CH 7: REVIEW

Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a dynamic combination of lipids, proteins, and carbohydrates. This model explains how membranes maintain flexibility and allow for the movement of embedded molecules.

• Phospholipid bilayer: The basic structure consists of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Proteins: Embedded within the bilayer, proteins serve various functions such as transport, signaling, and structural support.

• Carbohydrates: Often attached to proteins or lipids on the extracellular surface, they play roles in cell recognition.

• Cholesterol: Interspersed within the bilayer, cholesterol acts as a temperature buffer, maintaining membrane fluidity.

Example: The plasma membrane of animal cells contains cholesterol to prevent the membrane from becoming too rigid in cold temperatures or too fluid in warm temperatures.

Membrane Fluidity

Membrane fluidity refers to the ability of lipids and proteins to move laterally within the bilayer. Fluidity is influenced by several factors:

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Saturated vs. Unsaturated Fatty Acids: Unsaturated fatty acids (with double bonds) increase fluidity, while saturated fatty acids decrease it.

• Cholesterol: Acts as a buffer, preventing drastic changes in fluidity.

Equation:

Example: Membranes with more unsaturated fatty acids remain fluid at lower temperatures.

Types of Membrane Proteins

Membrane proteins are classified based on their location and function:

• Peripheral proteins: Attached to the surface of the membrane; involved in signaling and structural support.

• Integral proteins: Embedded within the membrane; often function as channels or transporters.

• Transmembrane proteins: Span the entire membrane; involved in transport and communication.

Example: Aquaporin is a transmembrane protein that facilitates water transport.

Functions of Membrane Proteins

Membrane proteins perform a variety of essential functions:

• Transport: Move substances across the membrane (e.g., channels, carriers).

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Signal transduction: Relay signals from the external environment to the cell interior.

• Cell-cell recognition: Allow cells to identify each other.

• Intercellular joining: Connect adjacent cells.

• Attachment to cytoskeleton and extracellular matrix (ECM): Maintain cell shape and stabilize membrane location.

Transport Across Membranes

Transport across cell membranes is essential for maintaining homeostasis. It can be classified as passive or active:

• Passive Transport: Does not require energy; substances move down their concentration gradient.

  • Diffusion: Movement of molecules from high to low concentration.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

  • Facilitated Diffusion: Movement of molecules via channel or carrier proteins (e.g., Aquaporin for water).

• Active Transport: Requires energy (usually ATP); moves substances against their concentration gradient.

  • Electrogenic pumps: Create membrane potential by moving ions (e.g., sodium-potassium pump in animals, proton pump in plants).

  • Cotransport: Coupled transport of two substances.

Equation:

Bulk Transport: Endocytosis and Exocytosis

Bulk transport involves the movement of large particles or volumes of fluid across the membrane:

• Endocytosis: Uptake of materials into the cell.

  • Phagocytosis: "Cell eating"; engulfment of large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

  • Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins.

• Exocytosis: Release of materials from the cell.

Example: White blood cells use phagocytosis to engulf pathogens.

Water Movement and Tonicity

Water movement across membranes is governed by tonicity, which describes the relative concentration of solutes:

• Isotonic solution: Equal solute concentration inside and outside the cell; no net water movement.

• Hypotonic solution: Lower solute concentration outside the cell; water enters the cell, which may swell.

• Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, which may shrink.

Equation:

Example: Plant cells become turgid in hypotonic solutions due to water uptake.

Comparison of Endocytosis Types

Endocytosis can be classified based on the mechanism and specificity of uptake:

Key Terms and Definitions

Below is a summary of important terms related to membrane structure and function:

Additional info: Academic context and definitions have been expanded for clarity and completeness. All equations are provided in LaTeX format as required.