Cell Membrane Lecture Notes

Overview of the Cell Membrane

• Structure: Plasma membrane, visible as thick dark lines under a microscope.
• Faces: Intracellular (inside) and extracellular (outside).
• Functions:
  • Sets cell boundaries.
  • Regulates interactions with other cells.
  • Controls passage of substances, selectively semi-permeable.

Composition of the Cell Membrane

• 98% of molecules are lipids.
  • 75% of these are phospholipids.
  • Phospholipids:
    • Amphipathic: hydrophilic (water-loving) heads and hydrophobic (water-hating) tails.
    • Arrangement: Forms a bilayer with tails inward to avoid water.
  • Cholesterol (20%): Stiffens membrane, acts as glue without being rigid.
  • Glycolipids (5%): Sugar fats contributing to glycocalyx on the cell surface.

Proteins in the Cell Membrane

• Account for 2% of molecules but half the weight.
• Types:
  • Integral Proteins: Penetrate the membrane, transmembrane.
  • Peripheral Proteins: Adhere to one side of the membrane.

Functions of Membrane Proteins

• Receptors: Bind to chemical signals.
• Second Messenger Systems: Communicate chemical messages.
• Enzymes: Catalyze reactions.
• Channel Proteins: Allow passage of hydrophilic solutes and water.
  • Gated Channels: Opened by ligands, voltage, or mechanical stress.
• Carrier Proteins: Move solutes across the membrane.
• Cell Identity Markers: Glycoproteins that tag cells.
• Cell Adhesion Molecules: Link cells to extracellular material.

Transport and Membrane Functionality

• Transport: Primary function, acting as a gatekeeper.
• Types of Transport:
  • Passive Transport: No energy (ATP) required.
    • Filtration: Physical pressure pushes particles.
    • Diffusion: Movement from high to low concentration.
    • Osmosis: Diffusion of water only.
  • Active Transport: Requires energy.
    • Carrier-mediated Mechanisms: Proteins assist in moving molecules.

Detailed Mechanisms of Transport

• Passive Transport:

  • Driven by concentration gradients (high to low).
  • Factors Affecting Diffusion:
    • Temperature, molecular weight, concentration gradient steepness, surface area, permeability.
  • Osmosis: Specific to water, uses aquaporins.

• Active Transport:

  • Primary Active Transport: Moves solutes against gradient, consumes ATP.
  • Vesicular Transport: Moves large particles via vesicles, involves endocytosis and exocytosis.
  • Co-transport: Simultaneous transport of two substances, one with and one against gradient.

Osmolarity and Tonicity

• Osmolarity: Concentration of solutes in a solution.
• Tonicity: Effect of a solution on cell volume.
  • Hypotonic: Causes cell to swell (low solute concentration outside).
  • Hypertonic: Causes cell to shrink (high solute concentration outside).
  • Isotonic: No net movement, equilibrium.

Cell Transport Processes

• Endocytosis: Cell takes in materials.
  • Phagocytosis: Engulfs solids.
  • Pinocytosis: Engulfs liquids.
  • Receptor-mediated: Specific binding.
• Exocytosis: Expels materials from the cell.
• Transcytosis: Moves substances across the cell.

These notes provide a comprehensive overview of the cell membrane's structure, composition, and function, focusing on transport mechanisms and protein roles in maintaining cell integrity and communication.