Cytology Review

The Study of Cells

  • Cells in the human body come in a variety of shapes, primarily categorized into three types:
    • Squamous Cells:
    • Characteristics: Relatively flat and thin in structure.
    • Cuboidal Cells:
    • Characteristics: Cube-shaped with equal lengths on all sides.
    • Appearance: May have rounded corners, appearing more ball-shaped or circular under a microscope.
    • Columnar Cells:
    • Characteristics: Tall and relatively thin.

Cytology

  • Definition: Cytology is the study of cells.
  • In the hierarchy of body organization, cells represent the first level that demonstrates the characteristics of life.
  • Components of Each Cell:
    • Cell Membrane: Encapsulates the cell.
    • Cytoplasm: Everything that exists inside the cell membrane.
    • Cytoskeleton: Network of proteins providing internal support for the cell.

Components of Cytoplasm

  • Primary Divisions:
    • Cytosol:
    • Definition: Fluid within the cell, also referred to as intracellular fluid (ICF).
    • Organelles: Small structures within the cell facilitating specific functions.

Functions of the Cell Membrane

  • Shape and Protection: Provides structure and safeguards the cell.
  • Communication: Facilitates cell-to-cell communication.
  • Semi-Permeability:
    • Definition: Selectively allows certain substances to enter or exit while preventing others.

Structure of the Cell Membrane

  • Composed of a bilayer of phospholipids, referred to as the phospholipid bilayer.
    • Phospholipids: Amphiphilic molecules with:
    • Hydrophilic Portion:
      • Attracted to water; constitutes the heads of phospholipids.
    • Hydrophobic Portion:
      • Repelled by water; comprises the fatty acid tails.
    • Arrangement:
    • Heads in contact with aqueous fluids; tails sandwiched away in the interior.
    • Fluidity: Phospholipids move independently, allowing the membrane to be flexible.

Selective Permeability

  • Substances that pass freely:
    • Small, neutral, lipid-soluble substances (e.g., vitamins, steroids).
  • Substances that cannot pass easily:
    • Larger or charged molecules (e.g., water, glucose, proteins, ions).
  • Cholesterol:
    • Found between phospholipids, providing membrane rigidity.
  • Glycolipids:
    • Combinations of carbohydrates and lipids that provide various cell functions.

Membrane Proteins

  • Types of Proteins:
    • Integral/Transmembrane Proteins:
    • Exposed to both the intracellular and extracellular environments.
    • Peripheral Proteins:
    • Associated with only one side of the membrane.
  • Functions of Membrane Proteins:
    • Receptors: Communicate and identify neurotransmitters and hormones.
    • Second Messengers: Relay signals from outside to inside the cell, inducing responses.
    • Enzymes: Break down chemical messengers and nutrients.
    • Channel Proteins:
    • Allow hydrophilic solutes to pass through; can be gated or constant.
    • Carrier Proteins: Facilitating large molecules transport; some require ATP.
    • Cell Identity Markers:
    • Glycoproteins that help the cell identify itself as ‘self’.
    • Cell Adhesion Molecules:
    • Proteins that connect adjacent cells in a tissue.

Extensions of Cell Surfaces

  • Microvilli:
    • Small projections increasing cell surface area, aiding in nutrient absorption.
  • Cilia:
    • Hair-like extensions, often numerous; types include:
    • Non-motile: Serve sensory functions.
    • Motile: Propel fluids (e.g., mucus in the respiratory tract).
  • Flagellum:
    • Long extension for sperm cell movement.

Membrane Structure Diagram

  • Phospholipid heads form internal and external surfaces, interacting with fluids.
  • The membrane's semi-permeability is attributed to the orientation of phospholipids.
  • Small, non-polar, and hydrophobic substances pass easily, while larger or charged substances usually require assistance.

Membrane Transport Mechanisms

  • Passive vs. Active Transport:
    • Passive Transport: Does not require ATP.
    • Types of Passive Transport:
      • Filtration: Uses hydrostatic pressure to move particles through membranes.
      • Simple Diffusion: Movement from high to low concentration (e.g., red dye in water).
      • Osmosis: Water diffusion, following similar principles to simple diffusion.
      • Carrier Mediated Transport: Uses transmembrane proteins.
      • Facilitated Diffusion: Passive movement through a transmembrane protein; does not require ATP.
    • Active Transport: Requires ATP to move substances against a concentration gradient.
    • Example: Active transport proteins moving substances from low to high concentration.
  • Vesicular Transport:
    • Active process requiring ATP to transport large molecules.
    • Endocytosis: Movement of substances into the cell.
    • Exocytosis: Movement of substances out of the cell.

Cytoplasm

  • Definition: Includes all cellular contents. Consists primarily of:
    • Cytosol: The intracellular fluid, mainly composed of water.
    • Organelles: Various structures responsible for specific cellular functions.

Organelles

  • Nucleus:
    • Control center containing DNA, typically large and centrally located.
    • Surrounded by a double membrane known as the nuclear envelope.
    • Contains a nucleolus:
    • Produces ribosomes.
  • Endoplasmic Reticulum (ER):
    • Network of channels known as cisternae.
    • Types:
    • Rough ER:
      • Contains ribosomes; continuous with the nuclear envelope; synthesizes proteins.
    • Smooth ER:
      • Lacks ribosomes; detoxifies cytoplasm.
  • Ribosomes:
    • Tiny organelles produced by the nucleolus, crucial for protein synthesis.
  • Golgi Complex:
    • Collection of flat sacs (cisternae) that finalize protein synthesis and carbohydrate modification.
    • Packages finished proteins in Golgi vesicles for transport or exocytosis.
  • Lysosomes:
    • Special types of vesicles containing digestive enzymes to break down particles; prevalent in white blood cells for bacteria digestion.
    • Contain about 50 different types of digestive enzymes.
  • Peroxisomes:
    • Resemble lysosomes but are formed from the endoplasmic reticulum; involved in detoxifying cells by producing hydrogen peroxide.
  • Mitochondria:
    • Oval-shaped organelles with internal folds (cristae) responsible for ATP synthesis from sugars, fats, and proteins. Often referred to as the powerhouse of the cell.
    • Energy-rich cells contain numerous mitochondria.
  • Centrioles:
    • Paired organelles arranged perpendicularly, containing microtubules that assist during cell division, positioning cellular components appropriately.

Conclusion

  • Understanding the structure and function of cells and their components is essential for grasping the fundamentals of human biology and physiology.