Chapter 7: Membrane Structure and Function

Overview

• Focus on membrane structure and function.
• Membranes are primarily composed of a phospholipid bilayer.
  • Amphipathic phospholipids: hydrophilic heads and hydrophobic tails.
  • Membranes have hydrophilic regions exposed to water and a hydrophobic region away from water.
• Membranes are described as a Fluid Mosaic Model.

Components of the Membrane

Phospholipid Bilayer

• Hydrophilic heads: face outward towards water.
• Hydrophobic tails: face inward, creating a water-free environment.
• Lipids are not covalently bonded, allowing lateral movement (fluidity).

Proteins and Other Molecules

• Membrane proteins act like icebergs, floating unless anchored:
  • Integral proteins: embedded within the bilayer, some are transmembrane.
  • Peripheral proteins: loosely attached to the surface.
• Cholesterol: buffers membrane fluidity, preventing it from becoming too rigid or too fluid.
• Glycoproteins and Glycolipids: part of the membrane, serve various functions.

Functions of Membrane Proteins

• Transport: move substances across the membrane.
• Enzymatic Activity: catalyze chemical reactions.
• Signal Transduction: transmit signals across the membrane.
• Cell-Cell Recognition: help cells identify each other.
• Intercellular Joining: connect adjacent cells.
• Attachment: anchor the cell to its surroundings.

Membrane Permeability

• Membranes are selectively permeable.
• Small nonpolar molecules (e.g., O2, CO2) cross easily.
• Polar molecules and ions need transport proteins.

Transport Mechanisms

Passive Transport

• Diffusion: movement from high to low concentration (e.g., dialysis).
• Osmosis: diffusion of water across a membrane.
• Facilitated Diffusion: requires transport proteins for polar molecules.

Active Transport

• Moves substances against concentration gradient.
• Requires energy (ATP).
• Sodium-Potassium Pump: Example of active transport.

Co-Transport

• Uses gradient created by active transport to move other substances.
• Example: Proton-sucrose co-transporter.

Bulk Transport

Exocytosis

• Vesicles fuse with plasma membrane to release contents.

Endocytosis

• Types:
  • Phagocytosis: "cell eating" of large particles.
  • Pinocytosis: "cell drinking" of fluids.
  • Receptor-Mediated Endocytosis: specific uptake triggered by ligands.

Tonicity and Osmoregulation

• Isotonic: no net water movement, ideal for animal cells.
• Hypotonic: water enters the cell, causing it to swell.
• Hypertonic: water leaves the cell, causing it to shrink.
• Osmoregulation: mechanisms to balance water and solute concentrations.