Cell: The Unit of Life – Complete NEET Notes

 

Introduction

The cell is the basic structural and functional unit of life. Every living organism is made up of one or more cells. The study of cells is known as Cytology. Understanding cell structure and functions is extremely important for NEET Biology, as several questions are asked from this chapter every year.

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Cell Theory

Cell Theory was proposed by:

• Matthias Schleiden (1838) – Studied plant cells.

• Theodor Schwann (1839) – Studied animal cells.

Main Points of Cell Theory

1. All living organisms are composed of cells and cell products.

2. Cells are the basic units of life.

Modification by Rudolf Virchow (1855)

• "Omnis cellula e cellula"

• Meaning: All cells arise from pre-existing cells.

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Types of Organisms Based on Cell Number

Unicellular Organisms

Organisms consisting of a single cell.

Examples:

• Amoeba

• Paramecium

• Bacteria

Multicellular Organisms

Organisms consisting of many cells.

Examples:

• Humans

• Plants

• Animals

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Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
DNA	Circular	Linear
Cell Size	Small (1-10 µm)	Large (10-100 µm)
Examples	Bacteria, Cyanobacteria	Plants, Animals, Fungi

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Cell Shape

Cell shape depends upon function.

Examples:

• RBC – Biconcave

• Neuron – Long and branched

• Muscle cell – Spindle-shaped

• WBC – Amoeboid

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Cell Envelope in Prokaryotes

Glycocalyx

Outer covering of bacteria.

Types:

1. Capsule – Thick and tough

2. Slime layer – Thin and loose

Functions:

• Protection

• Prevents desiccation

• Helps attachment

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Cell Wall

Composition

Made of Peptidoglycan.

Functions:

• Gives shape

• Protection

• Prevents bursting

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Plasma Membrane

Present beneath the cell wall.

Fluid Mosaic Model

Proposed by:

• Singer and Nicolson (1972)

Components:

• Lipids

• Proteins

Functions:

• Selectively permeable

• Transport of materials

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Cell Organelles

Endoplasmic Reticulum (ER)

Network of membranes present in cytoplasm.

Types

Rough Endoplasmic Reticulum (RER)

• Ribosomes present

• Protein synthesis

Smooth Endoplasmic Reticulum (SER)

• Ribosomes absent

• Lipid synthesis

• Detoxification

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Golgi Apparatus

Discovered by Camillo Golgi.

Structure:

• Flattened sacs called cisternae.

Functions:

• Packaging

• Modification

• Secretion

• Formation of lysosomes

Known as:
Post Office of the Cell

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Lysosomes

Discovered by Christian de Duve.

Contain:

• Hydrolytic enzymes

Function:

• Intracellular digestion

Known as:
Suicidal Bags of Cell

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Mitochondria

Discovered by Altmann.

Structure

• Double membrane

• Inner membrane forms cristae

• Matrix contains DNA and ribosomes

Functions:

• Cellular respiration

• ATP production

Known as:
Powerhouse of the Cell

Semi-autonomous Organelle

Because it contains:

• DNA

• Ribosomes

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Plastids

Present only in plant cells.

Types

Chloroplast

Green plastid containing chlorophyll.

Function:

• Photosynthesis

Chromoplast

Colored plastids.

Function:

• Provide color to flowers and fruits.

Leucoplast

Colorless plastids.

Function:

• Storage

Types:

• Amyloplast (starch)

• Elaioplast (oil)

• Aleuroplast (protein)

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Ribosomes

Discovered by George Palade.

Function:

• Protein synthesis

Known as:
Protein Factories of Cell

Types

• 70S (Prokaryotes)

• 80S (Eukaryotes)

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Centrosome and Centrioles

Present mainly in animal cells.

Function:

• Spindle formation during cell division.

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Vacuoles

Membrane called:
Tonoplast

Functions:

• Storage

• Osmotic balance

• Turgidity

Large central vacuole present in plant cells.

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Nucleus

Discovered by Robert Brown.

Components:

1. Nuclear membrane

2. Nucleoplasm

3. Nucleolus

4. Chromatin

Nucleolus Function

• Ribosome formation

Chromatin

DNA-protein complex.

Condenses during cell division to form chromosomes.

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Chromosomes

Discovered by Waldeyer.

Human chromosome number:

• 46 (23 pairs)

Structure:

• Chromatid

• Centromere

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Cytoskeleton

Network of protein fibers.

Types:

1. Microtubules

2. Microfilaments

3. Intermediate filaments

Functions:

• Shape

• Movement

• Mechanical support

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Cell Membrane Transport

Passive Transport

No energy required.

Examples:

• Diffusion

• Osmosis

Osmosis

Movement of water from higher water concentration to lower water concentration through a semipermeable membrane.

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Active Transport

Requires ATP.

Movement occurs against concentration gradient.

Example:

• Sodium-Potassium Pump

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Important NEET One-Liners

• Cell theory: Schleiden and Schwann

• Omnis cellula e cellula: Virchow

• Powerhouse of cell: Mitochondria

• Protein factory: Ribosome

• Suicidal bags: Lysosomes

• Post office of cell: Golgi apparatus

• Membrane model: Singer and Nicolson

• Nucleus discovered by: Robert Brown

• Ribosome discovered by: George Palade

• Golgi apparatus discovered by: Camillo Golgi

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Previous Year NEET Focus Areas

Most frequently asked topics:

1. Cell Theory

2. Fluid Mosaic Model

3. Mitochondria

4. Lysosomes

5. Ribosomes

6. Plastids

7. Cell Membrane Transport

8. Prokaryotic vs Eukaryotic Cells

9. Golgi Apparatus Functions

10. Chromosome Structure

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Quick Revision Table

Organelle	Function
Nucleus	Control center
Mitochondria	ATP production
Ribosome	Protein synthesis
Golgi Apparatus	Packaging and secretion
Lysosome	Digestion
Chloroplast	Photosynthesis
Vacuole	Storage
ER	Synthesis and transport

Conclusion

The cell is the fundamental unit of life and forms the basis of all biological activities. A strong understanding of cell structure, organelles, and their functions is essential for scoring well in NEET Biology. Focus on diagrams, organelle functions, and important scientists, as these areas are frequently tested in examinations.