🦠 Photosynthesis in Higher Plants Study Notes

13.1 What Do We Know?

Green plants are autotrophs — they synthesise their own food by photosynthesis, a physico-chemical process that uses light energy to build organic compounds. All other organisms are heterotrophs that depend on them.

Photosynthesis matters for two reasons: it is the primary source of all food on earth, and it releases oxygen into the atmosphere.

• Early experiments showed chlorophyll, light and CO₂ are all required.
• A variegated / partly-covered leaf tested for starch makes starch only in green, light-exposed parts.
• The half-leaf-in-KOH experiment (KOH absorbs CO₂) showed CO₂ is required — only the exposed half made starch.

13.2 Early Experiments

• Joseph Priestley (1770): a candle/mouse fouls air in a bell jar; a mint plant restores it. Plants restore to the air whatever burning candles and breathing animals remove. (He discovered O₂ in 1774.)
• Jan Ingenhousz: sunlight is essential; only green parts release O₂ (bubbles around green parts of an aquatic plant in light).
• Julius von Sachs (1854): glucose is produced as plants grow, stored as starch, in the green chloroplasts.
• T.W. Engelmann: prism + Cladophora + aerobic bacteria → first action spectrum; bacteria gathered in blue and red light.
• Cornelius van Niel: photosynthesis is light-dependent; hydrogen from an oxidisable compound reduces CO₂. In green plants H₂O is the donor → O₂ comes from water.

13.3 Where Does Photosynthesis Take Place?

It occurs in the green parts of plants — chiefly leaves, where mesophyll cells hold many chloroplasts aligned to catch light.

Inside the chloroplast: a membrane system of grana and stroma lamellae, plus the stroma (matrix).

• Membrane system → traps light energy, makes ATP & NADPH (light reactions / photochemical).
• Stroma → enzymatic synthesis of sugar → starch (dark / carbon reactions).

13.4 Pigments Involved

Paper chromatography of leaf pigments reveals four pigments:

Pigments absorb light at specific wavelengths. Chlorophyll a absorbs maximally in the blue and red regions — matching where photosynthesis (action spectrum) peaks, so it is the chief pigment.

Accessory pigments absorb extra wavelengths, transfer energy to chlorophyll a, and protect it from photo-oxidation. The absorption and action spectra roughly overlap but not one-to-one.

13.5 Light Reaction

The light (photochemical) phase includes light absorption, water splitting, O₂ release, and formation of ATP & NADPH.

Pigments are organised into two Light Harvesting Complexes (LHC) in Photosystem I (PS I) and Photosystem II (PS II) — named by order of discovery, not function.

• All pigments except one chlorophyll a = the antenna.
• The single chlorophyll a = the reaction centre.
• PS I: reaction centre P700 (peak 700 nm). PS II: P680 (peak 680 nm).

13.6 Electron Transport & the Z-Scheme

In PS II, P680 absorbs 680 nm light; excited electrons jump out, are caught by an acceptor, and pass downhill through cytochromes (ETS) to PS I. In PS I, P700 electrons (excited by 700 nm) pass to a higher-redox acceptor, then downhill to NADP⁺ → NADPH + H⁺. The whole path looks like a "Z" on a redox scale.

13.6.1 Splitting of water replaces PS II's lost electrons; it is on the inner (lumen) side of the thylakoid:

13.6.2 Photophosphorylation = ATP synthesis in light.

13.6.3 Chemiosmotic hypothesis: ATP synthesis is linked to a proton gradient across the thylakoid membrane; protons accumulate in the lumen (lowering its pH) because: (a) water splitting releases H⁺ in the lumen; (b) electron transport pumps H⁺ from stroma to lumen; (c) NADP⁺ reduction removes H⁺ from the stroma.

Protons flow back through the CF0 channel of ATP synthase; this causes a conformational change in CF1 (facing the stroma) that makes ATP. Chemiosmosis needs: a membrane, proton pump, proton gradient and ATP synthase.

13.7 Where ATP & NADPH Are Used — The Calvin Cycle

O₂ diffuses out; ATP & NADPH drive sugar synthesis in the stroma (biosynthetic phase — depends on light products, not light directly).

Melvin Calvin (using ¹⁴C) found the first product of CO₂ fixation in C3 plants is 3-PGA (3 carbons). The primary CO₂ acceptor is the 5-carbon RuBP.

The Calvin cycle has three stages:

• Carboxylation: RuBP + CO₂ →(RuBisCO) 2 × 3-PGA.
• Reduction: 2 ATP + 2 NADPH per CO₂ → triose phosphate / sugar.
• Regeneration: 1 ATP per turn regenerates RuBP.

13.8 The C4 Pathway

C4 plants (dry tropical) form a 4-carbon acid (OAA) first, but still use the Calvin cycle for biosynthesis. They have special Kranz anatomy: large bundle-sheath cells with many chloroplasts, thick gas-impervious walls, and no intercellular spaces.

Hatch–Slack pathway:

C4 special features: special leaf anatomy, tolerate higher temperatures, respond to high light, lack photorespiration, greater biomass productivity.

13.9 Photorespiration

RuBisCO (the most abundant enzyme in the world) binds both CO₂ and O₂ — competitively. In C3 plants, when O₂ binds, RuBP forms one PGA + one phosphoglycolate (2C).

• No sugar synthesised.
• No ATP or NADPH made.
• CO₂ is released and ATP is wasted.
• Biological function unknown.

C4 plants avoid it by concentrating CO₂ in bundle-sheath cells, so RuBisCO works as a carboxylase — explaining their higher productivity.

13.10 Factors Affecting Photosynthesis

Internal: number, size, age & orientation of leaves, mesophyll cells, chloroplasts, internal CO₂, chlorophyll amount. External: light, temperature, CO₂, water.

• Light: linear at low intensity; light saturation at ~10% of full sunlight; rarely limiting except in shade; excess light breaks down chlorophyll.
• CO₂: major limiting factor (0.03–0.04% of air). C4 saturate ~360 µl/L; C3 respond beyond 450 µl/L. Greenhouse tomato/bell-pepper are CO₂-enriched.
• Temperature: dark reactions are enzymatic & temperature-controlled; C4 have higher optima; tropical > temperate plants.
• Water: acts indirectly — stress closes stomata (less CO₂) and wilts leaves.