Discovery of Cells: Robert Hooke first observed and named “cells” in 1665 while examining thin slices of cork through a self-designed primitive microscope. He noted the cork’s honeycomb-like structure of many little compartments.

Significance of Hooke’s Observation: This was the first time someone observed that living things appear to consist of separate units, a fundamental discovery in the history of science. The term ‘cell’ is still used in biology today.

Further Discoveries (Chronological Order):

• Leeuwenhoek (1674): With an improved microscope, he discovered free-living cells in pond water for the first time.

• Robert Brown (1831): Discovered the nucleus within the cell.

• Purkinje (1839): Coined the term ‘protoplasm’ for the fluid substance of the cell.

• Schleiden (1838) and Schwann (1839): Proposed the initial Cell Theory, stating that all plants and animals are composed of cells, and the cell is the basic unit of life.

• Virchow (1855): Expanded the cell theory by suggesting that “all cells arise from pre-existing cells” (Omnis cellula e cellula).

• Electron Microscope (1940): Its invention allowed for the observation and understanding of the complex structure of the cell and its various organelles.

The Cell Theory (Modern Summary):

1. All living organisms are composed of cells.

2. The cell is the basic structural and functional unit of life.

3. All cells arise from pre-existing cells.

Unicellular vs. Multicellular Organisms:

• Unicellular Organisms: A single cell constitutes the whole organism (e.g., Amoeba, Chlamydomonas, Paramoecium, bacteria). These organisms perform all basic functions within that single cell.

• Multicellular Organisms: Many cells group together in a single body, assuming different functions to form various body parts (e.g., some fungi, plants, animals). Every multicellular organism originates from a single cell through cell division.

Diversity in Cell Shape and Size:

• Cells exhibit diverse shapes and sizes, which are related to the specific functions they perform.

• Some cells, like Amoeba, have changing shapes.

• Other cells have fixed and peculiar shapes (e.g., nerve cells).

Division of Labour within a Cell:

• Similar to multicellular organisms having different organs for different functions (e.g., heart for pumping blood, stomach for digestion), a single cell also exhibits division of labour.

• Specific components within the cell, known as cell organelles, perform specialized functions (e.g., making new material, clearing waste). These organelles collectively allow the cell to live and perform its functions.

Basic Components of a Cell: Most cells possess three fundamental components:

1. Plasma Membrane (Cell Membrane): The outermost covering of the cell that separates the cell contents from the external environment.

2. Nucleus: Generally located in the center, containing genetic material.

3. Cytoplasm: The jelly-like substance between the plasma membrane and the nucleus, containing cell organelles.

Plasma Membrane (Cell Membrane):

• Function: Regulates the entry and exit of substances, acting as a selectively permeable (or semi-permeable) membrane.

• Composition: Made up of lipids and proteins.

• Flexibility: Its flexible nature allows the cell to engulf food and other substances from its external environment (e.g., endocytosis in Amoeba).

• Transport Mechanisms:

  • Diffusion: Movement of substances (like CO2, O2) from a region of higher concentration to a region of lower concentration. This is important for gaseous exchange between the cell and its external environment.

  • Osmosis: The movement of water molecules through a selectively permeable membrane from a region of higher water concentration (dilute solution) to a higher water concentration to a region of lower water concentration (concentrated solution).

  • Effect of External Solutions on Cells (Osmosis):

    • Isotonic Solution: External solution has the same solute concentration as the cell. No net movement of water; cell maintains its size.

    • Hypotonic Solution: External solution is more dilute (higher water concentration) than the cell sap. Water enters the cell by osmosis, causing the cell to swell and potentially burst (especially in animal cells).

    • Hypertonic Solution: External solution is more concentrated (lower water concentration) than the cell sap. Water leaves the cell by osmosis, causing the cell to shrink (plasmolysis in plant cells).

Cell Wall (Exclusive to Plant Cells, Fungi, and Bacteria):

• Composition: Primarily composed of cellulose in plants, providing structural strength.

• Function: Provides rigidity and protection to plant cells. Allows plant cells to withstand very dilute (hypotonic) external media without bursting by exerting pressure against the swollen protoplast.

• Plasmolysis: Shrinkage of the protoplast away from the cell wall when a living plant cell loses water through osmosis. This phenomenon occurs when a plant cell is placed in a hypertonic solution.