Photosystem I (PSI): Light re-excites electrons at P700. These electrons are passed through a short chain to ferredoxin, and ultimately to NADP⁺ reductase, which reduces NADP⁺ to NADPH: NADP⁺ + 2 e⁻ + H⁺ → NADPH.
ATP Synthesis: The proton gradient built up in the thylakoid lumen drives ATP synthase (chemiosmosis), producing ATP as protons flow back into the stroma. This is called photophosphorylation.
Summary of Light Reactions:
- Inputs: H₂O, light, NADP⁺, ADP + Pᵢ
- Outputs: O₂ (released), ATP, NADPH
- Location: Thylakoid membrane
The Calvin Cycle (Stroma)
The Calvin cycle uses ATP and NADPH from the light reactions to fix CO₂ into carbohydrate.
Phase 1 — Carbon Fixation: CO₂ combines with ribulose-1,5-bisphosphate (RuBP, a 5-carbon sugar), catalyzed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase). The resulting 6-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate (3-PGA, 3C each).
Phase 2 — Reduction: ATP phosphorylates 3-PGA, and NADPH reduces it to glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar. For every 6 CO₂ fixed, 12 G3P molecules are produced. Two G3P molecules exit the cycle to make one glucose; the remaining 10 G3P molecules (30 carbons) are used to regenerate RuBP.
Phase 3 — Regeneration of RuBP: Using ATP, the 10 G3P molecules are rearranged through a series of reactions to regenerate 6 RuBP molecules (30 carbons), ready to accept more CO₂.
Net Calvin Cycle (per 6 CO₂): 18 ATP + 12 NADPH consumed → 1 glucose equivalent (2 G3P).
Photosynthesis vs. Cellular Respiration
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Equation | 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂ | C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP |
| Organelle | Chloroplast | Mitochondria |
| Energy conversion | Light → Chemical | Chemical → ATP |
| Electron source | H₂O (split at PSII) | NADH, FADH₂ (from glucose oxidation) |
| Final electron acceptor | NADP⁺ (→ NADPH) | O₂ (→ H₂O) |
| ATP synthesis | Photophosphorylation | Oxidative phosphorylation |
| Type of organism | Photoautotrophs | All organisms (nearly) |
| CO₂ role | Consumed (fixed) | Produced (released) |
Common Traps
- "Photosynthesis = CO₂ in, O₂ out; Respiration = O₂ in, CO₂ out": Both statements are correct, but plants do BOTH — they respire 24/7 and photosynthesize only in light. At night, plants are net O₂ consumers.
- "The Calvin cycle is the dark reactions": This is an outdated term. The Calvin cycle does NOT require darkness; it simply does not directly require light. Many enzymes of the Calvin cycle are actually light-activated.
- "PSI and PSII are numbered by order of action": They are numbered by order of discovery. PSII acts first in the pathway.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of a chloroplast as a solar-powered candy factory. The thylakoid membranes are solar panels (Photosystems I and II) that capture sunlight. When light hits, the panels use that energy to split water — releasing oxygen bubbles (that's what plants breathe out for us) — and to charge up two delivery trucks: ATP (the energy truck) and NADPH (the electron truck). These trucks drive over to the stroma — the factory floor — where the Calvin cycle machines use them to grab CO₂ from the air and snap the carbons together. After enough cycles, you get a G3P molecule, which pairs up to make glucose — the candy. Cellular respiration is the exact opposite: you eat the candy, break it down, and use the energy.
Key takeaways
- PSII before PSI: PSII was discovered second but functions first. Water splitting occurs at PSII.
- Cyclic vs. noncyclic electron flow: Noncyclic produces both ATP and NADPH (uses both photosystems). Cyclic (PSI only) produces ATP but NOT NADPH — used when the cell needs more ATP than NADPH.
- RuBisCO is the most abundant enzyme on Earth. It can also fix O₂ (photorespiration), which wastes energy — a problem in hot, dry conditions.
- C3, C4, CAM plants: C3 = standard Calvin cycle; C4 = spatial separation of CO₂ fixation (mesophyll) and Calvin cycle (bundle sheath); CAM = temporal separation (stomata open at night).
- Chlorophyll absorbs red and blue light; reflects green — hence plants look green.
Check yourself
3 review questions from the chapter. Try each one, then open the answer.
What are the products of the light reactions that are consumed by the Calvin cycle?
Show answer
ATP and NADPH. The light reactions produce these energy carriers, which the Calvin cycle uses to reduce 3-PGA to G3P (NADPH) and to phosphorylate intermediates and regenerate RuBP (ATP).
If a herbicide blocks the flow of electrons from PSII to the ETC, what specific outputs of the light reactions would be affected?
Show answer
All three major outputs would be affected. Without electron flow from PSII: (1) water cannot be split (no O₂ production), (2) the proton gradient would not be established (no ATP synthesis via chemiosmosis), and (3) electrons would not reach PSI (no NADPH production). The plant would essentially starve even in full sunlight.
During the Calvin cycle, how many CO₂ molecules must be fixed to produce one glucose molecule (C₆H₁₂O₆), and what is the cost in ATP and NADPH?
Show answer
Six CO₂ molecules must be fixed (since glucose has 6 carbons and each CO₂ contributes 1 carbon). The cost is 18 ATP (12 for reduction phase + 6 for RuBP regeneration) and 12 NADPH (all used in the reduction phase per 6 CO₂ fixed).
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Describe the structure of the chloroplast, identifying where the light reactions and Calvin cycle occur.
- Trace the path of electrons through Photosystems II and I, including the role of water splitting, the electron transport chain, and NADP⁺ reduction.
- Summarize the three phases of the Calvin cycle: carbon fixation, reduction, and regeneration of RuBP.
- Compare and contrast photosynthesis and cellular respiration across reactants, products, location, and energy flow.
Sources & references
- OpenStax Biology 2e, Chapter 8: "Photosynthesis"
- NCBI Bookshelf, Molecular Cell Biology, 4th edition, Section 16.1: "Photosynthesis"
- NIH, "The Calvin Cycle"
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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