Lecture Notes: Plasma Membrane and Cellular Transport

Introduction

• Previous Lecture Recap: Overview of cell organelles, importance of plasma membranes.
• Current Lecture Focus: Structure and function of plasma membranes, cellular transport processes.

Plasma Membrane Structure

• Function: Acts as a selectively permeable barrier.
• Structure:
  • Composed of a phospholipid bilayer.
  • Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.
  • Fluid Mosaic Model:
    • Membrane behaves like a fluid, allowing lateral movement of components.
    • Contains embedded proteins and cholesterol affecting fluidity.
  • Cholesterol's Role:
    • At warm temperatures, restrains movement.
    • At cool temperatures, maintains fluidity by preventing tight packing.

Membrane Proteins

• Types:
  • Peripheral Proteins: Bound to membrane surface.
  • Integral Proteins: Penetrate the hydrophobic core, can be transmembrane.
• Functions:
  • Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and extracellular matrix.
• Glycoproteins and Glycolipids: Involved in cell recognition.

Selective Permeability

• Molecules that Cross Easily: Small, nonpolar molecules like oxygen (O2), carbon dioxide (CO2), and fat-soluble vitamins.
• Transport Proteins:
  • Channel Proteins: Form hydrophilic channels for specific molecules.
  • Carrier Proteins: Bind substances and change shape to transport across membrane.

Types of Transport

Passive Transport

• No energy required, moves substances down concentration gradient.
• Simple Diffusion: Movement of molecules from high to low concentration.
• Osmosis: Diffusion of water across a selectively permeable membrane.
  • Tonicity: Impact of surrounding solution on cell water balance.
  • Isotonic: No net movement.
  • Hypertonic: Cell loses water.
  • Hypotonic: Cell gains water.
• Facilitated Diffusion: Uses transport proteins without energy.
  • Types include channel proteins and carrier proteins.

Active Transport

• Requires energy (ATP), moves substances against concentration gradient.
• Electrogenic Pumps:
  • Sodium-Potassium Pump: Exchanges 3 Na+ out and 2 K+ in.
  • Proton Pump: Moves protons against gradient, used in plants.
• Co-Transport: Uses potential energy from one ion's gradient to transport another substance.
• Bulk Transport: Moves large molecules via vesicles.
  • Exocytosis: Exports materials out of the cell.
  • Endocytosis: Imports materials into the cell, includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Conclusion

• Review the schematic provided for a visual summary of transport methods.
• Prepare for next lecture on energy and entropy in cellular processes.