Biology Notes on Cells and Cell Theory

Overview of Cells

  • Cells are the fundamental unit of life, responsible for all biological processes in organisms.

Syllabus Highlights

  • Understanding prokaryotic vs eukaryotic cells.
  • Differences between animal and plant cells.
  • Cell theory and structure of cell organelles.

2.1 What is a Cell?

  • Definition: A cell is the smallest unit capable of independent existence.
  • Life Span: Cells have life cycles; old cells die and new cells are formed, e.g., red blood cells last about 120 days.
  • Microscopic Nature: Cells cannot be seen without a microscope.

2.2 Microscope and Cell Discovery

  • Antony van Leeuwenhoek: First to construct simple microscopes with up to 200x magnification.
  • Robert Hooke: Developed compound microscopes and coined the term "cells" from observing cork.
  • Electron Microscopes: Provide magnification over 200,000x using electron beams.

2.3 Cell Theory

  • Formulated by Schleiden and Schwann:
    1. Cells are the basic unit of structure in living things.
    2. Cells are the functional units of life.
    3. All cells arise from pre-existing cells.
  • Examples: Movement in frogs is due to muscle cells; photosynthesis in mango trees occurs in leaf cells.

2.4 Cell Count in Organisms

  • Single-Celled Organisms: Bacteria, yeast, amoeba.
  • Few-Celled Organisms: Spirogyra, Volvox.
  • Multi-Celled Organisms: Humans (~37.2 trillion cells), mango trees (millions of cells).

2.5 Size and Shape of Cells

  • Smallest Cells: Bacteria (0.3–5.0 micrometer). Red blood cells (7 micrometer).
  • Longest Cells: Nerve cells.
  • Largest Cells: Ostrich eggs are the largest single cells.
  • Cells maintain small size for efficiency in diffusion and communication.

2.6 Cell Shapes and Functions

  • Shape Variation: Cells can be disc-like, cuboid, thread-like, etc., impacting their functions.
  • Examples:
    • Red blood cells: Biconcave for easy movement.
    • White blood cells: Amoeboid shape allows them to engulf pathogens.
    • Nerve cells: Long shape to conduct impulses.

2.7 Organelles Overview

  • Cell Membrane: Protects and regulates substance entry; selectively permeable.
  • Cell Wall (in Plants): Made of cellulose, provides structure and rigidity.
  • Cytoplasm: Semi-liquid medium where organelles reside; site of chemical reactions.
Major Organelles
  1. Endoplasmic Reticulum (ER)
    • Rough (with ribosomes) and Smooth (no ribosomes)
    • Functions: Synthesis and transport of proteins and lipids.
  2. Ribosomes
    • Sites of protein synthesis, can be free-floating or attached to ER.
  3. Mitochondria
    • Powerhouses of the cell; produce ATP through cellular respiration.
  4. Golgi Apparatus
    • Packages and secretes enzymes and hormones.
  5. Lysosomes
    • Digestive centers of the cell; contain enzymes to breakdown waste.
  6. Centrosome and Centrioles
    • Important for cell division in animal cells.
  7. Plastids (in Plants)
    • Chloroplasts (photosynthesis), Chromoplasts (color), Leucoplasts (storage).
  8. Nucleus
    • Control center of the cell; contains genetic material, regulates cellular activity.

2.8 Protoplasm

  • The living substance in a cell composed of cytoplasm and nucleus.

2.9 Distinction Between Plant and Animal Cells

  • Plant Cells: Cell wall, plastids (chloroplasts, etc.), large vacuoles.
  • Animal Cells: No cell wall, smaller vacuoles, centrosome present.

2.10 Prokaryotic vs Eukaryotic Cells

  • Prokaryotic: Simple cells without a defined nucleus, e.g., bacteria.
  • Eukaryotic: More complex cells with a well-defined nucleus, e.g., plants and animals.

2.11 Cellular Activity and Life Processes

  • All activities (movement, growth, reproduction, etc.) are driven by cellular functions. For example:
    • Growth: Due to increase in cell size and number.
    • Repair: Involves cell division.
    • Movement: Caused by muscle cell contractility.
    • Respiration: Variables are transported via blood cells.

Additional Notes

  • DNA Fingerprinting: Individual identification based on genetic variation in DNA.
  • Stem Cells: Undifferentiated cells with potential to become various cell types; crucial in medical treatments.