Plasma membrane and cell junctions

Plasma Membrane and Cell Junctions

Learning Objectives

• Composition of the Plasma Membrane: Draw and understand the structure.

• Functions of the Plasma Membrane: Discuss its various roles.

• Definition of Tissues: Understand the collection of joined cells.

• Types of Cell Junctions: Differentiate between and provide examples of communicating, occluding, and anchoring cell junctions.

Overview of Cells

• A cell is the basic functional unit of living organisms.

• Cells perform all necessary functions for life; some organisms are unicellular (e.g., protons).

• Estimated cell count in an adult human: More than $700,000,000,000,000$ (700 trillion).

• Human cells are specialized for specific functions, illustrated by:

  • Neurons: Specialized for relaying information.

  • Skeletal Muscle Cells: Specialized for contraction and movement.

Plasma Membrane

• Definition: The plasma membrane surrounds the cell, separating the intracellular environment from the extracellular environment.

  • Intracellular fluid (ICF): Inside the cell.

  • Extracellular fluid (ECF): Outside the cell.

• Electron Micrograph Representation:

  • Two dark lines represent plasma membranes; a wider center represents the extracellular fluid.

Composition of the Plasma Membrane

• Phospholipids: Major component of the plasma membrane.

  • Structure: Composed of a hydrophilic phosphate head and two hydrophobic fatty acid tails.

  • Hydrophilic: Molecules that dissolve well in water (e.g., phosphates).

  • Hydrophobic: Molecules that do not mix easily with water (e.g., fatty acids).

  • Orientation in the Plasma Membrane:

  • Hydrophilic heads face the ECF and ICF.

  • Hydrophobic tails face inward, creating a barrier that restricts water and hydrophilic substance movement.

Fluid Mosaic Model

• Concept: Plasma membrane is described as a fluid mosaic model.

  • Fluidity: Ability to change shape and composition.

  • Mosaic: Various molecules (lipids, proteins, carbohydrates, cholesterol) integrated into the membrane.

  • Functions:

  • Barrier for ICF and ECF.

  • Regulates material exchange via proteins that function as transport channels.

  • Enables communication between intracellular and extracellular environments.

  • Provides structural support and maintains cell shape.

Components of the Plasma Membrane

1. Lipids: Phospholipids and cholesterol.

   • Function: Form a barrier between ICF and ECF.

2. Proteins: Integral and peripheral proteins.

   • Functions:

   • Transport materials across the membrane.

   • Serve as receptors for signal transduction.

   • Structural roles for cell integrity.

3. Carbohydrates: Generally located on the extracellular side.

   • Function: Primarily structural and involved in immunity.

Relative Composition of the Plasma Membrane

• High Concentration: Lipids

• Moderate Concentration: Proteins

• Low Concentration: Carbohydrates

• Exact composition varies depending on cell type (e.g., muscle cells may have more proteins than neurons).

Tissues and Cell Junctions

• Definition of Tissues: A collection of cells joined together to perform a specific function.

• Types of Tissues in the Human Body:

  1. Connective Tissue: Bones, blood, adipose tissue (fat).

  2. Muscle Tissue: Skeletal muscle, smooth muscle.

  3. Nervous Tissue: Neurons and glial cells in the brain.

  4. Epithelial Tissue: Lining of organs (e.g., blood vessels, digestive tract).

Importance of Cell Junctions

• Cell Communication: Essential for functioning as a unit.

• Material Movement Control: Prevents or allows passage between cells.

• Cell Anchoring: Necessary for structural integrity and connection to the extracellular matrix.

Types of Cell Junctions

1. Communicating Cell Junctions: Gap Junctions

   • Function: Allow direct and rapid communication between cells.

   • Structure: Composed of proteins forming tunnels between two cells’ cytoplasm.

   • Example: Cardiac cells in the heart allow synchronized contraction through rapid sodium transfer between cells.

2. Occluding Cell Junctions: Tight Junctions

   • Function: Restrict movement of materials between cells.

   • Structure: Junctions that seal cells to prevent leakage of substances.

   • Example: Cells in the small intestine, controlling what enters the body while blocking harmful substances (e.g., bacteria).

3. Anchoring Cell Junctions: Desmosomes

   • Function: Hold cells together and provide structural stability under stress.

   • Structure: Made of various proteins linking adjacent cells like a zipper.

   • Example: Heart muscle cells remain bound together during repeated contractions.

Summary

• Understanding the composition and functions of the plasma membrane is crucial for comprehending cellular functions.

• Cell junctions are vital for tissue structure and coordination of cellular activities. Types include communicating (gap junctions), occluding (tight junctions), and anchoring (desmosomes).