Class 12 Biology Complete Notes - Human Physiology, Genetics, Ecology

Complete Class 12 Biology notes covering Human Physiology, Genetics, Evolution, Ecology, and Biotechnology for NEB exam. Includes diagrams, key concepts, and practice questions.

Subesh Yadav·June 15, 2026·Updated June 15, 2026·16 min read

Class 12 Biology Overview #

Class 12 Biology under NEB covers Human Physiology, Genetics & Evolution, Ecology, and Biotechnology. This is a crucial year for medical/biological career aspirants.

Unit 1: Human Physiology #

1.1 Digestion and Absorption #

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 capillaries
Fatty acids, glycerol: Re-esterified → Chylomicrons → Lymph (lacteals) → Blood
Water, electrolytes, vitamins: Passive/active transport

1.2 Breathing and Respiration #

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

Gas Exchange at Alveoli

O₂: Alveoli (PO₂ ~100 mmHg) → Blood (PO₂ ~40 mmHg) → Diffusion
CO₂: Blood (PCO₂ ~45 mmHg) → Alveoli (PCO₂ ~40 mmHg) → Diffusion
Driven 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 binding
Right shift (decreased affinity): ↑CO₂, ↑H⁺, ↑Temp, ↑2,3-BPG
Left 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 filling
Ventricular Systole (0.3s): Ventricles contract → AV valves close (lub), semilunar open
Ventricular Diastole (0.4s): Ventricles relax → Semilunar close (dub), AV open
Complete Cycle: 0.8s → 72 beats/min

Blood Pressure

Systolic: ~120 mmHg (ventricular systole)
Diastolic: ~80 mmHg (ventricular diastole)
Pulse Pressure: 40 mmHg
Mean Arterial Pressure: DP + 1/3(SP - DP) ≈ 93 mmHg

1.4 Excretion #

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

Urine Formation

Glomerular Filtration (GFR ~125 ml/min): Passive, non-selective (except proteins)
Tubular Reabsorption: 99% filtrate reabsorbed
Tubular Secretion: H⁺, K⁺, NH₄⁺, drugs, toxins
Concentration: Counter-current mechanism, ADH regulates water permeability

1.5 Locomotion and Movement #

Muscle Structure

Sarcomere: Functional unit (Z-line to Z-line)
Thin filament: Actin, troponin, tropomyosin
Thick filament: Myosin
Sliding 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

1.6 Neural Control and Coordination #

Neuron Structure

Dendrites: Receive signals
Cell body (soma): Integration
Axon: Transmits impulse
Synapse: 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⁺ overshoot
Refractory 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)

1.7 Chemical Coordination and Integration #

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 synthesis
Low glucose: α-cells → Glucagon → Glycogenolysis, Gluconeogenesis

Unit 2: Genetics and Evolution #

2.1 Principles of Inheritance #

Mendel's Laws

Law of Dominance: One allele masks other in heterozygote
Law of Segregation: Alleles separate during gamete formation
Law 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 daughters
Y-linked: Only males, father → all sons
Cytoplasmic: Maternal inheritance (mitochondria)

2.2 Molecular Basis of Inheritance #

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 stabilize
Elongation:
- 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 promoter
Elongation: 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 acid
Universal: Almost same in all organisms
Non-overlapping, commaless

Translation

Initiation: 30S + mRNA + fMet-tRNA + IFs → 70S initiation complex
Elongation: EF-Tu brings aa-tRNA → A site → Peptide bond (peptidyl transferase) → Translocation
Termination: 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 protein
Inducer: Allolactose (inactivates repressor)
CAP-cAMP: Positive control (glucose low → cAMP high)

2.3 Evolution #

Theories

Lamarckism: Inheritance of acquired characters (disproved)
Darwinism: Natural selection, survival of fittest
Mutation Theory (de Vries): Mutations cause new species
Modern 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), Vestigial
Embryology: Ontogeny recapitulates phylogeny (Haeckel)
Biochemistry: Universal genetic code, similar proteins (cytochrome c)
Biogeography: Continental drift, island endemism

Speciation

Allopatric: Geographic isolation → Reproductive isolation
Sympatric: Polyploidy, habitat differentiation, sexual selection
Reproductive 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

Unit 3: Ecology and Environment #

3.1 Organisms and Populations #

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)

3.2 Ecosystem #

Structure

Biotic: Producers (autotrophs), Consumers (heterotrophs), Decomposers (saprotrophs)
Abiotic: Light, temperature, water, soil, nutrients

Energy Flow

Unidirectional: Sun → Producers → Herbivores → Carnivores
10% Law (Lindemann): ~10% energy transferred to next trophic level
Pyramids: 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

3.3 Biodiversity and Conservation #

Biodiversity Levels

Genetic Diversity: Variation within species
Species Diversity: Variety of species in area
Ecosystem 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

3.4 Environmental Issues #

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 weather
Ozone Depletion: CFCs → O₃ destruction → UV radiation ↑
Deforestation: Habitat loss, soil erosion, climate change
Eutrophication: Nutrient enrichment → Algal bloom → O₂ depletion
Biomagnification: Non-degradable toxins accumulate up food chain (DDT, Hg)

Solid Waste Management

3Rs: Reduce, Reuse, Recycle
Composting: Organic waste → Manure
Incineration: Volume reduction, energy recovery
Landfill: Engineered, leachate collection, methane capture

Unit 4: Biotechnology #

4.1 Principles and Processes #

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)

Tools

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

4.2 Applications #

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 fingerprinting
Transgenic Animals: Models for disease, protein production (milk)

Agricultural

Bt Cotton: Cry protein from Bacillus thuringiensis → insect resistance
Golden Rice: β-carotene (vitamin A) biosynthesis genes
Herbicide Tolerance: EPSPS gene (Roundup Ready crops)
Virus Resistance: Coat protein gene

Industrial

Enzymes: Amylase, protease, lipase (detergents, food processing)
Biofuels: Ethanol from biomass
Bioplastics: PHB from bacteria
Bioremediation: Oil spills, heavy metals

Ethical Issues

GEAC (Genetic Engineering Approval Committee) regulates in India
Biosafety: Gene flow, superweeds, allergenicity, antibiotic resistance markers
Bioethics: Human cloning, designer babies, patenting life forms

Practice Questions #

Study Plan #

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

Conclusion #

Class 12 Biology is vast but highly structured. Focus on:

Diagrams - Draw and label perfectly
Flowcharts - For pathways (metabolic, hormonal, signal transduction)
Tables - Compare/contrast (hormones, blood groups, cycles)
Numericals - Genetics problems, population growth
Past Papers - Pattern recognition is key

Nothing in biology makes sense except in the light of evolution.
— Theodosius Dobzhansky

Good luck with your NEB Class 12 Biology preparation!

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