What are the products of the light reactions of photosynthesis?

The light reactions (occurring in the thylakoid membrane) produce: ATP (via photophosphorylation), NADPH (via reduction of NADP+ by electrons from Photosystem I), and O2 (released as a by-product of water splitting at Photosystem II). These products — ATP and NADPH — are used in the Calvin cycle (dark reactions) to fix CO2 into carbohydrate. O2 is released into the atmosphere.

What is the role of auxin in phototropism?

In phototropism, light from one direction causes auxin (IAA) to migrate from the illuminated side to the shaded side of the shoot tip. The higher auxin concentration on the dark side causes greater cell elongation on that side, bending the shoot toward the light. This was demonstrated by Went's experiments with oat coleoptiles (1926). Auxin acts by stimulating cell elongation through acidification of cell walls (acid growth hypothesis) — lowered pH activates expansins that loosen cell wall polymers.

Tamil Nadu Class 12 Biology — Plant Physiology Notes & Important Questions

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Overview: Plant Physiology in NEET

Plant Physiology contributes 8–10 questions in NEET Biology. Photosynthesis (especially the Z-scheme, Calvin cycle, C3 vs C4 plants), plant respiration (ATP yield), and phytohormones are the most tested areas.

1. Photosynthesis — Light Reactions

Where: Thylakoid Membrane (grana of chloroplast)

Light reactions convert light energy into chemical energy (ATP and NADPH) and release O2.

Photosystem II (P680)

Absorbs light at 680 nm
Water splitting (photolysis): 2H2O → 4H+ + 4e- + O2 (oxygen is released here)
Electrons passed to plastoquinone → cytochrome b6f complex → plastocyanin
ATP synthesis: H+ gradient drives ATP synthase (photophosphorylation) as electrons flow through cytochrome b6f

Photosystem I (P700)

Absorbs light at 700 nm
Electrons re-energised by light
NADP+ reduction: Electrons passed via ferredoxin to NADP+ reductase → NADP+ + 2H+ + 2e- → NADPH

Z-Scheme (Non-Cyclic Electron Flow)

H2O → PS II → Plastoquinone → Cytochrome b6f → Plastocyanin → PS I → Ferredoxin → NADP+

       ↑ light                                                    ↑ light

Products: O2 (from water splitting) + ATP (from cytochrome b6f step) + NADPH (from PS I)

Why Z-scheme? The energy level of electrons goes up (light at PS II) then down, then up again (light at PS I) — the path looks like a Z when drawn on an energy diagram.

Cyclic Electron Flow (PS I only)

Electrons from PS I → ferredoxin → back to cytochrome b6f → PS I
Produces only ATP (no NADPH, no O2 release)
Occurs when NADPH demand is low but ATP demand is high

Summary of Light Reaction Products

For every 2 water molecules split and 2 NADPH produced:

O2: 1 molecule (released)
ATP: 3 molecules
NADPH: 2 molecules

2. Dark Reactions — Calvin Cycle (C3 pathway)

Where: Stroma of chloroplast

The Calvin cycle uses ATP and NADPH from light reactions to fix CO2 into sugar.

Three Stages

Stage 1: Carbon Fixation

Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase) — the most abundant enzyme on Earth
CO2 + RuBP (5C) → 2 × 3-PGA (3-phosphoglycerate, 3C) — the first stable product
This is why these plants are called C3 plants

Stage 2: Reduction

3-PGA + ATP + NADPH → G3P (glyceraldehyde-3-phosphate, 3C)
G3P is the direct sugar product of the Calvin cycle
1 G3P per 3 turns; 2 G3P needed for glucose synthesis

Stage 3: Regeneration of RuBP

G3P → RuBP (requires ATP)
Regeneration is essential to keep the cycle running

Calvin Cycle Summary (per 3 CO2 fixed)

Input:  3 CO2 + 9 ATP + 6 NADPH + 3 H2O
Output: 1 G3P (net, used for biosynthesis) + 9 ADP + 8 Pi + 6 NADP+

6 turns of Calvin cycle → enough G3P to synthesise 1 molecule of glucose

3. C3, C4, and CAM Plants

C3 Plants

First product of CO2 fixation: 3-PGA (3 carbons)
CO2 fixation directly in mesophyll cells by RuBisCO
RuBisCO also oxygenates RuBP → photorespiration (wasteful process at high temperatures)
Less efficient in hot, dry conditions
Examples: Wheat, rice, pea, sunflower, soybean, potato, tomato

C4 Plants (Hatch-Slack Pathway)

Key adaptation: CO2 is pre-concentrated in bundle sheath cells, minimising photorespiration.

Two cell types: mesophyll cells and bundle sheath cells

CO2 in mesophyll cell:
  CO2 + PEP (3C) → Oxaloacetate (4C) — enzyme: PEP carboxylase
  Oxaloacetate → Malate/Aspartate (4C) — transported to bundle sheath

In bundle sheath cell:
  Malate → CO2 + Pyruvate (3C) (CO2 released at high concentration)
  CO2 fixed by RuBisCO → normal Calvin cycle
  Pyruvate returns to mesophyll → regenerated to PEP

Advantages of C4:

PEP carboxylase has much higher affinity for CO2 than RuBisCO
No photorespiration — RuBisCO surrounded by high CO2 in bundle sheath
More efficient at high light, high temperature, low CO2 (arid/tropical conditions)

Kranz anatomy: Bundle sheath cells form a distinct ring around vascular bundles — visible feature of C4 leaves.

Examples: Maize (corn), sugarcane, sorghum, pearl millet, Bermuda grass

CAM Plants (Crassulacean Acid Metabolism)

Desert plants (succulents) — adapt to minimise water loss
Stomata open at night → CO2 absorbed and stored as malic acid in vacuoles
Stomata close during day → CO2 released from malic acid internally → Calvin cycle runs
Very low photorespiration; highly water-efficient

Examples: Cactus, pineapple, agave, jade plant

Comparison Table

Feature	C3	C4	CAM
First stable product	3-PGA (3C)	Oxaloacetate (4C)	Oxaloacetate (4C)
CO2 fixation enzyme	RuBisCO	PEP carboxylase (initially)	PEP carboxylase
Stomata pattern	Open day	Open day	Open night
Photorespiration	High	Minimal	Minimal
Examples	Wheat, rice	Maize, sugarcane	Cactus, pineapple

4. Plant Respiration

Glycolysis (Cytoplasm — no mitochondria needed)

Glucose (6C) → 2 Pyruvate (3C)

Energy balance:
  Input: 2 ATP (activation)
  Output: 4 ATP (substrate-level phosphorylation) + 2 NADH
  Net: 2 ATP + 2 NADH

Also produces: 2 pyruvate (feed into Krebs cycle)

Pyruvate Oxidation (Mitochondrial matrix)

2 Pyruvate (3C) → 2 Acetyl-CoA (2C) + 2 CO2 + 2 NADH
(Enzyme: Pyruvate dehydrogenase complex)

Krebs Cycle / Citric Acid Cycle (Mitochondrial matrix)

Each turn processes one Acetyl-CoA (2 turns per glucose):

Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C)
  ↓ (series of reactions)
  → 2 CO2 released
  → 3 NADH + 1 FADH2 + 1 GTP (≈ ATP)
  → Oxaloacetate regenerated

Per glucose (2 turns): 6 NADH + 2 FADH2 + 2 GTP

Electron Transport Chain / ETC (Inner mitochondrial membrane)

NADH → Complex I → NADH dehydrogenase
FADH2 → Complex II → Succinate dehydrogenase
Electrons flow to Complex III (cytochrome bc1), then cytochrome c, then Complex IV (cytochrome oxidase)
O2 is the final electron acceptor → forms H2O

H+ pumped across inner membrane → drives ATP synthase (Complex V)
NADH yields ~2.5 ATP
FADH2 yields ~1.5 ATP

Total ATP from 1 Glucose (Aerobic Respiration)

Stage	ATP	NADH	FADH2
Glycolysis	2	2	—
Pyruvate oxidation	—	2	—
Krebs cycle (×2)	2	6	2
ETC (NADH: ×10 × 2.5; FADH2: ×2 × 1.5)	28	—	—
Total	~30–32	—	—

Note: Older textbooks state 36–38 ATP — modern measurements give 30–32 ATP due to proton leak and transport costs.

Fermentation (Anaerobic)

Lactic acid fermentation:

Pyruvate + NADH → Lactic acid + NAD+
(Enzyme: Lactate dehydrogenase)
Occurs in: muscle cells during intense exercise, lactic acid bacteria

Alcoholic fermentation:

Pyruvate → Acetaldehyde + CO2 (Enzyme: Pyruvate decarboxylase)
Acetaldehyde + NADH → Ethanol + NAD+ (Enzyme: Alcohol dehydrogenase)
Occurs in: yeast, plants under waterlogging

Purpose of fermentation: Regenerates NAD+ so glycolysis can continue and produce 2 ATP per glucose even without oxygen.

5. Transpiration and Water Movement

Cohesion-Tension Theory (Dixon and Joly)

The dominant mechanism of water movement from roots to leaves in tall plants:

1. Water evaporates from leaf cells (transpiration) → creates tension (negative pressure) in leaf xylem 2. Tension transmitted down xylem columns 3. Cohesion of water molecules (hydrogen bonds) keeps column intact — water pulled up as one continuous thread 4. Adhesion of water to xylem walls provides additional support

Analogy: Like drinking through a straw — you create suction at the top, and the continuous column of water moves up.

Root Pressure

Ions actively pumped into xylem → water follows by osmosis → positive pressure pushes water up
Observed as guttation (water droplets from leaf margins) in the morning
Significant only in small plants with low transpiration (humid conditions, night)

Stomatal Mechanism

Guard cells regulate stomatal opening:

Opening (light, low CO2, high K+): 1. K+ ions actively pumped into guard cells 2. Water follows by osmosis → guard cells become turgid 3. Unequal cell wall thickening causes guard cells to bend → stomatal pore opens

Closing (darkness, water stress, high CO2, ABA): 1. ABA (abscisic acid) signals guard cells to release K+ 2. Water leaves guard cells → cells become flaccid → stomata close

Factors Affecting Transpiration

Factor	Effect on Transpiration
Light	Increases (stomata open)
Temperature	Increases (greater evaporation)
Humidity	Decreases (less water potential gradient)
Wind	Increases (removes humid air from leaf surface)
CO2 concentration	High CO2 → stomata close → decreases
Water availability	Water stress → ABA → stomata close

6. Plant Growth Regulators (Phytohormones)

Auxin (IAA — Indole Acetic Acid)

Site of production: Shoot apex, young leaves, developing seeds

Main effects:

Phototropism: Moves to shaded side → greater cell elongation → bend toward light
Apical dominance: Apical bud produces auxin that suppresses lateral buds (removing apical bud releases lateral buds)
Root initiation: Low concentrations stimulate root formation on stem cuttings (used commercially)
Fruit development: Promotes fruit growth; prevents abscission (fruit/leaf drop)

Key principle: Auxin response is concentration-dependent:

Low concentration → promotes growth in roots, inhibits in shoots
High concentration → inhibits roots, promotes shoots

Gibberellin (GA)

Site of production: Young leaves, root tips, seeds

Main effects:

Stem elongation: Stimulates cell elongation and division in internodes (dwarf mutants lack gibberellin)
Seed germination: Promotes germination by stimulating amylase production (breaks down starch in seeds)
Bolting: Causes rapid stem elongation in rosette plants before flowering (spinach, cabbage)
Parthenocarpy: Fruit development without fertilisation (seedless grapes treated with GA)
Breaks dormancy in seeds and buds

Cytokinin

Site of production: Root apical meristem, developing seeds

Main effects:

Cell division (cytokinesis): Promotes mitosis — essential for growth
Delay of senescence: Keeps leaves green and metabolically active ("Richmond-Lang effect")
Lateral bud growth: Acts opposite to auxin — promotes branching
Nutrient mobilisation: Attracts nutrients to areas where cytokinin is applied (sink effect)

Abscisic Acid (ABA) — "Stress Hormone"

Site of production: Mature leaves, root caps, seeds

Main effects:

Stomatal closure: Causes guard cells to lose turgor → stomata close → reduces water loss under drought stress
Seed dormancy: Inhibits germination; levels fall during stratification (cold treatment) to break dormancy
Bud dormancy: Prepares plants for winter
Abscission: Promotes leaf and fruit drop (though ethylene is more important for abscission)

Memory: ABA is the "Abominable Brake of Activity" — it generally inhibits growth and promotes dormancy/stress responses.

Ethylene (C2H4)

Only gaseous plant hormone

Site of production: Ripening fruits, nodes of stems, aged tissues

Main effects:

Fruit ripening: Promotes ripening in climacteric fruits (banana, mango, apple, tomato) — commercial use: bananas ripened with ethylene gas
Leaf and fruit abscission: Promotes formation of abscission zone
Triple response in seedlings: Inhibition of elongation, increase in girth, horizontal growth (response to mechanical stress)
Epinasty: Downward bending of leaves

Practical application: Unripe fruits stored with ripe fruits ripen faster (ethylene from ripe fruit).

Summary Table: Phytohormones

Hormone	Promotes	Inhibits	Memory
Auxin	Elongation, root initiation, fruit growth	Lateral buds	"AUX — AUgment growth"
Gibberellin	Stem elongation, germination, bolting	Dwarf phenotype	"GA — Go Above"
Cytokinin	Cell division, delay aging	Apical dominance	"CYT — Cell division"
ABA	Dormancy, stomata closure	Germination, growth	"ABA — Always Brakes Activity"
Ethylene	Ripening, abscission	Elongation	"Eth — Ending growth"

7. Mineral Nutrition

Essential Elements

Macronutrients (required in large amounts): C, H, O, N, P, K, Ca, Mg, S

Micronutrients (required in trace amounts): Fe, Mn, Zn, Cu, B, Mo, Cl, Ni

Key Functions of Selected Elements

Element	Key Function	Deficiency symptom
Nitrogen (N)	Amino acids, nucleic acids, chlorophyll	Chlorosis (yellowing), stunted growth
Phosphorus (P)	ATP, DNA/RNA backbone, phospholipids	Purple coloration, poor root growth
Potassium (K)	Stomatal opening, enzyme activation	Brown leaf margins, wilting
Magnesium (Mg)	Centre of chlorophyll molecule	Interveinal chlorosis
Iron (Fe)	Electron carriers, chlorophyll synthesis	Interveinal chlorosis (young leaves)

Nitrogen Fixation

Atmospheric N2 (cannot be directly used by plants) must be converted to NH3/NO3-.

Type	Organism	Association
Symbiotic	Rhizobium	Root nodules of legumes (pea, soybean)
Free-living aerobic	Azotobacter, Beijerinckia	Soil
Free-living anaerobic	Clostridium	Soil
Symbiotic cyanobacteria	Anabaena with Azolla	Paddy fields
Free-living cyanobacteria	Nostoc, Anabaena	Soil and water

Nitrogenase enzyme: Converts N2 → NH3; requires anaerobic conditions (leghemoglobin in root nodules creates micro-anaerobic environment).

Board Examination Questions

1-Mark Questions

Q1: Name the enzyme that fixes CO2 in the Calvin cycle. RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase)

Q2: What is photorespiration? Photorespiration is the process by which RuBisCO adds O2 instead of CO2 to RuBP, producing phosphoglycolate (a 2C compound). This is wasteful because it releases CO2 without producing ATP or NADPH. It occurs at high temperatures and low CO2 concentrations — a major inefficiency in C3 plants.

Q3: Name the gas produced during lactic acid fermentation. No gas is produced in lactic acid fermentation. (Only alcoholic fermentation produces CO2.)

Q4: Which phytohormone is called the "stress hormone"? Abscisic Acid (ABA)

Q5: Name the bacterium responsible for nitrogen fixation in the root nodules of legumes. Rhizobium

2-Mark Questions

Q1: Distinguish between cyclic and non-cyclic photophosphorylation.

Answer

Feature	Cyclic	Non-cyclic
Photosystems involved	PS I only	PS II and PS I
Products	ATP only	ATP + NADPH + O2
Water splitting	Does not occur	Occurs
Electron path	Cyclic (returns to PS I)	Linear (ends at NADP+)
Occurs when	NADPH adequate, need ATP	Normal photosynthesis

Q2: What is the significance of Kranz anatomy in C4 plants?

Answer

Kranz anatomy refers to the arrangement of mesophyll cells in a ring (wreath) around the bundle sheath cells, which in turn surround the vascular bundle. This anatomy is significant because it allows two distinct compartments for the C4 pathway: CO2 is fixed by PEP carboxylase in outer mesophyll cells and transported as 4-carbon acids to inner bundle sheath cells, where CO2 is released at high concentration for the Calvin cycle. This compartmentation keeps CO2 levels high around RuBisCO in the bundle sheath, effectively eliminating photorespiration and increasing photosynthetic efficiency.

Q3: Compare the roles of auxin and ethylene in fruit development.

Answer

Auxin promotes fruit development and delays abscission — it maintains the connection between fruit and plant by preventing formation of the abscission zone. Used commercially to prevent fruit drop in orchards. Ethylene promotes fruit ripening in climacteric fruits — stimulates production of enzymes that break down starch to sugars, soften cell walls, and develop colour and aroma. Ethylene also promotes abscission (fruit and leaf drop). The balance shifts from auxin to ethylene as fruit matures: auxin dominant during growth, ethylene dominant during ripening.

Q4: Explain the role of leghemoglobin in nitrogen fixation.

Answer

Leghemoglobin is a red, haemoglobin-like protein present in root nodules of legumes. It has a very high affinity for oxygen. Nitrogenase, the enzyme that fixes N2 to NH3, is irreversibly inactivated by oxygen. Leghemoglobin acts as an oxygen buffer — it maintains very low free oxygen levels in the nodule (micro-aerobic conditions) while still allowing enough oxygen for the aerobic respiration needed to produce ATP for nitrogen fixation. Without leghemoglobin, nitrogenase would be destroyed and nitrogen fixation could not occur.

5-Mark Questions

Q1: Describe the Calvin cycle with a diagram represented as a text cycle. Why is it called a cycle?

Answer

The Calvin cycle (C3 pathway or reductive pentose phosphate pathway) occurs in the stroma of the chloroplast and uses the ATP and NADPH produced by light reactions to fix CO2 into organic molecules.

Stage 1 — Carbon Fixation: CO2 from the atmosphere combines with the 5-carbon acceptor RuBP (ribulose-1,5-bisphosphate) catalysed by RuBisCO, the key enzyme of the cycle. The unstable 6-carbon intermediate immediately splits into two molecules of 3-PGA (3-phosphoglycerate, a 3-carbon compound). For every 3 CO2 molecules fixed, 6 molecules of 3-PGA are produced.

Stage 2 — Reduction: 3-PGA is reduced using ATP and NADPH from the light reactions. ATP phosphorylates 3-PGA to 1,3-bisphosphoglycerate (1,3-BPG), and NADPH reduces it to G3P (glyceraldehyde-3-phosphate). G3P is the direct product — a 3-carbon sugar that is the building block for glucose and other organic compounds. For every 6 molecules of 3-PGA, 6 molecules of G3P are formed; one of the 6 G3P molecules exits the cycle as net fixed carbon (usable for biosynthesis).

Stage 3 — Regeneration of RuBP: The remaining 5 G3P molecules (15 carbons total) are rearranged using ATP to regenerate 3 RuBP molecules (15 carbons total), which are ready for another turn of the cycle.

Why it is called a cycle: The final stage regenerates RuBP — the CO2 acceptor — so the cycle can accept more CO2. Each turn of the cycle fixes one CO2 and is self-regenerating. Three turns fix 3 CO2 to produce one net G3P; six turns produce enough G3P (2 molecules) to synthesise one glucose.

Energy balance per glucose (6 turns): 18 ATP + 12 NADPH consumed.

Q2: Describe the five major plant growth regulators, giving one specific function and one commercial/agricultural application for each.

Answer

1. Auxin (IAA — Indole Acetic Acid): Produced at shoot apex and transported downward (polar auxin transport). Its primary function is to promote cell elongation by loosening cell walls (acid growth hypothesis — auxin stimulates H+ pumping, lowering cell wall pH, activating expansins). Commercial application: rooting powder containing IBA (indole butyric acid, a synthetic auxin) is used to stimulate root formation on stem cuttings in horticulture. Auxin also delays fruit drop, so synthetic auxins are sprayed on apple trees to prevent premature fruit abscission.

2. Gibberellin (GA): Produced in young leaves, root tips, and seeds. Primary function: stimulates stem elongation by promoting both cell division and elongation in internodes. Commercial application: GA sprays are used on Thompson seedless grapes to increase berry size and loosen clusters for higher-quality fruit. GA also induces barley seeds to produce amylase for malting in the brewing industry.

3. Cytokinin: Produced mainly in actively dividing tissues including root apical meristems and developing seeds. Primary function: promotes cytokinesis (cell division) — acts synergistically with auxin in tissue culture to regulate differentiation. Commercial application: Cytokinins delay senescence (aging) and are used to keep cut flowers and vegetables fresh during storage and transport (post-harvest treatment).

4. Abscisic Acid (ABA): Produced in mature leaves and roots under stress. Primary function: closes stomata during drought or water stress by triggering K+ efflux from guard cells. Commercial application: ABA is used to synchronise fruit ripening and abscission for easier mechanical harvesting. Also used in research to study stress responses and potential crop drought tolerance.

5. Ethylene: The only gaseous phytohormone; produced by ripening fruits, senescing tissues, and mechanically stressed plants. Primary function: promotes fruit ripening in climacteric fruits by stimulating the synthesis of ripening enzymes (pectinase breaks down pectin in cell walls, amylase converts starch to sugar). Commercial application: Ethephon (ethylene-releasing compound) is sprayed on pineapple fields to synchronise flowering and harvesting. Ripe fruits are stored near unripe ones to accelerate ripening. Conversely, fruits are stored in CO2-rich, ethylene-free atmospheres during long-distance transport to prevent premature ripening.

NEET Focus Points

1. Photosystems: PS II = P680 = water splitting = O2 release; PS I = P700 = NADPH production 2. Calvin cycle first product: 3-PGA (3C), not glucose; RuBisCO is the key enzyme 3. C3 vs C4: C3 first product = 3C (3-PGA); C4 first product = 4C (oxaloacetate); Maize, sugarcane = C4 4. ATP yield: Glycolysis = 2 ATP net; Krebs = 2 ATP per glucose; ETC provides the bulk (~26–28 ATP) 5. Phytohormones: ABA closes stomata; Ethylene = only gas = ripening; Cytokinin = cell division = delays aging; Gibberellin = elongation + germination 6. Nitrogen fixation: Rhizobium (legume nodules) is symbiotic; Azotobacter is free-living aerobic; Nitrogenase is O2-sensitive; leghemoglobin protects it

Frequently Asked Questions

In C3 plants, CO2 is first fixed into a 3-carbon compound (3-PGA) by RuBisCO in mesophyll cells. In C4 plants, CO2 is first fixed into a 4-carbon compound (oxaloacetate/malate) in mesophyll cells and then transported to bundle sheath cells where the Calvin cycle occurs — this concentrates CO2 around RuBisCO and minimises photorespiration. C3 examples: wheat, rice, pea. C4 examples: maize, sugarcane, sorghum. C4 plants are more efficient at high temperatures and low CO2.