The Fundamental Unit of Life

Robert Hooke and Discovery of Cells

  • In 1665, Robert Hooke examined a thin slice of cork through a self-designed microscope.

  • Observations:

    • Cork resembled a honeycomb structure consisting of numerous compartments.

    • Hooke referred to these compartments as 'cells', derived from the Latin word for 'a little room'.

  • Significance:

    • Marked the first observation of living things as composed of distinct units (cells).

    • The term 'cell' is still utilized in biology today.

Understanding Cells

5.1 What are Living Organisms Made Up of?

Activity 5.1: Observing Onion Cells

  • Materials Needed:

    • Onion bulb

    • Forceps

    • Watch-glass containing water

    • Glass slide

    • Drop of water

    • Safranin solution

    • Cover slip

    • Mounting needle

  • Procedure:

    • Peel the skin (epidermis) from the concave side of an onion bulb.

    • Place the peel in water to avoid folding or drying out.

    • Transfer the peel onto a glass slide with a drop of water, ensuring it lays flat.

    • Add a drop of safranin solution and gently place a cover slip, avoiding air bubbles.

  • Observation:

    • Examine the temporary mount under low and high power of a compound microscope.

    • Compare findings to Figure 5.2.

Types of Organisms

Unicellular vs. Multicellular Organisms

  • Unicellular Organisms:

    • Examples include Chlamydomonas, Paramecium, and bacteria (uni = single).

  • Multicellular Organisms:

    • Composed of multiple cells forming various body parts found in fungi, plants, and animals (multi = many).

  • Development:

    • All multicellular organisms originate from a single cell through cell division; all cells arise from pre-existing cells.

Activity 5.2: Understanding Cell Structure

  • Tasks:

    • Prepare temporary mounts of leaf peels and various onion sizes.

    • Answer the following questions:

    • (a) Do all cells have a similar shape or size?

    • (b) Is the structural similarity present?

    • (c) Are there differences among cells from different plant body parts?

    • (d) Identify similarities.

  • Observations indicate that onion bulb cells maintain structural similarities regardless of onion size.

Cell Organism Discoveries

  • Key Historical Figures:

    • Robert Hooke (1665): Cork cells.

    • Leeuwenhoek (1674): First to see free-living cells in pond water.

    • Robert Brown (1831): Discovery of the nucleus.

    • Purkinje (1839): Coined 'protoplasm'.

    • Schleiden and Schwann (1838, 1839): Formulated the cell theory that all organisms consist of cells.

    • Virchow (1855): Proposed that all cells arise from pre-existing cells.

    • Electron microscope (1940): Allowed detailed observation of cell structures.

Cell Structure and Function

5.2 What is a Cell Made Up of?

  • Each cell contains:

    • Plasma membrane

    • Nucleus

    • Cytoplasm

5.2.1 Plasma Membrane

  • Definition: Outermost covering separating cell contents from the external environment.

  • Characteristics:

    • Selectively permeable: Regulates material movement into and out of the cell.

    • Movement of Substances:

    • Diffusion: Movement of gases like O2 and CO2.

      • Example: CO2 moves out when high concentration inside the cell.

      • O2 enters the cell when levels are low.

    • Osmosis: Movement of water through the selectively permeable membrane.

      • Distinction between solutions:

      1. Hypotonic: Medium has higher water concentration than the cell. Cell swells as water enters.

      2. Isotonic: Same water concentration, no net water movement.

      3. Hypertonic: Medium has lower water concentration than the cell. Cell shrinks as water exits.

Activity 5.3: Osmosis Experiment

  • Task:

    1. De-shell an egg using dilute hydrochloric acid.

    2. Place in pure water - observe swelling (osmosis).

    3. Place in concentrated salt solution - observe shrinkage (water loss).

    4. Use dried fruits to demonstrate osmotic effects while soaked in water and salt solutions.

5.2.2 Cell Wall

  • Function: Rigid structure outside the plasma membrane, present in plant cells.

  • Composition: Mainly cellulose, providing structural strength.

  • Plasmolysis: Occurs when living cells lose water and contents shrink from the cell wall.

Activity 5.6: Observing Plasmolysis

  • Mount Rhoeo leaf peel; add a strong sugar/salt solution to observe plasmolysis under microscope.

5.2.3 Nucleus

  • Structure: Double-layered nuclear membrane with pores.

  • Contains chromosomes (DNA) crucial for hereditary information.

  • Chromatin: DNA in non-dividing cells appears as an entangled mass.

  • Nucleoid: Defined nuclear region in prokaryotes (no membrane).

  • Eukaryotes: Defined nucleus with membrane-bound organelles.

5.2.4 Cytoplasm

  • Definition: Fluid content inside the plasma membrane containing organelles.

  • Prokaryotic cells lack membrane-bound organelles, while eukaryotic cells possess them.

Cell Organelles

5.2.5 Overview of Key Organelles

  1. Endoplasmic Reticulum (ER):

    • Network of membrane-bound tubes and sheets (Rough ER and Smooth ER).

    • Functions: Protein synthesis (RER), lipid production (SER).

    • Role in intracellular transport.

  2. Golgi Apparatus:

    • Stacked membrane-bound vesicles.

    • Functions: Storage, modification, packaging of cell products.

    • Involved in lysosome formation.

  3. Lysosomes:

    • Sacs filled with digestive enzymes (produced by RER).

    • Functions: Digestion of foreign bodies and waste material.

    • Known as ‘suicide bags’ due to potential to digest own cell when damaged.

  4. Mitochondria:

    • Known as powerhouses of the cell due to ATP production.

    • Double membrane structure; outer membrane is porous, inner is folded.

    • Contain their own DNA and ribosomes.

  5. Plastids:

    • Present only in plant cells, includes chloroplasts (photosynthesis) and leucoplasts (storage).

    • Chloroplasts contain pigments and are crucial for photosynthesis.

  6. Vacuoles:

    • Storage sacs; significant in plant cells for maintaining turgidity.

    • Functions include storage of nutrients, waste, and maintaining cellular rigidity.

Cell Division

Processes

  1. Mitosis: Cell division for growth/repair (produces two identical daughter cells).

  2. Meiosis: Cell division for gamete formation (produces four non-identical cells with half the chromosomes).

    • Importance of chromosome reduction for reproduction.