Introduction to Cells

• Observation by Robert Hooke (1665): Hooke examined cork under a self-designed microscope, noting its resemblance to a honeycomb structure with compartments.
    - Term 'Cell' Origin: Hooke coined the term 'cell' from Latin meaning 'a little room'.
    - Significance: This observation marked the first time living organisms were identified as being composed of individual units or cells.

5.1 What are Living Organisms Made Up of?

• Activity 5.1:
    1. Peel the epidermis from a small piece of an onion bulb.
    2. Place the peel in water on a watch glass to prevent drying out.
    3. Transer a piece of the peel to a slide with a drop of water.
    4. Add a drop of safranin solution and cover with a slip, avoiding air bubbles.
    5. Observe under a compound microscope.

• Observation Questions:
    - Draw the observed structures.
    - Compare findings with Fig. 5.2 showing onion peel cells.

Unicellular vs. Multicellular Organisms

• Unicellular Organisms: Organisms like Chlamydomonas, Paramecium, and bacteria consist of a single cell (uni = single).

• Multicellular Organisms: Organisms composed of many cells (multi = many), like plants and animals, where cells work together for specific functions.

• Cell Division: All multicellular organisms originate from a single cell, which divides to produce similar cells.

• Activity 5.2: Try preparing temporary mounts of onion peels and observe differences in structure among cells from different parts.

Historical Contributions to Cell Theory

• Leeuwenhoek (1674): First observed free-living single cells in pond water.
• Robert Brown (1831): Discovered the nucleus.
• Purkinje (1839): Coined the term ‘protoplasm’.
• Schleiden & Schwann (1838-1839): Proposed the cell theory that all living organisms are composed of cells.
• Virchow (1855): Expanded cell theory to include that all cells arise from pre-existing cells.
• Electron Microscope (1940): Enabled detailed observation of cell structure and organelles.

5.2 What is a Cell Made Up of?

5.2.1 Plasma Membrane or Cell Membrane

• Definition: The plasma membrane is the outermost covering of a cell, separating it from the external environment.

• Functionality:
    - Selectively permeable, regulating the entry and exit of materials.
    - Facilitates diffusion (movement of substances from high to low concentration).

• Diffusion and Osmosis:
    - Diffusion: Example with CO₂ and O₂ exchange; movement happens from areas of higher concentration to lower.
    - Osmosis: Specifically relates to the diffusion of water across a selectively permeable membrane.

5.2.2 Cell Wall

• Structural Overview:
    - Present outside the plasma membrane in plant cells, composed mainly of cellulose.
    - Provides structural support and strength.
    - In plant cells, losing water leads to plasmolysis (cell contents shrink away from the cell wall).

5.2.3 Nucleus

• Composition: Contains a double-layered nuclear membrane with pores allowing material transfer to the cytoplasm.
    - Chromosomes: Located within the nucleus, composed of DNA and proteins, responsible for hereditary information (genes).
• Cell Division: Central role in cellular reproduction and development.
• Prokaryotic Cells: Lack a defined nucleus (referred to as nucleoid) and membrane-bound organelles.
• Eukaryotic Cells: Feature a nuclear membrane defining the nucleus.

5.2.4 Cytoplasm

• Definition: The viscous fluid inside the plasma membrane containing various organelles.
    - Organelles serve distinct functions necessary for maintaining cellular activities.
    - Prokaryotes: Lack membrane-bound organelles.

5.2.5 Cell Organelles

5.2.5 (i) Endoplasmic Reticulum (ER)

• Structure: A network of membrane-bound tubular structures.
    - Types:
      - Rough ER (RER): Studded with ribosomes, involved in protein synthesis.
      - Smooth ER (SER): Involved in lipid synthesis.
• Function: Acts as a transport system within cells for proteins and lipids.

5.2.5 (ii) Golgi Apparatus

• Structure: Stacks of membrane-bound vesicles known as cisterns, involved in modifying, packaging, and storing materials synthesized in the cell.
• Functions: Forms lysosomes and processes proteins and lipids.

5.2.5 (iii) Lysosomes

• Composition: Contain digestive enzymes.
• Function: Acts as the cellular waste disposal system, breaking down foreign materials and old organelles.
• Suicide Bags: Can digest their own cell material during metabolic disturbances.

5.2.5 (iv) Mitochondria

• Functionality: Known as the powerhouse of the cell; involved in energy production through ATP synthesis.
• Structure: Double-membrane with an inner folded membrane to increase surface area for reactions.

5.2.5 (v) Plastids

• Types: Chromoplasts (coloured, contain chlorophyll for photosynthesis) and leucoplasts (colourless, store nutrients).
• Importance: Central to photosynthesis and storage processes in plant cells.

5.2.5 (vi) Vacuoles

• Function: Storage of materials; large in plant cells, small in animal cells.
• Role in Plant Cells: Contributes to cell turgidity and stores vital substances like sugars and minerals.

Cell Division

• Types of Cell Division:
    - Mitosis: Involved in growth and repair; produces two identical daughter cells.
    - Meiosis: Produces gametes for reproduction; results in four cells with half the chromosome count of the parent cell.

Summary of Key Concepts

• Cells are the fundamental unit of life, enclosed by a plasma membrane.
• Plant cells have a rigid cell wall, enabling them to exist in hypotonic environments without bursting.
• The nucleus orchestrates the cell's functions and contains genetic material.
• Organelle functions vary but are essential for overall cellular performance.