1. Cell: The Basic Unit of Life

1.1 What is a Cell?

A cell is the basic structural and functional unit of life.
All living organisms are made up of one or more cells.
A cell performs essential life processes such as:
- Nutrition
- Respiration
- Growth
- Excretion
- Reproduction
- Response to stimuli

Examples

Bacteria → Unicellular
Amoeba → Unicellular
Human → Multicellular
Mango plant → Multicellular

1.2 Cell Size

Most cells are microscopic.
Mycoplasma → Among the smallest known free-living cells.
Ostrich egg → Commonly cited as the largest single cell.
Human RBC → approximately 7–8 µm in diameter.

2. Cell Theory

2.1 Development of Cell Theory

Scientist	Contribution
Robert Hooke	Observed cork cells in 1665 and coined the term “cell”
Anton van Leeuwenhoek	Observed living cells and microorganisms
Matthias Schleiden	Proposed that plants are made of cells
Theodor Schwann	Proposed that animals are made of cells
Rudolf Virchow	Proposed that new cells arise from pre-existing cells

2.2 Modern Cell Theory

All living organisms are composed of cells and cell products.
The cell is the basic structural and functional unit of life.
New cells arise from pre-existing cells.
Cells contain hereditary information that is transmitted to daughter cells.
Energy flow and metabolism occur within cells.

NEET High-Yield Fact

“Omnis cellula e cellula” → Rudolf Virchow

Meaning:

Every cell arises from a pre-existing cell.

3. Types of Cells

Cells are broadly divided into:

Prokaryotic cells
Eukaryotic cells

4. Prokaryotic Cells

4.1 General Characteristics

Usually small and structurally simple.
True nucleus is absent.
Nuclear membrane is absent.
DNA is present in a nucleoid region.
Membrane-bound organelles are absent.
Ribosomes are present.
Ribosomes are generally 70S.
Cell wall is generally present in bacteria.
Cell division generally occurs by binary fission.

Examples

Bacteria
Cyanobacteria
Mycoplasma
Archaea

4.2 Structure of a Typical Bacterial Cell

The main components are:

Glycocalyx → Cell wall → Plasma membrane → Cytoplasm → Nucleoid → Ribosomes

4.3 Glycocalyx

The outermost covering in some bacteria is called the glycocalyx.

It may occur as:

Capsule → Thick and organised
Slime layer → Loose and less organised

Functions

Protection
Prevents desiccation
Helps attachment to surfaces
May contribute to pathogenicity

4.4 Bacterial Cell Wall

Located outside the plasma membrane.
Mainly composed of peptidoglycan.
Provides:
- Shape
- Mechanical support
- Protection from osmotic lysis

4.5 Plasma Membrane

Present inside the cell wall.
Made mainly of lipids and proteins.
Selectively regulates movement of substances.
Some metabolic processes occur on the bacterial plasma membrane.

4.6 Nucleoid

Prokaryotes do not have a membrane-bound nucleus.
Their genetic material is concentrated in a region called the nucleoid.
Usually contains a single circular DNA molecule.

Plasmids

Small, circular, extra-chromosomal DNA molecules.
Replicate independently.
May carry useful genes.

Example

Some plasmids carry genes for antibiotic resistance.

4.7 Ribosomes in Prokaryotes

Ribosomes are non-membranous structures.
Prokaryotic ribosomes → 70S
70S ribosome consists of:
- 50S large subunit
- 30S small subunit

Function

Protein synthesis

4.8 Inclusion Bodies

Some prokaryotes store reserve materials in inclusion bodies.
They are generally not surrounded by a membrane.

Examples:

Phosphate granules
Glycogen granules
Cyanophycean granules

5. Eukaryotic Cells

5.1 General Characteristics

Generally larger and more complex than prokaryotic cells.
True nucleus is present.
Nuclear envelope is present.
Membrane-bound organelles are present.
Chromosomes are generally linear.
Cytoplasmic ribosomes are 80S.
Cytoskeleton is well developed.

Examples

Protists
Fungi
Plants
Animals

6. Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	Generally smaller	Generally larger
True nucleus	Absent	Present
Nuclear membrane	Absent	Present
DNA	Usually circular	Usually linear
Membrane-bound organelles	Absent	Present
Ribosome	70S	80S in cytoplasm
Cell division	Binary fission	Mitosis/Meiosis
Cytoskeleton	Simple	Well developed
Examples	Bacteria	Plants, animals

Important Exception

Mitochondria and chloroplasts of eukaryotic cells contain 70S ribosomes.

7. Plant Cell and Animal Cell

7.1 Plant Cell

A typical plant cell contains:

Cell wall
Plasma membrane
Cytoplasm
Nucleus
Plastids
Large central vacuole
Mitochondria
Endoplasmic reticulum
Golgi apparatus
Ribosomes
Peroxisomes

7.2 Animal Cell

A typical animal cell contains:

Plasma membrane
Cytoplasm
Nucleus
Mitochondria
Endoplasmic reticulum
Golgi apparatus
Lysosomes
Ribosomes
Centrosome with centrioles
Small vacuoles may occur

8. Plant Cell vs Animal Cell

Feature	Plant Cell	Animal Cell
Cell wall	Present	Absent
Plastids	Present	Absent
Large central vacuole	Usually present	Usually absent
Centrioles	Generally absent in higher plants	Usually present
Shape	Often fixed	Usually flexible
Nutrition	Mainly autotrophic	Heterotrophic

9. Cell Envelope

In bacteria, the cell envelope generally consists of:

Glycocalyx
Cell wall
Plasma membrane

Functions

Protection
Maintaining cell shape
Selective transport
Interaction with external environment

10. Plasma Membrane

10.1 Structure

The plasma membrane is the living boundary of the cell.

It is mainly composed of:

Lipids
Proteins
Carbohydrates

It is selectively permeable.

10.2 Fluid Mosaic Model

The Fluid Mosaic Model was proposed by Singer and Nicolson in 1972.

According to this model:

Lipids form a bilayer.
Proteins are embedded in or associated with the lipid bilayer.
Membrane components can move laterally.
The membrane is dynamic and flexible.

10.3 Phospholipid Bilayer

A phospholipid has:

Hydrophilic head
Hydrophobic tail

In the membrane:

Hydrophilic heads face the aqueous environments.
Hydrophobic tails face inward toward each other.

Important Term

Amphipathic molecule → Molecule having both hydrophilic and hydrophobic regions.

Phospholipids are amphipathic.

10.4 Membrane Proteins

Integral Proteins

Embedded deeply within the membrane.
Some span the entire lipid bilayer.
Important in transport and signalling.

Peripheral Proteins

Associated with the membrane surface.
Generally more loosely attached.

10.5 Functions of Plasma Membrane

Provides cell boundary.
Controls entry and exit of substances.
Helps in cell signalling.
Cell recognition.
Cell adhesion.
Endocytosis.
Exocytosis.

11. Transport Across Plasma Membrane

11.1 Simple Diffusion

Movement of molecules:

Higher concentration → Lower concentration

No ATP required.
Occurs along concentration gradient.

Example

Movement of gases such as O₂ and CO₂.

11.2 Osmosis

Osmosis is the movement of water through a selectively permeable membrane.

Water moves from a region of:

Higher water potential → Lower water potential

11.3 Facilitated Diffusion

Movement occurs along concentration/electrochemical gradient.
Transport proteins are involved.
Direct ATP expenditure is not required.

11.4 Active Transport

Movement occurs against concentration/electrochemical gradient.
Requires energy.
Transport proteins are involved.

Example

Na⁺/K⁺ pump

11.5 Endocytosis

Cell takes materials inside by forming membrane-bound vesicles.

Types include:

Phagocytosis → Cell eating
Pinocytosis → Cell drinking
Receptor-mediated endocytosis

11.6 Exocytosis

Materials are transported out of the cell through vesicles that fuse with the plasma membrane.

Examples

Hormone secretion
Enzyme secretion
Neurotransmitter release

12. Cell Wall

12.1 Plant Cell Wall

Plant cell wall is a rigid, non-living outer layer outside the plasma membrane.

Main components:

Cellulose
Hemicellulose
Pectin

Functions

Provides mechanical support.
Maintains cell shape.
Protects the cell.
Prevents osmotic bursting.
Helps maintain structural integrity of plants.

12.2 Layers of Plant Cell Wall

Middle Lamella

Lies between adjacent plant cells.
Mainly rich in calcium pectate.
Helps cement neighbouring cells together.

Primary Cell Wall

Present in young growing cells.
Relatively thin and flexible.
Allows cell expansion.

Secondary Cell Wall

Develops inside the primary wall in some mature cells.
Generally thicker and stronger.
May contain lignin.

12.3 Plasmodesmata

Plasmodesmata are cytoplasmic connections between neighbouring plant cells.

Functions

Cell-to-cell communication
Transport of substances
Coordination between neighbouring cells

13. Endomembrane System

The endomembrane system includes:

Endoplasmic reticulum
Golgi apparatus
Lysosomes
Vacuoles

Important

Mitochondria and chloroplasts are NOT part of the endomembrane system.

14. Endoplasmic Reticulum

ER is a network of membrane-bound tubules, sacs and channels.

Two main types:

Rough ER
Smooth ER

14.1 Rough Endoplasmic Reticulum — RER

RER has ribosomes attached to its surface.

Functions

Synthesis of proteins
Folding and modification of proteins
Production of membrane proteins
Transport of proteins

Example

Pancreatic cells have abundant RER because they produce many digestive enzymes.

14.2 Smooth Endoplasmic Reticulum — SER

SER lacks attached ribosomes.

Functions

Lipid synthesis
Steroid synthesis
Detoxification
Carbohydrate metabolism
Calcium ion storage

Example

Liver cells contain abundant SER involved in detoxification.

Specialised SER

In muscle cells, SER is called sarcoplasmic reticulum.

It stores and releases Ca²⁺ during muscle contraction.

15. Golgi Apparatus

Golgi apparatus consists of stacks of flattened membrane-bound sacs called cisternae.

It was discovered by Camillo Golgi.

15.1 Faces of Golgi Apparatus

Cis Face

Receiving face
Receives vesicles from ER.

Trans Face

Shipping/dispatching face
Sends modified products to their destinations.

15.2 Functions of Golgi Apparatus

Modification of proteins
Sorting
Packaging
Secretion
Formation of lysosomes
Formation of glycoproteins and glycolipids
Cell wall material synthesis in plants

Example

ER → Golgi → Modification → Sorting → Vesicle → Final destination

16. Lysosomes

Lysosomes are single-membrane-bound organelles containing hydrolytic enzymes.

Important enzymes

Proteases
Lipases
Nucleases
Carbohydrases

Functions

Intracellular digestion
Breakdown of foreign material
Destruction of worn-out organelles
Autophagy
Recycling of cellular components

Important Term

Lysosomes are traditionally called “suicidal bags” because of their powerful digestive enzymes.

However, lysosomes normally perform many essential cellular functions.

17. Vacuoles

Vacuoles are membrane-bound sacs involved mainly in storage and regulation.

Plant Vacuole

Usually very large.
Occupies a major portion of the cell.
Surrounded by a membrane called tonoplast.
Contains cell sap.

Cell sap may contain

Water
Sugars
Amino acids
Mineral ions
Pigments
Waste products

Functions

Storage
Maintaining turgor pressure
Osmoregulation
Waste storage
Maintaining cell volume

Example

A well-watered plant cell remains turgid partly because of its large vacuole.

18. Mitochondria

Mitochondria are double-membrane-bound organelles involved in aerobic respiration and ATP production.

They are often called the powerhouses of the cell.

19. Structure of Mitochondria

Outer Membrane

Smooth
Contains porins
Relatively permeable to many small molecules

Inner Membrane

Highly folded.
Folds are called cristae.
Contains components of the electron transport chain and ATP synthase.

Matrix

The internal fluid-filled region is called the matrix.

It contains:

Enzymes
Circular DNA
RNA
70S ribosomes

20. Functions of Mitochondria

Aerobic respiration
ATP production
TCA/Krebs cycle
Oxidative phosphorylation
Fatty acid oxidation
Other metabolic reactions

NEET High-Yield Facts

Mitochondria are semi-autonomous organelles.
They contain their own DNA.
Mitochondrial DNA is generally circular.
They contain 70S ribosomes.
Inner membrane contains the electron transport system.
ATP synthase is associated with the inner mitochondrial membrane.

21. Ribosomes

Ribosomes are non-membranous structures responsible for protein synthesis.

Prokaryotic Ribosome

70S = 50S + 30S

Eukaryotic Cytoplasmic Ribosome

80S = 60S + 40S

Mitochondria and Chloroplasts

Contain 70S ribosomes.

NEET Trick

70S → Prokaryotes + Mitochondria + Chloroplasts

80S → Eukaryotic cytoplasm

22. Plastids

Plastids are double-membrane-bound organelles found mainly in plant cells and algae.

Major types:

Chloroplasts
Chromoplasts
Leucoplasts

23. Chloroplast

Chloroplast is the major organelle of photosynthesis.

Structure

It contains:

Outer membrane
Inner membrane
Stroma
Thylakoids
Grana
Stroma lamellae

Grana

Stacks of thylakoids are called grana.

Stroma

The fluid matrix surrounding the thylakoid system is called stroma.

It contains:

Enzymes
DNA
Ribosomes
Starch grains

23.1 Major Functions

Photosynthesis
Light reactions
Carbon fixation
Synthesis of carbohydrates
Various biosynthetic reactions

Location of Photosynthetic Reactions

Reaction	Major location
Light reaction	Thylakoid membrane
Calvin cycle	Stroma

24. Chromoplasts

Chromoplasts contain coloured pigments other than chlorophyll in many plant parts.

Function

They contribute to yellow, orange and red colours.

Examples

Tomato
Carrot
Coloured flowers

25. Leucoplasts

Leucoplasts are colourless plastids mainly involved in storage.

Types

Plastid	Main storage
Amyloplast	Starch
Elaioplast	Oil
Proteinoplast/Aleuroplast	Protein

26. Microbodies

Microbodies are small, single-membrane-bound organelles containing specialised enzymes.

Important examples:

Peroxisomes
Glyoxysomes

27. Peroxisomes

Peroxisomes contain oxidative enzymes.

Important enzyme

Catalase

Catalase breaks down hydrogen peroxide:

2H₂O₂ → 2H₂O + O₂

Functions

Detoxification
Hydrogen peroxide metabolism
Fatty acid oxidation
Photorespiration in plants

28. Glyoxysomes

Glyoxysomes are specialised microbodies found particularly in germinating oil-rich seeds.

Function

They participate in pathways that convert stored fats into carbohydrates.

Examples

Castor
Groundnut
Other oil-rich seeds

NEET Fact

Glyoxysomes → Germinating oil seeds

29. Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments present in the cytoplasm.

Major components:

Microtubules
Microfilaments
Intermediate filaments

30. Microtubules

Microtubules are hollow structures made mainly of tubulin.

Functions

Maintain cell shape
Intracellular transport
Chromosome movement
Spindle formation
Formation of cilia and flagella

31. Microfilaments

Microfilaments are mainly made of actin.

Functions

Cell shape
Cell movement
Cytoplasmic streaming
Muscle contraction
Cytokinesis

32. Intermediate Filaments

Intermediate filaments are strong protein fibres.

Functions

Mechanical strength
Structural support
Maintenance of cell integrity

33. Cilia and Flagella

Cilia and flagella are specialised structures involved in movement.

Cilia

Shorter
Usually numerous
Beat in a coordinated manner

Example

Cilia of respiratory tract help move mucus.

Flagella

Usually longer
Usually fewer
Often involved in locomotion

Example

Human sperm has a flagellum.

34. Ultrastructure of Cilia and Flagella

The typical eukaryotic axoneme has a:

9 + 2 arrangement

Arrangement

9 peripheral microtubule doublets
2 central singlet microtubules

Important Protein

Dynein

Dynein arms help produce movement by promoting microtubule sliding.

Mechanism

Microtubule sliding → Bending → Movement

35. Basal Body

The basal body is present at the base of cilia and flagella.

Structure

Typical basal body contains:

9 peripheral triplets

Difference

Structure	Microtubule arrangement
Cilium/Flagellum axoneme	9 + 2
Basal body	9 triplets
Centriole	9 triplets

36. Centrioles

Centrioles are cylindrical, non-membranous structures.

They are commonly present in animal cells as part of the centrosome.

Structure

A centriole contains:

9 peripheral microtubule triplets

Functions

Organisation of spindle apparatus
Formation of basal bodies
Organisation associated with cilia and flagella formation

37. Nucleus

The nucleus is the major genetic control centre of a eukaryotic cell.

Main components

Nuclear envelope
Nucleoplasm
Chromatin
Nucleolus

38. Nuclear Envelope

The nuclear envelope consists of two membranes.

Outer Membrane

Continuous with endoplasmic reticulum.
Ribosomes may be attached.

Inner Membrane

Faces the nucleoplasm.

Nuclear Pores

Present in the nuclear envelope.
Allow controlled exchange between nucleus and cytoplasm.

39. Nucleoplasm

Nucleoplasm is the fluid matrix present inside the nucleus.

It contains:

Chromatin
Nucleolus
Various enzymes and molecules involved in nuclear processes

40. Chromatin

Chromatin consists mainly of:

DNA + Histone proteins + Other associated proteins

Two major forms

Euchromatin

Loosely packed
Lightly stained
Generally transcriptionally active

Heterochromatin

More condensed
Darkly stained
Generally less transcriptionally active

NEET Trick

Eu = Easy to transcribe

Hetero = Highly condensed

41. Nucleolus

Nucleolus is a dense, non-membrane-bound structure inside the nucleus.

Functions

rRNA synthesis
Ribosomal subunit assembly

Important

Nucleolus has no surrounding membrane.

42. Chromosomes

During cell division, chromatin becomes highly condensed and forms visible chromosomes.

Major components

DNA
Histones
Non-histone proteins

Centromere

The centromere is a specialised constricted region of a chromosome.

Function

Important for chromosome movement during cell division.
Provides the region associated with kinetochore formation.

43. Chemical Constituents of Living Cells

The major chemical components of cells are:

Inorganic substances

Water
Mineral ions
Gases

Organic substances

Carbohydrates
Proteins
Lipids
Nucleic acids

44. Water

Water is the most abundant inorganic component of most living cells.

Functions

Solvent
Medium for biochemical reactions
Transport
Temperature regulation
Maintains cellular structure

45. Biomolecules

Biomolecules are organic molecules associated with living organisms.

Major biomolecules:

Carbohydrates
Proteins
Lipids
Nucleic acids

46. Carbohydrates

Carbohydrates mainly contain:

Carbon
Hydrogen
Oxygen

Major functions

Energy source
Energy storage
Structural support
Cell recognition

47. Monosaccharides

These are the simplest carbohydrates.

Examples

Glucose
Fructose
Ribose
Deoxyribose

Important

Glucose → Important respiratory fuel
Ribose → RNA
Deoxyribose → DNA

48. Disaccharides

Two monosaccharides joined together form a disaccharide.

Disaccharide	Components
Sucrose	Glucose + Fructose
Maltose	Glucose + Glucose
Lactose	Glucose + Galactose

The monosaccharides are joined by a glycosidic bond.

49. Polysaccharides

Polysaccharides are polymers containing many monosaccharide units.

Polysaccharide	Major role
Starch	Storage carbohydrate in plants
Glycogen	Storage carbohydrate in animals
Cellulose	Plant cell wall
Chitin	Fungal cell wall and arthropod exoskeleton

NEET Trick

Plant storage → Starch

Animal storage → Glycogen

Plant wall → Cellulose

Fungal wall → Chitin

50. Proteins

Proteins are polymers of amino acids.

Main elements

Some proteins also contain sulphur.

51. Amino Acids

A typical α-amino acid contains:

Amino group → –NH₂
Carboxyl group → –COOH
Hydrogen atom
Variable R group
α-carbon

General structure:

NH₂–CH(R)–COOH

Examples

Glycine
Alanine
Lysine
Valine

52. Peptide Bond

Two amino acids can join through a condensation reaction.

The bond formed is called a peptide bond.

Peptide bond → –CO–NH–

Sequence

Amino acids → Peptide → Polypeptide → Functional protein

53. Levels of Protein Structure

Primary Structure

Exact amino acid sequence.
Determines the basic structure of the polypeptide.

Secondary Structure

Major forms:

α-helix
β-pleated sheet

Tertiary Structure

Overall three-dimensional folding of a single polypeptide chain.

Quaternary Structure

Association of multiple polypeptide chains.

Example

Haemoglobin → Four polypeptide subunits

54. Functions of Proteins

Function	Example
Enzyme	Amylase
Transport	Haemoglobin
Hormone	Insulin
Structural	Collagen
Defence	Antibodies
Movement	Actin, Myosin
Storage	Ferritin

55. Lipids

Lipids are generally hydrophobic or poorly soluble in water.

They mainly contain:

Carbon
Hydrogen
Oxygen

Some contain:

Phosphorus
Nitrogen

Functions

Long-term energy storage
Membrane formation
Hormone production
Insulation
Protection

56. Fatty Acids

Fatty acids contain:

Long hydrocarbon chain
Carboxyl group

Saturated Fatty Acids

No carbon-carbon double bond.

Unsaturated Fatty Acids

One or more carbon-carbon double bonds.

57. Triglycerides

A triglyceride contains:

1 glycerol + 3 fatty acids

The fatty acids are attached through ester bonds.

Main function

Long-term energy storage

58. Phospholipids

Phospholipids contain:

Glycerol
Fatty acids
Phosphate-containing group

They are amphipathic.

Importance

They form the basic lipid framework of biological membranes.

59. Steroids

Steroids contain characteristic fused carbon-ring structures.

Examples

Cholesterol
Testosterone
Estrogen
Cortisol

Functions

Membrane component
Hormonal functions
Precursors of steroid hormones

60. Nucleic Acids

Nucleic acids are polymers of nucleotides.

Two major types:

61. Nucleotide

A nucleotide contains:

Nitrogenous base + Pentose sugar + Phosphate

Nucleoside

Nitrogenous base + Pentose sugar

Therefore:

Nucleoside + Phosphate = Nucleotide

62. Nitrogenous Bases

Purines

Adenine
Guanine

Mnemonic:

Pure As Gold → Purines = A + G

Pyrimidines

Cytosine
Thymine
Uracil

Mnemonic:

CUT → Cytosine, Uracil, Thymine

63. DNA : Deoxyribonucleic Acid

DNA = Deoxyribonucleic Acid

Main functions

Stores genetic information.
Transfers hereditary information.
Acts as a template for RNA synthesis.

Structure

Usually double-stranded.
Strands are antiparallel.
Sugar → Deoxyribose.
Bases → A, T, G, C.

Base Pairing

A = T → 2 hydrogen bonds

G ≡ C → 3 hydrogen bonds

64. RNA : Ribonucleic Acid

RNA = Ribonucleic Acid

Characteristics

Usually single-stranded.
Sugar → Ribose.
Bases → A, U, G, C.

Major Types

RNA	Main role
mRNA	Carries genetic message
tRNA	Brings amino acids during translation
rRNA	Structural/catalytic component of ribosomes

65. DNA vs RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Pyrimidine	Thymine	Uracil
Structure	Usually double-stranded	Usually single-stranded
Main role	Genetic information storage	Gene expression and protein synthesis
Stability	Generally more stable	Generally less stable

66. Enzymes

Enzymes are biological catalysts that increase the rate of biochemical reactions without being permanently consumed.

Important

Most enzymes are proteins.

Exception

Some RNA molecules have catalytic activity.

These are called ribozymes.

67. Properties of Enzymes

Highly specific
Increase reaction rate
Required in small quantities
Not permanently consumed
Have optimum temperature
Have optimum pH
May require cofactors
Lower activation energy

68. Activation Energy

Chemical reactions require a minimum energy barrier called activation energy.

Enzymes accelerate reactions by lowering activation energy.

Important

Enzymes do not:

Change the overall free-energy change of the reaction.
Change the equilibrium constant.

69. Enzyme-Substrate Complex

The molecule on which an enzyme acts is called the substrate.

The specific region where the substrate binds is the active site.

Basic reaction:

Enzyme + Substrate → Enzyme-Substrate Complex → Product + Enzyme

70. Lock and Key Model

Proposed by Emil Fischer.

Concept

Enzyme active site → Lock
Substrate → Key

The substrate fits specifically into the active site.

71. Induced Fit Model

Associated with Daniel Koshland.

Concept

Active site is flexible.
Binding of substrate causes conformational adjustment.
This improves interaction between enzyme and substrate.

Comparison

Lock and Key	Induced Fit
Active site considered relatively rigid	Active site is flexible
Exact fitting	Conformational adjustment
Fischer	Koshland

72. Factors Affecting Enzyme Activity

72.1 Temperature

Increasing temperature generally increases enzyme activity initially.
Maximum activity occurs at an optimum temperature.
Excessive temperature may denature the enzyme.

Human enzymes

Many human enzymes have optimum activity around 37°C.

72.2 pH

Every enzyme has a characteristic optimum pH.

Examples

Enzyme	Approximate optimum pH
Pepsin	~2
Trypsin	~8

72.3 Substrate Concentration

Initially, increasing substrate concentration increases reaction rate.
At high substrate concentration, enzyme active sites become saturated.
Rate then approaches a maximum.

72.4 Enzyme Concentration

When substrate is available in excess:

More enzyme → Higher reaction rate

within the relevant range.

73. Cofactors

Some enzymes require non-protein components for activity.

These are called cofactors.

Types

Prosthetic groups
Coenzymes
Metal ions

74. Coenzymes

Coenzymes are organic cofactors.

Examples

NAD⁺
NADP⁺
FAD
Coenzyme A

75. Prosthetic Groups

A prosthetic group is a cofactor that is tightly bound to an enzyme.

76. Apoenzyme and Holoenzyme

Apoenzyme

Protein component of an enzyme.

Holoenzyme

Complete, active enzyme:

Apoenzyme + Cofactor = Holoenzyme

NEET Formula

Holoenzyme = Apoenzyme + Cofactor

77. Enzyme Classification

Enzymes are classified according to the type of reaction they catalyse.

Major classes include:

Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases

The modern EC system also recognises Translocases as a seventh class.

78. Oxidoreductases

Catalyse oxidation-reduction reactions.

Examples

Oxidases
Dehydrogenases

79. Transferases

Transfer functional groups from one molecule to another.

Example

Kinases

80. Hydrolases

Break chemical bonds using water.

Examples

Proteases
Lipases
Nucleases

81. Lyases

Catalyse addition or removal of groups without typical hydrolysis or oxidation-reduction reactions.

Example

Decarboxylases

82. Isomerases

Catalyse rearrangement of atoms within the same molecule.

Example

Phosphoglucose isomerase

83. Ligases

Join two molecules, generally using energy from ATP or another nucleotide triphosphate.

Example

DNA ligase

84. Cell Cycle

The cell cycle is the sequence of events through which a cell grows, duplicates its DNA and divides.

Main phases:

Interphase
M phase

85. Interphase

Interphase is an active phase, not simply a “resting phase”.

It includes:

G₁ → S → G₂

Major activities

Cell growth
DNA replication
Protein synthesis
Organelle duplication
Preparation for cell division

86. G₁ Phase

G₁ = First Gap Phase

Events

Cell growth
RNA synthesis
Protein synthesis
Organelle development
Normal metabolism

87. S Phase

S = Synthesis Phase

Major event

DNA replication

Important NEET Point

During S phase:

DNA content doubles.
Each chromosome becomes associated with two sister chromatids.
Chromosome number itself does not simply double because of DNA replication.

88. G₂ Phase

Events

Further cell growth
Protein synthesis
Preparation for mitosis
Spindle-related proteins are produced

89. M Phase

M phase includes:

Karyokinesis → Nuclear division
Cytokinesis → Cytoplasmic division

90. G₀ Phase

Some differentiated cells temporarily or permanently leave the active cell cycle and enter G₀ phase.

Example

Some highly differentiated cells may remain in a non-dividing state for long periods.

91. Mitosis

Mitosis is a type of cell division in which one parent cell generally produces two genetically similar daughter cells.

It is called equational division because chromosome number is generally maintained.

Major functions

Growth
Development
Tissue repair
Cell replacement
Asexual reproduction in some organisms
Genetic continuity

92. Stages of Mitosis

Mitosis occurs through:

Prophase → Metaphase → Anaphase → Telophase

Memory Trick

PMAT

93. Prophase

Major events

Chromatin condenses into visible chromosomes.
Each chromosome consists of two sister chromatids.
Nucleolus disappears.
Nuclear envelope progressively breaks down.
Spindle formation begins.

94. Metaphase

Major events

Chromosomes are highly condensed.
Chromosomes align at the equatorial plate.
Spindle fibres attach to kinetochores.

NEET Fact

Metaphase → Chromosomes at equator

95. Anaphase

Major event

Centromeres divide.

Sister chromatids separate.
They become daughter chromosomes.
Daughter chromosomes move toward opposite poles.

NEET Fact

Sister chromatids separate → Anaphase

96. Telophase

Major events

Chromosomes reach opposite poles.
Chromosomes decondense.
Nuclear envelope reforms.
Nucleolus reappears.
Two daughter nuclei form.

97. Cytokinesis

Cytokinesis is the division of cytoplasm.

Animal Cells

A cleavage furrow develops.

Plant Cells

A cell plate develops.

Animal Cell	Plant Cell
Cleavage furrow	Cell plate
Constriction from outside inward	Cell plate develops from centre outward

98. Significance of Mitosis

Maintains chromosome number.
Enables growth.
Replaces damaged cells.
Helps tissue repair.
Maintains genetic stability.
Supports asexual reproduction in many organisms.

99. Meiosis

Meiosis is a specialised cell division in which chromosome number is reduced to half.

It is called reduction division.

Major features

Two successive nuclear divisions.
One round of DNA replication.
Produces haploid cells from a diploid starting cell.
Important in sexual reproduction.
Generates genetic variation.

Divisions

Meiosis I
Meiosis II

100. Meiosis I

Meiosis I separates homologous chromosomes.

Stages:

Prophase I → Metaphase I → Anaphase I → Telophase I

101. Prophase I

Prophase I is the longest and most complex phase of meiosis.

It has five sub-stages:

Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis

Memory Trick

Le → Zy → Pa → Di → Dia

102. Leptotene

Chromosomes begin to condense.
Chromosomes appear as thin threads.

103. Zygotene

Major event

Synapsis

Homologous chromosomes pair with each other.

The paired homologous chromosomes form a:

Bivalent

104. Pachytene

Major event

Crossing over

Exchange of genetic material occurs between non-sister chromatids of homologous chromosomes.
Crossing over increases genetic variation.

105. Diplotene

Major events

Homologous chromosomes begin to separate.
They remain attached at points called chiasmata.
Synaptonemal complex disappears.

Chiasmata

Visible points of contact associated with crossing over.

106. Diakinesis

Major events

Maximum chromosome condensation.
Chiasmata undergo terminalisation.
Nuclear envelope breaks down.
Spindle formation occurs.

107. Metaphase I

Major events

Homologous chromosome pairs arrange at the equatorial plate.
Each pair is called a bivalent.

NEET Fact

Metaphase I → Bivalents at equator

108. Anaphase I

Major event

Homologous chromosomes separate.

Important

Centromeres do NOT normally divide during Anaphase I.

Therefore:

Sister chromatids remain together.
Homologous chromosomes move to opposite poles.

109. Telophase I

Chromosomes reach opposite poles.
Nuclear envelopes may reform depending on the organism.
Two haploid nuclei/cells may form.

Interkinesis

A short interval may occur between Meiosis I and II.

DNA replication does NOT occur during interkinesis.

110. Meiosis II

Meiosis II resembles mitosis.

Stages:

Prophase II → Metaphase II → Anaphase II → Telophase II

Anaphase II

Centromeres divide.

Sister chromatids separate and move toward opposite poles.

111. Result of Meiosis

One diploid starting cell generally produces:

Four haploid daughter cells

Importance

Reduces chromosome number by half.
Produces gametes or meiotic products.
Maintains chromosome number across generations after fertilisation.
Generates genetic variation.

112. Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
DNA replication	Once	Once
Daughter cells	Usually 2	Usually 4
Chromosome number	Maintained	Reduced to half
Synapsis	Absent	Present in Prophase I
Crossing over	Normally absent	Present in Prophase I
Genetic variation	Low/limited	High
Main role	Growth, repair	Sexual reproduction
Homologous chromosomes	Do not pair	Pair in Prophase I
Centromere division	Anaphase	Anaphase II

113. High-Yield Meiosis Comparison

Mitosis

2n → 2n + 2n

Chromosome number is maintained.

Meiosis

2n → n + n + n + n

Chromosome number is reduced.

Most Important Concept

Meiosis I → Homologous chromosomes separate

Meiosis II → Sister chromatids separate

114. Major Cell Organelles — One-Glance Revision

Organelle	Membrane	Major Function
Nucleus	Double	Genetic control
Ribosome	None	Protein synthesis
RER	Single	Protein synthesis/transport
SER	Single	Lipid synthesis/detoxification
Golgi apparatus	Single	Modification/packaging
Lysosome	Single	Intracellular digestion
Vacuole	Single	Storage/turgor
Mitochondria	Double	ATP production
Chloroplast	Double	Photosynthesis
Peroxisome	Single	Oxidative reactions
Glyoxysome	Single	Fat-to-carbohydrate conversion
Centriole	None	Spindle/basal body organisation

115. Double-Membrane Organelles

Major double-membrane organelles:

Nucleus
Mitochondria
Plastids

Special Feature

Mitochondria and chloroplasts possess:

Their own DNA
Their own ribosomes
Some capacity for independent protein synthesis

Hence, they are called semi-autonomous organelles.

116. Single-Membrane Organelles

Important examples:

Endoplasmic reticulum
Golgi apparatus
Lysosomes
Vacuoles
Peroxisomes
Glyoxysomes

117. Non-Membranous Structures

Important non-membranous structures:

Ribosomes
Centrioles
Cytoskeleton
Nucleolus
Chromatin

118. Important Biological Bonds

Bond	Major occurrence
Glycosidic bond	Carbohydrates
Peptide bond	Proteins
Ester bond	Triglycerides
Phosphodiester bond	Nucleic acids
Hydrogen bond	DNA base pairing

119. Important Biomolecules — Quick Table

Biomolecule	Basic unit	Major functions
Carbohydrates	Monosaccharides	Energy, storage, structure
Proteins	Amino acids	Enzymes, structure, transport
Lipids	Fatty acids/glycerol in many lipids	Energy, membranes, hormones
Nucleic acids	Nucleotides	Genetic information

120. Important Cell Biology Scientists

Scientist	Contribution
Robert Hooke	Coined term “cell” after observing cork
Leeuwenhoek	Observed living cells/microorganisms
Schleiden	Plant cell theory
Schwann	Animal cell theory
Virchow	New cells arise from pre-existing cells
Camillo Golgi	Golgi apparatus
Singer & Nicolson	Fluid mosaic model
Watson & Crick	DNA double-helix model

121. NEET High-Yield Facts

Cell

Cell → Basic structural and functional unit of life.
Hooke → Coined the term “cell”.
Virchow → Omnis cellula e cellula.

Cell Types

Prokaryotes → No true nucleus.
Eukaryotes → True nucleus.
Prokaryotic ribosome → 70S.
Eukaryotic cytoplasmic ribosome → 80S.
Mitochondria/chloroplasts → 70S ribosomes.

Cell Membrane

Fluid mosaic model → Singer and Nicolson.
Phospholipids → Amphipathic.
Plasma membrane → Selectively permeable.

Cell Wall

Plant cell wall → Cellulose, hemicellulose, pectin.
Middle lamella → Calcium pectate-rich.
Plasmodesmata → Cell-to-cell connections.

Endomembrane System

RER → Protein synthesis.
SER → Lipid synthesis and detoxification.
Golgi → Modification, sorting and packaging.
Lysosome → Intracellular digestion.
Vacuole → Storage and turgor.

Energy and Photosynthesis

Mitochondria → ATP production.
Cristae → Inner mitochondrial membrane folds.
Chloroplast → Photosynthesis.
Grana → Stacks of thylakoids.
Light reactions → Thylakoid membrane.
Calvin cycle → Stroma.

Cytoskeleton

Microtubules → Tubulin.
Microfilaments → Actin.
Intermediate filaments → Mechanical support.
Cilia/flagella → Usually 9 + 2.
Centriole → 9 triplets.

Nucleus

Nuclear envelope → Double membrane.
Nucleolus → rRNA synthesis and ribosome assembly.
Euchromatin → Less condensed, generally active.
Heterochromatin → More condensed, generally less active.

Biomolecules

Starch → Plant storage carbohydrate.
Glycogen → Animal storage carbohydrate.
Cellulose → Plant cell wall.
Chitin → Fungal cell wall/arthropod exoskeleton.
Protein → Polymer of amino acids.
Peptide bond → Joins amino acids.
Triglyceride → Glycerol + 3 fatty acids.
DNA → Deoxyribose + thymine.
RNA → Ribose + uracil.

Enzymes

Enzyme → Biological catalyst.
Enzyme lowers → Activation energy.
Active site → Substrate binding/catalytic region.
Apoenzyme + cofactor → Holoenzyme.
Catalytic RNA → Ribozyme.
Lock and key → Fischer.
Induced fit → Koshland.

Cell Cycle

Interphase → G₁ + S + G₂.
S phase → DNA replication.
M phase → Nuclear + cytoplasmic division.
G₀ → Non-dividing/quiescent state.

Mitosis

Mitosis → Equational division.
Sequence → PMAT.
Metaphase → Chromosomes at equator.
Anaphase → Sister chromatids separate.
Animal cytokinesis → Cleavage furrow.
Plant cytokinesis → Cell plate.

Meiosis

Meiosis → Reduction division.
Prophase I → Longest and most complex phase.
Zygotene → Synapsis.
Pachytene → Crossing over.
Diplotene → Chiasmata visible.
Anaphase I → Homologous chromosomes separate.
Anaphase II → Sister chromatids separate.
Meiosis → Usually four haploid cells.

122. Final NEET Rapid Revision Map

Cell Structure

Cell

→ Plasma membrane

→ Cytoplasm

→ Nucleus

→ Cell organelles

Major Organelles

Ribosome

→ Protein synthesis

RER

→ Protein synthesis

SER

→ Lipid synthesis + Detoxification

Golgi

→ Modification + Sorting + Packaging

Lysosome

→ Intracellular digestion

Vacuole

→ Storage + Turgor

Mitochondria

→ ATP production

Chloroplast

→ Photosynthesis

Peroxisome

→ H₂O₂ metabolism

Cytoskeleton

→ Shape + Movement + Transport

Centriole

→ Spindle/basal body organisation

Nucleus

→ Genetic control

Biomolecules

Carbohydrates

→ Energy + Storage + Structure

Proteins

→ Amino acids → Peptide bonds

Lipids

→ Energy + Membrane + Hormones

Nucleic acids

→ Nucleotides → DNA/RNA

Enzymes

Enzyme

→ Biological catalyst

→ Lowers activation energy

→ Active site

→ Substrate

→ Product

Apoenzyme + Cofactor = Holoenzyme

Cell Cycle

G₁ → S → G₂ → M

Mitosis

PMAT → 2 daughter cells

Meiosis

Meiosis I → Homologous chromosomes separate

Meiosis II → Sister chromatids separate

One DNA replication + Two divisions → Four haploid cells

Cell Structure and Function — Complete NEET Biology Notes

1. Cell: The Basic Unit of Life

1.1 What is a Cell?

Examples

1.2 Cell Size

2. Cell Theory

2.1 Development of Cell Theory

2.2 Modern Cell Theory

NEET High-Yield Fact

3. Types of Cells

4. Prokaryotic Cells

4.1 General Characteristics

Examples

4.2 Structure of a Typical Bacterial Cell

4.3 Glycocalyx

Functions

4.4 Bacterial Cell Wall

4.5 Plasma Membrane

4.6 Nucleoid

Plasmids

Example

4.7 Ribosomes in Prokaryotes

Function

4.8 Inclusion Bodies

5. Eukaryotic Cells

5.1 General Characteristics

Examples

6. Prokaryotic vs Eukaryotic Cells

Important Exception

7. Plant Cell and Animal Cell

7.1 Plant Cell

7.2 Animal Cell

8. Plant Cell vs Animal Cell

9. Cell Envelope

Functions

10. Plasma Membrane

10.1 Structure

10.2 Fluid Mosaic Model

10.3 Phospholipid Bilayer

Important Term

10.4 Membrane Proteins

Integral Proteins

Peripheral Proteins

10.5 Functions of Plasma Membrane

11. Transport Across Plasma Membrane

11.1 Simple Diffusion

Example

11.2 Osmosis

11.3 Facilitated Diffusion

11.4 Active Transport

Example

11.5 Endocytosis

11.6 Exocytosis

Examples

12. Cell Wall

12.1 Plant Cell Wall

Functions

12.2 Layers of Plant Cell Wall

Middle Lamella

Primary Cell Wall

Secondary Cell Wall

12.3 Plasmodesmata

Functions

13. Endomembrane System

Important

14. Endoplasmic Reticulum

14.1 Rough Endoplasmic Reticulum — RER

Functions

Example

14.2 Smooth Endoplasmic Reticulum — SER

Functions

Example

Specialised SER

15. Golgi Apparatus

15.1 Faces of Golgi Apparatus

Cis Face

Trans Face

15.2 Functions of Golgi Apparatus

Example

16. Lysosomes