Botany वनस्पतिशास्त्र

Quick summary

Plant cells are built from a few kinds of large organic molecules – carbohydrates, proteins, lipids and nucleic acids – along with water and minerals. This topic covers the structure, classification and functions of carbohydrates, amino acids and proteins, and lipids, which form the base for studying enzymes, photosynthesis and respiration.

Key points

• Carbohydrates are polyhydroxy aldehydes or ketones with the general formula (CH₂O)n.
• Monosaccharides: simple sugars – trioses (glyceraldehyde), pentoses (ribose, deoxyribose), hexoses (glucose – aldose; fructose – ketose).
• Disaccharides: two monosaccharides joined by a glycosidic bond – sucrose (glucose + fructose; the transport sugar of plants, non-reducing), maltose (glucose + glucose), lactose.
• Polysaccharides: starch (storage; amylose – unbranched α-1,4; amylopectin – branched with α-1,6), cellulose (structural; β-1,4 glucose; most abundant organic compound on Earth), inulin (fructose polymer in dahlia tubers), hemicellulose, pectin.
• Amino acids have an amino group (–NH₂), a carboxyl group (–COOH) and an R-group on one carbon. 20 standard amino acids make proteins. They are joined by peptide bonds.
• Protein structure: primary (sequence), secondary (α-helix, β-pleated sheet held by hydrogen bonds), tertiary (3-D folding: disulphide bonds, ionic and hydrophobic forces), quaternary (several polypeptide chains, e.g. RuBisCO with 8 large + 8 small subunits).
• Protein functions in plants: enzymes, storage proteins in seeds (gluten in wheat), structural proteins, transport and pigment-binding proteins.
• Simple lipids: fats and oils = triglycerides (glycerol + 3 fatty acids, ester bonds); waxes (cuticle).
• Compound lipids: phospholipids (membranes), glycolipids (chloroplast membranes).
• Derived lipids: sterols, carotenoids.
• Lipids: insoluble in water, soluble in organic solvents.
• Saturated fatty acids have no double bonds (palmitic, stearic); unsaturated have double bonds (oleic, linoleic) – plant oils are mostly unsaturated and liquid.
• Nucleic acids (DNA, RNA) are polymers of nucleotides (sugar + phosphate + base).

Important terms

• Glycosidic bond – link between two sugar units.
• Reducing sugar – sugar with a free aldehyde or ketone group that reduces Benedict's/Fehling's solution.
• Peptide bond – –CO–NH– link between two amino acids.
• Denaturation – loss of a protein's 3-D shape by heat, pH or chemicals.
• Triglyceride – ester of glycerol with three fatty acids.

Key facts

• Tests: iodine – blue-black for starch; Benedict's – brick red for reducing sugar; Biuret – violet for protein; Sudan III – red for oil.

5 important questions

Q1. Distinguish between starch and cellulose.

Starch is a storage polysaccharide of α-glucose with α-1,4 (and α-1,6) bonds; it gives blue-black colour with iodine. Cellulose is a structural polysaccharide of β-glucose with β-1,4 bonds forming straight fibres; it gives no colour with iodine.

Q2. What is a peptide bond?

A peptide bond is the –CO–NH– covalent bond formed between the carboxyl group of one amino acid and the amino group of the next, with loss of a water molecule.

Q3. Classify carbohydrates with examples and functions.

Monosaccharides, disaccharides (reducing and non-reducing), polysaccharides (storage and structural); structure of glucose; role of sucrose in transport.

Q4. Describe the levels of protein structure.

Primary, secondary (α-helix, β-sheet), tertiary, quaternary – bonds involved at each level, examples, and the meaning of denaturation.

Q5. Write an account of plant lipids.

Simple, compound and derived lipids; triglyceride formation; saturated and unsaturated fatty acids; roles in storage, membranes, cuticle; commercial oils.

Exam tip

Learn the bond type for each polymer and the four colour tests – they are frequent short-answer and practical questions.

Quick summary

Enzymes are biological catalysts, mostly proteins, that speed up chemical reactions in cells without being used up. Every step of photosynthesis, respiration and growth is run by enzymes. This topic covers the nature, properties, classification, mechanism of action, factors affecting activity and inhibition of enzymes.

Key points

• The term enzyme was given by W. Kühne (1878). J. B. Sumner (1926) first crystallised an enzyme (urease, from jack bean) and showed it is a protein.
• Most enzymes are proteins; some RNA molecules (ribozymes) also act as catalysts.
• Holoenzyme = apoenzyme (protein part) + cofactor. Cofactors may be metal ions (Mg²⁺, Zn²⁺, Fe²⁺), coenzymes (NAD⁺, NADP⁺, FAD, coenzyme A – often from vitamins) or tightly bound prosthetic groups (haem).
• Enzymes lower the activation energy of a reaction; they do not change the equilibrium.
• Active site – the small pocket where the substrate binds.
• Lock and key model (Emil Fischer, 1894): substrate fits the rigid active site exactly.
• Induced fit model (Daniel Koshland, 1958): the active site changes shape slightly to fit the substrate.
• Properties: high specificity, high efficiency (turnover number), work in small amounts, sensitive to temperature and pH, reversible action in many cases.
• Factors: temperature (optimum ~25–40 °C for most plant enzymes; denatured at high temperature), pH (each enzyme has an optimum), substrate concentration (rate rises then becomes constant – Vmax), enzyme concentration, inhibitors.
• Michaelis–Menten constant (Km): substrate concentration at which the rate is half of Vmax. Low Km = high affinity.
• Inhibition: competitive (inhibitor resembles substrate, binds active site; e.g. malonate inhibits succinate dehydrogenase; Km increases, Vmax unchanged), non-competitive (binds elsewhere; Vmax decreases), allosteric/feedback inhibition (end product inhibits the first enzyme of a pathway).
• IUB classification – six classes (below).

Important terms

• Apoenzyme – protein part of an enzyme, inactive alone.
• Coenzyme – organic, loosely bound non-protein helper (NAD⁺).
• Activation energy – minimum energy needed to start a reaction.
• Isoenzymes – different molecular forms of an enzyme with the same action (e.g. lactate dehydrogenase).
• Allosteric site – regulatory site other than the active site.

Formulas

• v = rate of reaction, Vmax = maximum rate, [S] = substrate concentration, Km = Michaelis constant (same unit as [S], e.g. mM).
• Michaelis–Menten equation: v = Vmax [S] / (Km + [S])
• When [S] = Km, v = Vmax/2.

5 important questions

Q1. What is a holoenzyme?

A holoenzyme is the complete active enzyme made of a protein part (apoenzyme) and a non-protein cofactor such as a metal ion or coenzyme.

Q2. What is Km?

Km (Michaelis constant) is the substrate concentration at which the reaction rate is half of its maximum. A low Km shows high affinity of the enzyme for its substrate.

Q3. Explain the mechanism of enzyme action.

Activation energy concept with a graph; enzyme–substrate complex (E + S ⇌ ES → E + P); lock and key vs induced fit models with diagrams.

Q4. Discuss the factors affecting enzyme activity.

Temperature, pH, substrate and enzyme concentration (with graphs), activators and inhibitors; Michaelis–Menten curve.

Q5. Describe the IUB classification of enzymes.

Six classes with reaction type and examples, the four-digit EC number (e.g. EC 3.2.1.1 for α-amylase).

Exam tip

Draw the substrate-concentration curve with Vmax and Km marked, and remember the six classes with the order "Ox-Trans-Hydro-Ly-Iso-Lig".

Quick summary

Plants need mineral elements from the soil for normal growth. These are divided into macronutrients and micronutrients based on the amount needed. This topic covers criteria of essentiality, hydroponics, roles and deficiency symptoms of elements, mechanism of mineral absorption and the nitrogen cycle including biological nitrogen fixation.

Key points

• Julius von Sachs (1860) grew plants in nutrient solutions without soil – hydroponics (soilless culture).
• Criteria of essentiality (Arnon and Stout, 1939): the element must be needed for completing the life cycle; it cannot be replaced by another element; it must be directly involved in plant metabolism.
• About 17 elements are essential for most higher plants.
• Macronutrients (needed in large amounts, more than 10 mmol per kg dry matter): C, H, O, N, P, K, S, Ca, Mg.
• Micronutrients (trace elements): Fe, Mn, Cu, Mo, Zn, B, Cl, Ni.
• C, H and O come from air and water; others from soil.
• Absorption of minerals occurs by passive (diffusion, ion exchange) and active (carrier proteins, using ATP) methods.
• Nitrogen cycle steps: nitrogen fixation, ammonification, nitrification, nitrate assimilation, denitrification.
• Biological nitrogen fixation (N₂ → NH₃) is done by the enzyme nitrogenase, which works only in the absence of oxygen.
• In legume root nodules, Rhizobium fixes nitrogen; leghaemoglobin (pink pigment) protects nitrogenase from oxygen.

Important terms

• Chlorosis – yellowing of leaves due to loss of chlorophyll (deficiency of N, K, Mg, S, Fe, Mn, Zn, Mo).
• Necrosis – death of tissue (deficiency of Ca, Mg, Cu, K).
• Mobile elements – N, P, K, Mg; deficiency appears first in older leaves.
• Immobile elements – Ca, S, Fe, B; deficiency appears first in young leaves.
• Nitrification – NH₃ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter).
• Denitrification – NO₃⁻ → N₂ (Pseudomonas, Thiobacillus).
• Toxicity – excess of a mineral that reduces dry weight by about 10%.

Key facts

• Overall nitrogenase reaction: N₂ + 8e⁻ + 8H⁺ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pi.

5 important questions

Q1. What is hydroponics?

Hydroponics is the method of growing plants in a balanced nutrient solution without soil. It was started by Sachs (1860) and helps find essential elements and grow crops in small spaces.

Q2. What is leghaemoglobin? Give its role.

Leghaemoglobin is a pink, oxygen-binding pigment in legume root nodules. It keeps oxygen level low around nitrogenase, protecting the enzyme while still supplying oxygen for bacterial respiration.

Q3. Explain the criteria of essentiality of mineral elements.

• Given by Arnon and Stout.
• Absence prevents completion of life cycle.
• Specific; cannot be replaced by another element.
• Directly involved in metabolism.
• List macro and micronutrients with examples.

Q4. Describe the formation of root nodules in legumes.

• Root secretes chemicals; Rhizobium multiplies near root hairs.
• Root hair curls around bacteria.
• Infection thread carries bacteria into the cortex.
• Cortical and pericycle cells divide to form the nodule.
• Bacteria become bacteroids; leghaemoglobin and nitrogenase formed.
• N₂ fixed to NH₃, used to make amino acids.

Q5. Explain the nitrogen cycle.

• Fixation: lightning, industrial (Haber process), biological.
• Ammonification by decomposers.
• Nitrification by Nitrosomonas and Nitrobacter.
• Assimilation of nitrate by plants; animals eat plants.
• Denitrification returns N₂ to air.
• Draw a cycle diagram with arrows.

Exam tip

Learn the deficiency-symptom table well; questions like "whiptail is caused by deficiency of …" are common.

Quick summary

Photosynthesis is the process by which green plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. It occurs in chloroplasts and has two stages: the light reactions and the dark (carbon-fixing) reactions. This topic covers pigments, photosystems, the Z-scheme, photophosphorylation, the Calvin cycle, C₄ pathway, CAM, photorespiration and limiting factors.

Key points

• Overall equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (in light, with chlorophyll). Photosynthesis is प्रकाशसंश्लेषण in Marathi.
• Van Niel showed that the oxygen released comes from water, not CO₂ (later confirmed with ¹⁸O by Ruben and Kamen).
• Site: chloroplast. Light reactions in thylakoid membranes (grana); dark reactions in the stroma.
• Pigments: chlorophyll a (main pigment, reaction centre), chlorophyll b, carotenoids (carotenes and xanthophylls) as accessory pigments.
• Photosystem I (PS I) has reaction centre P700; Photosystem II (PS II) has P680.
• Emerson effect (enhancement effect): the rate of photosynthesis is higher when red light (680 nm) and far-red light (above 680 nm) are given together. This proved two photosystems.
• Photolysis of water at PS II: 2H₂O → 4H⁺ + O₂ + 4e⁻ (needs Mn²⁺ and Cl⁻).
• Non-cyclic photophosphorylation (Z-scheme) uses both PS II and PS I; produces ATP, NADPH and O₂.
• Cyclic photophosphorylation uses only PS I; produces only ATP.
• ATP is made by chemiosmosis (Peter Mitchell) through ATP synthase as protons flow from the thylakoid lumen to the stroma.
• Calvin cycle (C₃ cycle), worked out by Melvin Calvin with ¹⁴C in Chlorella: carboxylation, reduction, regeneration. First stable product is 3-phosphoglyceric acid (3-PGA). Enzyme: RuBisCO.
• C₄ pathway (Hatch–Slack pathway): first product is oxaloacetic acid (OAA); CO₂ acceptor is PEP; enzyme PEP carboxylase. Found in maize, sugarcane, sorghum. Leaves show Kranz anatomy.
• CAM (Crassulacean Acid Metabolism): stomata open at night; CO₂ stored as malic acid; used in the day (e.g. Bryophyllum, pineapple, Opuntia).

Important terms

• Action spectrum – rate of photosynthesis at different wavelengths (peaks in blue and red).
• Absorption spectrum – light absorbed by a pigment at different wavelengths.
• Quantasome – unit of photosynthesis on thylakoid membranes (older term).
• Photorespiration – light-dependent uptake of O₂ and release of CO₂ when RuBisCO acts as an oxygenase; involves chloroplast, peroxisome and mitochondria; common in C₃ plants and wasteful.
• Kranz anatomy – bundle sheath cells with large chloroplasts surrounded by mesophyll cells, like a wreath.
• Blackman's law of limiting factors (1905) – when a process depends on several factors, its rate is limited by the factor in shortest supply.

Formulas

• Overall: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂
• Carboxylation: RuBP (5C) + CO₂ → 2 × 3-PGA (3C), by RuBisCO.
• For one glucose (6 turns of Calvin cycle): 18 ATP and 12 NADPH are used.
• C₄ plants need 30 ATP per glucose (extra ATP for regenerating PEP).

5 important questions

Q1. What is the Emerson enhancement effect?

When far-red light (above 680 nm) and shorter red light are given together, the rate of photosynthesis is more than the sum of the rates with each alone. It shows that two photosystems work together.

Q2. Differentiate between cyclic and non-cyclic photophosphorylation.

Cyclic uses only PS I; electrons return to P700; only ATP is formed, no O₂ or NADPH. Non-cyclic uses PS II and PS I; electrons go to NADP⁺; ATP, NADPH and O₂ are formed.

Q3. Describe the Calvin cycle.

• Carboxylation: CO₂ + RuBP → 2 molecules of 3-PGA (RuBisCO).
• Reduction: 3-PGA → 1,3-bisphosphoglycerate → glyceraldehyde-3-phosphate using ATP and NADPH.
• Regeneration: some G3P regenerates RuBP using ATP.
• Some G3P forms glucose/sucrose/starch.
• Six turns fix 6 CO₂ for one glucose; 18 ATP and 12 NADPH used.

Q4. Explain the C₄ pathway and its advantages.

• In mesophyll cells: PEP + CO₂ → OAA (PEP carboxylase) → malate.
• Malate moves to bundle sheath cells; decarboxylated to release CO₂ and pyruvate.
• CO₂ enters Calvin cycle in bundle sheath.
• Pyruvate returns to mesophyll; PEP regenerated using ATP.
• Advantages: little photorespiration, high efficiency at high temperature and light, better water use.

Q5. Explain the Z-scheme of light reaction.

• PS II absorbs light; P680 excited; electrons to pheophytin → plastoquinone → cytochrome b₆f → plastocyanin.
• Electrons from water replace those lost (photolysis, O₂ released).
• PS I absorbs light; P700 excited; electrons → ferredoxin → NADP⁺ reductase → NADPH.
• Proton gradient drives ATP synthesis.
• Energy diagram looks like the letter Z.

Exam tip

Learn the first stable products (3-PGA in C₃, OAA in C₄) and the ATP/NADPH count for one glucose. Draw the Z-scheme with carriers in the right order.

Quick summary

Sugars made in leaves must reach roots, fruits, seeds and growing tips. This long-distance movement of organic food through the phloem is called translocation. This topic covers the evidence that phloem carries food, the source–sink concept, phloem loading and unloading, and the theories of the mechanism, especially Münch's pressure flow (mass flow) hypothesis.

Key points

• Food moves in the sieve tubes of phloem, helped by companion cells. Sieve tube elements have no nucleus at maturity and are joined end to end through sieve plates.
• Main sugar translocated is sucrose (non-reducing, stable, highly soluble). Some plants also carry raffinose, stachyose or sugar alcohols. Amino acids, hormones and some minerals also move in phloem.
• Girdling (ringing) experiment (Malpighi): removing a ring of bark (with phloem) causes swelling above the ring as food collects there; roots below starve.
• Radioactive tracers: leaves given ¹⁴CO₂ – the labelled sugar appears in sieve tubes.
• Aphid stylet technique: aphids feed on sieve tubes; the cut stylet exudes phloem sap rich in sucrose.
• Evidence for phloem transport:
• Source – organ that makes or releases sugar (mature leaf, germinating storage organ). Sink – organ that uses or stores it (roots, fruits, young leaves, tubers). Direction is from source to sink, so it can be upward or downward (bidirectional).
• Phloem loading – sucrose enters sieve tube–companion cell complex in the leaf; often active, by a sucrose–H⁺ symport powered by a proton pump (ATP). It may be apoplastic or symplastic (via plasmodesmata).
• Phloem unloading – sucrose leaves sieve tubes at the sink; it may be converted to starch or hexoses.
• Sugar loaded at source → water potential falls → water enters from xylem by osmosis → high turgor pressure.
• Sugar unloaded at sink → water leaves → low pressure.
• Sap flows in bulk from high to low pressure along the sieve tube.
• Pressure flow hypothesis (E. Münch, 1930):
• Other theories: diffusion (too slow), protoplasmic streaming (de Vries), electro-osmotic theory – less accepted.
• Speed of translocation is about 0.5–1 m per hour.

Important terms

• Translocation – movement of organic solutes through phloem.
• Source / sink – supplier and user of sugars.
• Phloem loading – transfer of sugars into the sieve tubes.
• Symport – carrier moving two substances in the same direction.
• P-protein and callose – substances that seal sieve plates after injury.

Key facts

5 important questions

Q1. Why is sucrose the main form of sugar translocated?

Sucrose is non-reducing and chemically stable, so it does not react on the way; it is highly soluble and carries a lot of energy per molecule.

Q2. What is the girdling experiment?

Removing a complete ring of bark (including phloem) from a stem. Food collects and the bark swells above the ring, while the root side starves – showing food moves down through phloem.

Q3. Explain the pressure flow hypothesis with a diagram.

Münch (1930); source loading, osmotic water entry, high turgor; sink unloading; bulk flow; model of two osmometers joined by a tube; merits and objections (bidirectional flow, energy need).

Q4. Describe phloem loading and unloading.

Apoplastic and symplastic routes; sucrose–proton symport; role of companion cells (transfer cells); unloading at sinks and fate of sucrose.

Q5. Give experimental evidence that phloem translocates food.

Girdling, radioactive ¹⁴C tracer, aphid stylet, analysis of sap, blocking of sieve plates by callose.

Exam tip

Draw the Münch two-bulb model or a leaf–stem–root diagram with arrows showing sugar and water movement – it earns most of the marks in a long answer.

Quick summary

Respiration is the breakdown of food (mainly glucose) in living cells to release energy in the form of ATP. Aerobic respiration uses oxygen; anaerobic respiration does not. This topic covers glycolysis, link reaction, Krebs cycle, electron transport chain, fermentation, respiratory quotient and the energy balance sheet.

Key points

• Respiration (श्वसन) is a catabolic, energy-releasing process occurring in all living cells.
• Overall aerobic equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
• Glycolysis (EMP pathway – Embden, Meyerhof, Parnas): in the cytoplasm; glucose → 2 pyruvate; net gain of 2 ATP and 2 NADH. Common to aerobic and anaerobic respiration.
• Link reaction (oxidative decarboxylation): pyruvate → acetyl CoA + CO₂ + NADH in the mitochondrial matrix (enzyme complex pyruvate dehydrogenase).
• Krebs cycle (TCA / citric acid cycle), by Hans Krebs (1937), in the mitochondrial matrix. Acetyl CoA (2C) + OAA (4C) → citric acid (6C). Per turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP/GTP.
• Electron transport chain (ETS) on the inner mitochondrial membrane; oxygen is the final electron acceptor, forming water.
• Oxidative phosphorylation forms ATP by chemiosmosis through ATP synthase (F₀–F₁ particles).
• Anaerobic respiration / fermentation: pyruvate → ethanol + CO₂ (alcoholic fermentation, yeast) or → lactic acid (lactic acid bacteria, muscles). Net gain only 2 ATP per glucose.
• Pentose phosphate pathway (HMP shunt) is an alternative route producing NADPH and 5-carbon sugars.
• Respiration is called an amphibolic pathway because it is both catabolic and anabolic (its intermediates are used to make fats, amino acids).

Important terms

• Respiratory quotient (RQ) – ratio of CO₂ given out to O₂ taken in.
• Substrate-level phosphorylation – direct ATP formation from a substrate (e.g. in glycolysis, Krebs cycle).
• Oxidative phosphorylation – ATP formation using energy of electron transport.
• Pasteur effect – decrease in sugar breakdown when oxygen is supplied to cells doing anaerobic respiration.
• Cytochrome oxidase – last enzyme of ETS (complex IV) that passes electrons to O₂.

Formulas

• RQ = volume of CO₂ evolved ÷ volume of O₂ consumed.
• Carbohydrate: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, RQ = 6/6 = 1.
• Fat (tripalmitin): 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O, RQ = 102/145 ≈ 0.7.
• Organic acid (oxalic acid): 2(COOH)₂ + O₂ → 4CO₂ + 2H₂O, RQ = 4/1 = 4 (more than 1).
• Anaerobic: RQ is infinite (CO₂ released, no O₂ used).
• ATP balance (aerobic, one glucose): modern value about 30–32 ATP (taking 1 NADH ≈ 2.5 ATP and 1 FADH₂ ≈ 1.5 ATP); older textbooks give 36–38 ATP (taking NADH = 3, FADH₂ = 2).

5 important questions

Q1. What is RQ? Give its value for carbohydrates and fats.

RQ is the ratio of the volume of CO₂ released to the volume of O₂ used in respiration. It is 1 for carbohydrates and about 0.7 for fats.

Q2. Why is respiration called an amphibolic pathway?

Respiration breaks down food (catabolism), but its intermediates like acetyl CoA and α-ketoglutarate are used to make fatty acids and amino acids (anabolism). Hence it is both catabolic and anabolic.

Q3. Describe glycolysis.

• Occurs in cytoplasm; no O₂ needed.
• Glucose phosphorylated twice using 2 ATP → fructose-1,6-bisphosphate.
• Split into two 3C molecules (G3P and DHAP).
• Oxidation forms 2 NADH; substrate-level phosphorylation gives 4 ATP.
• End product: 2 pyruvate; net gain 2 ATP + 2 NADH.

Q4. Explain the Krebs cycle.

• Acetyl CoA + OAA → citric acid (citrate synthase).
• Citric acid → isocitric acid → α-ketoglutaric acid (CO₂, NADH).
• α-KG → succinyl CoA (CO₂, NADH) → succinic acid (GTP/ATP).
• Succinic → fumaric (FADH₂) → malic → OAA (NADH).
• Two turns per glucose.

Q5. Calculate the RQ of tripalmitin and explain its meaning.

• 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O.
• RQ = 102 ÷ 145 = 0.70 (approx.).
• RQ less than 1 means fats need more oxygen because they have less oxygen in their molecule than carbohydrates.

Exam tip

Write the place of each stage (cytoplasm, matrix, inner membrane). If asked for total ATP, mention both the older (38) and modern (about 30–32) values with the reason.

Quick summary

Plants store energy as oils in seeds such as groundnut, sunflower and castor. When these seeds germinate, the oil is broken down and changed into sugar for the growing seedling. This topic covers the breakdown of fats (lipolysis and β-oxidation), the glyoxylate cycle that turns fat into sugar, and a short outline of fatty acid synthesis.

Key points

• Lipolysis: the enzyme lipase hydrolyses triglycerides into glycerol + 3 fatty acids. In oil seeds this happens in oleosomes (oil bodies).
• Glycerol is phosphorylated to glycerol-3-phosphate and oxidised to dihydroxyacetone phosphate (DHAP), which joins glycolysis or gluconeogenesis.
• Activation of fatty acid: fatty acid + CoA + ATP → fatty acyl-CoA (by acyl-CoA synthetase).
• β-oxidation (Knoop, 1904): fatty acyl-CoA is shortened by 2 carbons per cycle, releasing acetyl-CoA. Each cycle has four steps: oxidation (FAD → FADH₂), hydration, oxidation (NAD⁺ → NADH) and thiolysis.
• In plants, β-oxidation takes place mainly in glyoxysomes (in germinating oily seeds) and peroxisomes, not mainly in mitochondria as in animals. In glyoxysomes the first oxidation passes electrons to O₂, forming H₂O₂, which catalase breaks down.
• A fatty acid with n carbons (even) needs (n/2) – 1 cycles and gives n/2 acetyl-CoA. Palmitic acid (C16): 7 cycles, 8 acetyl-CoA.
• Glyoxylate cycle (Kornberg and Krebs, 1957): a modified Krebs cycle in glyoxysomes that skips the two decarboxylation steps, so carbon is saved.
• Two special enzymes: isocitrate lyase (isocitrate → succinate + glyoxylate) and malate synthase (glyoxylate + acetyl-CoA → malate).
• Net result: 2 acetyl-CoA → 1 succinate. Succinate goes to mitochondria → malate → oxaloacetate → PEP (by PEP carboxykinase) → sucrose by gluconeogenesis in the cytosol.
• Animals lack the glyoxylate cycle, so they cannot make sugar from fat; oily seedlings can.
• Fatty acid synthesis occurs in plastids: acetyl-CoA → malonyl-CoA (acetyl-CoA carboxylase); fatty acid synthase adds 2 C at a time using NADPH, forming palmitate (C16) and stearate (C18).

Important terms

• Lipase – enzyme hydrolysing fats into glycerol and fatty acids.
• β-oxidation – stepwise removal of two-carbon units from fatty acids at the β-carbon.
• Glyoxysome – microbody containing glyoxylate-cycle enzymes in fat-storing seedlings.
• Gluconeogenesis – formation of glucose/sucrose from non-carbohydrate sources.
• Oleosome – oil body storing triglycerides in seeds.

Formulas

• Number of β-oxidation cycles for an even-carbon fatty acid = (n/2) – 1; acetyl-CoA formed = n/2.
• Worked example: stearic acid (C18) → cycles = 18/2 – 1 = 8; acetyl-CoA = 18/2 = 9.
• Glyoxylate cycle (net): 2 Acetyl-CoA + NAD⁺ → Succinate + 2 CoA + NADH + H⁺

5 important questions

Q1. Name the two key enzymes of the glyoxylate cycle.

Isocitrate lyase and malate synthase. They are present in glyoxysomes and are absent in animals.

Q2. How many acetyl-CoA molecules are formed from palmitic acid?

Palmitic acid has 16 carbons, so it gives 16/2 = 8 acetyl-CoA in 7 cycles of β-oxidation.

Q3. Describe β-oxidation of fatty acids.

Activation to acyl-CoA; entry into glyoxysome/peroxisome; four reactions of each cycle; product acetyl-CoA; energy yield; difference between plant and animal sites.

Q4. Explain the glyoxylate cycle and its significance.

Location, steps with the two special enzymes, net reaction, conversion of succinate to sucrose; significance in germinating oily seeds; draw the cycle.

Q5. Write a note on the conversion of fats into carbohydrates in germinating seeds.

Lipolysis in oil bodies → β-oxidation in glyoxysomes → glyoxylate cycle → mitochondria (succinate → oxaloacetate) → cytosol (PEP → sucrose); role of three organelles.

Exam tip

Learn the "three-organelle route" (oil body → glyoxysome → mitochondrion → cytosol) and practise the β-oxidation count for C16 and C18 – easy marks.

Quick summary

Plant growth regulators (plant hormones) are organic substances produced in small amounts that control growth and development. Auxins, gibberellins and cytokinins mostly promote growth; abscisic acid and ethylene mostly inhibit it or cause ageing. This topic covers their discovery, physiological effects and uses in agriculture, plus photoperiodism and vernalisation.

Key points

• Plant hormones are also called phytohormones (वनस्पती संप्रेरके).
• Auxins: first discovered. Charles and Francis Darwin (1880) observed bending of canary grass coleoptile towards light. F. W. Went (1928) isolated auxin from Avena coleoptile tips using agar blocks (Avena curvature test). Natural auxin: indole-3-acetic acid (IAA). Synthetic: 2,4-D, NAA, IBA.
• Gibberellins (GA): discovered by E. Kurosawa (1926) from the fungus Gibberella fujikuroi, which causes bakanae (foolish seedling) disease of rice. More than 100 gibberellins are known; GA₃ is the most common.
• Cytokinins: kinetin discovered by Skoog and Miller (1955) from autoclaved herring sperm DNA. Natural cytokinin zeatin from maize. Found in coconut milk.
• Abscisic acid (ABA): a growth inhibitor; called the stress hormone. Causes stomatal closure, seed and bud dormancy.
• Ethylene: a gaseous hormone; promotes fruit ripening, senescence and abscission. Ethephon releases ethylene.

Important terms

• Apical dominance – the apical bud suppresses lateral buds; controlled by auxin.
• Parthenocarpy – seedless fruit formation without fertilisation (auxins, gibberellins).
• Bolting – sudden elongation of internodes before flowering in rosette plants (gibberellin), e.g. cabbage.
• Richmond–Lang effect – delay of leaf senescence by cytokinins.
• Photoperiodism – response of plants to the relative length of day and night (Garner and Allard, 1920). Plants are short-day (e.g. Xanthium, chrysanthemum), long-day (e.g. wheat, radish) or day-neutral (e.g. tomato, maize).
• Vernalisation – promotion of flowering by a period of low temperature.
• Florigen – hypothetical flowering hormone (Chailakhyan).

Key facts

• In tissue culture: high auxin/cytokinin ratio → roots; low ratio → shoots (Skoog and Miller).

5 important questions

Q1. What is apical dominance?

Apical dominance is the suppression of the growth of lateral buds by the apical bud. It is due to auxin from the shoot tip. Removing the tip (as in tea plucking or hedge cutting) allows lateral branches to grow.

Q2. Why is ABA called the stress hormone?

During stress like drought, ABA level rises and causes stomata to close, reducing water loss. It also induces dormancy, helping plants survive bad conditions.

Q3. Describe the discovery and physiological effects of gibberellins.

• Kurosawa (1926) – bakanae disease of rice due to Gibberella fujikuroi.
• Seedlings grow abnormally tall and weak.
• Effects: internode elongation, bolting, breaking seed dormancy, parthenocarpy, α-amylase formation in cereal grains, reversal of dwarfism.
• Uses: sugarcane yield, grape size, malting industry.

Q4. Explain the role of auxins in plants.

• Cell elongation in shoots; phototropism and geotropism.
• Apical dominance.
• Root initiation in stem cuttings.
• Prevents abscission of leaves and fruits.
• Parthenocarpy (tomato).
• Synthetic auxins as weedicides (2,4-D).
• Describe Went's Avena coleoptile experiment briefly.

Q5. What is photoperiodism? Classify plants on its basis.

• Response of flowering to day length (Garner and Allard).
• Short-day plants flower when the day is shorter than a critical length (actually need a long dark period).
• Long-day plants flower when the day is longer than a critical length.
• Day-neutral plants flower regardless.
• Site of perception: leaves; pigment phytochrome is involved.

Exam tip

Make a one-line table of each hormone: discoverer, first source, two effects, one use. It answers most short questions.

Quick summary

Plants cannot walk, but they do move – whole cells swim, organs bend towards light, and leaves fold at night or on touch. This topic covers the types of plant movements: movements of locomotion, tropic movements (growth towards or away from a stimulus), nastic movements (not directed by the stimulus) and autonomic movements, with their mechanisms and examples.

Key points

• Movements of locomotion – whole organism or cell moves: ciliary/flagellar (Chlamydomonas, antherozoids), amoeboid (slime moulds), cytoplasmic streaming (cyclosis in Hydrilla leaf cells).
• Movements of curvature – a fixed organ bends. Two kinds: growth movements (permanent, due to unequal growth) and turgor (variation) movements (reversible, due to change in turgor).
• Autonomic (spontaneous) movements: caused by internal factors – nutation (zig-zag growth of the stem tip), circumnutation (spiral growth of tendrils, twiners), and the jerky leaflet movement of the telegraph plant (Desmodium gyrans).
• Paratonic (induced) movements: caused by external stimuli.
• Phototropism: shoot positive, root negative to light. Explained by Cholodny–Went theory – auxin moves to the shaded side, cells there elongate more, so the shoot bends towards light. Went's Avena coleoptile curvature test.
• Geotropism (gravitropism): root positive, shoot negative. Statoliths (starch grains, amyloplasts) in root-cap cells settle and signal gravity; auxin collects on the lower side, which inhibits root cell growth and promotes shoot cell growth.
• Hydrotropism (roots towards water), thigmotropism (tendrils coil around supports), chemotropism (pollen tube towards ovule).
• Tropic movements – direction decided by the stimulus:
• Photonasty: flowers opening in light (sunflower) or at night (Cestrum, night queen).
• Thermonasty: tulip and crocus flowers open with warmth.
• Nyctinasty (sleep movements): leaflets of tamarind, Cassia, Oxalis fold at night.
• Seismonasty/thigmonasty: Mimosa pudica leaflets fold within seconds of touch; insectivorous Drosera tentacles and Dionaea (Venus flytrap) lobes close on prey.
• Nastic movements – direction not decided by the stimulus:
• In Mimosa pudica, touch sends an electrical signal to the pulvinus; K⁺ and water leave the lower motor cells, they lose turgor and the leaf droops. Recovery takes 10–15 minutes.
• Hygroscopic movements (physical, in dead tissue): opening of capsules, Funaria peristome teeth, elaters of Equisetum.

Important terms

• Tropism – directional growth movement in response to a one-sided stimulus.
• Nastic movement – non-directional movement in response to a diffuse stimulus.
• Pulvinus – swollen leaf base with motor cells that cause turgor movements.
• Statolith – starch grain that senses gravity.
• Nutation – zig-zag growth of the stem tip.

Key facts

5 important questions

Q1. Differentiate between tropic and nastic movements.

Tropic movements are directional growth movements; the organ bends towards or away from the stimulus (phototropism). Nastic movements are non-directional, often turgor movements, and the direction is fixed by the organ's structure (Mimosa leaf folding).

Q2. What are statoliths?

Statoliths are dense starch grains (amyloplasts) in root-cap and shoot endodermis cells. They settle under gravity and help the plant sense the direction of gravity.

Q3. Describe the mechanism of seismonastic movement in Mimosa pudica.

Stimulus at leaflet; electrical and chemical signal; pulvinus structure; loss of K⁺ and water from lower motor cells; drooping; recovery; diagram of pulvinus.

Q4. Explain phototropism with the Cholodny–Went theory.

Darwin and Boysen-Jensen experiments; Went's agar block test; lateral auxin transport to the shaded side; differential growth; diagrams.

Q5. Classify plant movements with examples.

Locomotion vs curvature; autonomic vs paratonic; growth vs turgor; tropic vs nastic; hygroscopic – a flow chart with examples.

Exam tip

A flow chart of plant movements, plus the Mimosa pulvinus diagram, covers most questions in this topic. Remember: tropic = direction from stimulus, nastic = direction from structure.

Quick summary

Plants often grow in unfavourable conditions – too little water, too much salt, extreme heat or cold, flooding or pollution. Such conditions are called stress. This topic covers types of stress, how they harm plants, and the avoidance and tolerance mechanisms plants use, including the roles of abscisic acid, osmolytes, heat-shock proteins and antioxidants.

Key points

• Stress – any external factor that reduces growth, development or yield below the plant's potential. Biotic stress comes from pathogens, pests and weeds; abiotic stress from water, temperature, salt, light, flooding, heavy metals and pollutants.
• Stress resistance has two strategies: avoidance (prevents the stress reaching the tissues) and tolerance (tissues withstand it).
• Effects: stomatal closure, less photosynthesis, wilting, reduced cell expansion, early leaf fall.
• ABA (abscisic acid) rises in leaves and closes stomata – the "stress hormone".
• Adaptations: deep roots, thick cuticle, sunken stomata, leaf rolling, CAM pathway, and accumulation of osmolytes such as proline, glycine betaine and sugars (osmotic adjustment).
• Drought (water stress):
• High Na⁺ and Cl⁻ cause osmotic stress (physiological drought) and ion toxicity.
• Halophytes survive by excluding salt at roots, secreting it through salt glands (Avicennia), storing it in vacuoles, or succulence (Salicornia, Suaeda).
• Salt stress:
• Heat stress: proteins denature, membranes become leaky. Plants make heat-shock proteins (HSPs) that act as molecular chaperones; transpirational cooling helps.
• Cold and freezing stress: chilling damages membranes of tropical plants (banana, mango); freezing forms ice crystals. Tolerance by cold acclimation (hardening), unsaturated membrane lipids, antifreeze proteins and sugars.
• Flooding (anaerobic) stress: roots get little oxygen; plants form aerenchyma, adventitious roots and use fermentation. Ethylene promotes aerenchyma.
• Oxidative stress is common to most stresses: reactive oxygen species (ROS) such as superoxide and H₂O₂ build up. Antioxidant enzymes superoxide dismutase (SOD), catalase and peroxidase, and antioxidants like ascorbate and glutathione remove them.
• Breeding and biotechnology produce stress-tolerant varieties (e.g. submergence-tolerant rice with the Sub1 gene).

Important terms

• Osmolyte (compatible solute) – small molecule like proline that lowers water potential without harming the cell.
• Halophyte / glycophyte – salt-tolerant plant / salt-sensitive plant.
• Acclimation – gradual increase in tolerance after exposure to mild stress.
• Heat-shock proteins – chaperones made under high temperature to protect proteins.
• ROS – reactive oxygen species that damage lipids, proteins and DNA.

Key facts

5 important questions

Q1. Why is ABA called the stress hormone?

Because its level rises quickly under drought, salinity and cold, and it triggers protective responses such as stomatal closure and synthesis of protective proteins.

Q2. What is the role of proline in stress?

Proline accumulates in the cytoplasm under drought and salt stress. It lowers the cell's water potential (osmotic adjustment), protects proteins and membranes, and helps remove ROS.

Q3. Describe the effects of water stress and adaptations to it.

Effects on growth, stomata, photosynthesis, enzymes; avoidance (roots, cuticle, leaf rolling) and tolerance (osmolytes, ABA, CAM, dehydrins).

Q4. Explain the mechanisms of salt tolerance in plants.

Osmotic and ionic phases; exclusion, compartmentation in vacuoles, salt glands, succulence; examples of halophytes in mangroves.

Q5. Write a note on oxidative stress and antioxidant defence.

Formation of ROS; damage to lipids, proteins and DNA; enzymes SOD, catalase, peroxidase; non-enzymatic antioxidants.

Exam tip

Use the table to answer "short notes" quickly, and always link each stress to one hormone or molecule: drought – ABA/proline, heat – HSPs, flooding – ethylene/aerenchyma.