📖 Chapter BN04 · NDA Class 11–12 Level🎯 NDA Level : High Priority
Plant Physiology is a reliably tested chapter in NDA Biology. Questions appear on the overall equation of photosynthesis, the difference between light and dark reactions, factors limiting photosynthesis, aerobic vs anaerobic respiration ATP yield, and the five plant hormones with their specific functions. The chapter is compact, formula-rich, and high-scoring for students who memorise systematically.
📌 What to expect in NDA (based on 2022–2025 pattern): (1) Overall equation of photosynthesis — raw materials, products, and conditions; (2) Light reaction vs Dark reaction (Calvin cycle) — location, inputs, outputs; (3) Factors affecting the rate of photosynthesis (Blackman's Law of Limiting Factors); (4) Aerobic vs anaerobic respiration — ATP yield comparison; fermentation products; (5) Plant hormones — each hormone, its source, function, and a key applied example; (6) Photosynthesis vs Respiration comparison (direction of gas exchange, energy).
Topics at a Glance
① Photosynthesis
Equation, light & dark reactions, factors
② Plant Respiration
Aerobic, anaerobic, ATP yield, vs photosynthesis
③ Plant Hormones
Auxin, Gibberellin, Cytokinin, ABA, Ethylene
1. Photosynthesis
1.1
What is Photosynthesis & Overall Equation
The equation with conditions above/below the arrow is the single most tested fact in this chapter
Photosynthesis is the process by which green plants (and some bacteria) convert light energy into chemical energy stored as glucose, using carbon dioxide and water. It is an anabolic (building up), endergonic (energy-absorbing) process — the opposite of respiration.
6CO₂ + 6H₂O──Sunlight, Chlorophyll──▶C₆H₁₂O₆ + 6O₂Carbon dioxide + Water → Glucose + Oxygen | Requires: light energy + chlorophyll
📌 Raw materials: CO₂ (from air, enters via stomata) + H₂O (from soil, absorbed by roots)
📌 Products: Glucose (C₆H₁₂O₆) + O₂ (released as byproduct)
📌 Site: Chloroplasts (specifically thylakoid membranes for light reactions; stroma for dark reactions)
📌 Energy conversion: Light energy → Chemical energy (stored in glucose)
Fig. 1 — Labelled chloroplast diagram. Light reactions occur in the thylakoid membrane (grana); Dark reactions (Calvin cycle) occur in the stroma. Chloroplasts contain their own 70S ribosomes and circular DNA.
1.2
Light Reaction vs Dark Reaction (Calvin Cycle)
Location, inputs, outputs — each column is a potential NDA MCQ option
☀️ Light Reaction (Light-Dependent)
🌘 Dark Reaction — Calvin Cycle (Light-Independent)
Location: Thylakoid membrane (grana)
Location: Stroma of chloroplast
Requires: Light (sunlight) — cannot proceed in dark
Requires: No direct light needed — but needs ATP & NADPH from light reaction
Inputs: H₂O + Light energy + ADP + NADP⁺
Inputs: CO₂ + ATP + NADPH
Outputs: ATP + NADPH + O₂ (O₂ released from water splitting)
Outputs: Glucose (C₆H₁₂O₆) + ADP + NADP⁺
Key process: Photolysis of water (2H₂O → 4H⁺ + 4e⁻ + O₂); Photophosphorylation (ADP → ATP)
Key process: CO₂ fixation by RuBisCO enzyme; C3 pathway; Regeneration of RuBP
Pigments involved: Chlorophyll a, Chlorophyll b, Carotenoids (in Photosystem I & II)
🧠 Memory Aid for Light vs Dark Reactions: Light reaction = "SPLIT and MAKE energy" — Splits water (photolysis), makes ATP & NADPH, releases O₂ Dark reaction = "USE energy to BUILD glucose" — Uses ATP + NADPH to fix CO₂ into glucose (no light needed directly)
Key link molecule: ATP and NADPH produced in light reaction are consumed in dark reaction.
📌 Photosystems I & II — NDA Statement Questions: Photosystem II (P680): absorbs light at 680 nm; splits water (photolysis); releases O₂; produces ATP Photosystem I (P700): absorbs light at 700 nm; reduces NADP⁺ → NADPH (used in Calvin cycle)
Note: PS II acts first despite being numbered II — NDA has asked "Which photosystem releases O₂?" → PS II
1.3
Factors Affecting Photosynthesis
Blackman's Law of Limiting Factors — the factor in shortest supply limits the overall rate
The rate of photosynthesis is controlled by the factor that is present in the least favourable amount — this is Blackman's Law of Limiting Factors (1905). Even if all other conditions are optimal, a single limiting factor will cap the rate.
☀️ Light Intensity
💔 CO₂ Concentration
💧 Water Availability
🌡️ Temperature
🌿 Chlorophyll Content
☀️ Light Intensity
As light ↑, photosynthesis rate ↑ up to a point
Light saturation point: beyond which rate does not increase (CO₂ or temp becomes limiting)
Compensation point: rate of photosynthesis = rate of respiration (net gas exchange = 0)
Red and blue light most effective; green light least (reflected → plant appears green)
Very high light can cause photoinhibition (damage to reaction centres)
💔 CO₂ Concentration
Strongest limiting factor under normal conditions (CO₂ in air = only 0.04%)
As CO₂ ↑, rate ↑ until another factor limits
C3 plants (wheat, rice): RuBisCO fixes CO₂ directly; more sensitive to CO₂ levels
C4 plants (maize, sugarcane): CO₂ concentrating mechanism; less affected by low CO₂
Greenhouses add CO₂ to boost crop yield
🌡️ Temperature
Dark reactions (enzyme-driven) are temperature-sensitive
Light reactions less sensitive to temperature (photochemical processes)
Low temperature: enzyme activity ↓ → rate of dark reaction ↓
🌿 Chlorophyll & Other Factors
More chlorophyll → higher light absorption → higher rate
Mineral deficiency (Mg for chlorophyll, Fe for electron carriers) → rate ↓
Water: raw material for light reaction; water stress → stomata close → CO₂ entry ↓
Wavelength of light: 680 nm (red) and 450–500 nm (blue) most effective (action spectrum peaks)
🌿 C3 vs C4 Plants — NDA Tested
C3 plants: first stable product is 3-PGA (3 carbon) — wheat, rice, oats, sunflower, potato
C4 plants: first stable product is OAA (4 carbon) — maize/corn, sugarcane, sorghum
C4 plants more efficient in high light, high temp, low CO₂ (tropical plants)
CAM plants: stomata open at night; Crassulacean acid metabolism — cacti, succulents
C4 enzyme: PEP carboxylase (higher CO₂ affinity than RuBisCO)
PYQTopic-Wise PYQs — Photosynthesis
Q1. The oxygen released during photosynthesis comes from:
A. Carbon dioxide
B. Glucose
C. Water
D. ATP
Answer: C — Water. During the light reaction, water molecules are split (photolysis) by the energy of sunlight: 2H₂O → 4H⁺ + 4e⁻ + O₂. The O₂ released comes entirely from water, not from CO₂. This was proven by Calvin and Ruben using radioactive isotopes (H₂¹⁸O experiment).
Q2. Dark reactions of photosynthesis take place in the:
A. Thylakoid membrane
B. Stroma of chloroplast
C. Outer membrane of chloroplast
D. Grana
Answer: B — Stroma of chloroplast. The Calvin cycle (dark reactions) takes place in the stroma — the fluid-filled space surrounding the thylakoids inside the chloroplast. The thylakoid membranes are the site of light reactions (light absorption, photolysis, ATP and NADPH production).
Q3. Which of the following is the first stable product of carbon dioxide fixation in C3 plants?
A. Oxaloacetic acid (OAA)
B. Phosphoglyceric acid (3-PGA)
C. Phosphoenolpyruvate (PEP)
D. Glucose-6-phosphate
Answer: B — 3-PGA (3-Phosphoglyceric acid). In C3 plants, CO₂ combines with RuBP (5-carbon) via RuBisCO enzyme to form an unstable 6-carbon compound that immediately splits into two molecules of 3-PGA (3-carbon). This is the first stable product — the "C3" in C3 plants refers to this 3-carbon compound. In C4 plants, OAA (4-carbon) is the first stable product.
Q4. Which photosystem is involved in the splitting of water during photosynthesis?
A. Photosystem I (P700)
B. Photosystem II (P680)
C. Both PS I and PS II equally
D. Neither — water splits spontaneously
Answer: B — Photosystem II (P680). Despite its name, Photosystem II acts first in the sequence. It absorbs light at 680 nm and uses that energy to split water molecules (photolysis), releasing O₂. The electrons released replace those lost by P680. Photosystem I (P700) then reduces NADP⁺ to NADPH using electrons from PS II.
TRICKY🧐 Photosynthesis Traps
⚠️ "Dark reactions cannot occur in light." True or False?
False. Dark reactions are called "dark" because they do not require light directly — not because they only happen in darkness. They proceed equally well in light or dark, as long as ATP and NADPH (produced by light reactions) are available. They are better called Light-Independent Reactions or the Calvin Cycle. The name "dark reactions" is technically misleading and NDA has used this distinction in statement-true/false questions.
⚠️ "Chlorophyll absorbs all wavelengths of light equally." True or False?
False. Chlorophyll absorbs maximally in the red (around 680 nm) and blue (around 450 nm) regions of the visible spectrum. It reflects green light — which is why plants appear green. The graph of photosynthesis rate vs wavelength is the action spectrum; the graph of absorption vs wavelength is the absorption spectrum. NDA may ask: "Why does a plant look green?" → Because chlorophyll reflects green light.
⚠️ "RuBisCO is important in photosynthesis — where does it act?"
RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) acts in the stroma during the Calvin cycle (dark reaction). It catalyses the fixation of CO₂ onto RuBP. It is the most abundant protein/enzyme on Earth. Note: RuBisCO can also act as an oxygenase (fixing O₂ instead of CO₂), leading to photorespiration — which wastes energy and is more common in C3 plants. C4 plants avoid this by concentrating CO₂ around RuBisCO.
2. Plant Respiration
2.1
Mechanism of Respiration in Plants
All four stages of aerobic respiration + anaerobic pathways + ATP yield — NDA tests ATP numbers and fermentation products
Plants respire 24 hours a day (unlike photosynthesis which requires light). During the day, photosynthesis produces more O₂ and consumes more CO₂ than respiration, so the net effect is O₂ release. At night, only respiration occurs — O₂ is consumed, CO₂ is released.
Glucose (6C) → 2 Pyruvate (3C). Occurs in cytoplasm. Does NOT require O₂ (anaerobic step). Net yield: 2 ATP + 2 NADH. Also called EMP pathway. Present in ALL living cells — most ancient metabolic pathway.
LINK REACTION (Pyruvate Oxidation)
Occurs in mitochondrial matrix. 2 Pyruvate → 2 Acetyl-CoA + 2CO₂. Each pyruvate loses one carbon as CO₂. Produces 2 NADH. Requires O₂ (aerobic). Links glycolysis to Krebs cycle.
KREBS CYCLE (TCA Cycle)
Occurs in mitochondrial matrix. Each Acetyl-CoA (2C) enters the cycle. Per glucose: 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂ released. Discovered by Hans Krebs (1937) — Nobel Prize. Also called Citric Acid Cycle (first product = citric acid, 6C).
ETC (Oxidative Phosphorylation)
Occurs on inner mitochondrial membrane (cristae). NADH and FADH₂ donate electrons → proton gradient → ATP synthase makes ATP. O₂ is the final electron acceptor → forms H₂O. Yield: ~34 ATP. This is where most ATP is made. Inhibited by cyanide (blocks electron flow).
ANAEROBIC (Fermentation)
Occurs in cytoplasm when O₂ is absent. Only glycolysis occurs → 2 ATP (net). Two pathways: (1) Lactic acid fermentation: Pyruvate → Lactic acid (in muscles under anaerobic stress, in Lactobacillus). (2) Alcoholic fermentation: Pyruvate → Ethanol + CO₂ (in yeast, plants in waterlogged soil). Yeast used in bread and alcohol industry.
Glycolysis: 2 ATP (net)
Link + Krebs: 2 ATP direct
ETC: ~34 ATP
Total Aerobic: ~36–38 ATP
Anaerobic only: 2 ATP
📌 Fermentation Products (NDA Direct Questions): Alcoholic fermentation: Pyruvate → Ethanol + CO₂ (enzyme: pyruvate decarboxylase + alcohol dehydrogenase; in yeast) Lactic acid fermentation: Pyruvate → Lactic acid (enzyme: lactate dehydrogenase; in muscle cells during vigorous exercise, in Lactobacillus)
Lactic acid accumulation = muscle fatigue and soreness after exercise
2.2
Photosynthesis vs Respiration — Key Comparison
The most tested comparison table in NDA Plant Physiology — know every row
Feature
🌿 Photosynthesis
⚡ Respiration
Energy
Stores energy (endergonic — light → glucose)
Releases energy (exergonic — glucose → ATP)
Raw materials
CO₂ + H₂O
Glucose + O₂
Products
Glucose + O₂
CO₂ + H₂O + ATP
Gas exchange
Takes in CO₂; releases O₂
Takes in O₂; releases CO₂
Occurs in
Green parts only (chloroplasts)
All living cells (mitochondria + cytoplasm)
Time
Only when light is available
24 hours a day (continuous)
Weight of organic matter
Increases (anabolism)
Decreases (catabolism)
RQ (Respiratory Quotient)
Not applicable
RQ = 1 for glucose; <1 for fats; >1 for organic acids
Process type
Anabolic (building up)
Catabolic (breaking down)
🧠 Compensation Point — NDA Statement Trap:
The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration. At this point, net gas exchange = zero (no net O₂ release or CO₂ uptake is apparent). Plants below the compensation point for extended periods will starve — they consume their own stored food. Shade plants have a lower compensation point than sun plants.
PYQTopic-Wise PYQs — Plant Respiration
Q5. The net ATP yield from the complete aerobic oxidation of one molecule of glucose is approximately:
A. 2 ATP
B. 8 ATP
C. 38 ATP
D. 100 ATP
Answer: C — 38 ATP (approximately 36–38). Glycolysis: 2 ATP; Link reaction + Krebs: 2 ATP; Electron Transport Chain: ~34 ATP. Total ≈ 38 ATP. Anaerobic respiration (fermentation) yields only 2 ATP — 19× less efficient than aerobic respiration. NDA accepts 36 or 38 as correct.
Q6. During anaerobic respiration in yeast, pyruvate is converted to:
A. Lactic acid only
B. Ethanol and CO₂
C. Acetyl-CoA and CO₂
D. Glucose and water
Answer: B — Ethanol and CO₂. In yeast (and plant roots in waterlogged conditions), anaerobic respiration causes pyruvate to be converted to ethanol (ethyl alcohol) + carbon dioxide. This is alcoholic fermentation, exploited in bread-making (CO₂ makes dough rise) and alcohol production (ethanol). In animal muscle cells, pyruvate → lactic acid instead.
Q7. Glycolysis — the first stage of respiration — takes place in the:
A. Mitochondrial matrix
B. Inner mitochondrial membrane
C. Cytoplasm (cytosol)
D. Nucleus
Answer: C — Cytoplasm (cytosol). Glycolysis occurs in the cytoplasm and does not require oxygen — it is the only stage of respiration that can proceed anaerobically. It is therefore present in virtually all living organisms (prokaryotes and eukaryotes). The subsequent stages (Link reaction, Krebs cycle, ETC) all occur inside the mitochondria.
TRICKY🧐 Respiration Traps
⚠️ "Plants only photosynthesise during the day and only respire at night." True or False?
False. Plants respire continuously — day and night. During the day, photosynthesis produces far more O₂ than respiration consumes, so the net effect is O₂ release. At night, only respiration occurs, so CO₂ is released. The compensation point is when PS rate = respiration rate (no net gas exchange). It is a myth (but a popular one!) that plants release CO₂ at night and O₂ only during the day — they always release CO₂ from respiration; during the day it is masked by photosynthesis.
⚠️ "Lactic acid and alcohol are both produced during anaerobic respiration in humans." True or False?
False. In human muscles under anaerobic conditions (intense exercise), only lactic acid is produced. Alcoholic fermentation (ethanol + CO₂) occurs in yeast and plant cells. Humans do not have the enzyme pyruvate decarboxylase (required for ethanol production). NDA has tested: "In which organisms does alcoholic fermentation occur?" → Yeast + some plant roots under waterlogging.
3. Plant Growth & Development — Plant Hormones
3.1
Five Plant Hormones — Functions & Applied Examples
Hormone → function → specific applied example — the complete NDA format for plant hormone MCQs
Plant hormones (phytohormones) are chemical messengers produced in one part of the plant and transported to another where they regulate growth and development. Unlike animal hormones, they are not produced by specific glands.
🌿 Auxin (IAA)
Indole-3-Acetic Acid — First plant hormone discovered (Went, 1926)
Promotes cell elongation (cell wall loosening)
Phototropism: auxin moves to shaded side → shaded side elongates → plant bends toward light
Triple response in seedlings: thickening, shortening, horizontal growth
Applied use: ripening chambers for commercial banana/mango; acetylene used similarly; Ethephon (ethylene-releasing compound)
Hormone
Type
Key Function
Key Applied Example
NDA Trap
Auxin (IAA)
Promoter
Cell elongation, phototropism, apical dominance
2,4-D weedkiller; rooting powder
High auxin inhibits root growth
Gibberellin (GA)
Promoter
Stem elongation, breaks seed dormancy
Increases grape size; barley malting
"Foolish seedling" disease — excess GA
Cytokinin
Promoter
Cell division, delays leaf senescence
Tissue culture; fresh-cut flowers
Promotes lateral buds (opposes auxin)
ABA
Inhibitor
Stomatal closure, seed dormancy
Drought resistance; storage
ONLY inhibitory hormone; antagonises GA
Ethylene
Promoter/Inhibitor
Fruit ripening, abscission
Banana ripening chambers
Only GASEOUS plant hormone (C₂H₄)
📌 "Foolish Seedling" Disease — Bakanae Disease (NDA Historical Fact):
Caused by the fungus Gibberella fujikuroi infecting rice seedlings. Infected plants grow abnormally tall and thin (foolish = bakanae in Japanese) before dying. Japanese scientists studying this disease in the 1920s isolated the substance responsible — which led to the discovery of gibberellins. This is one of NDA's most tested "discovery of hormone" questions.
PYQTopic-Wise PYQs — Plant Hormones
Q8. Which plant hormone is responsible for causing phototropism (bending of plants toward light)?
A. Gibberellin
B. Cytokinin
C. Ethylene
D. Auxin (IAA)
Answer: D — Auxin (IAA). When light hits a plant from one side, auxin migrates to the shaded side of the shoot. Higher auxin concentration on the shaded side causes greater cell elongation there, making the shoot curve toward the light. This was first demonstrated by F.W. Went (1926) using oat coleoptile (Avena) tips — the experiment that led to the discovery of auxin.
Q9. The only gaseous plant hormone is:
A. Abscisic acid
B. Cytokinin
C. Ethylene
D. Gibberellin
Answer: C — Ethylene (C₂H₄). Ethylene is the only plant hormone that exists as a gas at physiological temperatures. It promotes fruit ripening, flower and fruit abscission, and the "triple response" in seedlings. Commercially, ethylene gas (or Ethephon, a liquid that releases ethylene) is used to ripen bananas, mangoes, and tomatoes uniformly.
Q10. Abscisic acid (ABA) is called the "stress hormone" because it:
A. Promotes rapid cell division under stress
B. Causes stomata to close during water stress/drought
C. Increases photosynthesis rate during heat stress
D. Stimulates lateral root growth in drought
Answer: B. ABA is synthesised in the leaves during water stress/drought conditions. It signals guard cells to release K⁺ → guard cells lose water → stomata close → transpiration water loss is reduced. This is the plant's primary rapid response to drought. ABA also maintains seed dormancy, preventing premature germination in unfavourable conditions.
Q11. Gibberellin was first discovered from which organism?
A. Yeast (Saccharomyces cerevisiae)
B. The fungus Gibberella fujikuroi infecting rice
C. Avena (oat) coleoptile tips
D. Tobacco tissue culture
Answer: B — Gibberella fujikuroi infecting rice. Japanese plant pathologist Eiichi Kurosawa (1926) discovered that rice plants infected with this fungus grew abnormally tall ("foolish seedling" / Bakanae disease). The substance causing excessive elongation was isolated and named gibberellin. Auxin was discovered from Avena coleoptile tips (Went). Cytokinin was discovered in tobacco tissue culture (Skoog).
TRICKY🧐 Plant Hormone Traps
⚠️ "Auxin always promotes growth." True or False?
False — it depends on concentration AND the organ. Auxin promotes shoot growth at concentrations that inhibit root growth. At high concentrations, auxin actually inhibits root elongation. The optimal auxin concentration for root growth is 10⁻¹⁰ M; for shoots it is 10⁻⁶ M. This differential sensitivity is why auxin causes phototropism (shoot curves toward light) while roots respond differently. NDA uses this to frame: "At high concentration, auxin inhibits growth in which organ?" → Roots.
⚠️ "Cytokinin and Auxin always oppose each other." True or False?
False — it depends on the ratio. Cytokinin and Auxin interact in tissue culture to determine cell fate: High cytokinin:auxin ratio → shoot differentiation; Low cytokinin:auxin ratio (high auxin) → root differentiation; Equal ratio → callus (undifferentiated mass). They oppose each other in apical dominance (auxin promotes apical bud; cytokinin promotes lateral buds) but cooperate in cell division. The ratio concept — discovered by Skoog and Miller — is a classic NDA higher-difficulty question.
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