Biology Chapter on Cells and Hooke's Discovery

Robert Hooke's Observation of Cells

• Robert Hooke, in 1665, observed cork through a microscope, noticing compartments resembling a honeycomb.
• He named these compartments "cells," drawing from the Latin word for "a little room."
• This was a crucial moment in science, marking the first observation of living things as comprised of discrete units.

Living Organisms and Cells

• Living things consist of either single cells (unicellular) or multiple cells (multicellular).
• Examples of unicellular organisms include Chlamydomonas, Paramecium, and bacteria.
• Multicellular organisms, such as some fungi, plants, and animals, consist of many cells grouped together to form various body parts.
• All cells originate from pre-existing cells.

Observing Cells in Onion Peel

• To observe cells, a thin peel from an onion is prepared on a glass slide.
• Safranin solution is used to stain the cells for better visibility under a microscope.
• Onion cells are observed under different microscope magnifications to study structure.

Cell Shape and Structure

• Cells vary in shape and size depending on their specific function.
• Cells may change their shape (e.g., Amoeba) or have a fixed shape.
• Cells from various parts of a plant body might be different in structure.
• However, all cells display similarities in basic structural features.

Cell Theory

• The cell theory, proposed by Schleiden and Schwann in 1838-1839, states that all plants and animals are composed of cells, and the cell is the fundamental unit of life.
• Virchow (1855) expanded the theory by stating that all cells arise from pre-existing cells.
• Improved microscopes, including the electron microscope (1940), allowed further understanding of cells.

Cell Components

• Plasma Membrane (Cell Membrane):
  • The outer boundary of the cell, separating its internal contents from the external environment.
  • Selectively permeable, regulating what enters and exits the cell.
  • Allows movement of some substances (e.g., CO2, O2) via diffusion.
  • Movement of water through the membrane is called osmosis.
  • Osmosis depends on relative solute concentrations (hypotonic, isotonic, hypertonic solutions).
  • The membrane is flexible and composed of lipids and proteins.
  • Endocytosis allows cells to engulf material from their environment.
• Cell Wall:
  • Found in plant cells, fungi, and bacteria.
  • Rigid outer layer outside the plasma membrane. Primarily composed of cellulose in plants.
  • Provides structural support and prevents bursting in hypotonic environments.
  • Can undergo plasmolysis (shrinkage) when cells lose water.
• Nucleus:
  • A central, typically spherical or oval structure in eukaryotic cells.
  • Contains genetic material (DNA) organized as chromosomes.
  • The nucleus directs cell activities and plays a role in reproduction.
  • Surrounded by a double membrane (nuclear membrane) with pores.
  • In some organisms (e.g., bacteria), the nuclear region is undefined (nucleoid).
• Cytoplasm:
  • The fluid contents of the cell, enclosed by the plasma membrane.
  • Contains cell organelles, which perform various functions.
  • Membrane-bound organelles are absent in prokaryotic cells.
• Cell Organelles:
  • Endoplasmic Reticulum (ER):
    • Network of membrane tubes and sheets, aids in transport of material.
    • Rough ER (RER) has ribosomes for protein synthesis.
    • Smooth ER (SER) synthesizes lipids and detoxifies substances.
  • Golgi Apparatus:
    • Stacks of flattened membrane sacs. Modifies, packages, and transports cellular products
  • Lysosomes:
    • Contain digestive enzymes to break down waste material and foreign substances.
    • "Suicide bags" – can also digest the cell if damaged.
  • Mitochondria:
    • "Powerhouses" of the cell, with inner folds that generate ATP (energy).
    • Contain their own DNA and ribosomes for protein synthesis.
  • Plastids:
    • Found only in plant cells.
    • Chromoplasts (e.g., chloroplasts) contain pigments for photosynthesis.
    • Leucoplasts store substances like starch.
  • Vacuoles:
    • Storage sacs for liquids or solids.
    • Larger in plant cells (central vacuole).

Prokaryotic vs Eukaryotic Cells

• Prokaryotic cells lack a defined nucleus and membrane-bound organelles (e.g., bacteria).
• Eukaryotic cells have a defined nucleus and membrane-bound organelles (e.g., plants, animals).

Cell Division

• Cell division (mitosis) creates identical daughter cells for growth and repair.
• Cell division (meiosis) produces gametes with half the number of chromosomes.