Photosynthesis in Higher Plants

In this chapter: early experiments; site and pigments; light reaction, photosystems and the Z-scheme; cyclic and non-cyclic photophosphorylation; chemiosmotic hypothesis; Calvin cycle; C4 pathway and Kranz anatomy; photorespiration; factors affecting photosynthesis.

Early experiments

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in the presence of light and chlorophyll)

Where and with what

Photosynthesis takes place mainly in the chloroplasts of mesophyll cells. The grana (thylakoid membranes) carry out the light reactions; the stroma carries out the enzymatic carbon reactions (earlier called dark reactions, though they depend on products of the light reactions).

Paper chromatography of leaf pigments separates: chlorophyll a (bright or blue-green), chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange). Chlorophyll a is the chief pigment; its absorption peaks are in the blue and red regions, matching the action spectrum. The other pigments are accessory pigments: they absorb a wider range of light, pass energy to chlorophyll a, and protect it from photo-oxidation.

The light reaction

The light reaction includes light absorption, splitting of water, release of oxygen and formation of ATP and NADPH. Pigments are organised into two photosystems, each with a light-harvesting complex (LHC, antenna) of hundreds of pigment molecules around a reaction centre of chlorophyll a.

• PS I: reaction centre absorbs maximally at 700 nm: P700.
• PS II: reaction centre absorbs maximally at 680 nm: P680.

The Z-scheme (non-cyclic electron flow)

1. PS II absorbs 680 nm light; electrons in P680 are excited and passed to an electron acceptor.
2. They move down an electron transport chain (including cytochromes) to PS I.
3. PS I absorbs 700 nm light; its excited electrons go to another acceptor and finally reduce NADP⁺ to NADPH + H⁺.
4. PS II replaces its lost electrons by splitting water: 2H₂O → 4H⁺ + O₂ + 4e⁻. This happens on the inner side of the thylakoid membrane (lumen side) and releases oxygen as a by-product.

When the carriers are drawn on a redox potential scale, the electron path looks like the letter Z, which gives the scheme its name.

Photophosphorylation

Chemiosmotic hypothesis (how ATP is made)

ATP synthesis is linked to a proton gradient across the thylakoid membrane. Protons accumulate inside the lumen because: (1) water splitting releases H⁺ in the lumen; (2) as electrons move through the photosystems, protons are carried across the membrane into the lumen; (3) NADP reductase, on the stroma side, removes H⁺ from the stroma when reducing NADP⁺. Protons flow back into the stroma through ATP synthase, which has two parts: CF₀ (a channel in the membrane) and CF₁ (protruding into the stroma). The energy released makes ATP.

The Calvin cycle (C3 pathway)

The cycle was worked out by Melvin Calvin using radioactive ¹⁴C in algae. The first stable product is 3-phosphoglyceric acid (PGA), a 3-carbon compound, which is why it is called the C3 pathway. It occurs in all photosynthetic plants.

1. Carboxylation: CO₂ combines with RuBP (ribulose-1,5-bisphosphate, a 5-carbon sugar) to form two molecules of 3-PGA. The enzyme is RuBisCO (RuBP carboxylase-oxygenase).
2. Reduction: PGA is reduced to carbohydrate using ATP and NADPH.
3. Regeneration: RuBP is regenerated so the cycle can continue; this needs one ATP.

For every CO₂ fixed: 3 ATP and 2 NADPH. For one glucose (6 turns): 18 ATP and 12 NADPH.

The C4 pathway (Hatch and Slack)

This pathway is found in plants adapted to dry tropical regions, such as maize and sorghum. The first stable product is oxaloacetic acid (OAA), a 4-carbon acid.

• Kranz anatomy: large bundle sheath cells form layers around vascular bundles; they have many chloroplasts, thick walls impervious to gases, and no intercellular spaces.
• In mesophyll cells, the primary CO₂ acceptor is PEP (phosphoenolpyruvate, 3C), and the enzyme is PEP carboxylase. Mesophyll cells lack RuBisCO.
• OAA forms malic acid or aspartic acid, which move to the bundle sheath cells. There they release CO₂ and a 3C molecule, which returns to the mesophyll to regenerate PEP.
• The released CO₂ enters the Calvin cycle in the bundle sheath, which is rich in RuBisCO but lacks PEP carboxylase.
• C4 plants tolerate higher temperatures, show higher productivity and have no photorespiration.

Photorespiration

RuBisCO can bind O₂ as well as CO₂. When it binds O₂ (its oxygenase activity), RuBP forms one PGA and one phosphoglycolate. This pathway produces no sugar and no ATP; instead it releases CO₂ while using ATP. It is a wasteful process in C3 plants. C4 plants avoid it by concentrating CO₂ around RuBisCO in the bundle sheath, so the enzyme works as a carboxylase.

Factors affecting photosynthesis

Blackman's law of limiting factors (1905): if a chemical process is affected by more than one factor, its rate is determined by the factor nearest its minimal value.

• Light: rate increases linearly with light at low intensities; light saturation occurs at about 10% of full sunlight. Very high light can break down chlorophyll.
• CO₂: the major limiting factor; atmospheric concentration is very low (about 0.03 to 0.04%). Increasing it up to about 0.05% raises the rate. C3 plants respond to higher CO₂ and saturate only beyond about 450 µL L⁻¹; C4 plants saturate at about 360 µL L⁻¹. Greenhouse crops like tomato and bell pepper are grown in CO₂-enriched air for higher yields.
• Temperature: carbon reactions are enzymatic, so they are temperature-controlled. C4 plants have a higher optimum temperature.
• Water: water stress closes stomata, reducing CO₂ supply; leaves wilt, reducing surface area and metabolic activity.

Common traps: (1) Oxygen comes from water (van Niel). (2) Cyclic photophosphorylation gives only ATP and occurs in stroma lamellae. (3) In C4 plants, mesophyll has PEP carboxylase but no RuBisCO; bundle sheath has RuBisCO. (4) Water splitting occurs on the lumen side and is linked to PS II.

NEET focus

• Experiments and scientists; action spectrum.
• P680 vs P700; Z-scheme sequence; photolysis of water.
• Cyclic vs non-cyclic photophosphorylation; CF₀/CF₁.
• ATP and NADPH counts for the Calvin cycle; C4 enzymes and cell types.
• Photorespiration; Blackman's law; CO₂ saturation values.

Practice questions