CYTOLOGY

Chapter 03: Cytology - The Study of Cells

Introduction to Cytology

  • Cells were first described in 1665 by Robert Hooke.

  • By the mid-1800s, the cell theory emerged, stating:

    • All living things are composed of cells.

    • All cells come from pre-existing cells.

    • Cells are the smallest units of life that perform all vital physiological functions.

Types of Cells

Prokaryotic Cells

  • Domains: Bacteria and Archaea.

  • Characteristics:

    • Single-celled organisms.

    • Size: Less than 10 µm.

    • Lack a nucleus.

    • Lack membrane-bound organelles.

    • Contain 70S ribosomes.

Eukaryotic Cells

  • Domain: Eukarya.

  • Includes four kingdoms: Plants, Animals, Fungi, and 'Protists'.

  • Characteristics:

    • Can be single or multicellular.

    • Size: 10 to 100 µm.

    • Have a nucleus.

    • Have membrane-bound organelles.

    • Contain 80S ribosomes.

The Cell: Basic Functional Unit of Life

Cell (Plasma) Membrane

  • Composed of phospholipids that form a double-layered membrane.

    • Non-polar (hydrophobic) tails face away from the watery fluid inside and outside the cell.

Fluid Mosaic Model

  • Illustrates that the phospholipid bilayer contains various molecules that are in constant motion:

    • Important molecules include:

      • Phospholipids

      • Cholesterol molecules

      • Carbohydrates

      • Proteins

  • Main Functions:

    1. Physical isolation from the environment.

    2. Regulation of exchanges with the environment.

    3. Response to environmental changes.

  • Cell Membrane is selectively permeable:

    • Hydrophilic molecules allow for transport of polar substances, while hydrophobic molecules pass easily across the membrane.

Transport Mechanisms

Passive Processes

  • Movement of substances across the cell membrane without energy expenditure:

    • Diffusion: Movement from high concentration to low concentration.

    • Facilitated Diffusion: Special diffusion aided by carrier or channel proteins without ATP usage.

    • Osmosis: Diffusion of water from high water concentration to low water concentration.

  • Types of solutions:

    • Hypotonic: Lower solute concentration.

    • Hypertonic: Higher solute concentration.

    • Isotonic: Equal solute concentration.

Red Blood Cell Dynamics

  • Changes in solute concentrations affect physiological and clinical states, influencing osmoregulation through various systems (urinary, respiratory, integumentary).

Facilitated Diffusion

  • Types of channel proteins:

    • Leak Channels: Always open, allowing ions like K+.

    • Gated Channels: Open or close to control ions movement, such as Na+ ions.

  • Carrier proteins have specific binding sites for certain molecules like glucose.

Active Transport vs. Passive Transport

Active Transport

  • Involves energy (ATP) to move substances against their concentration gradient:

    • Sodium-Potassium Exchange Pump: Example of active transport.

Bulk Transport

  • Exocytosis: Release of materials via vesicles (requires ATP).

  • Endocytosis: Intake of materials via vesicles (requires ATP).

    • Types include:

      • Phagocytosis (solids, non-selective)

      • Pinocytosis (liquids, non-selective)

      • Receptor-Mediated (specific substances like Na+).

The Cytoplasm

  • Separated from extracellular fluid by the plasma membrane, consisting of:

    • Cytosol: Intracellular fluid with higher protein and K+ ion concentrations than extracellular fluid.

    • Organelles: Specific function performing structures varying by cell type.

Ribosomes

  • Function as protein factories found in all cell types (eukaryotes and prokaryotes).

  • Responsible for protein synthesis using mRNA as a template:

    • In eukaryotes, ribosomes can be free-floating in cytoplasm or bound to the rough endoplasmic reticulum (RER).

Cytoskeleton and Cellular Structures

  • Cytoskeleton: Provides internal support and shape.

  • Cilia and Flagella: Extensions for movement or to increase surface area.

  • Centrosome: Organizes microtubules during cell division.

Endomembrane System (EMS)

  • Includes a network of membranous organelles interconnected by transport vesicles:

    1. Vesicles

    2. Endoplasmic Reticulum (ER)

    3. Golgi Apparatus

    4. Lysosomes

    5. Peroxisomes

  • Smooth and rough ER differentiate based on ribosome presence.

Golgi Apparatus

  • Processes and distributes proteins and lipids received from the ER.

Lysosomes and Peroxisomes

  • Lysosomes: Contain digestive enzymes to break down waste material and cellular debris.

  • Peroxisomes: Contain enzymes that detoxify harmful substances.

Mitochondria: Energy Factories

  • Double-membrane organelle crucial for ATP production through cellular respiration:

    • Features include:

      • Outer and inner membranes with cristae.

      • Contains circular DNA and 70S ribosomes.

The Nucleus: The Genetic Control Center

  • Doubled-membrane organelle containing the cell's DNA organized into chromosomes.

  • Site of transcription, where mRNA is synthesized from DNA, allowing communication to ribosomes for protein synthesis.