Cell: The Unit of Life (Class 11, NEET-Oriented Notes)

Definition of Cell

The cell is the fundamental structural and functional unit of life. It is the smallest unit capable of performing all life processes. Cells can exist independently (unicellular organisms) or as part of a multicellular organism.

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Discovery of the Cell

Scientist	Discovery
Robert Hooke (1665)	Observed cork under a microscope and coined the term “cell.”
Anton van Leeuwenhoek (1674)	First to observe living cells (bacteria, protozoa) using a simple microscope.
Robert Brown (1831)	Discovered the nucleus in plant cells.
Purkinje (1839)	Coined the term “protoplasm” for cell fluid.
Schleiden & Schwann (1838-1839)	Proposed the Cell Theory.
Rudolf Virchow (1855)	Stated “Omnis cellula e cellula” (cells arise from pre-existing cells).

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Cell Theory

Proposed by Matthias Schleiden (botanist) and Theodor Schwann (zoologist) and later modified by Rudolf Virchow.

Postulates of Cell Theory:

1. All living organisms are composed of cells.
2. Cells are the basic units of structure and function in all organisms.
3. All cells arise from pre-existing cells.

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Types of Organisms Based on Cell Structure

Organism Type	Example	Characteristics
Unicellular	Amoeba, Bacteria	Single cell performs all life functions.
Multicellular	Humans, Plants	Multiple specialized cells working together.

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Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid present)	Present with nuclear membrane
DNA	Circular, not associated with histones	Linear, associated with histone proteins
Membrane-bound organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Cell Wall	Present (made of peptidoglycan in bacteria)	Present in plants (cellulose) and fungi (chitin), absent in animals
Cell Division	Binary fission	Mitosis and meiosis
Examples	Bacteria, Cyanobacteria	Animals, Plants, Fungi, Protists

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Similarities Between Prokaryotic and Eukaryotic Cells

• Both have cell membranes, cytoplasm, ribosomes, and DNA as genetic material.
• Both carry out essential life processes like metabolism and reproduction.

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Shape, Size, and Function of Cells

• Shape: Varies from spherical (RBCs) to spindle-shaped (muscle cells), elongated (nerve cells), or irregular (Amoeba).
• Size: Ranges from smallest (Mycoplasma, 0.1 µm) to largest (Ostrich egg, 18 cm).
• Function: Specialized for different roles (e.g., nerve cells for conduction, muscle cells for contraction).

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Detailed Explanation of the Eukaryotic Cell

Eukaryotic cells have a well-defined nucleus and membrane-bound organelles. They include plant and animal cells and are more complex than prokaryotic cells.

Feature	Plant Cell	Animal Cell
Cell Wall	Present (cellulose)	Absent
Plastids	Present (chloroplast, chromoplast, leucoplast)	Absent
Vacuole	Large and central	Small and temporary
Centrosome	Absent	Present
Lysosomes	Rare	Present

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Components of Eukaryotic Cells

1. Plasma Membrane
2. Cell Wall (Plants)
3. Cytoplasm
4. Nucleus
5. Ribosomes
6. Mitochondria
7. Plastids (Plants)
8. Endoplasmic Reticulum (ER)
9. Golgi Apparatus
10. Lysosomes
11. Vacuoles
12. Microbodies
13. Centrosome & Centrioles (Animals)

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Detailed Explanation of Cell Components

1. Plasma Membrane

• Structure: Phospholipid bilayer with embedded proteins.
• Functions:
• Selectively permeable (controls entry/exit of molecules).
• Involved in cell communication and signaling.

Lipid Movement in Plasma Membrane

• Lateral movement (frequent, within the same layer).
• Flip-flop movement (rare, phospholipid moves across layers).

Transport Across Membrane

Type	Description
Passive Transport	No energy required (Diffusion, Osmosis, Facilitated diffusion).
Active Transport	Requires ATP (Sodium-Potassium Pump).

Osmosis

• Hypotonic solution → Water enters, cell swells.
• Hypertonic solution → Water leaves, cell shrinks.
• Isotonic solution → No net movement of water.

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2. Cell Wall (Only in Plants)

• Composition: Cellulose (plants), Chitin (fungi), Peptidoglycan (bacteria).
• Functions:
• Provides rigidity and protection.
• Prevents excessive water intake.

Different Layers of Cell Wall

1. Primary Wall – Thin, flexible.
2. Secondary Wall – Thick, rigid.
3. Middle Lamella – Holds adjacent cells together.

Plasmodesmata

• Cytoplasmic connections between plant cells for material exchange.

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3. Ribosomes

• Structure: RNA + proteins, 70S (prokaryotic), 80S (eukaryotic).
• Function: Protein synthesis.

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4. Pits

• Thin areas in plant cell walls for material transport.

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5. Mitochondria

• Powerhouse of the cell (ATP production).
• Double-membraned, has its own DNA & ribosomes.
• Inner membrane forms cristae (site of ATP production).

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6. Plastids (Only in Plants)

• Chloroplasts (Photosynthesis).
• Chromoplasts (Color pigments).
• Leucoplasts (Storage of starch, proteins, fats).

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7. Endomembrane System

Includes Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, Vacuoles (functionally interconnected).

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8. Endoplasmic Reticulum (ER)

Type	Function
Smooth ER	Lipid synthesis, detoxification.
Rough ER	Protein synthesis (has ribosomes).

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9. Golgi Apparatus

• Modifies, sorts, and packages proteins.
• Forms lysosomes.

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10. Lysosomes

• Digestive system of the cell (contains hydrolytic enzymes).
• Types:
• Primary Lysosome – Inactive enzymes.
• Secondary Lysosome – Active digestion.

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11. Vacuoles

• Storage of water, ions, nutrients (large in plants).

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12. Microbodies

• Includes Peroxisomes (detoxification) and Glyoxysomes (fat metabolism in plants).

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13. Centrosome (Animal Cells)

• Contains centrioles (involved in cell division).

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These notes cover all key points from NCERT in a structured, detailed, and exam-oriented manner for NEET preparation.