☀️ Photosynthesis
Learn photosynthesis light reactions and Calvin cycle for CUET Agriculture. Chlorophyll, photosystems I-II, C3 vs C4 and CAM pathways.
Definition
Photosynthesis is the process by which plants use sunlight, CO₂ gas, and water to synthesize sugars (carbohydrates) in the presence of chlorophyll. This is a biochemical, anabolic process — meaning it builds complex molecules (glucose) from simple ones (CO₂ and H₂O) using light energy.
Overall Equation:
12 H₂O + 6 CO₂ → (Chlorophyll + Light) → C₆H₁₂O₆ + 6H₂O + 6O₂
NOTE
Notice that 12 water molecules are used and 6 water molecules are produced — the net consumption is 6 H₂O. The oxygen released comes from the splitting of water, not from CO₂ (this was proven by Van Niel using isotope studies).
Important Discoveries
| Discovery | Scientist |
|---|---|
| Stephan Hales | Father of plant physiology |
| Von Niel | First to state that O₂ in photosynthesis comes from H₂O, not CO₂ |
| F.F. Blackman | Law of Limiting Factors |
| Peter Mitchell | Chemiosmotic theory (ATP synthesis) |
| Robert Hill | Light reaction (Hill reaction) discovery |
| Hatch and Slack | C₄ pathway discovery |
| Calvin and Benson | C₃ cycle (Calvin cycle) discovery |
Chloroplast Structure
The chloroplast is the organelle where photosynthesis takes place. Understanding its structure is key to understanding where each reaction occurs.
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Definition
Photosynthesis is the process by which plants use sunlight, CO₂ gas, and water to synthesize sugars (carbohydrates) in the presence of chlorophyll. This is a biochemical, anabolic process — meaning it builds complex molecules (glucose) from simple ones (CO₂ and H₂O) using light energy.
Overall Equation:
12 H₂O + 6 CO₂ → (Chlorophyll + Light) → C₆H₁₂O₆ + 6H₂O + 6O₂
NOTE
Notice that 12 water molecules are used and 6 water molecules are produced — the net consumption is 6 H₂O. The oxygen released comes from the splitting of water, not from CO₂ (this was proven by Van Niel using isotope studies).
Important Discoveries
| Discovery | Scientist |
|---|---|
| Stephan Hales | Father of plant physiology |
| Von Niel | First to state that O₂ in photosynthesis comes from H₂O, not CO₂ |
| F.F. Blackman | Law of Limiting Factors |
| Peter Mitchell | Chemiosmotic theory (ATP synthesis) |
| Robert Hill | Light reaction (Hill reaction) discovery |
| Hatch and Slack | C₄ pathway discovery |
| Calvin and Benson | C₃ cycle (Calvin cycle) discovery |
Chloroplast Structure
The chloroplast is the organelle where photosynthesis takes place. Understanding its structure is key to understanding where each reaction occurs.
- Chloroplasts are found in green cells (mainly mesophyll cells of leaves)
- They are double membrane-bound organelles containing:
- Stroma (matrix) — the fluid-filled interior; site of dark reactions (Calvin cycle)
- Thylakoids — flattened membrane sacs where light-dependent reactions occur
- Grana — stacks of thylakoids (singular: granum); the primary site of light reactions
- Stroma lamellae — unstacked thylakoid membranes that connect different grana
- Ribosomes and DNA — chloroplasts have their own genetic material and can make some of their own proteins
Photosynthetic Pigments
Pigments are molecules that absorb specific wavelengths of light. In photosynthesis, they are found in the thylakoid membranes of chloroplasts. There are three main types:
(1) Chlorophyll (Green Pigment)
Discovered by Willstatter, Stoll, and Fischer.
Five types exist: chl-a, chl-b, chl-c, chl-d, chl-e — but chl-a and chl-b are the most important in higher plants.
| Property | Chl-a | Chl-b |
|---|---|---|
| Molecular formula | C₅₅H₇₂O₅N₄Mg | C₅₅H₇₀O₆N₄Mg |
| Side group | −CH₃ (methyl) | −CHO (aldehyde) |
| Head structure | Porphyrin ring with Mg²⁺ in center | Porphyrin ring with Mg²⁺ in center |
| Tail part | Phytol chain (hydrophobic — anchors into membrane) | Phytol chain (hydrophobic) |
| Porphyrin ring | Present (head region) | Present (head region) |
- Chl-a is the primary pigment (reaction center pigment) — it directly participates in light reactions by donating excited electrons
- Chl-b is an accessory pigment — it absorbs light energy and transfers it to chl-a (broadening the range of light wavelengths the plant can use)
- Plants absorb blue and red wavelengths most efficiently for photosynthesis
- Plants appear green because they reflect green light (they do not use it)
- PAR (Photosynthetically Active Radiation) spectrum: 400-700 nm
(2) Carotenoids
Discovered by Wackenroder. These are yellow-orange pigments with two types:
- (a) Carotene — orange-red color (e.g., gives carrots their color; also found as rhodopsin in animal eyes)
- (b) Xanthophyll — yellow color (responsible for autumn leaf colors when chlorophyll breaks down)
TIP
Carotenoids serve a dual purpose: they act as accessory pigments (harvesting light and passing energy to chlorophyll), and they provide photoprotection by quenching harmful excess energy that could damage the photosynthetic apparatus.
(3) Phycobilins
Discovered by Kyoge Brady. These are found mainly in algae:
- (a) Phycoerythrin — red pigment (found in red algae/Rhodophyceae — gives them their characteristic red color)
- (b) Phycocyanin — blue pigment (found in blue-green algae/BGA/Cyanobacteria)
Key Points about Light Absorption:
- Sunlight photons are absorbed by pigments as packets of energy called photons (quanta)
- One photon excites one chlorophyll molecule
- Photosynthesis requires a minimum of 8 photons of light for one CO₂ molecule to be fixed
- One photon releases one electron from the photosystem
Photosynthetic Apparatus
Each photosystem is a complex of pigment molecules organized into two components:
- Reaction Center: A single chl-a molecule that actually performs the photochemistry (donates the excited electron)
- Antenna Complex: A collection of accessory pigments (chl-b, chl-c, chl-d, chl-e, phycobilins, carotenoids) that harvest light from a wide range of wavelengths and funnel the energy to the reaction center — like a satellite dish collecting signals
Photosystem I (PS-I) — P₇₀₀
- Reaction center absorbs light at 700 nm wavelength
- Found in both grana and stroma lamellae
- Involved in both cyclic and non-cyclic photophosphorylation
Photosystem II (PS-II) — P₆₈₀
- Reaction center absorbs light at 680 nm wavelength
- Found only in grana (not in stroma lamellae)
- Involved only in non-cyclic photophosphorylation
- Site of water splitting (photolysis of water) — this is where O₂ is released
IMPORTANT
Despite the numbering, PS-II acts first in the non-cyclic pathway (Z-scheme). PS-II was discovered after PS-I, hence the seemingly reversed numbering.
Light Reactions (Photochemical Phase)
Discovered by Robert Hill (Hill reaction). These reactions occur on the thylakoid membranes and require light energy directly.
Red Drop Effect
- When plants are given only red light (above 680 nm), only PS-I functions (because PS-II cannot absorb wavelengths above 680 nm)
- Output (sugar production) decreases sharply — this is called the Red Drop Effect
Emerson Enhancement Effect
- When both shorter wavelength light (below 680 nm) is provided along with far-red light, both PS-I and PS-II work together
- Output is more than the sum of individual outputs when each light is used alone
- This experiment proved the existence of two photosystems working in series (like two engines working together)
(1) Cyclic Photophosphorylation
- Only PS-I (P₇₀₀) is involved
- Occurs in stroma lamellae (where only PS-I is found)
- Electrons from PS-I are excited by light, pass through an electron transport chain, and return back to PS-I — hence "cyclic"
- Produces only ATP (no NADPH₂ is produced)
- No photolysis of water occurs (no O₂ evolution)
- Called "cyclic" because electrons complete a circle and return to their origin
Electron flow: PS-I (P₇₀₀) → Fe-S → Ferredoxin → Cyt-b₆ → Cyt-b₆f → Plastocyanin (PC) → back to PS-I
Dark Reactions (Carbon Fixation)
The "dark reactions" (more accurately called light-independent reactions) use the ATP and NADPH₂ produced during light reactions to fix CO₂ into sugar. They occur in the chloroplast stroma.
C₃ Cycle / Calvin Cycle / Benson Cycle
- Occurs in C₃ plants (the majority of plants on Earth)
- Discovered by Calvin and Benson using radioactive tracer technique with ¹⁴C (radioactive carbon isotope) and the alga Chlorella
- Occurs in the chloroplast stroma of mesophyll cells
- Key enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) — the most abundant enzyme on Earth (making up about 50% of all leaf protein!)
- CO₂ acceptor: RuBP (Ribulose bisphosphate, a 5-carbon compound)
Three Phases:
| Phase | Description |
|---|---|
| 1. Carboxylation | CO₂ is fixed by RuBisCO onto RuBP (5C) → produces 2 molecules of PGA (3-phosphoglyceric acid, 3C) |
| 2. Reduction | PGA (3C) is reduced to G3P (glyceraldehyde-3-phosphate) using ATP and NADPH₂ — this is where light reaction products are consumed |
| 3. Regeneration | RuBP (5C) is regenerated from G3P molecules to continue the cycle — requires ATP |
Key Facts:
- First stable product: PGA (3-phosphoglyceric acid) — a 3C compound (hence the name "C₃ cycle")
- For one glucose molecule, the cycle must run 6 times (fixing 6 CO₂ molecules)
- Each cycle uses: 3 ATP + 2 NADPH₂ per CO₂ molecule
- For one glucose: 18 ATP + 12 NADPH₂ = total 54 ATP equivalent
- Occurs in mesophyll cells only — no special anatomy required
Photorespiration (C₂ Cycle / Warburg Effect)
Photorespiration is a wasteful process that undoes some of the work of photosynthesis. It happens when RuBisCO mistakenly binds to O₂ instead of CO₂.
- The term "photorespiration" was coined by Krotkov
- Discovered by Decker and Tio
- Occurs only in C₃ plants when CO₂ concentration is low and O₂ concentration is high
- First product: Phosphoglycolate (a 2C compound; hence "C₂ cycle")
- Catalyzed by RuBisCO enzyme acting as an oxygenase (instead of its normal carboxylase activity)
Pathway: Chloroplast → Peroxisome → Mitochondria (involves three organelles!)
Key Facts:
- No ATP is produced — energy is wasted
- CO₂ is released without energy gain — this directly reduces photosynthetic efficiency
- Occurs when O₂ concentration is high and CO₂ is low (hot, dry conditions when stomata partially close)
- C₃ plants lose up to 25-30% of fixed carbon through photorespiration
- C₄ plants avoid this by concentrating CO₂ in bundle sheath cells, ensuring RuBisCO always encounters high CO₂
WARNING
Photorespiration is not true respiration — it does not produce ATP or useful energy. It is essentially a "mistake" by RuBisCO that wastes fixed carbon. This is why C₄ plants are more efficient than C₃ plants in hot, tropical environments.
Factors Affecting Photosynthesis
Blackman's Law of Limiting Factors
"When any process is affected by multiple factors simultaneously, the rate of the process is determined by the factor present in the minimum quantity (the most limiting factor)."
This means that even if light and water are abundant, if CO₂ is low, the rate of photosynthesis will be limited by CO₂ availability.
Key Factors:
| Factor | Optimum Range |
|---|---|
| Light | Blue and red wavelengths (400-700 nm); green is not used |
| Temperature | 10-35°C for most plants |
| CO₂ concentration | 0.03-0.05% (natural atmospheric level) |
| Water | Adequate soil moisture; stomata close under water stress |
| O₂ | High O₂ inhibits photosynthesis (Warburg effect/photorespiration) |
Additional Important Notes (Click to expand)
- Photosynthesis produces sugar/sucrose that is stored as starch granules
- Only 1% of total water absorbed by plants is used in photosynthesis; the rest is lost through transpiration
- Only about 1-2% of light energy is actually captured and used in photosynthesis
- For one sugar molecule, the C₃ cycle runs 6 times, consuming 18 ATP and 12 NADPH₂ (= 54 ATP total)
- RuBisCO is the most abundant protein/enzyme on Earth
- In photosynthesis, fixing one CO₂ requires 3 ATP and 2 NADPH₂ in the Calvin cycle
- Photorespiration produces no ATP — it is purely wasteful
Key Facts for Exam Revision
| Fact | Detail |
|---|---|
| Photosynthesis equation | 12H₂O + 6CO₂ → C₆H₁₂O₆ + 6H₂O + 6O₂ |
| PAR range | 400-700 nm |
| Most abundant enzyme | RuBisCO |
| C₃ first stable product | PGA (3C) |
| C₄ first stable product | OAA (4C) |
| CAM first product | OAA (4C) |
| C₂ cycle product | Phosphoglycolate (2C) |
| Z-scheme produces | ATP + NADPH₂ + O₂ |
| Cyclic produces | Only ATP |
| Calvin cycle per glucose | 18 ATP + 12 NADPH₂ |
| Chlorophyll discoverers | Willstatter, Stoll, Fischer |
| Chl-a vs Chl-b | −CH₃ vs −CHO side group |
| PS-I wavelength | 700 nm |
| PS-II wavelength | 680 nm |
| Light absorbed for photosynthesis | Blue and red (not green) |
| Carotenoids dissolving in | Organic solvents (lipid-soluble) |
| Water splitting occurs at | PS-II |
| Kranz anatomy found in | C₄ plants |
| Photorespiration absent in | C₄ and CAM plants |
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Photosynthesis equation | 12H₂O + 6CO₂ → C₆H₁₂O₆ + 6H₂O + 6O₂ (chlorophyll + light) |
| O₂ source | Comes from H₂O (not CO₂); proved by Von Niel |
| Father of plant physiology | Stephan Hales |
| Chloroplast structure | Stroma (dark reactions), Grana/thylakoids (light reactions), stroma lamellae (connect grana) |
| Chlorophyll discoverers | Willstatter, Stoll, Fischer |
| Chl-a vs Chl-b side group | −CH₃ (methyl) vs −CHO (aldehyde) |
| Chl-a role | Primary/reaction center pigment; directly participates in light reactions |
| PAR range | 400-700 nm |
| Absorbed wavelengths | Blue and red; green is reflected (why plants look green) |
| Carotenoids | Discovered by Wackenroder; Carotene (orange-red) + Xanthophyll (yellow); accessory + photoprotection |
| Phycobilins | Phycoerythrin (red, in red algae) + Phycocyanin (blue, in BGA); discovered by Kyoge Brady |
| Photons per CO₂ fixed | Minimum 8 photons |
| PS-I (P₇₀₀) | Absorbs 700 nm; in grana + stroma lamellae; cyclic + non-cyclic |
| PS-II (P₆₈₀) | Absorbs 680 nm; only in grana; non-cyclic only; site of water splitting |
| Red Drop Effect | Only red light (>680 nm) → only PS-I works → output drops sharply |
| Emerson Enhancement Effect | Both wavelengths together → output > sum of individual → proved two photosystems |
| Cyclic photophosphorylation | Only PS-I; in stroma lamellae; produces only ATP; no NADPH₂, no O₂ |
| Non-cyclic / Z-Scheme | Both PS-I + PS-II; in grana; produces ATP + NADPH₂ + O₂ |
| Z-Scheme electron flow | H₂O → PS-II → Pheophytin → PQ → Cyt-b₆f → PC → PS-I → Fe-S → Fd → NADPH₂ |
| First e⁻ acceptor in PS-II | Pheophytin (chlorophyll without Mg²⁺) |
| Photolysis of water | 2H₂O → O₂ + 4H⁺ + 4e⁻ at PS-II |
| C₃ / Calvin Cycle | By Calvin & Benson; in stroma of mesophyll cells; enzyme: RuBisCO; CO₂ acceptor: RuBP (5C) |
| C₃ first stable product | PGA (3C) — 3-phosphoglyceric acid |
| Calvin cycle phases | Carboxylation → Reduction → Regeneration |
| Calvin cycle per glucose | Runs 6 times; uses 18 ATP + 12 NADPH₂ |
| RuBisCO | Most abundant enzyme on Earth (~50% of leaf protein) |
| C₄ / Hatch-Slack pathway | In sugarcane, maize, sorghum; two cell types: mesophyll + bundle sheath; Kranz anatomy |
| C₄ first stable product | OAA (4C); CO₂ acceptor: PEP (3C); enzyme: PEP carboxylase |
| C₄ — no photorespiration | CO₂ concentrated in bundle sheath → RuBisCO acts only as carboxylase |
| CAM cycle | By Rego Rensaum & Thomas; in succulents (cactus, pineapple); stomata open at night (scotactive) |
| CAM separation | Temporal (same cell, different times) vs C₄ = spatial (different cells) |
| Photorespiration | Coined by Krotkov; discovered by Decker & Tio; only in C₃ plants; first product: Phosphoglycolate (2C) |
| Photorespiration pathway | Chloroplast → Peroxisome → Mitochondria; no ATP produced; wastes 25-30% of fixed carbon |
| Blackman's Law | Rate limited by factor in minimum quantity (limiting factor) |
| Light energy captured | Only 1-2% of total light; only 1% of absorbed water used in photosynthesis |
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