Photosynthesis

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Photosynthesis is the process by which green plants, algae, and certain bacteria convert light energy into chemical energy stored in organic compounds (primarily glucose). The overall equation is:

$$6CO_2 + 6H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2$$

This occurs in chloroplasts, which contain the pigment chlorophyll. Photosynthesis has two main stages: the light-dependent reactions (occurring in the thylakoid membranes) and the light-independent reactions (the Calvin cycle, occurring in the stroma).

Chloroplast Structure:

• Thylakoid: Flattened disc-shaped sacs stacked into grana; contains chlorophyll pigments and the photosynthetic electron transport chain
• Granum: A stack of thylakoids
• Stroma: Fluid matrix surrounding thylakoids; contains RuBisCO enzyme and Calvin cycle enzymes
• Stroma lamellae (intergranal lamellae): Connecting channels between grana

Light-Dependent Reactions (Non-Cyclic Photophosphorylation):

Water is split (photolysis): $2H_2O \rightarrow 4H^+ + 4e^- + O_2$ (oxygen is released as a by-product)

Electron flow through Photosystem II → Plastoquinone (PQ) → Cytochrome b₆f complex → Plastocyanin (PC) → Photosystem I → Ferredoxin → NADP⁺ reductase

ATP is generated via chemiosmosis as protons flow through ATP synthase (photophosphorylation). This produces approximately 1.3 ATP per photon pair processed.

Calvin Cycle (C₃ Pathway) — Light-Independent Reactions:

The Calvin cycle fixes CO₂ into organic molecules using ATP and NADPH from the light reactions:

1. Carbon Fixation: $CO_2 + RuBP \xrightarrow{\text{RuBisCO}} 2 \times 3-PGA$ (3-phosphoglycerate)
2. Reduction: $2 \times 3-PGA + 2ATP + 2NADPH \rightarrow 2 \times G3P$ (glyceraldehyde-3-phosphate) + 2ADP + 2NADP^+ + 2Pi$
3. Regeneration: $5 \times G3P + 3ATP \rightarrow 3RuBP$

Net: 3CO₂ → 1 G3P (then 2 G3P → 1 glucose). For one glucose: 6CO₂ → 12 G3P → 2 G3P (to glucose) + 10 G3P (regenerate 6 RuBP)

⚡ Exam Tip (MDCAT): RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant enzyme on Earth. It catalyses both carboxylation (normal photosynthesis) and oxygenation (photorespiration). In hot, dry conditions when stomata close and O₂ builds up, RuBisCO acts as an oxygenase, consuming RuBP and releasing CO₂ — this is photorespiration, which is wasteful. C₃ plants (rice, wheat, barley) are most affected by this. C₄ plants (maize, sugarcane) have a spatial solution: they fix CO₂ into a 4-carbon compound (oxaloacetate) in mesophyll cells, then transport it to bundle sheath cells where CO₂ is released at high concentration near RuBisCO, minimising photorespiration.

⚡ MDCAT Memory Trick: C₃ plants = “Cool climate” plants (wheat, rice, oats). C₄ plants = “Hot, dry” climates (maize, sorghum, sugarcane). CAM plants = “Arid” conditions (cactus, pineappple) — they open stomata at night and fix CO₂ into malic acid, which is decarboxylated during the day when stomata are closed.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding of plant physiology.

The Two Photosystems:

Photosystem II (PSII, P680): Absorbs light at 680 nm. Water splitting occurs here. Produces O₂. Releases electrons that replace those lost from the reaction centre chlorophyll P680.

Photosystem I (PSI, P700): Absorbs light at 700 nm. Uses electrons to reduce NADP⁺ to NADPH. Can operate cyclically (producing ATP only) or non-cyclically (producing both ATP and NADPH).

Non-Cyclic vs Cyclic Photophosphorylation:

Feature	Non-Cyclic	Cyclic
Electron source	Water (via PSII)	PSI only
Products	ATP + NADPH + O₂	ATP only
NADP⁺ reduction	Yes	No
O₂ evolution	Yes	No
Occurs when	Normal light conditions	ATP needed but NADPH abundant

Factors Affecting Photosynthesis:

The rate of photosynthesis is limited by the factor that is in shortest supply (Liebig’s Law of the Minimum):

1. Light intensity: As light increases, rate increases until another factor limits it
2. CO₂ concentration: Ambient ~0.04% (400 ppm); increasing to 0.1% increases rate (but too high can close stomata)
3. Temperature: Optimal for most C₃ plants ~25–30°C; RuBisCO activity drops above 40°C
4. Water availability: Water stress causes stomatal closure, reducing CO₂ intake

The Blackman’s principle of limiting factors: If one factor is limiting, increasing others has no effect until the limiting factor is addressed.

Chlorophyll and Accessory Pigments:

Chlorophyll a is the primary photosynthetic pigment, absorbing blue-violet (430 nm) and red-orange (662 nm) light. Chlorophyll b absorbs at 453 nm and 642 nm. Accessory pigments (carotenoids, xanthophylls) absorb light at wavelengths chlorophyll does not absorb well (400–500 nm, blue-green), and they protect against photo-oxidation.

⚡ Common MDCAT Error: Students often confuse the locations of light reactions vs Calvin cycle. Light reactions occur in the thylakoid membrane (specifically within the granum), while the Calvin cycle occurs in the stroma. Also note that the light reactions produce ATP and NADPH (which are used in the Calvin cycle) as well as O₂ (which is released). The Calvin cycle produces glucose (or G3P, which can be converted to glucose) but consumes ATP and NADPH.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Detailed Z-Scheme of Electron Transport:

H₂O → PSII (P680) → PQ → Cyt b₆f → PC → PSI (P700) → Fd → NADP⁺ reductase → NADPH
              ↓                              ↓
         ATP Synthase                   (cyclic e⁻ flow back to PQ)
              ↓
            ATP

The proton gradient is built up in the thylakoid lumen (inside). Protons are pumped from the stroma into the lumen at three sites: (1) water splitting at PSII, (2) PQ oxidation at cytochrome b₆f, (3) NADP⁺ reduction withdraws protons from the stroma. This creates a gradient of ~3 pH units (lumen pH ~4, stroma pH ~7). ATP synthase allows protons back through, releasing energy to phosphorylate ADP.

C₄ Photosynthesis — The Hatch and Slack Pathway:

In C₄ plants (maize, sugarcane), the initial CO₂ fixation occurs in mesophyll cells: $$CO_2 + PEP \xrightarrow{\text{PEP carboxylase}} OAA \rightarrow Malate$$ Malate is transported to bundle sheath cells, where it is decarboxylated: $$Malate + NADP^+ \rightarrow CO_2 + NADPH + pyruvate$$ The released CO₂ is then fixed by RuBisCO via the normal Calvin cycle. The C₄ pathway concentrates CO₂ around RuBisCO, suppressing photorespiration.

C₄ plants have Kranz anatomy: distinct mesophyll and bundle sheath cells, each with their own chloroplasts.

CAM Photosynthesis:

Crassulacean Acid Metabolism (cactus, pineapple) is adapted to arid conditions:

• Night: Stomata open; CO₂ fixed into oxaloacetate → malate; stored in vacuoles
• Day: Stomata closed; malate decarboxylated to release CO₂; enters Calvin cycle

Photorespiration (C₂ Cycle):

When O₂ concentration is high relative to CO₂ at the active site of RuBisCO: $$RuBP + O_2 \xrightarrow{\text{RuBisCO (oxygenase activity)}} 3-PGA + 2-PG$$ (2-phosphoglycolate)

2-phosphoglycolate is toxic and must be recycled at considerable energy cost. The C₂ cycle consumes ATP and releases previously fixed CO₂ — wasting up to 25% of photosynthetically fixed carbon under hot, dry conditions.

Uses of Photosynthesis Products in Plants:

The G3P (glyceraldehyde-3-phosphate) produced by the Calvin cycle is:

• Converted to glucose → starch (storage) or cellulose (cell wall synthesis)
• Used to synthesise amino acids (when combined with nitrogen from the soil)
• Converted to fatty acids and lipids
• Combined with nitrate to form proteins

⚡ MDCAT Exam Pattern: Questions on photosynthesis frequently ask about the fate of specific atoms or groups. Remember: the O₂ released comes from water, not from CO₂ (this was proven using the radioactive isotope ¹⁸O in experiments by Ruben and Kamen, 1941). The hydrogen in glucose comes from water; the carbon in glucose comes from CO₂. Common MDCAT questions include identifying the role of specific chlorophyll pigments, calculating ATP/NADPH requirements per glucose, and distinguishing C₃ from C₄ plants.

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