Class 12 Biology under NEB covers Human Physiology, Genetics & Evolution, Ecology, and Biotechnology. This is a crucial year for medical/biological career aspirants.
Digestive System Organs
Organ Function Enzymes/Secretions Mouth Mechanical digestion, initial starch digestion Salivary amylase (ptyalin) Pharynx/Oesophagus Transport Peristalsis Stomach Protein digestion, storage Pepsin, HCl, mucus, intrinsic factor Small Intestine Major digestion & absorption Pancreatic juice, bile, intestinal juice Large Intestine Water absorption, faeces formation Bacteria, mucus Liver Bile production, detox, metabolism Bile salts, bilirubin Pancreas Digestive enzymes, hormones Amylase, lipase, trypsin, insulin, glucagon
Digestion of Major Nutrients
Nutrient Enzyme Site Products Carbohydrates Salivary amylase, pancreatic amylase, maltase, sucrase, lactase Mouth, SI Glucose, fructose, galactose Proteins Pepsin, trypsin, chymotrypsin, carboxypeptidase, aminopeptidase, dipeptidase Stomach, SI Amino acids Fats Gastric lipase, pancreatic lipase, bile (emulsification) Stomach, SI Fatty acids, glycerol, monoglycerides Nucleic acids Nuclease, nucleotidase, nucleosidase SI Nitrogenous bases, pentose, phosphate
Exam Tip
Memorize: Carbohydrates → Monosaccharides; Proteins → Amino acids; Fats → Fatty acids + Glycerol
Absorption
Monosaccharides, amino acids: Active transport → Blood capillariesFatty acids, glycerol: Re-esterified → Chylomicrons → Lymph (lacteals) → BloodWater, electrolytes, vitamins: Passive/active transport
Respiratory System
Structure Function Nose/Nasal cavity Filter, warm, humidify air Pharynx/Larynx Air passage, voice production Trachea Air passage, C-rings prevent collapse Bronchi/Bronchioles Air distribution Alveoli Gas exchange (site of respiration) Diaphragm/Intercostal muscles Breathing mechanics
Mechanism of Breathing
Phase Diaphragm Intercostal Muscles Thoracic Volume Pressure Air Flow Inspiration Contracts (flattens) External contract Increases Decreases (below atm) In Expiration Relaxes (domes) Internal contract Decreases Increases (above atm) Out
Gas Exchange & Transport
O₂: Alveoli (PO₂ ~100 mmHg) → Blood (PO₂ ~40 mmHg) → DiffusionCO₂: Blood (PCO₂ ~45 mmHg) → Alveoli (PCO₂ ~40 mmHg) → DiffusionDriven by partial pressure gradients
Gas Transport in Blood O₂ 98.5% bound to Hb (oxyhemoglobin), 1.5% dissolved in plasma CO₂ 70% as HCO₃⁻, 23% bound to Hb (carbaminohemoglobin), 7% dissolved
Oxygen-Hemoglobin Dissociation Curve
Sigmoidal shape → Cooperative bindingRight shift (decreased affinity): ↑CO₂, ↑H⁺, ↑Temp, ↑2,3-BPGLeft shift (increased affinity): Opposite conditions, fetal Hb
1.3 Body Fluids and Circulation#
Blood Components
Component Percentage Function Plasma 55% Water (90%), proteins (albumin, globulins, fibrinogen), nutrients, hormones, wastes RBCs 45% O₂ transport (Hb), CO₂ transport WBCs <1% Immunity (neutrophils, lymphocytes, monocytes, eosinophils, basophils) Platelets <1% Clotting
Blood Groups (ABO & Rh)
Blood Group Antigen on RBC Antibody in Plasma Can Donate To Can Receive From A A Anti-B A, AB A, O B B Anti-A B, AB B, O AB A, B None AB A, B, AB, O (Universal recipient) O None Anti-A, Anti-B A, B, AB, O (Universal donor) O
Cardiac Cycle
Atrial Systole (0.1s): Atria contract → 30% ventricular fillingVentricular Systole (0.3s): Ventricles contract → AV valves close (lub), semilunar openVentricular Diastole (0.4s): Ventricles relax → Semilunar close (dub), AV openComplete Cycle: 0.8s → 72 beats/min
Blood Pressure
Systolic: ~120 mmHg (ventricular systole)Diastolic: ~80 mmHg (ventricular diastole)Pulse Pressure: 40 mmHgMean Arterial Pressure: DP + 1/3(SP - DP) ≈ 93 mmHg
Nephron Structure & Function
Nephron Part Function Glomerulus Filtration (BP forces plasma into Bowman's capsule) Proximal Convoluted Tubule Reabsorption: 65% Na⁺, H₂O, glucose, amino acids, HCO₃⁻ Loop of Henle Counter-current multiplier: Creates medullary gradient Distal Convoluted Tubule Regulated reabsorption (aldosterone: Na⁺; ADH: H₂O) Collecting Duct Final concentration, urea recycling
Glomerular Filtration (GFR ~125 ml/min): Passive, non-selective (except proteins)Tubular Reabsorption: 99% filtrate reabsorbedTubular Secretion: H⁺, K⁺, NH₄⁺, drugs, toxinsConcentration: Counter-current mechanism, ADH regulates water permeability
Muscle Structure
Sarcomere: Functional unit (Z-line to Z-line)Thin filament: Actin, troponin, tropomyosinThick filament: MyosinSliding filament theory: Myosin heads bind actin → Power stroke → ADP/Pi release → ATP binds → Detachment
Muscle Type Striated Control Location Skeletal Yes Voluntary Bones Cardiac Yes Involuntary Heart Smooth No Involuntary Viscera, vessels
Neuron Structure
Dendrites: Receive signalsCell body (soma): IntegrationAxon: Transmits impulseSynapse: Junction with next neuron/effector
Nerve Impulse
Resting Potential (-70mV): Na⁺ out, K⁺ in (Na⁺/K⁺ pump)Depolarization: Na⁺ influx (voltage-gated Na⁺ channels)Repolarization: K⁺ efflux (voltage-gated K⁺ channels)Hyperpolarization: K⁺ overshootRefractory Period: Na⁺ channels inactivated
Synaptic Transmission
Action potential → Ca²⁺ influx → Vesicle fusion → Neurotransmitter release
NT binds receptors → Ion channels open → Post-synaptic potential
Excitatory (EPSP) or Inhibitory (IPSP)
Summation at axon hillock → New action potential
Central Nervous System
Brain Part Function Cerebrum Higher functions (thought, memory, speech, voluntary movement) Cerebellum Balance, coordination, posture Medulla Oblongata Vital centers (respiration, cardiac, vasomotor) Pons Relay, respiratory rhythm Hypothalamus Homeostasis (temp, hunger, thirst, endocrine) Thalamus Sensory relay (except olfaction)
Endocrine Glands
Gland Hormone Function Hypothalamus Releasing/inhibiting hormones Controls pituitary Anterior Pituitary GH, TSH, ACTH, FSH, LH, PRL Controls other glands Posterior Pituitary ADH, Oxytocin Water balance, uterine contraction Thyroid T₃, T₄, Calcitonin Metabolism, Ca²⁺ regulation Parathyroid PTH ↑ Blood Ca²⁺ Adrenal Cortex Cortisol, Aldosterone, Androgens Stress, Na⁺/K⁺ balance Adrenal Medulla Adrenaline, Noradrenaline Fight-or-flight Pancreas Insulin, Glucagon Blood glucose regulation Gonads Testosterone, Estrogen, Progesterone Reproduction, secondary sexual chars
Blood Glucose Regulation
High glucose: β-cells → Insulin → GLUT4 translocation → Glucose uptake → Glycogen synthesisLow glucose: α-cells → Glucagon → Glycogenolysis, Gluconeogenesis
Mendel's Laws
Law of Dominance: One allele masks other in heterozygoteLaw of Segregation: Alleles separate during gamete formationLaw of Independent Assortment: Genes on different chromosomes assort independently
Genetic Terminology
Term Definition Gene Unit of heredity (DNA segment) Allele Alternative form of gene Genotype Genetic constitution (e.g., TT, Tt, tt) Phenotype Observable trait Homozygous Same alleles (TT or tt) Heterozygous Different alleles (Tt) Dominant Expressed in heterozygote Recessive Expressed only in homozygous
Monohybrid Cross
P: TT (tall) × tt (dwarf)F₁: All Tt (tall)F₂: TT : Tt : tt = 1 : 2 : 1 (genotype), 3 : 1 (phenotype)
Dihybrid Cross
P: Round Yellow (RRYY) × Wrinkled Green (rryy)F₁: All RrYy (Round Yellow)F₂: 9:3:3
(Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)
Non-Mendelian Inheritance
Type Description Example Incomplete Dominance Heterozygote intermediate Snapdragon: RR (red) × rr (white) → Rr (pink) Co-dominance Both alleles expressed ABO blood groups (IAIB = AB) Multiple Alleles >2 alleles in population ABO (IA, IB, i) Polygenic Inheritance Multiple genes, additive Skin color, height Pleiotropy One gene, multiple effects Sickle cell anemia Epistasis One gene masks another Bombay phenotype (hh masks ABO)
Sex Determination
System Male Female Examples XX-XY XY XX Humans, Drosophila XX-XO XO XX Grasshoppers ZZ-ZW ZZ ZW Birds, butterflies Haplodiploidy Haploid Diploid Bees, ants, wasps
Sex-Linked Inheritance
X-linked recessive: More common in males (hemophilia, color blindness)X-linked dominant: Rare, affected fathers → all daughtersY-linked: Only males, father → all sonsCytoplasmic: Maternal inheritance (mitochondria)
DNA Structure
Double helix (Watson & Crick, 1953)Antiparallel strands (5'→3' and 3'→5')Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)Chargaff's Rules: A=T, G=C, A+G = T+C
DNA Replication
Initiation: Origin of replication, helicase unwinds, SSB proteins stabilizeElongation:
Leading strand: Continuous 5'→3'
Lagging strand: Discontinuous (Okazaki fragments), RNA primers
Termination: RNA primers removed, DNA ligase joins fragments
Transcription
Initiation: RNA polymerase + sigma factor binds promoterElongation: 5'→3', reads template 3'→5'Termination: Rho-dependent or independent
Genetic Code
Triplet code: 64 codons (61 sense, 3 stop)Degenerate: Multiple codons per amino acidUniversal: Almost same in all organismsNon-overlapping, commaless
Translation
Initiation: 30S + mRNA + fMet-tRNA + IFs → 70S initiation complexElongation: EF-Tu brings aa-tRNA → A site → Peptide bond (peptidyl transferase) → TranslocationTermination: Stop codon → Release factor → Release polypeptide
Gene Expression Regulation (Lac Operon)
Structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase)Promoter, Operator, CAP site Regulator gene: lacI → Repressor proteinInducer: Allolactose (inactivates repressor)CAP-cAMP: Positive control (glucose low → cAMP high)
Theories
Lamarckism: Inheritance of acquired characters (disproved)Darwinism: Natural selection, survival of fittestMutation Theory (de Vries): Mutations cause new speciesModern Synthesis: Mutation + Recombination + Natural Selection + Genetic Drift + Gene Flow
Evidence for Evolution
Fossil Record: Transitional forms (Archaeopteryx, horse evolution)Comparative Anatomy: Homologous (common ancestry), Analogous (convergent), VestigialEmbryology: Ontogeny recapitulates phylogeny (Haeckel)Biochemistry: Universal genetic code, similar proteins (cytochrome c)Biogeography: Continental drift, island endemism
Speciation
Allopatric: Geographic isolation → Reproductive isolationSympatric: Polyploidy, habitat differentiation, sexual selectionReproductive Isolating Mechanisms:
Pre-zygotic: Temporal, habitat, behavioral, mechanical, gametic
Post-zygotic: Hybrid inviability, sterility, breakdown
Human Evolution
Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens
Population Attributes
Attribute Description Density N/area or volume Natality Birth rate Mortality Death rate Age Distribution Pre-reproductive, reproductive, post-reproductive Sex Ratio Males
Dispersion Clumped, uniform, random
Population Growth
Model Equation Shape Exponential dN/dt = rN J-shaped Logistic dN/dt = rN(1-N/K) S-shaped (carrying capacity K)
Life History Strategies
r-selected: High r, many offspring, little care, unstable environments (insects, bacteria)K-selected: Low r, few offspring, high care, stable environments (mammals, birds)
Structure
Biotic: Producers (autotrophs), Consumers (heterotrophs), Decomposers (saprotrophs)Abiotic: Light, temperature, water, soil, nutrients
Energy Flow
Unidirectional: Sun → Producers → Herbivores → Carnivores10% Law (Lindemann): ~10% energy transferred to next trophic levelPyramids: Numbers (may invert), Biomass (may invert), Energy (always upright)
Nutrient Cycling
Cycle Key Processes Human Impact Carbon Photosynthesis, respiration, decomposition, combustion Fossil fuels → ↑CO₂ Nitrogen Fixation, nitrification, assimilation, ammonification, denitrification Fertilizers → eutrophication Phosphorus Weathering, uptake, decomposition, sedimentation Detergents, fertilizers
Biodiversity Levels
Genetic Diversity: Variation within speciesSpecies Diversity: Variety of species in areaEcosystem Diversity: Variety of ecosystems
Biodiversity Hotspots
Criteria: ≥1500 endemic vascular plants, ≥70% habitat loss
India: Himalaya, Indo-Burma, Western Ghats, Sundaland
Conservation
Type Method In-situ National parks, wildlife sanctuaries, biosphere reserves, sacred groves Ex-situ Botanical gardens, seed banks, tissue culture, cryopreservation, zoos
IUCN Categories
EX, EW, CR, EN, VU, NT, LC, DD
Pollution
Type Sources Effects Control Air Vehicles, industry, burning Respiratory, acid rain, global warming Catalytic converters, scrubbers, CNG Water Sewage, industry, agriculture Waterborne diseases, eutrophication, bioaccumulation STP, ETP, bioremediation Soil Waste, chemicals, deforestation Reduced fertility, groundwater contamination Composting, phytoremediation Noise Traffic, industry Hearing loss, stress Barriers, silencers, zoning
Major Environmental Issues
Climate Change: Greenhouse effect → Global warming → Sea level rise, extreme weatherOzone Depletion: CFCs → O₃ destruction → UV radiation ↑Deforestation: Habitat loss, soil erosion, climate changeEutrophication: Nutrient enrichment → Algal bloom → O₂ depletionBiomagnification: Non-degradable toxins accumulate up food chain (DDT, Hg)
Solid Waste Management
3Rs: Reduce, Reuse, RecycleComposting: Organic waste → ManureIncineration: Volume reduction, energy recoveryLandfill: Engineered, leachate collection, methane capture
Recombinant DNA Technology Steps
Isolation of DNA Cutting with Restriction Enzymes (palindromic sequences, sticky/blunt ends)Ligation (DNA ligase joins vector + insert)Transformation (CaCl₂, heat shock, electroporation)Selection (Antibiotic resistance, blue-white screening)Expression (Inducible promoters)
Tool Function Restriction Enzymes Cut DNA at specific sequences DNA Ligase Join DNA fragments Vectors Plasmids, phages, cosmids, BACs, YACs Competent Host E. coli, yeast, mammalian cells PCR Amplify specific DNA sequence Gel Electrophoresis Separate DNA by size
Medical
Insulin: Human insulin via recombinant DNA (E. coli/yeast)Vaccines: Hepatitis B (recombinant HBsAg in yeast)Gene Therapy: Replace defective genes (ADA deficiency, SCID)Molecular Diagnosis: PCR, ELISA, DNA probes, RFLP, DNA fingerprintingTransgenic Animals: Models for disease, protein production (milk)
Agricultural
Bt Cotton: Cry protein from Bacillus thuringiensis → insect resistanceGolden Rice: β-carotene (vitamin A) biosynthesis genesHerbicide Tolerance: EPSPS gene (Roundup Ready crops)Virus Resistance: Coat protein gene
Industrial
Enzymes: Amylase, protease, lipase (detergents, food processing)Biofuels: Ethanol from biomassBioplastics: PHB from bacteriaBioremediation: Oil spills, heavy metals
Ethical Issues
GEAC (Genetic Engineering Approval Committee) regulates in IndiaBiosafety: Gene flow, superweeds, allergenicity, antibiotic resistance markersBioethics: Human cloning, designer babies, patenting life forms
NEB Class 12 Biology Practice Questions Human Physiology:
Describe cardiac cycle with pressure changes
Explain oxygen dissociation curve and Bohr effect
Describe counter-current mechanism in Loop of Henle
Mechanism of hormone action (steroid vs peptide)
Sliding filament theory of muscle contraction
Genetics:
Mendel's laws with examples
Incomplete dominance vs co-dominance
ABO blood group inheritance
Lac operon regulation
DNA replication mechanism
Lac operon regulation
Ecology:
Energy flow in ecosystem, 10% law
Nitrogen cycle with diagram
Succession: Hydrosere vs Xerosere
Biodiversity hotspots in India
Causes and effects of eutrophication
Biotechnology:
Steps in recombinant DNA technology
Applications of PCR
Bt cotton mechanism
Ethical issues in genetic engineering
Week Focus Resources 1-2 Human Physiology (Digestion, Respiration, Circulation) NCERT + diagrams 3-4 Human Physiology (Excretion, Neural, Endocrine) Flowcharts, tables 5 Genetics & Evolution Punnett squares, pedigrees 6 Molecular Biology Central dogma, operons 7 Ecology Cycles, pyramids, biodiversity 8 Biotechnology Steps, applications, ethics 9-10 Full Revision + Past Papers 5 years NEB papers
Class 12 Biology is vast but highly structured. Focus on:
Diagrams - Draw and label perfectlyFlowcharts - For pathways (metabolic, hormonal, signal transduction)Tables - Compare/contrast (hormones, blood groups, cycles)Numericals - Genetics problems, population growthPast Papers - Pattern recognition is key
Nothing in biology makes sense except in the light of evolution.
Good luck with your NEB Class 12 Biology preparation!
