Cell : The Unit of Life

1. Introduction to the Cell

• The cell is the fundamental structural and functional unit of all living beings.
• It is the smallest part of the body of an organism capable of independent existence and essential life functions.
• Every cell has its own lifespan; old cells continually die and are replaced by new ones (e.g., red blood cells live for about 120 days).

2. Invention of the Microscope

• Antony van Leeuwenhoek: Constructed the first simple microscope using a single biconvex lens, magnifying objects up to 200 times.
• Robert Hooke: Developed the compound microscope (using two lenses). He examined a thin slice of cork and observed tiny "box-like" compartments, coining the term "cells".
• Electron Microscope: A modern invention using beams of electrons bent by magnets, offering a magnification of over 200,000 times.

3. The Cell Theory

• Proposed by Matthias Schleiden (a botanist, 1838) and Theodor Schwann (a zoologist, 1839).
• Rudolf Virchow added a crucial point in 1858, stating that all cells arise from pre-existing cells.
• Three Major Points of the Cell Theory:
  • The cell is the smallest structural unit of all living things.
  • The cell is the functional unit of all living things.
  • All cells arise from pre-existing cells.

4. Cell Quantity, Size, and Efficiency

• Quantity: Organisms can be single-celled (e.g., Amoeba, Bacteria), few-celled (e.g., Spirogyra), or multi-celled (e.g., humans, trees).
• Size Extremes:
  • Smallest: Bacteria and human red blood cells.
  • Longest: Nerve cells.
  • Largest: Birds' eggs (an ostrich egg is the largest single cell).
• Efficiency of Small Size: Cells remain small to maintain a high surface area-to-volume ratio (for greater diffusion of nutrients, gases, and wastes) and to allow rapid communication across different regions of the cell.

5. Cell Shapes and Functional Requirements

• Human Red Blood Cells: Circular and biconcave to pass easily through narrow capillaries and transport oxygen.
• White Blood Cells: Amoeboid shape allows them to squeeze out through capillary walls.
• Nerve Cells: Long shape helps conduct impulses from distant body parts to the brain.
• Muscle Cells: Long and contractile to pull or squeeze structural parts.
• Guard Cells: Bean-shaped to properly open and close stomatal pores in plant leaves.

6. Basic Structure of a Cell

• A generalized cell consists of three essential parts: Cell Membrane, Cytoplasm, and Nucleus.
• Cell Membrane (Plasma Membrane): Outer living, semi-permeable boundary made of lipoproteins. Controls entry and exit of specific substances.
• Cell Wall (Plants Only): Non-living layer surrounding the cell membrane. Made of cellulose, gives shape and rigidity, and is freely permeable.
• Cytoplasm: A semi-liquid, partially transparent substance where many chemical reactions occur. Contains both living organelles and non-living inclusions.

7. Cell Organelles (Living Parts)

• Endoplasmic Reticulum (ER): An irregular network of double membranes acting as a supportive framework and pathway for material distribution. Can be rough (with ribosomes) or smooth.
• Ribosomes: Minute granules scattered in cytoplasm or attached to ER; they are the sites of protein synthesis.
• Mitochondria: The "powerhouses" of the cell. Double-walled structures with finger-like inner projections called cristae. They release energy in the form of ATP during cellular respiration. They possess their own DNA.
• Golgi Apparatus: Delivery system of the cell consisting of flattened sacs (cisternae) and vesicles. Involved in packaging and secreting enzymes and hormones. (Called dictyosomes in plants).
• Lysosomes: Intracellular digestive centers containing enzymes. Rapidly destroy damaged cells, thus termed "suicide bags."
• Centrosome and Centrioles (Animals Only): Located near the nucleus, these initiate and regulate cell division, forming spindle fibers.
• Plastids (Plants Only): Have three main types:
  • Leucoplasts: Colorless; store starch (e.g., in potatoes).
  • Chromoplasts: Variously colored (yellow, orange, red); found in petals and fruits to attract pollinators.
  • Chloroplasts: Green, contain chlorophyll; capture solar energy for photosynthesis. Have their own DNA.

8. Non-living Inclusions

• Granules: Small particles containing stored food such as starch, glycogen, and fats.
• Vacuoles: Clear spaces filled with water and substances in solution (cell sap). Prominent and large in plant cells (covered by a tonoplast layer); small or absent in animal cells.

9. The Nucleus and Protoplasm

• Nucleus: The most important and largest cell organelle. It regulates cell life processes and plays a key role in cell division and heredity.
• Contains delicate nuclear membrane, dense nucleoplasm, nucleolus (produces ribosomes), and chromatin fibres.
• Chromosomes: Formed when chromatin network thickens during cell division. Made of DNA and carry hereditary traits via genes. E.g., Human body cells have 46 chromosomes (23 pairs).
• Protoplasm: The total living substance of a cell, composed of both the Cytoplasm and the Nucleus. It consists of various elements formatted into specific compounds (water, proteins, fats, carbohydrates).

10. Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells: Primitive cells without a well-defined nuclear membrane (nucleoid), featuring a single strand of DNA, small ribosomes, and lacking other membranous organelles. (e.g., Bacteria, Blue-green algae).
• Eukaryotic Cells: Advanced cells with a well-defined nucleus enclosed in a nuclear membrane, chromosomes with DNA around proteins, larger ribosomes, and multiple organelles. (e.g., Euglena, all higher plants and animals).

11. Plant Cells vs. Animal Cells

• Plant Cells: Have a distinct cellulose cell wall, prominent central vacuoles, and usually contain plastids. The cytoplasm is mostly pushed to the periphery, and they lack a centrosome. Generally larger in size.
• Animal Cells: Have no cell wall or plastids. Contain a centrosome (important for division). Vacuoles are small and temporary. Cytoplasm densely fills the entire cell. Generally smaller in size.

12. Cellular Activity as the Basis of Life

• Every activity of a living organism is the outcome of specialized cellular function.
• Examples include:
  • Growth & Repair: Achieved through cell division.
  • Movement: Result of contractility of muscle cells.
  • Protection: White blood cells devour germs and secrete antibodies.
  • Sensory Perception: Sensory cells enable seeing, hearing, tasting, and feeling.
  • Thermoregulation: Cells generate heat, and skin cells cool the body via sweat.
  • Reproduction: Inherited traits are passed down via specific germ cells (egg and sperm).