Chapter 7: Membrane Structure and Function

Overview

• Focus on the structure and function of cell membranes.
• Membranes are composed of a phospholipid bilayer.
  • Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.
  • Bilayer structure: Hydrophilic heads face outward towards water, hydrophobic tails face inward, away from water.
• Membranes are fluid and mosaic.

Membrane Fluidity

• Fluid Mosaic Model: Membranes are fluid due to the lateral movement of phospholipids and proteins.
  • Phospholipids can move laterally but rarely flip-flop.
  • Cholesterol plays a role in maintaining fluidity:
    • High temperatures: Cholesterol restrains movement of phospholipids.
    • Low temperatures: Prevents tight packing of phospholipids.
• Unsaturated vs. Saturated Fatty Acids:
  • Unsaturated: More fluid (kinked tails prevent tight packing).
  • Saturated: Less fluid (straight tails pack tightly).

Membrane Proteins

• Integral proteins: Embedded in the hydrophobic core of the bilayer.
• Peripheral proteins: Loosely bound to the surface of the membrane.
• Functions of Membrane Proteins:
  • Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and ECM.
• Transmembrane proteins: Span the entire membrane, can be channels or carriers.

Selective Permeability

• Membranes allow some substances to cross more easily than others.
• Small, nonpolar molecules (e.g., O2, CO2) cross easily.
• Large, polar molecules and ions need assistance from transport proteins.

Transport Mechanisms

Passive Transport

• Diffusion: Movement from high to low concentration; does not require energy.
  • Simple Diffusion: Direct movement across the membrane.
  • Facilitated Diffusion: Requires transport proteins (channels or carriers).

Active Transport

• Moves substances against concentration gradient.
• Requires energy (usually ATP).
• Examples:
  • Sodium-Potassium Pump: Pumps Na+ out and K+ into the cell against their gradients.

Osmosis and Tonicity

• Osmosis: Diffusion of water across a membrane.
• Tonicity: Ability of a solution to cause a cell to gain or lose water.
  • Isotonic: No net water movement.
  • Hypotonic: Cell gains water (can burst in animal cells, becomes turgid in plant cells).
  • Hypertonic: Cell loses water (shrivels in animal cells, plasmolyzes in plant cells).

Bulk Transport

• Exocytosis: Vesicles fuse with membrane to release contents outside the cell.
• Endocytosis: Cell takes in macromolecules by forming vesicles.
  • Phagocytosis: "Cellular eating"; engulfs solid particles.
  • Pinocytosis: "Cellular drinking"; engulfs liquids.
  • Receptor-Mediated Endocytosis: Targeted uptake via receptor binding.

Important Concepts

• Membrane Potential: Voltage difference across a membrane due to ion distribution.
• Co-transport: Active transport of one solute drives transport of another.
  • Example: Proton pump creates a gradient used to transport sucrose.

Summary

• The membrane's structure is crucial for its diverse functions, including selective permeability and communication between cells and their environment.
• Understanding the fluid mosaic model, types of transport, and the role of proteins and lipids is essential for comprehending cellular processes.