Cell - The Unit of Life

The cell is the basic structural and functional unit of life. All living organisms are composed of cells, which can be unicellular or multicellular. This chapter focuses on the discovery of cells, their types, structure, and functions, which form the foundation of biological understanding.

Cell Theory and Discovery

1. Discovery of Cells

• The first observation of cells was made by Robert Hooke (1665) in cork tissue.

• Anton van Leeuwenhoek later observed living cells under a microscope.

2. Cell Theory

• Proposed by Schleiden and Schwann (1838-39) and later modified by Rudolf Virchow (1855), who added that "Omnis cellula e cellula" (all cells arise from pre-existing cells).

• The three main principles of cell theory:

  1. All living organisms are composed of one or more cells.

  2. The cell is the basic unit of structure and function in all organisms.

  3. All cells arise from pre-existing cells.

Types of Cells

Cells are classified based on their structural complexity.

1. Prokaryotic Cells

• Found in bacteria and archaea.

• Lack a well-defined nucleus; DNA is present in the nucleoid.

• Membrane-bound organelles are absent.

• Have a cell wall, plasma membrane, ribosomes, and flagella.

• Reproduce mainly by binary fission.

2. Eukaryotic Cells

• Found in plants, animals, fungi, and protists.

• Possess a true nucleus enclosed by a nuclear membrane.

• Contain membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi bodies.

• Show compartmentalization of cellular functions.

• Reproduce by mitosis or meiosis.

Cell Structure and Organelles

Eukaryotic cells contain various organelles that perform specific functions.

1. Plasma Membrane (Cell Membrane)

• Semi-permeable membrane that controls the movement of substances.

• Composed of a phospholipid bilayer with embedded proteins (Fluid Mosaic Model by Singer and Nicolson).

2. Cell Wall (Only in Plants, Fungi, and Bacteria)

• Provides rigidity and protection.

• Composed of cellulose in plants, chitin in fungi, and peptidoglycan in bacteria.

3. Nucleus

• Control center of the cell containing genetic material (DNA).

• Enclosed by a double membrane (nuclear envelope) with pores for material exchange.

• Contains chromatin (DNA + proteins) and nucleolus (site of ribosome formation).

4. Cytoplasm

• Semi-fluid matrix that contains organelles.

• Site of metabolic activities.

5. Endoplasmic Reticulum (ER)

• Rough ER (RER): Has ribosomes; involved in protein synthesis.

• Smooth ER (SER): No ribosomes; involved in lipid synthesis and detoxification.

6. Ribosomes

• Smallest organelles, not membrane-bound.

• Composed of RNA and protein.

• Sites of protein synthesis.

7. Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids.

• Forms lysosomes and secretory vesicles.

8. Lysosomes

• Contain digestive enzymes for intracellular digestion.

• Play a role in autophagy (self-digestion of worn-out cell parts).

9. Mitochondria

• Powerhouse of the cell, generates ATP through cellular respiration.

• Contains its own DNA and ribosomes, capable of self-replication.

10. Plastids (Only in Plants)

• Chloroplasts: Perform photosynthesis; contain chlorophyll.

• Chromoplasts: Store pigments.

• Leucoplasts: Store starch, oil, or protein.

11. Vacuoles

• Fluid-filled sacs for storage and maintaining cell turgor pressure.

• Large in plant cells; small in animal cells.

12. Cytoskeleton

• Network of microfilaments, microtubules, and intermediate filaments.

• Provides shape, support, and intracellular transport.

Cell Division

1. Mitosis

• Produces two identical daughter cells.

• Occurs in somatic cells.

• Important for growth and repair.

2. Meiosis

• Produces four genetically unique daughter cells.

• Occurs in gametes (sperm and egg cells).

• Leads to genetic variation.

This chapter is fundamental to understanding the structure and function of living organisms and serves as the basis for advanced topics in biology.