Photosynthesis in Higher Plants — NEET notes

Photosynthesis Overview

Photosynthesis is the process by which green plants, algae, and cyanobacteria convert light energy into chemical energy (glucose) using CO₂ and water. It occurs in chloroplasts, primarily in mesophyll cells of leaves. Chlorophyll a is the primary photosynthetic pigment; accessory pigments (chlorophyll b, carotenoids, xanthophylls) broaden the absorption spectrum. The absorption spectrum shows which wavelengths a pigment absorbs; the action spectrum shows which wavelengths actually drive photosynthesis — and their close match proves chlorophyll is the key player.

6CO_2 + 12H_2O light C_6H_ 12 O_6 + 6H_2O + 6O_2

The Overall Equation

The balanced equation of photosynthesis shows that water appears on both sides. The oxygen released comes from water (not CO₂), proven by isotope experiments.

6CO_2 + 12H_2O light, chlorophyll C_6H_ 12 O_6 + 6H_2O + 6O_2

Notice the equation uses 12H₂O on the left, not 6. Water appears on both sides because 6 new water molecules are produced during the light reactions. This balanced form was proposed by C.B. van Niel based on his work with photosynthetic bacteria.

• O₂ comes from H₂O, NOT from CO₂ — proven by Ruben and Kamen using heavy oxygen isotope ( ^ 18 O )
• When H_2^ 18 O was supplied → ^ 18 O_2 was released. When C^ 18 O_2 was supplied → no ^ 18 O_2
• Photosynthesis is essentially: CO₂ reduction (gains H) + H₂O oxidation (loses H)
• Van Niel's generalized equation: 2H_2A + CO_2 arrow 2A + CH_2O + H_2O (where H₂A is the hydrogen donor)

Where It Happens

Photosynthesis occurs in chloroplasts, specifically in the thylakoid membranes (light reactions) and stroma (Calvin cycle). Mesophyll cells of leaves are the primary site.

The chloroplast is a double-membrane organelle with an elaborate internal membrane system called thylakoids. Thylakoids are stacked into grana (singular: granum) and connected by stroma lamellae (intergranal thylakoids). The fluid-filled space inside the chloroplast but outside the thylakoids is the stroma.

• Thylakoid membrane — site of light reactions (PS I, PS II, ETC, ATP synthase embedded here)
• Thylakoid lumen — H⁺ ions accumulate here during light reactions (creates proton gradient)
• Stroma — site of Calvin cycle (dark reactions); contains RuBisCO, NADPH, ATP
• Grana thylakoids — stacked, tightly packed; contain mostly PS II
• Stroma lamellae — unstacked, connect grana; contain mostly PS I
• Mesophyll cells have the highest density of chloroplasts — that's why leaves are green

Photosynthetic Pigments

Chlorophyll a is the primary pigment (reaction centre). Accessory pigments (chlorophyll b, carotenoids, xanthophylls) absorb different wavelengths and transfer energy to chlorophyll a, broadening the usable light spectrum.

Leaf pigments can be separated by paper chromatography (or thin-layer chromatography). The separated bands reveal multiple pigments, each absorbing different wavelengths of light. Together, they form the antenna complex (light-harvesting complex) that funnels energy to the reaction centre.

• Chlorophyll a (blue-green) — the primary pigment; ONLY pigment that can directly participate in light reactions at the reaction centre
• Chlorophyll b (yellow-green) — accessory pigment; absorbs light and transfers energy to Chl a
• Carotenoids (yellow-orange) — absorb blue-violet light; also photoprotective (prevent photo-oxidation of chlorophyll)
• Xanthophylls (yellow) — oxygen-containing carotenoids; accessory light harvesters
• Phycobilins (phycoerythrin, phycocyanin) — found in red algae and cyanobacteria, NOT in higher plants

Absorption Spectrum vs Action Spectrum

The absorption spectrum shows wavelengths absorbed by a pigment. The action spectrum shows wavelengths that actually drive photosynthesis. Their close match proves chlorophyll is the key pigment, but the slight mismatch proves accessory pigments also contribute.

An absorption spectrum is a graph of absorbance vs wavelength for a single purified pigment. An action spectrum is a graph of the rate of photosynthesis vs wavelength for a whole plant/leaf. Comparing the two reveals which pigments actually drive the process.

• The action spectrum closely matches the absorption spectrum of chlorophyll a — proving Chl a is the primary driver
• But the action spectrum is slightly broader than Chl a's absorption spectrum — because accessory pigments (Chl b, carotenoids) absorb additional wavelengths and transfer energy to Chl a
• Engelmann's experiment with *Spirogyra* and aerobic bacteria: bacteria clustered at blue and red regions of the spectrum (where O₂ evolution was highest), confirming the action spectrum
• Photosynthesis is maximum in red light and minimum in green light

Quick recall

Why is the action spectrum slightly broader than the absorption spectrum of chlorophyll a alone?

Because accessory pigments (Chl b, carotenoids, xanthophylls) absorb wavelengths that Chl a cannot and transfer the energy to Chl a. So the leaf uses a wider range of light than Chl a alone would.

Light Reactions

The light reactions occur on thylakoid membranes and convert light energy into chemical energy (ATP + NADPH). Two photosystems (PS I and PS II) work together in the Z-scheme of non-cyclic electron flow. Water is split (photolysis) at PS II, releasing O₂. Electrons travel from PS II → plastoquinone → cytochrome b6f → plastocyanin → PS I → ferredoxin → NADP⁺ reductase. ATP is made by chemiosmosis as H⁺ flows through ATP synthase. Cyclic photophosphorylation involves only PS I and produces ATP but no NADPH.

2H_2O PS II, light 4H^+ + 4e^- + O_2 (Photolysis of water)

Photosystem I & II

PS I (P700) and PS II (P680) are pigment-protein complexes embedded in thylakoid membranes. PS II splits water and feeds electrons to PS I. PS I reduces NADP⁺ to NADPH.

Each photosystem has two components: (1) an antenna complex (light-harvesting complex) with hundreds of pigment molecules that absorb photons and funnel energy inward, and (2) a reaction centre with a special pair of chlorophyll a molecules that actually loses an electron when excited.

Z-Scheme

In non-cyclic electron flow, electrons travel from H₂O → PS II → PQ → Cyt b6f → PC → PS I → Fd → NADP⁺ reductase → NADPH. When plotted on a redox potential diagram, this path looks like a Z (or N) — hence the name.

The Z-scheme (proposed by Hill and Bendall, 1960) describes the path of electrons through both photosystems. It's called 'Z' because when you plot the redox potential (y-axis) against the sequence of carriers (x-axis), the path goes: up (PS II excited) → down (ETC to PS I) → up (PS I excited) → down (to NADPH). This makes a Z or N shape.

• Step 1: PS II absorbs light → P680 gets excited → P680* donates electron to pheophytin
• Step 2: Electron travels: pheophytin → plastoquinone (PQ) → cytochrome b₆f complex → plastocyanin (PC)
• Step 3: PS I absorbs light → P700 gets excited → P700* donates electron to ferredoxin (Fd)
• Step 4: Ferredoxin passes electron to NADP⁺ reductase → NADP⁺ + 2e⁻ + H⁺ → NADPH
• Step 5: PS II replaces its lost electron by splitting water (photolysis): 2H_2O → 4H^+ + 4e^- + O_2
• The electron flow is: H_2O → PS II → PQ → Cyt b_6f → PC → PS I → Fd → NADP^+

Photolysis of Water

PS II splits water molecules using light energy, releasing O₂, H⁺ ions, and electrons. This is the source of all oxygen released during photosynthesis. The reaction requires the manganese-containing oxygen-evolving complex (OEC).

2H_2O OEC, light O_2 + 4H^+ + 4e^-

When P680 donates its electron to pheophytin, it becomes P680⁺ — one of the strongest oxidizing agents in biology. P680⁺ is so desperate for electrons that it rips them from water molecules. This water-splitting is catalyzed by the oxygen-evolving complex (OEC), also called the water-splitting complex, which contains a cluster of 4 manganese (Mn) ions.

• Photolysis occurs on the inner side (lumen side) of the thylakoid membrane
• H⁺ ions released into the thylakoid lumen contribute to the proton gradient
• This is why the lumen becomes acidic (pH ~4) compared to the stroma (pH ~8)
• The O₂ we breathe is a byproduct of this water-splitting, not the main purpose
• Mn, Cl⁻, and Ca²⁺ are essential cofactors for the OEC — deficiency of Mn impairs photolysis

Cyclic vs Non-Cyclic Photophosphorylation

Non-cyclic uses both PS I and PS II, produces ATP + NADPH + O₂. Cyclic uses only PS I, produces only ATP (no NADPH, no O₂). Cyclic photophosphorylation provides the extra ATP needed for the Calvin cycle.

The Calvin cycle needs 3 ATP : 2 NADPH per CO₂ fixed. Non-cyclic photophosphorylation alone doesn't produce enough ATP relative to NADPH. Cyclic photophosphorylation tops up the ATP supply without generating more NADPH — balancing the ATP:NADPH ratio.

• In cyclic flow, excited electrons from PS I go to ferredoxin, then BACK to PS I via Cyt b₆f and plastocyanin
• The electron transport through Cyt b₆f still pumps H⁺ into the lumen → still makes ATP
• Cyclic photophosphorylation is believed to be the more primitive (evolutionarily older) process
• Some bacteria ONLY perform cyclic photophosphorylation (they have just one photosystem)
• Stroma lamellae have only PS I — so cyclic is the only option there

Chemiosmosis

H⁺ ions accumulate in the thylakoid lumen (from photolysis + ETC pumping), creating a proton gradient. H⁺ flows back to the stroma through ATP synthase (CF₀-CF₁ complex), driving ATP synthesis. This is the same principle as in mitochondria.

Chemiosmosis (proposed by Peter Mitchell, 1961 — Nobel Prize 1978) is the mechanism of ATP synthesis driven by a proton (H⁺) gradient across a membrane. In chloroplasts, the thylakoid membrane separates a low-pH lumen from a high-pH stroma.

• Three sources of H⁺ in the lumen: (1) Photolysis of water, (2) PQ shuttles H⁺ from stroma to lumen, (3) NADP⁺ reduction on stroma side removes H⁺ from stroma
• Lumen pH drops to ~4; stroma pH rises to ~8 → ΔpH ≈ 4 across the thylakoid membrane
• H⁺ flows DOWN its concentration gradient through ATP synthase (CF₀-CF₁ complex)
• CF₀ = transmembrane channel (lets H⁺ through); CF₁ = catalytic head (makes ATP)
• ATP is released on the stroma side — right where the Calvin cycle needs it

Calvin Cycle (C3 Pathway)

The Calvin cycle (discovered by Melvin Calvin) occurs in the stroma and fixes atmospheric CO₂ into organic molecules. It has three stages: carbon fixation (CO₂ + RuBP → 2 × 3-PGA, catalyzed by RuBisCO), reduction (3-PGA → G3P using ATP + NADPH), and regeneration (G3P → RuBP using ATP). For every glucose molecule, the cycle turns 6 times, using 18 ATP and 12 NADPH. The first stable product is 3-phosphoglyceric acid (3-PGA), a 3-carbon compound — hence 'C3 pathway.'

6CO_2 + 18ATP + 12NADPH RuBisCO C_6H_ 12 O_6 + 18ADP + 12NADP^+ + 18P_i

Carbon Fixation

CO₂ is fixed by RuBisCO (the most abundant enzyme on Earth) onto ribulose-1,5-bisphosphate (RuBP, 5C) to form an unstable 6C intermediate that immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA, 3C).

CO_2 + RuBP (5C) RuBisCO unstable 6C arrow 2 3-PGA (3C)

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that fixes CO₂. It's the most abundant protein on Earth — making up about 16% of total chloroplast protein. Why so much? Because RuBisCO is extremely SLOW (only ~3 CO₂ molecules per second), so plants compensate by making enormous quantities of it.

• RuBP (ribulose-1,5-bisphosphate) is the 5C CO₂ acceptor
• 3-PGA (3-phosphoglyceric acid) is the first stable product of the Calvin cycle
• RuBisCO has DUAL activity: carboxylase (fixes CO₂) and oxygenase (fixes O₂ → photorespiration)
• The 6C intermediate is so unstable it immediately splits — you never see it accumulate
• Calvin used radioactive ^ 14 C -labelled CO₂ to trace the pathway — Nobel Prize 1961

Reduction Phase

3-PGA is first phosphorylated by ATP and then reduced by NADPH to form glyceraldehyde-3-phosphate (G3P). This is where the light reaction products (ATP + NADPH) are actually used. G3P is the real sugar output of the Calvin cycle.

This is the phase where the chemical energy from light reactions is stored in sugar molecules. Each 3-PGA molecule is phosphorylated by ATP (forming 1,3-bisphosphoglyceric acid) and then reduced by NADPH to form G3P (glyceraldehyde-3-phosphate, also called PGAL or triose phosphate).

3-PGA ATP 1,3-bisPGA NADPH G3P (triose phosphate)

• Each 3-PGA uses 1 ATP (phosphorylation) + 1 NADPH (reduction) to become G3P
• Per CO₂ fixed: 2 ATP + 2 NADPH consumed in this phase (since 2 × 3-PGA are made per CO₂)
• G3P is a 3-carbon sugar phosphate — the actual output molecule of the Calvin cycle
• For 6 CO₂ (one glucose): 12 G3P molecules are made, but only 2 are net gain (the rest regenerate RuBP)
• NADPH is oxidized back to NADP⁺ and returns to the light reactions for recharging

Regeneration Phase

Out of every 6 G3P molecules produced (from 3 CO₂), 5 are rearranged to regenerate 3 RuBP molecules. This consumes 3 ATP. Only 1 G3P is net output — it takes 6 turns (and 6 net G3P = 2 × 3C = one 6C glucose) to make one glucose.

For every 3 CO₂ molecules that enter the cycle, 6 G3P molecules are produced. But 5 of these 6 G3P molecules must be recycled to regenerate 3 molecules of RuBP (the CO₂ acceptor), keeping the cycle going. Only 1 G3P is net gain per 3 CO₂ fixed.

• 5 G3P (5 × 3C = 15C) are rearranged into 3 RuBP (3 × 5C = 15C) — carbon is conserved
• This rearrangement requires 3 ATP (to phosphorylate ribulose-5-phosphate → RuBP)
• The regeneration involves a complex series of reactions (transketolase, aldolase, etc.) — NCERT doesn't require the details
• For 1 glucose (6C): cycle turns 6 times → 6 CO₂ → 12 G3P → 10 recycled + 2 net G3P → 1 glucose

Quick recall

For the Calvin cycle to produce one molecule of glucose, how many turns, ATP, and NADPH are needed?

6 turns of the cycle (one per CO₂). 18 ATP (12 for reduction + 6 for regeneration). 12 NADPH (all used in the reduction phase).

C4 Pathway (Hatch-Slack Pathway)

C4 plants (maize, sugarcane, sorghum) have a special anatomical feature called Kranz anatomy — bundle sheath cells surrounding vascular bundles with a distinct layer of mesophyll cells outside. CO₂ is first fixed by PEP carboxylase (not RuBisCO) in mesophyll cells into oxaloacetate (4C) — hence 'C4.' The 4C acid is shuttled to bundle sheath cells, where CO₂ is released and re-fixed by RuBisCO in the Calvin cycle. This CO₂ concentrating mechanism eliminates photorespiration.

CO_2 + PEP (3C) PEP carboxylase OAA (4C) arrow Malate/Aspartate (4C)

Kranz Anatomy

C4 plants have a distinctive wreath-like arrangement: large, thick-walled bundle sheath cells (with chloroplasts) surround each vascular bundle, and a layer of mesophyll cells surrounds the bundle sheath. This spatial separation is key to the C4 mechanism.

Kranz means 'wreath' in German — named for the wreath-like ring of bundle sheath cells surrounding vascular bundles. This anatomy is absent in C3 plants. The two cell types have different chloroplasts and different enzymatic machinery, creating a two-compartment CO₂-fixing system.

• Bundle sheath cells are gas-tight (minimal direct contact with intercellular air spaces)
• This creates a sealed compartment where CO₂ concentration can be pumped up to high levels
• High CO₂ around RuBisCO in bundle sheath = RuBisCO acts only as carboxylase, NOT oxygenase
• C4 plants: maize, sugarcane, sorghum, Amaranthus, crabgrass — mostly tropical grasses

The C4 Cycle Steps

In mesophyll cells: CO₂ + PEP → OAA (PEP carboxylase). OAA → malate (in most C4 plants). Malate moves to bundle sheath cells. There, malate is decarboxylated → CO₂ + pyruvate. CO₂ enters Calvin cycle. Pyruvate returns to mesophyll and is converted back to PEP (costs 2 ATP).

• Step 1 (Mesophyll): CO₂ + PEP (3C) → OAA (4C) — enzyme: PEP carboxylase (PEPcase)
• Step 2 (Mesophyll): OAA → Malate (reduced by NADPH) — in maize-type C4 plants
• Step 3: Malate moves to bundle sheath cells through plasmodesmata
• Step 4 (Bundle sheath): Malate → Pyruvate (3C) + CO₂ — decarboxylation (NADPH regenerated)
• Step 5 (Bundle sheath): Released CO₂ enters the Calvin cycle (RuBisCO fixes it as usual)
• Step 6: Pyruvate returns to mesophyll cells → converted back to PEP using 2 ATP (by pyruvate phosphate dikinase)

The C4 pathway is essentially a CO₂ concentrating pump. It costs extra energy (2 ATP per CO₂ to regenerate PEP), but the payoff is huge in hot, dry conditions — no photorespiration means much higher net photosynthesis.

Quick recall

How many extra ATP does the C4 pathway cost per CO₂ molecule compared to C3?

2 extra ATP per CO₂ (for regenerating PEP from pyruvate). So C4 uses 18 + 12 = 30 ATP per glucose vs 18 ATP for C3. But in hot conditions, this cost is worth it because C3 plants waste even more energy on photorespiration.

Why C4 Avoids Photorespiration

By concentrating CO₂ in bundle sheath cells, C4 plants ensure RuBisCO only sees high CO₂ and low O₂. This completely suppresses the oxygenase activity of RuBisCO, eliminating photorespiration. C4 plants thrive in hot, bright, dry conditions where C3 plants would waste 20-50% of fixed carbon.

RuBisCO can't tell CO₂ and O₂ apart very well. At normal atmospheric levels, it fixes O₂ about 25% of the time (photorespiration). C4 plants solve this by physically separating initial CO₂ fixation (mesophyll) from the Calvin cycle (bundle sheath). The C4 cycle pumps CO₂ into bundle sheath cells, raising the CO₂:O₂ ratio so high that RuBisCO virtually never encounters O₂.

• Bundle sheath cells have no PS II → no water splitting → no O₂ production inside these cells
• CO₂ is actively pumped in by the C4 cycle → very high CO₂ concentration around RuBisCO
• High CO₂ + low O₂ = RuBisCO acts exclusively as carboxylase (no oxygenase activity)
• Result: zero photorespiration in C4 plants
• C4 plants have higher photosynthetic efficiency, especially at high temperature and high light
• C4 plants: sugarcane, maize, sorghum achieve 2-3× higher productivity than C3 crops like wheat, rice

Photorespiration

Photorespiration is a wasteful process in C3 plants where RuBisCO fixes O₂ instead of CO₂, producing phosphoglycolate (2C) instead of 3-PGA. This 2C compound is processed through the glycolate pathway (chloroplast → peroxisome → mitochondria), releasing CO₂ without producing ATP or NADPH. It wastes up to 25-50% of fixed carbon in C3 plants. It increases at high temperature, high O₂, low CO₂, and high light. C4 plants avoid it entirely through their CO₂ concentrating mechanism.

RuBP + O_2 RuBisCO (oxygenase) Phosphoglycolate (2C) + 3-PGA (3C)

RuBisCO's Oxygenase Activity

RuBisCO evolved when Earth's atmosphere had high CO₂ and almost no O₂. It never needed to distinguish between the two. Now with 21% O₂, RuBisCO sometimes fixes O₂ onto RuBP, producing a useless 2C compound instead of useful 3-PGA.

RuBisCO's full name reveals its dual nature: Ribulose-1,5-bisphosphate carboxylase/oxygenase. When it acts as carboxylase (fixes CO₂), it produces 2 molecules of 3-PGA (useful). When it acts as oxygenase (fixes O₂), it produces 1 molecule of 3-PGA + 1 molecule of phosphoglycolate (2C) (wasteful).

RuBP (5C) + O_2 RuBisCO 3-PGA (3C) + Phosphoglycolate (2C)

• RuBisCO's active site can bind either CO₂ or O₂ — they compete for the same site
• At 25°C, RuBisCO fixes CO₂ about 3× faster than O₂. But at higher temperatures, its affinity for O₂ increases
• Phosphoglycolate (2C) is toxic and must be salvaged through the glycolate pathway
• The name 'photorespiration' is misleading — it's NOT real respiration (no ATP produced)
• It's called 'photo-respiration' because it requires light (RuBP is only made during photosynthesis) and releases CO₂

Glycolate Pathway

Phosphoglycolate from RuBisCO's oxygenase activity is processed through chloroplast → peroxisome → mitochondria. Some carbon is recovered as 3-PGA, but one CO₂ is lost. No ATP is generated. This three-organelle relay is unique to photorespiration.

• Chloroplast: Phosphoglycolate (2C) → dephosphorylated to glycolate → exported to peroxisome
• Peroxisome: Glycolate → glyoxylate → glycine (amino acid). H₂O₂ produced (broken down by catalase)
• Mitochondria: 2 glycine molecules → serine + CO₂ + NH₃ (this is where carbon is LOST)
• Back to peroxisome: Serine → glycerate → returns to chloroplast → 3-PGA (partial carbon recovery)
• Net result: 2 phosphoglycolate (2 × 2C = 4C) → 1 molecule of 3-PGA (3C) + 1 CO₂ (lost)

The glycolate pathway involves three organelles working together: chloroplast, peroxisome, and mitochondria. This is the only metabolic pathway in plants that requires all three. Out of every 2 carbons entering as phosphoglycolate, only 1.5 carbons are recovered. The remaining 0.5 carbon is lost as CO₂ — pure waste.

C3 vs C4 Plants

C3 and C4 plants differ in leaf anatomy, initial CO₂ fixation enzyme, first stable product, and susceptibility to photorespiration. C4 plants are more efficient in tropical conditions, but C3 plants dominate in cooler, wetter climates.

• CO₂ compensation point = CO₂ concentration at which photosynthesis = respiration (net = 0)
• C4 plants have a MUCH lower compensation point → they can photosynthesize even at very low CO₂
• At high temperatures, C4 plants outperform C3 by 2-3× because they avoid photorespiration
• At cool temperatures (<25°C), C3 plants are actually more efficient (no extra ATP cost of C4 cycle)
• Most crop plants are C3 (wheat, rice, barley, potato). Only a few crops are C4 (maize, sugarcane, millets)

CAM Plants (Crassulacean Acid Metabolism)

CAM plants (cacti, pineapple, Bryophyllum) fix CO₂ at NIGHT when stomata are open (to avoid water loss). CO₂ is fixed by PEP carboxylase into OAA → malic acid and stored in vacuoles. During the DAY, stomata close, malic acid is decarboxylated to release CO₂, which enters the Calvin cycle. This is TEMPORAL separation (day vs night) — unlike C4 plants which use SPATIAL separation (mesophyll vs bundle sheath cells). CAM plants are extremely water-efficient but slow-growing.

Night: CO_2 + PEP PEPcase OAA arrow Malic acid (stored in vacuole)

CAM Mechanism

CAM plants open stomata at night (cool, humid — minimal water loss) to fix CO₂ via PEP carboxylase. The resulting malic acid is stored in large vacuoles, making cells acidic at night. During the day, stomata close, malic acid is decarboxylated, and CO₂ feeds the Calvin cycle.

• Night (stomata OPEN): CO₂ enters → PEP carboxylase fixes CO₂ + PEP → OAA → malic acid
• Malic acid is stored in large vacuoles → cell sap becomes very acidic at night
• Day (stomata CLOSED): Malic acid is released from vacuoles → decarboxylated → CO₂ + pyruvate
• Released CO₂ enters the Calvin cycle in the same cell (RuBisCO fixes it as usual)
• Pyruvate is converted back to PEP → ready for the next night cycle
• Same cell does both jobs (NO Kranz anatomy needed) — the separation is in TIME, not space

The name Crassulacean Acid Metabolism comes from the family Crassulaceae (succulents like *Bryophyllum*) where this pathway was first discovered. The 'acid' refers to the accumulation of malic acid at night — if you taste a succulent leaf at night, it's noticeably more sour than during the day.

CAM vs C4

Both CAM and C4 use PEP carboxylase for initial CO₂ fixation and both avoid photorespiration. But C4 separates the two fixations in SPACE (mesophyll vs bundle sheath), while CAM separates them in TIME (night vs day). C4 plants are fast-growing; CAM plants are slow but extremely water-efficient.

• Both C4 and CAM use PEP carboxylase as the initial CO₂-fixing enzyme
• Both produce OAA (4C) as the first stable product of initial fixation
• Both eventually run the Calvin cycle with RuBisCO
• Both eliminate photorespiration by concentrating CO₂ around RuBisCO
• The KEY difference: C4 = same time, different place. CAM = same place, different time

Quick recall

A plant opens its stomata only at night, fixes CO₂ into malic acid, and stores it in vacuoles. Is this C4 or CAM?

CAM. The giveaway is stomata opening at night and storing malic acid in vacuoles. C4 plants open stomata during the day and separate fixations spatially (two cell types), not temporally.

Factors Affecting Photosynthesis

The rate of photosynthesis is controlled by multiple factors simultaneously — but at any given moment, only ONE factor limits the rate. Blackman's Law of Limiting Factors (1905) states that 'when a process depends on several factors, its rate is determined by the factor nearest its minimum value.' The major factors are light intensity, CO₂ concentration, temperature, and water availability. Understanding their individual effects and interactions is crucial for both NEET and agriculture.

Rate of photosynthesis = f(Light, CO_2, Temperature, Water) — limited by the factor at minimum

Blackman's Law of Limiting Factors

When a chemical process is governed by more than one factor, its rate is limited by the factor that is nearest to its minimum value. Increasing other factors beyond sufficiency won't help until the limiting factor is addressed.

F.F. Blackman (1905) proposed the Law of Limiting Factors: 'When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor.' In simpler terms — a chain is only as strong as its weakest link.

• At any given time, photosynthesis rate is determined by the factor closest to its minimum
• Increasing a non-limiting factor has no effect on the rate (plateau on graph)
• When the limiting factor is increased, rate rises until another factor becomes limiting (new plateau)
• This explains the plateau shape of photosynthesis rate vs light/CO₂ graphs
• Real-world application: in greenhouses, farmers optimize ALL factors (supplemental light, CO₂ enrichment, optimal temperature) for maximum yield

Effect of Light Intensity

As light intensity increases, photosynthesis rate increases linearly (light is the limiting factor), then plateaus when light saturation is reached and CO₂ or enzyme capacity becomes limiting. Beyond the saturation point, very high light can actually damage photosystems (photoinhibition).

• Light compensation point: intensity where photosynthesis = respiration (net CO₂ exchange = 0)
• Below compensation point → respiration > photosynthesis → plant loses carbon
• Light saturation point: intensity beyond which increasing light gives no further increase in rate
• At saturation, light is no longer limiting — CO₂ concentration or temperature becomes the bottleneck
• C4 plants have a higher light saturation point than C3 plants (they can use more intense light)
• Sun plants (heliophytes) have higher saturation points than shade plants (sciophytes)

Light affects the light reactions directly — more photons = more electrons excited in PS II and PS I = more ATP and NADPH produced. But once the Calvin cycle enzymes are running at maximum speed (all RuBisCO molecules occupied), extra light doesn't help.

Effect of CO₂ Concentration

Atmospheric CO₂ is only ~0.04% (400 ppm) — well below what most plants can use. Increasing CO₂ increases photosynthesis rate until a saturation point. CO₂ enrichment in greenhouses can boost yield by 30-50%. C4 plants saturate at lower CO₂ than C3 because PEP carboxylase has higher CO₂ affinity.

In natural conditions, CO₂ is the most common limiting factor because its atmospheric concentration (0.04%) is far below what RuBisCO can process at full capacity. Increasing CO₂ directly accelerates the Calvin cycle by providing more substrate for RuBisCO.

• Current atmospheric CO₂: ~0.04% (400 ppm) — far below the saturation point for most C3 plants
• C3 plants show continued increase in rate up to ~0.05% CO₂; beyond that, rate plateaus
• C4 plants saturate at lower CO₂ because PEP carboxylase has a much higher affinity for CO₂ than RuBisCO
• CO₂ enrichment in greenhouses (to ~0.05-0.1%) increases crop yield significantly
• At very high CO₂, stomata tend to close → reduces water loss but eventually limits CO₂ entry

Effect of Temperature

Photosynthesis rate increases with temperature (enzymes work faster), reaches an optimum (~25-35°C for most C3 plants, higher for C4), then drops sharply as enzymes denature. Temperature also affects the CO₂/O₂ solubility ratio and RuBisCO's oxygenase activity.

Both light reactions and the Calvin cycle involve enzymes. Like all enzymes, they have a temperature optimum. Below the optimum, reaction rates increase with temperature (Q₁₀ effect). Above the optimum, enzymes begin to denature and rates plummet.

• C3 plants: optimum temperature ~20-25°C (temperate species)
• C4 plants: optimum temperature ~30-45°C (tropical species) — adapted to heat
• At high temperature: O₂ solubility decreases less than CO₂ solubility → CO₂/O₂ ratio drops → more photorespiration in C3
• At high temperature: RuBisCO's oxygenase activity increases relative to carboxylase activity
• Above ~40-45°C: enzymes (especially RuBisCO activase) begin to denature → photosynthesis crashes
• Light reactions are less temperature-sensitive than the Calvin cycle (photochemical vs enzymatic)

Effect of Water

Water stress affects photosynthesis primarily by causing stomatal closure (reducing CO₂ entry), not by limiting the water used as substrate. Even a small water deficit triggers ABA-mediated stomatal closure, cutting off CO₂ supply.

Water is a substrate in the light reactions (photolysis), but the amount used is tiny compared to the water lost through transpiration. The main effect of water stress on photosynthesis is indirect — through stomatal closure.

• Water stress → guard cells lose turgor → stomata close → CO₂ entry is blocked
• With stomata closed, internal CO₂ drops → Calvin cycle slows → photosynthesis rate falls
• Abscisic acid (ABA) is the stress hormone that triggers stomatal closure during drought
• Water is also needed to maintain turgor for leaf expansion (wilted leaves intercept less light)
• Only about 1% of water absorbed by roots is used in photosynthesis; 99% is transpired
• Severe drought can damage thylakoid membranes and PS II — reducing photosynthetic capacity even after rewatering