Topic 3: The Cell

1. Concept of the Cell

A cell is the basic structural and functional unit of all living organisms. Organisms can be:

• Unicellular: Single-celled (e.g., Amoeba, Paramecium, Euglena, bacteria).
• Multicellular: Composed of millions to trillions of specialized cells (e.g., flowering plants, humans).

2. The Light Microscope

The light microscope uses glass lenses and visible light to illuminate and magnify transparent biological specimens.

          [Eyepiece Lens (Ocular)]
                    │
              [Body Tube]
                    │
         [Revolving Nosepiece]
                    │
         [Objective Lenses (x4, x10, x40)]
                    │
      [Stage & Stage Clips (Specimen)]
                    │
          [Iris Diaphragm & Condenser]
                    │
            [Mirror / Light Source]
                    │
                 [Base]

Essential Microscope Parts and Functions

• Eyepiece Lens: Contains magnifying lenses that enlarge the image formed by the objective lens.
• Objective Lenses: Mounted on a rotating nosepiece (Low power $\times 4$, Medium $\times 10$, High power $\times 40$) to magnify specimen details.
• Coarse Adjustment Knob: Rapidly raises or lowers the body tube to achieve approximate focus.
• Fine Adjustment Knob: Slowly moves the body tube to bring the image into sharp, distinct focus.
• Stage & Stage Clips: Flat platform supporting the glass slide and specimen.
• Condenser & Iris Diaphragm: Focuses light onto specimen and regulates the aperture of light passing through.

$$\text{Total Magnification} = \text{Eyepiece Magnification} \times \text{Objective Lens Magnification}$$

3. Cell Structure: Plant vs. Animal Cells

4. Structure and Function of Cell Organelles

1. Nucleus

• Enclosed by a double nuclear membrane with nuclear pores.
• Contains chromatin (DNA and histone proteins) and a nucleolus (synthesizes ribosomes).
• Function: Controls all metabolic activities and carries genetic instructions for inheritance.

2. Mitochondria (Singular: Mitochondrion)

• Sausage-shaped organelle with a smooth outer membrane and folded inner membrane forming cristae.
• Filled with an enzyme-rich matrix.
• Function: Site of aerobic cellular respiration to generate ATP energy. Cells with high metabolic demands (sperm cells, muscle cells, liver cells) contain dense populations of mitochondria.

3. Ribosomes

• Tiny, non-membrane bound spherical granules found freely in cytoplasm or attached to Rough Endoplasmic Reticulum (RER).
• Function: Sites of protein synthesis.

4. Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; transports newly synthesized proteins.
• Smooth ER (SER): Lacks ribosomes; synthesizes and transports lipids and steroid hormones, and detoxifies poisons.

5. Golgi Apparatus (Golgi Bodies)

• Stack of flattened, membrane-bound sacs (cisternae) producing secretory vesicles.
• Function: Packages, chemically modifies, and sorts proteins and lipids for secretion or intracellular delivery.

6. Lysosomes

• Membrane-bound sacs filled with powerful hydrolytic enzymes.
• Function: Destroy worn-out organelles (autophagy) and digest foreign pathogens engulfed by phagocytic cells.

5. Levels of Organization in Multicellular Organisms

[Organelle] ──► [Cell] ──► [Tissue] ──► [Organ] ──► [Organ System] ──► [Organism]

1. Cell: Basic unit (e.g., red blood cell, root hair cell).
2. Tissue: A group of specialized cells of similar structure working together to perform a specific function (e.g., xylem tissue, epithelial tissue).
3. Organ: Different tissues organized to execute a major physiological process (e.g., leaf, heart, kidney, stomach).
4. Organ System: A group of interrelated organs functioning together (e.g., circulatory system, digestive system).
5. Organism: An individual living entity made of integrated organ systems (e.g., human, maize plant).