Photosynthesis is the process by which green plants (and other autotrophs) convert light energy into chemical energy stored in glucose, using carbon dioxide and water.
Overall equation: 6CO2 + 12H2O + light energy → C6H12O6 + 6H2O + 6O2
Note: Van Niel showed that oxygen released comes from water, not CO2 (demonstrated using photosynthetic bacteria using H2S instead of water).
Site of Photosynthesis
- Photosynthesis occurs in chloroplasts — present mainly in mesophyll cells of leaves. A chloroplast has:
- Outer and inner membranes (double envelope)
- Stroma — fluid-filled space where dark reactions occur
- Thylakoids — flattened membrane sacs; stacked into grana (singular: granum)
- Grana are connected by stroma lamellae (intergranal thylakoids)
Photosynthetic Pigments
- Pigments absorb specific wavelengths. They are found in the thylakoid membranes.
- Chlorophyll a (blue-green): primary pigment; directly involved in light reactions; absorbs red and blue light best
- Chlorophyll b (yellow-green): accessory pigment
- Carotenoids (carotene — orange; xanthophylls — yellow): accessory pigments; protect chlorophyll from photo-oxidation; absorb wavelengths not absorbed well by chlorophylls
Action spectrum: Graph of rate of photosynthesis vs wavelength — peaks in red and blue regions.
Absorption spectrum: Graph of light absorption by a pigment vs wavelength.
The similarity between action and absorption spectra of chlorophyll confirms that chlorophyll is the main pigment for photosynthesis.
Emerson Enhancement Effect: Photosynthesis rate with two wavelengths of light simultaneously is greater than the sum of rates with each wavelength alone, suggesting two photosystems work together.
Light Reactions (in Thylakoid Membranes)
Light reactions convert light energy to chemical energy (ATP and NADPH).
Photosystem I (PS I): reaction centre P700 (absorbs 700 nm wavelength)
Photosystem II (PS II): reaction centre P680 (absorbs 680 nm wavelength)
- 1.Z-scheme (Non-cyclic photophosphorylation):
- 2.PS II absorbs light → P680 is excited → loses electrons to electron acceptor
- 3.Electrons pass through electron transport chain (plastoquinone, cytochrome b6f complex, plastocyanin) to PS I → ATP is synthesised via chemiosmosis (photophosphorylation)
- 4.PS I absorbs light → P700 excited → electrons pass to ferredoxin → NADP reductase reduces NADP+ to NADPH
- 5.PS II replaces lost electrons by photolysis (splitting) of water: 2H2O → 4H+ + 4e- + O2. This is the source of evolved oxygen.
Cyclic photophosphorylation: Only PS I is involved. Electrons from PS I cycle back through the electron transport chain to PS I. Only ATP is produced — no NADPH, no oxygen evolution.
Chemiosmosis: H+ ions accumulate in the thylakoid lumen (from water splitting and NADPH formation). This gradient drives H+ through ATP synthase (CF0-CF1 complex), synthesising ATP from ADP + Pi.
Dark Reactions — Calvin Cycle (in Stroma)
The Calvin cycle (C3 pathway) uses ATP and NADPH from light reactions to fix CO2 into carbohydrate.
Stage 1 — Carbon fixation: CO2 combines with a 5-carbon acceptor, RuBP (ribulose-1,5-bisphosphate), catalysed by enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) → unstable 6C compound → two molecules of 3-phosphoglycerate (3-PGA or 3C compound).
Stage 2 — Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
Stage 3 — Regeneration of RuBP: Most G3P is used to regenerate RuBP using ATP. Some G3P exits to form glucose.
To fix 3 CO2 and form 1 G3P net: 9 ATP and 6 NADPH are consumed. To form 1 glucose (6 carbons): 18 ATP and 12 NADPH needed.
C4 Pathway (Hatch-Slack Pathway)
Some plants (maize, sugarcane, sorghum) are adapted to hot, bright conditions. They initially fix CO2 into 4-carbon compounds (oxaloacetate → malate/aspartate) in mesophyll cells using enzyme PEP carboxylase (high affinity for CO2, no oxygenase activity). These C4 acids are transported to bundle sheath cells where CO2 is released and enters the Calvin cycle. This concentrates CO2 around RuBisCO, minimising photorespiration.
Photorespiration: In C3 plants, RuBisCO can fix O2 instead of CO2 at high O2/CO2 ratios (hot, bright conditions), releasing CO2 without producing ATP or NADPH — a wasteful process. C4 plants largely avoid this.
Common mistakes
- Oxygen produced in photosynthesis comes from water (photolysis), NOT from CO2.
- The Calvin cycle is called dark reactions but it is NOT restricted to darkness — it occurs in both light and dark but requires products of light reactions.
- Cyclic photophosphorylation involves only PS I and produces only ATP, not NADPH.
- RuBisCO is present in mesophyll in C3 plants but in bundle sheath cells in C4 plants.
Summary
Photosynthesis has two stages: light-dependent reactions in thylakoids (produce ATP, NADPH, O2) and the Calvin cycle in stroma (fix CO2 using ATP and NADPH to produce G3P). C4 plants have a modified pathway to concentrate CO2 and reduce photorespiration in hot climates.