Chapter 3.1: Cell - The Unit of Life

1. Historical Aspects & Cell Theory

The study of cells, or cytology, began with the invention of the microscope.

• Robert Hooke (1665): First observed cells in a piece of cork and coined the term "cell".
• Anton van Leeuwenhoek (1674): First observed and described live cells (bacteria, protozoa).
• Robert Brown (1831): Discovered the nucleus.
• Matthias Schleiden (1838) & Theodor Schwann (1839): Proposed the Cell Theory. Schleiden, a botanist, concluded that all plants are composed of cells. Schwann, a zoologist, concluded that all animals are composed of cells.
• Rudolf Virchow (1855): Modified the cell theory with his famous statement, "Omnis cellula-e cellula" (all cells arise from pre-existing cells).

Modern Cell Theory Postulates:

1. All living organisms are composed of cells and products of cells.
2. All cells arise from pre-existing cells.
3. The cell is the structural and functional unit of all living organisms.
4. Cells contain hereditary information (DNA) which is passed from cell to cell during division.
5. All cells are basically the same in chemical composition and metabolic activities.

Cell Size and Shape

• Cells vary greatly in size and shape.
• Size:
  • Smallest cells: Mycoplasma (0.3 µm in length).
  • Largest isolated single cell: Egg of an ostrich.
  • Human red blood cells are about 7.0 µm in diameter.
• Shape:
  • The shape of the cell may vary with the function they perform. They can be disc-like, polygonal, columnar, cuboid, thread-like, or even irregular.

2. General Structure of a Eukaryotic Cell

Eukaryotic cells are characterized by the presence of a true nucleus and other membrane-bound organelles. They include all protists, fungi, plants, and animals.

Differences between Plant and Animal Cells

Differences between Prokaryotic and Eukaryotic Cells

3. Cell Envelope

The cell envelope consists of the cell wall and the cell membrane.

Cell Wall

• A rigid, non-living outer covering found in plant cells, fungi, and some protists.
• Functions:
  • Provides definite shape to the cell.
  • Protects the cell from mechanical damage and infection.
  • Helps in cell-to-cell interaction.
  • Acts as a barrier to undesirable macromolecules.
• Ultrastructure:
  • Primary Wall: The first formed wall, capable of growth.
  • Secondary Wall: Formed on the inner side of the primary wall as the cell matures.
  • Middle Lamella: A layer mainly of calcium pectate which holds or glues the different neighbouring cells together.
• Plasmodesmata: Cytoplasmic bridges that connect the cytoplasm of neighboring plant cells, allowing for communication and transport between them.

Plasma Membrane (Cell Membrane)

• Structure: Described by the Fluid Mosaic Model (proposed by Singer and Nicolson in 1972).
  • It is a quasi-fluid lipid bilayer (mainly phosphoglycerides) with proteins embedded in it.
  • Lipids: Have a polar head (hydrophilic) and a nonpolar tail (hydrophobic).
  • Proteins:
    • Integral proteins: Partially or totally buried in the membrane.
    • Peripheral proteins: Lie on the surface of the membrane.
• Functions:
  • Selective Permeability: Regulates the movement of molecules into and out of the cell.
  • Transport:
    • Passive Transport: No energy required. Movement is along the concentration gradient.
      • Simple Diffusion: Movement of neutral solutes across the membrane.
      • Osmosis: Movement of water.
      • Facilitated Diffusion: Movement of substances with the help of carrier proteins. This can be:
        • Uniport: Moves one molecule across the membrane. (e.g., GLUT1 transporting glucose).
        • Symport: Moves two different molecules in the same direction. (e.g., Na+/glucose cotransport).
        • Antiport: Moves two different molecules in opposite directions. (e.g., Na+/Ca2+ exchanger).
    • Active Transport: Requires energy (ATP) to move molecules against the concentration gradient. (e.g., Na+/K+ pump).

4. Cell Organelles

Endomembrane System

A group of organelles whose functions are coordinated. It includes the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles.

Endoplasmic Reticulum (ER)

• A network of tiny tubular structures (cisternae, tubules, and vesicles) scattered in the cytoplasm.
• Types:
  • Rough ER (RER): Bears ribosomes on its surface. Actively involved in protein synthesis and secretion.
  • Smooth ER (SER): Ribosomes are absent. It is the major site for synthesis of lipids and steroidal hormones.

Golgi Apparatus (Golgi Complex)

• Consists of flattened, disc-shaped sacs or cisternae stacked parallel to each other.
• Functions:
  • Processing, packaging, and transport of materials synthesized in the ER.
  • Formation of glycoproteins and glycolipids.
  • Site of synthesis of cell wall materials in plants.

Lysosomes

• Membrane-bound vesicular structures containing hydrolytic enzymes (lipases, proteases, carbohydrases).
• They are involved in the digestion of macromolecules and are also known as "suicidal bags" because they can digest the cell's own components.

Vacuoles

• Membrane-bound space found in the cytoplasm, enclosed by a membrane called the tonoplast.
• In plant cells, the vacuole can occupy up to 90% of the cell volume.
• It contains water, sap, excretory products, and other materials not useful for the cell.

Mitochondria

• Double membrane-bound structures, known as the "powerhouses" of the cell.
• Structure:
  • Outer membrane: Smooth and permeable.
  • Inner membrane: Folded into numerous cristae, which increase the surface area. It is selectively permeable.
  • Matrix: The inner compartment, containing enzymes for the Krebs cycle, a circular DNA molecule, and 70S ribosomes.
• Function: Sites of aerobic respiration and ATP synthesis.

Plastids

• Found in all plant cells and in euglenoids.
• Types:
  • Chloroplasts: Contain chlorophyll and carotenoid pigments. They are the site of photosynthesis. Structurally, they have a double membrane, and an internal system of thylakoids (flattened sacs) arranged in stacks called grana, embedded in a fluid called stroma.
  • Chromoplasts: Contain fat-soluble carotenoid pigments like carotene and xanthophylls. They impart yellow, orange, or red color to parts of the plant (e.g., flowers, fruits).
  • Leucoplasts: Colourless plastids that store nutrients.
    • Amyloplasts: Store starch.
    • Elaioplasts: Store oils and fats.
    • Aleuroplasts: Store proteins.

Ribosomes

• Granular structures composed of ribonucleic acid (RNA) and proteins, without any membrane.
• They are the site of protein synthesis.
• Eukaryotic ribosomes are 80S, with two subunits: a large 60S and a small 40S subunit.

Microbodies

• Membrane-bound minute vesicles that contain various enzymes.
• Peroxisomes: Contain enzymes for peroxide biosynthesis and degradation.
• Glyoxysomes: Found in fat-storing tissues of plants, contain enzymes for the glyoxylate cycle.

Cytoskeleton

• A network of filamentous proteinaceous structures in the cytoplasm.
• Functions: Mechanical support, motility, maintenance of the shape of the cell.
• Components:
  • Microtubules: Hollow tubes made of tubulin protein. Form the spindle fibers during cell division.
  • Microfilaments: Solid rods made of actin protein. Involved in muscle contraction and amoeboid movement.
  • Intermediate Filaments: Rope-like fibers made of various proteins. Provide mechanical strength.

Cilia and Flagella

• Hair-like outgrowths of the cell membrane. Cilia are small and numerous, while flagella are longer and fewer.
• Structure: The core, called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has a 9 + 2 array (nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules).

Centrosome and Centrioles

• Centrosome is an organelle usually containing two cylindrical structures called centrioles, arranged perpendicular to each other.
• They are made up of nine evenly spaced peripheral fibrils of tubulin protein.
• Function: Help in cell division by forming the spindle fibres in animal cells.

Nucleus

• A large, spherical organelle that contains the genetic material (DNA) of the cell.
• Components:
  • Nuclear Envelope: A double membrane with pores that control the passage of materials between the nucleus and the cytoplasm.
  • Nucleoplasm: The matrix of the nucleus.
  • Nucleolus: A dense, spherical structure within the nucleus, which is the site of ribosomal RNA (rRNA) synthesis.
  • Chromatin: A network of nucleoprotein fibres (DNA and histone proteins). During cell division, chromatin condenses to form chromosomes.
• Chromosomes:
  • Structure: A typical chromosome consists of two identical sister chromatids joined at a primary constriction called the centromere.
  • Satellite: Some chromosomes have a non-staining secondary constriction at a constant location, which gives the appearance of a small fragment called the satellite.
  • Types based on Centromere Position:
    • Metacentric: Centromere in the middle, forming two equal arms.
    • Sub-metacentric: Centromere slightly away from the middle, resulting in one shorter arm and one longer arm.
    • Acrocentric: Centromere situated close to its end, forming one extremely short and one very long arm.
    • Telocentric: Centromere at the terminal end.