Topic 6

Topic 6

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Photosynthesis converts light energy into chemical energy (glucose) using CO₂ and H₂O, releasing O₂ as a byproduct. Chloroplasts are the organelle of function, with grana (thylakoid stacks) hosting light reactions and stroma housing the Calvin cycle.

Core equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Two stages: (1) Light reactions — occur in thylakoid membrane; water splits (photolysis), ATP and NADPH produced via photophosphorylation (cyclic produces ATP only; non-cyclic produces ATP + NADPH). (2) Calvin cycle — occurs in stroma; Rubisco enzyme fixes CO₂ to RuBP, ultimately producing glucose.

C3 vs C4: C3 plants (rice, wheat) use Calvin cycle only; C4 plants (maize, sugarcane) add PEP carboxylation in mesophyll → 4-carbon acids → bundle sheath → Calvin cycle. Photorespiration occurs when O₂ competes with CO₂ at Rubisco in C3 plants, wasting energy.

Key exam points: O₂ is released from water, not CO₂. C4 plants have Kranz anatomy. Quantum yield = 1 O₂ per 8–10 photons. CAM plants fix CO₂ at night.

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Chloroplast Architecture

The chloroplast is bounded by a double membrane. Inside, the stroma (fluid matrix) contains ribosomes, DNA, and the enzymes of the Calvin cycle. Immersed in the stroma is a network of thylakoid membranes, stacked into grana (singular: granum). Each thylakoid is a flattened vesicle whose membrane houses chlorophyll pigments, electron carriers, and the ATP synthase complex (CF₀-CF₁).

Light Reactions (Thylakoid Membrane)

Two photosystems work in series:

• Photosystem II (PSII): Absorbs light at ~680 nm. Chlorophyll P680 is excited, releasing electrons that pass through plastoquinone (PQ), cytochrome b₆f complex, and plastocyanin (PC) to Photosystem I. Water is split by the oxygen-evolving complex (OEC): 2H₂O → 4H⁺ + 4e⁻ + O₂. Protons accumulate in the thylakoid lumen, creating a gradient that drives ATP synthesis (photophosphorylation).
• Photosystem I (PSI): Absorbs light at ~700 nm. Chlorophyll P700 passes electrons via ferredoxin (Fd) to NADP⁺ reductase, reducing NADP⁺ to NADPH.

Non-cyclic photophosphorylation channels both PSII and PSI electrons to produce ATP + NADPH. Cyclic photophosphorylation routes PSI electrons back via Fd → PQ → cyt b₆f, producing ATP only — useful when NADPH accumulates faster than needed.

Calvin Cycle (Stroma)

Three phases, catalyzed entirely by stromal enzymes:

Phase	Reaction	Key Enzyme
Carboxylation	RuBP + CO₂ → 2 × 3-PGA	Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase)
Reduction	3-PGA + ATP → 1,3-BPGA + ADP; 1,3-BPGA + NADPH → G3P + NADP⁺	3-PGA kinase, glyceraldehyde-3-P dehydrogenase
Regeneration	G3P → RuBP (5-carbon)	Phosphoribulokinase, others

6 CO₂ + 18 ATP + 12 NADPH → 1 glucose + 18 ADP + 12 NADP⁺ + 18 Pi. For every 6 CO₂ molecules fixed, 2 G3P exit as net gain; 10 G3P recycle to regenerate 6 RuBP.

C4 Pathway

In C4 plants (maize, sorghum, sugarcane), spatial separation prevents photorespiration:

1. Mesophyll cell: Phosphoenolpyruvate (PEP) carboxylase (affinity for CO₂ 60× greater than Rubisco) fixes CO₂ → oxaloacetate (OAA, 4C) → converted to malate or aspartate.
2. Bundle sheath cell: 4-carbon acid is decarboxylated; CO₂ is concentrated and refixed by Rubisco through the standard Calvin cycle.

Kranz anatomy — distinct concentric layers of bundle sheath cells surrounding vascular tissue — is the structural hallmark. C4 plants outperform C3 at high light intensity, high temperature (>30°C), and low atmospheric CO₂ because PEP carboxylase is insensitive to O₂.

Photorespiration

Rubisco catalyzes both carboxylation (CO₂ + RuBP → 2 × 3-PGA) and oxygenation (O₂ + RuBP → 3-PGA + phosphoglycolate). Photorespiration oxidizes fixed carbon through the glycolate pathway, releasing previously fixed CO₂ and consuming O₂ without producing ATP — a net loss, especially in hot, dry conditions when stomata close and O₂:CO₂ ratio rises in the leaf.

Feature	C3 Plants	C4 Plants
Primary CO₂ acceptor	RuBP	PEP
Key enzyme	Rubisco	PEP carboxylase
Photorespiration	High	Low/near zero
Optimum temperature	20–25°C	30–40°C
Examples	Rice, wheat, potato	Maize, sugarcane, sorghum

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Chlorophyll Structure and Light Harvesting

Chlorophyll a (C₅₅H₇₂O₅N₄Mg) has a porphyrin head (contains Mg at centre, absorbs red 660–673 nm and blue 430 nm light) and a phytol tail (hydrophobic tail anchoring in thylakoid membrane). Chlorophyll b (C₅₅H₇₀O₆N₄Mg) absorbs blue 450–660 nm, extending the spectrum. Accessory pigments — carotenoids (carotene, xanthophyll) — absorb in the 400–500 nm range and dissipate excess excitation energy as heat, protecting against photo-oxidative damage.

The light-harvesting complex (LHC) arranges ~200 pigment molecules around each reaction centre. Energy migrates by Förster resonance energy transfer (FRET) to the reaction centre chlorophyll within ~10⁻¹¹ seconds — faster than photochemistry can degrade it.

Z-Scheme: Detailed Electron Flow

The Z-scheme maps the redox potential of electron carriers from H₂O (+0.82 V) at PSII to NADP⁺ (−0.32 V) at PSI:

H₂O → OEC → P680* → pheophytin → PQ → cyt b₆f → PC → P700* → Fd → NADP⁺ reductase → NADPH.

Photophosphorylation coupling: The proton gradient (ΔpH ≈ 3.5 units across thylakoid membrane) drives ATP synthase (CF₁ catalytic head, CF₀ proton channel) to synthesize ATP from ADP + Pi. Approximately 4 protons must pass through CF₁ per ATP synthesized. Non-cyclic flow yields ~3 ATP per 2 electrons (2 H₂O split → 1 O₂ + 4 e⁻ + 4 H⁺), plus 2 NADPH.

Regulation of Calvin Cycle

• Light activation: Rubisco activase uses ATP to remove inhibitory sugar phosphates from Rubisco’s active site; thioredoxin (reduced by PSI electrons) activates Calvin cycle enzymes via disulfide reduction.
• pH gradient: Protons entering the stroma via ATP synthase lower stromal pH, activating several Calvin cycle enzymes.
• Mg²⁺: H⁺ efflux through CF₀ drives Mg²⁺ counter-transport into the stroma, matching charge balance and activating enzymes.
• G3P feedback: Accumulating triose phosphates inhibit early-cycle enzymes, preventing runaway fixation.

CAM Plants: Temporal Separation

Crassulacean Acid Metabolism (e.g., cactus, pineapple) separates carboxylation temporally: PEP carboxylase fixes CO₂ into malic acid at night (cool, stomata open), stored in vacuoles. During the day, stomata close and malate is decarboxylated; CO₂ is released and refixed by Rubisco in the Calvin cycle. Water-use efficiency is extremely high — advantageous in arid habitats.

Factors Limiting Photosynthesis Rate

Law of limiting factors (Blackmann, 1905): The rate is limited by the factor at its minimum value, not at its optimum.

• Light intensity: At low light, rate is light-limited (more photons → more electrons). At high light, other factors (CO₂, temperature, Rubisco capacity) become limiting. Light compensation point (LCP) = rate of photosynthesis = rate of respiration. Saturation point = no further increase with increasing light.
• CO₂ concentration: CO₂ response curve rises steeply; C3 saturation occurs ~800–1000 ppm (ambient ~400 ppm). C4 plants saturate earlier because PEP carboxylase has high affinity.
• Temperature: Rate doubles per 10°C rise (Q₁₀ ≈ 2) within the physiological range; above ~40°C, enzyme denaturation and increased photorespiration reduce net gain.
• O₂ concentration: Increasing O₂ above 2% activates Rubisco’s oxygenase activity, raising photorespiration — relevant in enclosed or poorly ventilated growing environments.
• Water availability: Stomatal closure under drought reduces CO₂ uptake; measured as transpiration rate or stomatal conductance (gs).

Common Mistakes to Avoid

• O₂ source: Students incorrectly state O₂ is derived from CO₂. Controlled ¹⁸O-labeling experiments confirm O₂ comes exclusively from H₂O (water splitting at PSII).
• Cyclic vs. non-cyclic confusion: Cyclic produces only ATP; non-cyclic produces both ATP and NADPH. Many students forget this distinction.
• Rubisco dual function: Rubisco catalyzes carboxylation (productive) and oxygenation (wasteful) simultaneously. It cannot be described as acting only in C3 plants — it is present in all plants; C4 plants simply localize it to bundle sheath cells where CO₂ is concentrated.
• Net vs. gross photosynthesis: The rate measured experimentally is net photosynthesis = gross photosynthesis − respiration. At the light compensation point, net rate = zero, but gross photosynthesis continues.
• Kranz anatomy definition: Kranz means “wreath” — the concentric arrangement of bundle sheath cells around vascular tissue in C4 leaves. This structural feature is diagnostic and must be distinguished from C3 leaf anatomy.

Practice Prompts

1. A C3 plant and a C4 plant are grown together under conditions of 40°C, high light, and 350 ppm CO₂. Predict which plant will show higher photosynthetic rate and explain the biochemical basis, including the roles of PEP carboxylase and Rubisco. What change in atmospheric CO₂ would reduce the advantage of the C4 plant?
2. Describe the Z-scheme of non-cyclic electron transport, specifying the wavelengths absorbed by PSII and PSI, the exact splitting reaction of water (including products), and how the proton gradient drives ATP synthesis. Include the stoichiometry: how many photons are required per O₂ molecule evolved and per NADPH produced?

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.