Fundamental Unit of Life: Cells

Discovery of Cells

• Robert Hooke (1665): Observed cork structure; coined the term "cells."
• Leeuwenhoek (1674): Discovered free-living cells in pond water.
• Robert Brown (1831): Discovered the cell nucleus.
• Purkinje (1839): Coined the term protoplasm.
• Cell Theory: Proposed by Schleiden and Schwann; expanded by Virchow.
• Electron Microscope (1940): Enabled detailed study of cell structure.

Unicellular vs Multicellular Organisms

• Unicellular organisms: Single-celled, e.g., Amoeba.
• Multicellular organisms: Many cells with different functions.
• Cell Division: Cells replicate from pre-existing cells.

Observations with Compound Microscope

• Onion Peel Experiment: Observe cells under a microscope.
• Human Cells: Various cell types in the human body (sperm, bone, muscle, etc.).

Cell Structure

5.2.1 Plasma Membrane

• Definition: Outer cell covering; selectively permeable.
• Function: Regulates entry/exit of substances via diffusion and osmosis.
• Diffusion: Movement from high to low concentration (e.g., CO2, O2).
• Osmosis: Movement of water across the membrane.

5.2.2 Cell Wall

• Found in: Plants, fungi, bacteria.
• Composition: Mainly cellulose.
• Function: Provides structural strength; withstands hypotonic solutions.

5.2.3 Nucleus

• Parts: Nuclear membrane, chromosomes, nucleolus.
• Function: Contains genetic material (DNA); controls cell activities.
• Prokaryotes vs Eukaryotes: No nuclear membrane in prokaryotes.

5.2.4 Cytoplasm

• Definition: Fluid inside the plasma membrane.
• Contains: Organelles like ER, Golgi apparatus, mitochondria.

5.2.5 Cell Organelles

• Endoplasmic Reticulum (ER):
  • Types: Rough (RER) with ribosomes, Smooth (SER).
  • Functions: Protein/lipid synthesis, intracellular transport.
• Golgi Apparatus: Modifies, packages, and distributes cellular products.
• Lysosomes: Enzyme sacs for digestion; "suicide bags."
• Mitochondria: "Powerhouses"; ATP production.
• Plastids: Chloroplasts (photosynthesis), leucoplasts (storage) in plants.
• Vacuoles: Storage, turgidity, and rigidity in plant cells.

Cell Division

• Types: Mitosis (growth and repair), Meiosis (gamete formation).
• Mitosis: Produces identical cells; maintains chromosome number.
• Meiosis: Reduces chromosome number by half; forms gametes.

Summary Points

• Cells are the basic unit of life, with a plasma membrane, nucleus, cytoplasm, and organelles.
• Plant cells have a rigid cell wall; animal cells do not.
• Organelles perform specialized functions essential for cell survival.
• Important Organelles with DNA: Mitochondria and plastids.
• Cells divide for growth, repair, and reproduction.