Cell: The Unit of Life 🔬 | Class 11 Notes | NEET | NCERT

Introduction to the Cell

• The cell is the basic structural and functional unit of life.
• All living organisms are made of cells (unicellular or multicellular).
• Term “Cell” was first coined by Robert Hooke (1665) after observing cork cells under a microscope.
• Anton van Leeuwenhoek was the first to observe living cells (bacteria, protozoa).
• Purkinje (1839) coined the term protoplasm for the fluid content of the cell.

Cell Theory

Proposed by Schleiden (1838) and Schwann (1839).

• Schleiden: All plants are made of cells.
• Schwann: All animals are made of cells.
• Together → “Cell is the basic unit of structure and function in living organisms.”

Later, Rudolf Virchow (1855) modified it:

• Omnis cellula e cellula → “All cells arise from pre-existing cells.”

Modern Cell Theory states:

1. All living organisms are composed of one or more cells.
2. Cell is the structural and functional unit of life.
3. All cells arise from pre-existing cells.
4. Cells contain genetic material (DNA/RNA).
5. Energy flow (metabolism) occurs within cells.

General Structure of a Cell

Every cell has three major components:

1. Cell membrane (Plasma membrane)

• Outer boundary of the cell.
• Selectively permeable.
• Composed of phospholipid bilayer + proteins (Fluid Mosaic Model – Singer & Nicolson, 1972).

2. Cytoplasm

• Gel-like fluid inside cell.
• Contains cell organelles and inclusions.
• Site of most metabolic reactions.

3. Nucleus (in eukaryotes)

• Double membrane bound.
• Contains DNA in chromosomes.
• Control center of the cell.

Types of Cells

1. Prokaryotic Cells

Found in bacteria, archaea, cyanobacteria.

Features:

• Size: Small (1–10 µm).
• Nucleus absent → DNA is in a region called nucleoid (not membrane bound).
• No membrane-bound organelles.
• Cell wall made of peptidoglycan (in bacteria).
• Ribosomes present (70S type – smaller).
• Reproduce mainly by binary fission.
• Example: E. coli.

Diagram to sketch: Bacterial cell – showing nucleoid, ribosomes, plasma membrane, cell wall, capsule, pili, flagella.

2. Eukaryotic Cells

Found in protists, fungi, plants, and animals.

Features:

• Size: Larger (10–100 µm).
• True nucleus with double membrane.
• Complex internal structure with membrane-bound organelles.
• DNA organized into chromosomes.
• Ribosomes larger (80S type; except in mitochondria and chloroplast – 70S).

Important Cell Organelles

1. Endoplasmic Reticulum (ER)

• Rough ER (with ribosomes) – protein synthesis.
• Smooth ER – lipid and steroid synthesis, detoxification.

2. Golgi Apparatus

• Packaging and modification of proteins.
• Formation of lysosomes.

3. Mitochondria

• “Powerhouse of the cell.”
• Site of cellular respiration (ATP production).
• Double membrane; has its own DNA & ribosomes (semi-autonomous).

4. Lysosomes

• Contain hydrolytic enzymes.
• Involved in intracellular digestion (“suicidal bags”).

5. Plastids (in plants only)

• Chloroplasts (photosynthesis).
• Chromoplasts (pigment storage).
• Leucoplasts (storage of starch, oil, proteins).

6. Vacuoles

• Storage of water, salts, wastes.
• Large in plant cells; small in animal cells.

7. Cytoskeleton

• Microtubules + microfilaments.
• Provides shape and movement.

8. Centrioles (in animal cells only)

• Involved in cell division → spindle formation.

Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent (nucleoid present)	True nucleus with double membrane
Cell Size	Small (1–10 µm)	Large (10–100 µm)
DNA	Circular, naked	Linear, associated with histones
Organelles	No membrane-bound organelles	Membrane-bound organelles present
Ribosomes	70S	80S (cytoplasm), 70S (mitochondria/chloroplast)
Cell Division	Binary fission	Mitosis and meiosis
Cell Wall	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi)
Example	Bacteria, Cyanobacteria	Protists, Fungi, Plants, Animals

Key Points for Exams

• Hooke – “Cell.”
• Schleiden & Schwann – Cell theory.
• Virchow – All cells arise from pre-existing cells.
• Prokaryotes: Simple, nucleoid, 70S ribosomes.
• Eukaryotes: Complex, true nucleus, organelles, 80S ribosomes.
• Mitochondria and chloroplast → semi-autonomous, own DNA.

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