Chapter 7: Membrane Structure and Function

Introduction

• Covers the structure and function of cellular membranes.
• Focus on the phospholipid bilayer and its components.

Phospholipid Bilayer

• Structure: Made of phospholipids with hydrophilic heads and hydrophobic tails.
  • Hydrophilic heads face watery environments inside and outside the cell.
  • Hydrophobic tails face each other, creating a water-free interior.
• Properties:
  • Amphipathic molecules: Part hydrophilic (heads) and part hydrophobic (tails).
  • Fluidity: Phospholipids can move laterally within the layer.
  • Not covalently bonded to neighbors, allowing for lateral movement.

Fluid Mosaic Model

• Describes the membrane as a fluid structure with a "mosaic" of various proteins.
• Components:
  • Membrane proteins (integral and peripheral).
  • Cholesterol and glycolipids.
• Proteins can float freely unless anchored.

Membrane Fluidity

• Factors:
  • Temperature: Affects membrane fluidity.
  • Cholesterol: Acts as a fluidity buffer.
    • High temperatures: Cholesterol restrains movement.
    • Low temperatures: Prevents tight packing of phospholipids.
• Unsaturated vs. Saturated Fatty Acids:
  • Unsaturated fatty acids increase fluidity.
  • Saturated fatty acids decrease fluidity, making the membrane more solid.

Membrane Proteins

• Types:
  • Peripheral Proteins: Loosely attached, easily removed.
  • Integral Proteins: Embedded in the bilayer, can span the membrane (transmembrane).
• Functions:
  • Transport, enzymatic activity, signal transduction, cell recognition, intercellular joining, attachment to cytoskeleton and ECM.

Selective Permeability

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

Transport Proteins

• Channel Proteins: Provide a corridor for specific molecules.
• Carrier Proteins: Change shape to transport substances across the membrane.

Passive Transport

• Diffusion: Movement of molecules from high to low concentration.
• Types: Simple diffusion and facilitated diffusion (through transport proteins).

Active Transport

• Movement against concentration gradient.
• Requires energy (ATP).
• Sodium-Potassium Pump: Exchanges Na+ out of the cell for K+ into the cell.

Osmosis and Tonicity

• Osmosis: Diffusion of water across a selectively permeable membrane.
• Tonicity:
  • Isotonic: No net water movement.
  • Hypotonic: Cell gains water and may burst.
  • Hypertonic: Cell loses water and shrivels.

Plant vs. Animal Cells in Tonic Environments

• Plant Cells:
  • Prefer hypotonic environments (turgid state).
  • In hypertonic environments, cells become plasmolyzed.
• Animal Cells:
  • Prefer isotonic environments to remain stable.
  • Hypotonic environments cause lysis.
  • Hypertonic environments cause crenation.

Osmoregulation

• Mechanisms for controlling water balance.
• Example: Paramecium uses contractile vacuole to expel excess water.

Co-transport

• Uses a gradient of one substance to drive active transport of another.
• Example: Proton-sucrose co-transporter.

Bulk Transport

• Exocytosis: Release of substances from a cell.
• Endocytosis: Intake of substances into a cell.
  • Phagocytosis: Engulfing large particles.
  • Pinocytosis: Engulfing liquid.
  • Receptor-mediated endocytosis: Triggered by specific receptors.

Conclusion

• Membrane structure plays a crucial role in cellular function and regulation of the internal environment.