The cell is the fundamental structural and functional unit of life. All living organisms—from unicellular bacteria to complex multicellular humans—are made up of cells. Every function essential to life—growth, metabolism, reproduction, response to stimuli—begins at the cellular level.

Cell Structure and Function - NEET 2026 Complete Notes

1. Cell Theory and the Cell as the Basic Unit of Life

Historical Development:

• Robert Hooke (1665): Discovered dead cork cells, coined term “cell”
• Leeuwenhoek: Observed living cells in pond water
• Schleiden (Botanist) & Schwann (Zoologist): Proposed Cell Theory
• Rudolf Virchow (1855): “Omnis cellula e cellula” → all cells arise from pre-existing cells

Modern Cell Theory:

1. All living organisms are composed of cells
2. Cell is the basic structural and functional unit of life
3. All cells arise from pre-existing cells
4. Cells carry genetic material (DNA) passed during division
5. All metabolic reactions occur within the cell

2. Types of Cells: Prokaryotic vs Eukaryotic

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent (nucleoid)	True nucleus with envelope
Size	Small (1–10 µm)	Larger (10–100 µm)
Organelles	No membrane-bound organelles	Present
Cell Wall	Peptidoglycan	Cellulose (plants), chitin (fungi)
DNA	Circular, naked	Linear, histone-bound
Examples	Bacteria, Cyanobacteria	Plants, animals, fungi, protists

🧠 NEET Tip: Cyanobacteria have chlorophyll a and perform oxygenic photosynthesis

3. Plant Cell vs Animal Cell

Component	Plant Cell	Animal Cell
Cell wall	Present (cellulose)	Absent
Plastids	Present	Absent
Vacuole	Large central	Small/absent
Centrioles	Absent	Present
Lysosomes	Rare	Common
Shape	Fixed (rectangular)	Irregular (rounded)

4. Cell Envelope, Cell Wall, Cell Membrane

Cell Envelope (in Prokaryotes):

3-layered structure:

1. Glycocalyx (capsule/slime layer)
2. Cell Wall – Peptidoglycan
3. Plasma Membrane

In Gram-negative bacteria: Outer membrane + thin wall

In Gram-positive: Thick wall, no outer membrane

Cell Membrane (Plasma Membrane):

• Structure: Fluid Mosaic Model (Singer & Nicolson, 1972)
• Composed of phospholipid bilayer + proteins (integral/peripheral) + cholesterol

Functions:

• Selective permeability
• Transport (active, passive)
• Signal transduction
• Endocytosis (phagocytosis/pinocytosis)
• Cell recognition

Cell Wall (Plant Cells, Fungi, Bacteria):

• Plants: Made of cellulose, hemicellulose, pectin, lignin
• Layers: Middle lamella → Primary wall → Secondary wall
• Plasmodesmata: Cytoplasmic channels between plant cells

In fungi: Chitin

In bacteria: Peptidoglycan

5. Cell Organelles: Structure & Function

A. Endomembrane System

(Functional interconnection – ER, Golgi, lysosome, vacuoles)

1. Endoplasmic Reticulum (ER)

Type	Structure	Function
RER	Ribosomes attached	Protein synthesis & transport
SER	Smooth surface	Lipid synthesis, detoxification, steroid hormones, Ca²⁺ storage

Found abundantly in liver cells (SER) and pancreas (RER)

2. Golgi Apparatus

• Series of cisternae (flattened stacked membranes)
• Cis face: Receiving; Trans face: Maturing

Functions:

• Protein modification & packaging
• Forms lysosomes
• Glycosylation & secretion

3. Lysosomes

• Single membrane-bound organelles
• Contain acid hydrolases (optimal pH ~5.0)
• Formed from Golgi
• Function: Intracellular digestion, autolysis

Abundant in phagocytic WBCs (macrophages)

4. Vacuoles

• Plant cell vacuole: Large, central, filled with cell sap (water, ions, sugars)
• Tonoplast: Selectively permeable membrane
• Functions: Turgor pressure, storage, waste disposal

In protists: Contractile vacuoles (osmoregulation)

6. Energy Organelles

1. Mitochondria – “Powerhouse of the Cell”

• Double membrane-bound
• Inner membrane → Cristae
• Matrix contains circular DNA, ribosomes, enzymes
• Site of aerobic respiration (Krebs cycle, ETC)
• ATP produced via oxidative phosphorylation

Self-replicating → Has own DNA → Semi-autonomous

2. Plastids (Only in Plants)

Type	Pigment	Function
Chloroplast	Chlorophyll a & b	Photosynthesis
Chromoplast	Carotenoids (orange/yellow)	Attract pollinators
Leucoplast	None	Storage (amyloplast – starch, elaioplast – fat)

Chloroplast structure:

• Double membrane
• Stroma + Grana (thylakoid stacks)
• Contains circular DNA, ribosomes

3. Ribosomes – Protein Factories

Type	Found In	Size
70S	Prokaryotes, mitochondria, chloroplast	50S + 30S
80S	Cytoplasm of eukaryotes	60S + 40S

• Not membrane-bound
• Composed of rRNA + proteins
• Site of translation

4. Microbodies

• Single membrane
• Contain enzymes
• Peroxisomes: β-oxidation of fats, catalase (breaks H₂O₂)
• Glyoxysomes (in germinating seeds): Fat → sugars

7. Cytoskeleton, Cilia, Flagella, Centrioles

A. Cytoskeleton

• Network of microtubules + microfilaments
• Maintains cell shape, motility, intracellular transport

B. Cilia and Flagella

• Hair-like projections (9+2 arrangement of microtubules)

Feature	Cilia	Flagella
Number	Numerous	Few
Length	Short	Long
Function	Movement of fluid/substance	Locomotion

Basal body: 9+0 arrangement → forms base of cilia/flagella

C. Centrioles

• Present only in animal cells
• 9 triplet microtubules (9×3 = 27)
• Involved in spindle formation during mitosis
• Form basal bodies of cilia/flagella

8. Nucleus – The Control Center of the Cell

The nucleus is a large, membrane-bound organelle found in all eukaryotic cells. It houses the cell’s genetic material (DNA) and serves as the command center that regulates all cellular activities—from gene expression to cell division. Prokaryotes lack a true nucleus. Instead, their genetic material is located in a region called the nucleoid (no nuclear envelope).

Detailed Structure of the Nucleus

Component	Structure	Function
Nuclear Envelope	A double membrane structure with outer and inner membranes; the outer membrane is continuous with Rough ER	Separates nucleoplasm from cytoplasm; nuclear pores allow regulated exchange of materials (like RNA, ribosomal subunits)
Nuclear Pores	Large protein complexes (∼100 nm diameter) embedded in the envelope	Facilitate bidirectional transport of molecules (mRNA, proteins, ATP)
Nucleoplasm	Also called karyolymph – semi-fluid matrix containing ions, enzymes, nucleotides	Suspends chromatin and nucleolus; site of many nuclear enzymatic reactions
Chromatin	A complex of DNA + histone proteins arranged as long fibers; highly dynamic	Carries genetic blueprint; condenses into chromosomes during mitosis
Nucleolus	Dense, non-membranous structure rich in rRNA and proteins	Site of rRNA synthesis and ribosome subunit assembly

Chromatin Types – Structural & Functional Differences

Type	Appearance	Function
Euchromatin	Lightly stained in EM	Transcriptionally active (loosely packed)
Heterochromatin	Darkly stained, dense	Transcriptionally inactive (tightly packed)

Ruled by need: DNA is dynamically shifted between euchromatin (for gene expression) and heterochromatin (for protection/storage).

Functions of the Nucleus

• Genetic Information Storage: Houses the entire genome in the form of DNA; controls hereditary transmission from one generation to another.
• Transcription: Carries out DNA to mRNA synthesis via RNA polymerase inside the nucleus.
• Ribosome Biogenesis: Nucleolus assembles ribosomal RNA with proteins to form ribosome subunits, which are exported through nuclear pores.
• Regulation of Cell Cycle: Nucleus governs entry into mitosis/meiosis through complex checkpoint signaling (e.g., cyclins, CDKs).
• DNA Replication: Prior to cell division, the DNA duplicates inside the nucleus to ensure equal genetic inheritance.

Key NEET Highlights & MCQ Revision Pointers

Ribosome Types:

• 70S ribosomes = Found in prokaryotes, and inside mitochondria & chloroplasts (supporting endosymbiotic theory)
• 80S ribosomes = Found in eukaryotic cytoplasm and RER

Semi-Autonomous Organelles:

• Mitochondria and Chloroplasts contain:
• Their own DNA (circular)
• 70S ribosomes
• Ability to replicate independently

🧬 NEET MCQ: These organelles originated from free-living prokaryotes (endosymbiosis theory)

Golgi Apparatus vs ER:

• Golgi Body: Modifies, packages, and ships proteins (especially glycoproteins, lipids)
• Rough ER (RER): Synthesizes proteins with attached 80S ribosomes
• Smooth ER (SER): Lipid synthesis, detoxification (esp. in liver)

Lysosomes:

• Contain acid hydrolases
• Function in autophagy, breakdown of worn-out organelles
• Called "suicide bags" of the cell

Peroxisomes:

• Enzyme: Catalase
• Function: Break down hydrogen peroxide (H₂O₂) into water and oxygen
• Also involved in fatty acid oxidation

Plant vs Animal Cell Differentiators:

Plant Cells	Animal Cells
Plastids present	Absent
Cell wall (cellulose)	Absent
Large central vacuole	Small/temporary vacuoles
No centrioles	Centrioles present

Mitochondrial Cristae:

• Inner folds of mitochondria
• Increase surface area for electron transport chain (ETC)
• Site of ATP synthesis via oxidative phosphorylation

Potential Basic Level NEET MCQs from This Section:

Q. Which of the following organelles has 70S ribosomes and its own DNA?

A. Golgi body

B. Mitochondria ✅

C. Lysosome

D. Endoplasmic reticulum

Q. Which nuclear structure is involved in ribosome formation?

A. Chromatin

B. Nucleoplasm

C. Nucleolus ✅

D. Centriole

Q. In electron microscopy, euchromatin appears:

A. Dark and condensed

B. Light and less condensed ✅

C. Dark and uncoiled

D. Absent in nucleus

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