Photosynthesis in Higher Plants — NCERT Class 11 Biology

Early experiments on photosynthesis:

1. Jan van Helmont (1648): Planted a willow sapling in a pot with 91 kg of soil. After 5 years, the tree gained 75 kg but the soil lost only 57 g. He concluded that plants obtain their mass from water, not soil. (Partial conclusion — he did not account for CO₂ from air.)

2. Joseph Priestley (1771): Performed the candle and mint plant experiments. He showed that a burning candle or a mouse in a sealed bell jar extinguished/died, but when a mint plant was placed in the jar, the candle continued burning and the mouse survived. He concluded that plants restore something to the air that burning or respiration removes (later identified as oxygen).

3. Jan Ingenhousz (1779): Showed that sunlight is essential for plants to produce oxygen. In the dark, plants do not release oxygen; in sunlight, they do. He also showed that only the green parts of plants produce oxygen.

4. Theodore de Saussure (1804): Showed that the increase in plant mass during photosynthesis was due to both water and CO₂ absorbed from the air.

5. Julius von Sachs (1854): Showed that starch is produced in green plant parts during photosynthesis, and that chlorophyll is necessary for the process.

Chlorophyll is the primary photosynthetic pigment. Its roles: 1. Light absorption: Chlorophyll a and b absorb light mainly in the blue (430–470 nm) and red (640–680 nm) regions of the visible spectrum and reflect green light (which is why leaves appear green). 2. Reaction centre: Chlorophyll a (P680 in PS-II, P700 in PS-I) acts as the reaction centre pigment that converts light energy into chemical energy by exciting electrons. 3. Energy transfer: Accessory pigments (chlorophyll b, carotenoids) absorb light at different wavelengths and transfer the energy to chlorophyll a at the reaction centre (resonance transfer). 4. Electron excitation: When a chlorophyll molecule absorbs a photon, an electron is excited to a higher energy state and is transferred to an electron acceptor, initiating the electron transport chain.

Chloroplast structure:

• Double membrane envelope: Outer membrane (permeable) and inner membrane (selectively permeable) separated by intermembrane space.
• Stroma: The gelatinous matrix inside the inner membrane; contains enzymes for the Calvin cycle, DNA, ribosomes, starch grains, and lipid droplets.
• Thylakoids: Flattened membranous sacs inside the stroma; the site of light reactions. Thylakoid membrane contains photosynthetic pigments, electron carriers, and ATP synthase.
• Grana (singular: granum): Stacks of thylakoids (like coins in a stack); each chloroplast has 40–60 grana.
• Stroma lamellae (frets): Flat membranous connections between grana, linking thylakoids of different grana.
• Lumen: The interior space of the thylakoid; protons accumulate here during light reactions.

Light reactions (photochemical phase) occur in the thylakoid membranes. They involve: 1. Absorption of light by photosynthetic pigments (chlorophyll, carotenoids). 2. Photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂ (at PS-II). 3. Electron transport through the electron transport chain (ETC). 4. Synthesis of ATP by photophosphorylation (chemiosmotic mechanism). 5. Synthesis of NADPH by NADP⁺ reductase.

The Z-scheme (non-cyclic electron flow): The path of electrons through both photosystems is called the Z-scheme because the overall energy diagram is shaped like the letter 'Z':

1. PS-II (P680): Light excites P680 → P680 (excited state). The excited electron is transferred to the primary electron acceptor. 2. Electron donor: P680⁺ (oxidised) receives an electron from water via the oxygen-evolving complex (photolysis: H₂O → 2H⁺ + 2e⁻ + ½O₂). 3. Electron transport chain: Electron moves from PS-II primary acceptor → plastoquinone (PQ) → cytochrome b₆f complex → plastocyanin (PC). 4. PS-I (P700): Light excites P700 → P700. Electron transferred to its primary acceptor. 5. Ferredoxin (Fd): Electron from PS-I goes to ferredoxin. 6. NADP⁺ reductase: Ferredoxin-NADP⁺ reductase (FNR) transfers electrons to NADP⁺ + H⁺ → NADPH.

The chemiosmotic hypothesis was proposed by Peter Mitchell (1961). It explains how ATP is synthesised during photosynthesis (and respiration) using a proton gradient.

In photosynthesis: 1. Proton accumulation in thylakoid lumen: Protons (H⁺) accumulate inside the thylakoid lumen from three sources:

2. Proton gradient: This creates a proton gradient (higher H⁺ inside lumen, lower in stroma) — the lumen becomes more acidic (ΔpH ≈ 3 units). 3. ATP synthase: Protons flow back from the lumen to the stroma through the CF₀-CF₁ ATP synthase complex (also called ATP synthase). CF₀ is the transmembrane channel; CF₁ is the catalytic head facing the stroma. 4. ATP synthesis: The energy released by the proton flow (proton motive force) drives the conformational change in CF₁, which synthesises ATP from ADP + Pi.

• Splitting of water (2H₂O → 4H⁺ + 4e⁻ + O₂) on the inner side of PS-II.
• Transport of electrons through the cytochrome b₆f complex, which pumps protons from the stroma into the lumen.
• NADP⁺ reductase removes H⁺ from the stroma (reducing stromal H⁺).

This is called photophosphorylation because it is powered by light-driven proton accumulation.

The C4 pathway (Hatch-Slack pathway, 1966) is a CO₂ concentration mechanism found in tropical plants (maize, sugarcane, sorghum, amaranthus) that minimises photorespiration.

C4 pathway mechanism: 1. Mesophyll cells: CO₂ is fixed by PEP carboxylase combining CO₂ with phosphoenolpyruvate (PEP, 3C) to form oxaloacetate (OAA, 4C). 2. OAA is converted to malate (or aspartate) and transported to bundle sheath cells. 3. Bundle sheath cells: Malate is decarboxylated, releasing CO₂, which enters the Calvin cycle (RuBisCO fixes it into 3-PGA). The 3C product (pyruvate) returns to mesophyll cells. 4. Pyruvate is phosphorylated back to PEP using ATP, and the cycle continues.

Key differences from C3:

Photorespiration is the uptake of O₂ and release of CO₂ in the presence of light, catalysed by RuBisCO acting as an oxygenase rather than a carboxylase.

Process: 1. Under high O₂, low CO₂, and high temperature conditions, RuBisCO binds O₂ instead of CO₂. 2. RuBP (5C) + O₂ → 3-PGA (3C) + 2-phosphoglycolate (2C). 3. The 2-phosphoglycolate enters a pathway through peroxisomes and mitochondria, ultimately releasing CO₂ but producing no ATP and no sugar. 4. The process consumes ATP and involves three organelles: chloroplast → peroxisome → mitochondria.

Why it is wasteful:

• No ATP is synthesised (in fact, ATP is consumed).
• No sugar is produced.
• CO₂ is released (opposite of photosynthesis).
• It reduces photosynthetic efficiency by up to 25% in C3 plants.
• It diverts RuBisCO from productive carboxylation.

Conditions favouring photorespiration: High temperature, high light intensity, low CO₂/O₂ ratio (dry, hot conditions when stomata close, reducing CO₂ intake).

C4 plants minimise photorespiration by concentrating CO₂ in bundle sheath cells (where RuBisCO is located), maintaining a high CO₂/O₂ ratio that favours carboxylation over oxygenation.

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Conditions: Light energy and chlorophyll. The equation shows that CO₂ and H₂O are the raw materials, and glucose (C₆H₁₂O₆) and O₂ are the products. Note that the O₂ released comes from the photolysis of water (not from CO₂), as proven by Ruben and Kamen using isotopic oxygen (¹⁸O).

Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate (Pi) using light energy, occurring in the thylakoid membrane during photosynthesis. It is analogous to oxidative phosphorylation in respiration.

Non-cyclic photophosphorylation: 1. Light is absorbed by PS-II (P680), exciting an electron from P680 to the primary electron acceptor. 2. The electron deficit in P680⁺ is replenished by electrons from water splitting (2H₂O → 4H⁺ + 4e⁻ + O₂), catalysed by the oxygen-evolving complex. 3. The excited electron moves through plastoquinone (PQ) → cytochrome b₆f complex → plastocyanin (PC). The cytochrome b₆f complex pumps protons from the stroma into the lumen. 4. Light is also absorbed by PS-I (P700), exciting an electron from P700 to its primary acceptor. 5. The electron from PS-I passes to ferredoxin (Fd) and then to NADP⁺ reductase, which reduces NADP⁺ to NADPH. 6. The proton gradient across the thylakoid membrane drives ATP synthesis via ATP synthase (CF₀-CF₁).

Products: 1 ATP + 1 NADPH + ½O₂ per electron pair. Requires 4 photons (2 for PS-II, 2 for PS-I).

The Calvin cycle (discovered by Melvin Calvin, Benson, and Bassham using ¹⁴C labeling and autoradiography) occurs in the stroma of chloroplasts. It has three phases:

Phase 1: Carboxylation (CO₂ fixation)

• CO₂ combines with ribulose-1,5-bisphosphate (RuBP, 5C), catalysed by RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
• This forms an unstable 6C intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA, 3C).
• This is the first stable product of CO₂ fixation in C3 plants.

Phase 2: Reduction

• 3-PGA is phosphorylated by ATP to form 1,3-bisphosphoglycerate (1,3-BPGA).
• 1,3-BPGA is reduced by NADPH to form glyceraldehyde-3-phosphate (G3P).
• For every 3 CO₂ molecules fixed: 6 G3P are produced, but 5 G3P are used to regenerate RuBP, and only 1 G3P is the net gain (used for glucose synthesis).

Phase 3: Regeneration of RuBP

• 5 G3P (15 carbons) are converted back to 3 RuBP (15 carbons) using 3 ATP.
• This ensures the cycle continues.

Overall: 3 CO₂ + 9 ATP + 6 NADPH → 1 G3P (net) + 9 ADP + 8 Pi + 6 NADP⁺ Two G3P molecules combine to form one glucose molecule.

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that catalyses the first step of the Calvin cycle — the carboxylation of RuBP to form 3-PGA.

It is called the most abundant protein on earth because: 1. It constitutes about 16–25% of the total soluble protein in chloroplasts. 2. It is estimated to be the most abundant single protein in the biosphere, with approximately 500 million tonnes present globally. 3. Every photosynthetic organism (plants, algae, cyanobacteria) contains it. 4. Its abundance compensates for its relatively low catalytic efficiency (slow turnover rate of ~3 CO₂ per second).

RuBisCO has dual activity: it can act as both a carboxylase (fixing CO₂) and an oxygenase (binding O₂, leading to photorespiration). The ratio of carboxylation to oxygenation depends on the CO₂/O₂ ratio and temperature.

Kranz anatomy (German: Kranz = wreath/crown) is a specialised leaf anatomy found in C4 plants:

1. Bundle sheath cells form a wreath-like arrangement around the vascular bundles (hence the name). They are large, thick-walled, and contain chloroplasts (often agranal — without grana, or with reduced grana). 2. Mesophyll cells are arranged in a radial pattern around the bundle sheath, like spokes of a wheel. 3. The bundle sheath cells are deeply embedded and have fewer intercellular air spaces, which helps maintain high CO₂ concentration.

Relation to C4 pathway:

• The C4 pathway is a spatial separation of the initial CO₂ fixation (in mesophyll cells) from the Calvin cycle (in bundle sheath cells).
• Mesophyll cells: PEP carboxylase fixes CO₂ into oxaloacetate (OAA). OAA is converted to malate and transported to bundle sheath cells.
• Bundle sheath cells: Malate is decarboxylated, releasing a high concentration of CO₂ around RuBisCO. This suppresses photorespiration because the high CO₂/O₂ ratio favours carboxylation.
• The Kranz anatomy creates a CO₂ concentration mechanism that makes C4 photosynthesis more efficient under hot, dry conditions.

Blackman's Law of Limiting Factors: If a process is influenced by several factors, the rate of the process is determined by the factor that is nearest its minimum value (the limiting factor). For example, if light is limiting, increasing CO₂ will not increase the rate until light is also increased.

External factors:

1. Light: Light intensity affects the rate of photosynthesis up to a saturation point. At low light, rate is proportional to intensity (linear phase). Beyond the saturation point, rate plateaus. Very high light can damage chlorophyll (photo-inhibition). Light quality matters — red and blue light are most effective.

2. CO₂ concentration: Higher CO₂ increases the rate of photosynthesis (more substrate for RuBisCO). Atmospheric CO₂ ≈ 0.04%, but C3 plants benefit from increased CO₂ up to about 0.05%. C4 plants are saturated at lower CO₂ levels.

3. Temperature: Photosynthesis increases with temperature up to an optimum (25–35°C for C3, 30–40°C for C4). Beyond the optimum, enzymes denature and stomata close, reducing the rate. Dark reactions are more temperature-sensitive than light reactions.

4. Water: Water is a raw material for photosynthesis. Water stress causes stomata to close, reducing CO₂ intake and thus photosynthesis. Even a slight water deficit can significantly reduce the rate.

Internal factors:

1. Chlorophyll content: More chlorophyll = more light absorption = higher rate. 2. Leaf area and orientation: Larger leaf area captures more light. 3. Age of the leaf: Young and mature leaves photosynthesise maximally; old leaves have reduced chlorophyll and enzyme activity. 4. Internal CO₂ concentration: Depends on stomatal opening and mesophyll resistance. 5. Accumulation of products: Build-up of glucose/sucrose can feedback-inhibit photosynthesis.

(See Intext Page 225 Q2 for detailed answer.)

Photorespiration is the process where RuBisCO acts as an oxygenase, combining O₂ with RuBP instead of CO₂:

• RuBP + O₂ → 3-PGA + 2-phosphoglycolate
• 2-phosphoglycolate is salvaged through peroxisomes and mitochondria, releasing CO₂ but producing no sugar or ATP.
• It involves three organelles: chloroplast → peroxisome → mitochondria.
• It is wasteful because it consumes ATP, releases CO₂, and produces no photosynthate.
• Minimised in C4 plants by CO₂ concentration in bundle sheath cells.

(See Intext Page 218 Q2 for detailed answer.)

The chemiosmotic hypothesis (Peter Mitchell, 1961; Nobel Prize 1978) explains how the proton gradient across the thylakoid membrane drives ATP synthesis: 1. Protons accumulate in the thylakoid lumen (from water splitting and cytochrome b₆f pumping). 2. A proton gradient (ΔpH ≈ 3) is established across the thylakoid membrane (lumen is more acidic). 3. Protons flow back to the stroma through ATP synthase (CF₀-CF₁). 4. The proton motive force drives conformational changes in CF₁, synthesising ATP from ADP + Pi.

Significance: It provides a universal mechanism for ATP synthesis in both photosynthesis (thylakoid membrane) and respiration (inner mitochondrial membrane), unifying bioenergetics.

If bundle sheath cells are removed from a C4 plant: 1. The Calvin cycle cannot operate because RuBisCO is located in bundle sheath cells (not in mesophyll cells in C4 plants). 2. The CO₂ concentration mechanism would fail — there would be no site for decarboxylation of malate to release concentrated CO₂. 3. Photorespiration would increase because without the CO₂-concentrating mechanism, RuBisCO in any remaining tissue would be exposed to low CO₂/high O₂ conditions. 4. Photosynthesis would drastically decrease or stop, as the C4 pathway is a two-cell system that requires both mesophyll and bundle sheath cells. 5. The plant would likely die because it cannot fix CO₂ into sugar efficiently.

The Calvin cycle is called the C3 pathway because the first stable product of CO₂ fixation is a 3-carbon compound — 3-phosphoglycerate (3-PGA).

In the Calvin cycle:

• CO₂ + RuBP (5C) → unstable 6C intermediate → 2 × 3-PGA (3C)

This contrasts with the C4 (Hatch-Slack) pathway, where the first stable product is a 4-carbon compound — oxaloacetate (OAA).

Plants that use only the Calvin cycle are called C3 plants (e.g., wheat, rice, potato). They are so named because 3-PGA is their first identifiable product of CO₂ assimilation.

ATP and NADPH (produced during the light reactions) are used in the reduction phase of the Calvin cycle:

1. ATP is used in two places:

• Phosphorylation of 3-PGA: 3-PGA + ATP → 1,3-bisphosphoglycerate (1,3-BPGA) + ADP
• Regeneration of RuBP: 5 G3P are phosphorylated using 3 ATP to regenerate 3 RuBP molecules.

2. NADPH is used in one place:

• Reduction of 1,3-BPGA: 1,3-BPGA + NADPH → G3P + NADP⁺ + Pi

Overall, for every 3 CO₂ fixed:

• 9 ATP are consumed (6 for reduction phase, 3 for regeneration).
• 6 NADPH are consumed.

These energy-rich molecules are the link between light reactions and dark reactions — without them, the Calvin cycle cannot proceed.

C4 plants do not show significant photorespiration because of their CO₂ concentration mechanism:

1. PEP carboxylase in mesophyll cells has a higher affinity for CO₂ than RuBisCO and no oxygenase activity. It efficiently fixes CO₂ even when stomata are partially closed. 2. The 4-carbon product (malate/aspartate) is transported to bundle sheath cells, where it is decarboxylated to release a high concentration of CO₂ around RuBisCO. 3. This high CO₂ concentration in bundle sheath cells maintains a high CO₂/O₂ ratio, which strongly favours the carboxylase activity of RuBisCO over its oxygenase activity. 4. Thus, RuBisCO in C4 plants almost exclusively fixes CO₂ rather than O₂, effectively suppressing photorespiration. 5. Additionally, the Kranz anatomy provides spatial separation of the initial CO₂ fixation (mesophyll) from the Calvin cycle (bundle sheath), which further minimises O₂ exposure to RuBisCO.

This is why C4 plants are more efficient in hot, dry conditions where C3 plants would suffer significant photorespiration losses.