Photosynthesis in Higher Plants

Physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. Basis of life on earth.

• Joseph Priestley (1770): Bell jar experiment. Discovered Oxygen is essential for life (released by plants).
• Jan Ingenhousz (1779): Showed that sunlight is essential. Only green parts release oxygen.
• T.W. Engelmann (1843-1909): Prism experiment on Cladophora (Algae). Bacteria accumulated in Blue & Red light regions (First Action Spectrum).
• Cornelius van Niel (1897-1985): Purple & Green sulfur bacteria. Showed photosynthesis is a light-dependent reaction where H2 from suitable oxidisable compound reduces CO2 to carbs.
  Equation: 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2.

Chloroplast: Aligns along walls of mesophyll cells. Contains Grana (Light reaction) and Stroma (Dark reaction).

Pigments (Separated by Paper Chromatography)

• Chlorophyll a (Bright/Blue green): Chief pigment. Reaction center.
• Chlorophyll b (Yellow green): Accessory pigment.
• Xanthophylls (Yellow): Accessory pigment.
• Carotenoids (Yellow to Yellow-orange): Accessory pigment. Protects Chl a from photo-oxidation.

Includes Light absorption, Water splitting, Oxygen release, and formation of ATP & NADPH.

The Two Photosystems

• PS I (P700): Reaction center absorbs light at 700 nm. Cycles electrons in Cyclic Photophosphorylation.
• PS II (P680): Reaction center absorbs light at 680 nm. Associated with water splitting (Photolysis).

Electron Transport (Z-Scheme)

1. PS II absorbs light → e- uphill to acceptor → downhill to ETS → PS I.
2. H2O splitting complex (Mn, Cl, Ca) associated with PS II releases O2. (2H2O → 4H+ + O2 + 4e-).
3. PS I electrons → uphill to acceptor → downhill to NADP+ → NADPH.

Synthesis of ATP is linked to development of a proton gradient across thylakoid membrane.

• Protons accumulate in Lumen (Splitting of water + H+ from Stroma via PQ).
• Breakdown of gradient creates energy. H+ moves through F0 of ATPase to Stroma.
• Outcome: ATP and NADPH produced in Stroma.

Occurs in Stroma. ATP and NADPH are used to synthesize sugars.

A. Calvin Cycle (C3 Pathway)

Primary Acceptor: RuBP (5C). Enzyme: RuBisCO. First Product: 3-PGA (3C).

1. Carboxylation: RuBP + CO2 → 3-PGA.
2. Reduction: Uses 2 ATP + 2 NADPH per CO2 molecule.
3. Regeneration: Uses 1 ATP to regenerate RuBP.
4. Total for 1 Glucose: 6 turns. 18 ATP + 12 NADPH consumed.

B. C4 Pathway (Hatch & Slack)

Plants adapted to dry tropical regions (Maize, Sorghum). Kranz Anatomy (Bundle Sheath Cells). No Photorespiration.

• Mesophyll: PEP (3C) + CO2 → OAA (4C). Enzyme: PEPcase. No RuBisCO.
• Bundle Sheath: Decarboxylation of C4 acid releases CO2 → enters Calvin Cycle (RuBisCO present).

Wasteful process in C3 plants. RuBisCO binds O2 instead of CO2 when O2 is high.

• RuBP + O2 → Phosphoglycerate (3C) + Phosphoglycolate (2C).
• No ATP or Sugar synthesis. Release of CO2 with use of ATP.
• Absent in C4 plants (Mechanism increases CO2 at enzyme site).

A physico-chemical process by which green plants use light energy to synthesize organic compounds.

Performed bell jar experiments in 1770s. Revealed essential role of air (oxygen) in growth.

Showed that only green parts of plants release oxygen in presence of sunlight.

Described the first action spectrum of photosynthesis using Cladophora and aerobic bacteria.

Demonstrated that photosynthesis is a light-dependent reaction where hydrogen from oxidisable compound reduces CO2.

Oxygen evolved by green plants comes from H2O, not from CO2. (Proved by radioisotope techniques).

Found mainly in mesophyll cells. Align parallel to walls for partial light, perpendicular for high light.

The chief pigment associated with photosynthesis. Shows maximum absorption in blue and red regions.

Chl b, Xanthophylls, Carotenoids. Protect Chl a from photo-oxidation and widen light absorption spectrum.

Single Chlorophyll a molecule forms the reaction center in Photosystems (LHC).

Reaction center absorbs light at 700 nm (P700).

Reaction center absorbs light at 680 nm (P680).

Electron transport scheme. Uphill oxidation (PSII) -> Downhill (ETS) -> Uphill (PSI) -> Downhill (NADPH).

Associated with PS II. Occurs on inner side of thylakoid membrane. Releases 4H+, O2, 4e-.

Process of synthesis of ATP from ADP and iP in presence of light.

Occurs in Stroma lamellae (lack PS II and NADP reductase). Only ATP synthesised. No NADPH.

ATP synthesis linked to proton gradient across membrane. Protons accumulate in Lumen.

1. Water splitting in lumen. 2. H+ transport from stroma by PQ. 3. NADP reductase uses H+ from stroma.

F0 is transmembrane channel. F1 protrudes to stroma. H+ diffusion releases energy for ATP.

Biosynthetic phase. Occurs in Stroma. Not directly light driven but depends on ATP/NADPH.

3-Phosphoglyceric Acid (3-PGA), a 3-carbon compound. Hence C3 pathway.

Ribulose bisphosphate (RuBP). A 5-carbon ketose sugar.

Ribulose bisphosphate carboxylase-oxygenase. Most abundant enzyme in the world.

1. Carboxylation (CO2 fix). 2. Reduction (ATP/NADPH use). 3. Regeneration (ATP use).

For 1 Glucose: Input 6 CO2, 18 ATP, 12 NADPH.

Hatch and Slack pathway. Found in dry tropical plants (Maize, Sorghum). More efficient.

Large sheath cells around vascular bundles (wreath). Thick walls, no intercellular spaces.

Oxaloacetic Acid (OAA), a 4-carbon compound. Formed in Mesophyll cells.

Primary CO2 acceptor in C4 is PEP. Enzyme is PEP carboxylase. Mesophyll lacks RuBisCO.

Wasteful process in C3 plants where RuBisCO binds O2. Releases CO2, uses ATP. No Sugars.

C4 plants lack photorespiration. Can tolerate higher temps. Greater productivity.

Blackman (1905). Rate is determined by the factor occurring at sub-optimal level.

Occurs at 10% of full sunlight. Light is rarely a limiting factor in nature.

C3 organisms sat. at 450 uL/L. C4 sat. at 360 uL/L. Atmosphere has 0.03-0.04%.

Tomatoes / Bell pepper interact to higher CO2 by increasing yield (C3 Plants).