Membrane Structure and Transport: Quick Review

Membrane Structure

• Phospholipid bilayer- Selectively permeable

  • Contains proteins and other molecules

  • Allows hydrophobic molecules and small polar molecules to pass through, but not large polar molecules or ions.

• Fluid mosaic model- Fluid combination of phospholipids, proteins, and other molecules.

  • Phospholipids in lateral motion, rarely flip.

    • Lateral Movement: enables fluidity and flexibility.

    • Rare Flipping: maintains membrane stability and asymmetry.

  • Membrane fluidity regulated by cholesterol.

    • Acts as a 'fluidity buffer.'

    • High temperatures: reduces fluidity.

    • Low temperatures: increases fluidity.

Membrane Proteins

• Integral membrane proteins- Penetrate hydrophobic interior.

  • Transmembrane proteins span entire bilayer.

    • Functions on both sides of the membrane.

    • Act as channels or carriers.

  • Hydrophobic and hydrophilic sections.

    • Hydrophobic amino acids interact with lipid tails.

    • Hydrophilic amino acids exposed to aqueous environments.

  • Involved in transporting molecules.

• Peripheral membrane proteins- Loosely bound to the surface.

  • Hydrophilic.

  • Cell-to-cell recognition.

    • Glycoproteins on the surface.

    • Immune response.

Membrane Protein Functions

• Enzymes catalyze reactions.

  • Accelerate biochemical reactions.

• Signal receptors bind ligands.

  • Initiate cell signaling pathways.

• Transport ions and molecules.

  • Channels and carriers to facilitate diffusion or active transport.

• Cell recognition via glycoproteins.

  • Glycoproteins for cell identification.

• Cell junctions (gap, desmosomes, tight).

  • Gap: communication between cells.

  • Desmosomes: strong adhesion.

  • Tight: prevent leakage.

• ECM holds tissues together.

  • Provides structural support.

  • Regulates cell behavior.

Membrane Transport

• Passive Transport- No energy required.

  • Molecules move from high to low concentration.

  • Simple diffusion: small, hydrophobic molecules (lipids, CO2, O2).

    • Example: Oxygen diffusing into cells for respiration.

  • Facilitated diffusion: via channel or carrier proteins.

    • Selective for various molecules.

• Active Transport- Energy (ATP) required.

  • Molecules move from low to high concentration.

  • Examples: proton pump, Na^+/K^+ pump.

Osmosis

• Water movement across a semipermeable membrane.

• Tonicity: ability of solution to cause water gain/loss.

  • Isotonic: no net water movement.

  • Hypertonic: cell loses water.

    • Plasmolysis in plant cells.

  • Hypotonic: cell gains water.

    • Lysis in animal cells.

• Osmoregulation: control of water balance.

  • Maintains homeostasis.

Facilitated Diffusion

• Channel proteins: transmembrane, allow specific ions/molecules.

• Aquaporins: for water transport.

  • Very rapid water movement.

• Carrier proteins: shape change to transport molecules.

  • Binding site for specific solute.

Active Transport Pumps

• Move substances against concentration gradients using ATP.- Sodium/potassium pump (Na^+/K^+).

  • Maintains electrochemical gradient.

  • 3 Na^+ out, 2 K^+ in.

  • Essential for muscle contractions, neuron firings, kidney filtering, etc.

Bulk Transport

• Endocytosis: import via vesicles.

  • Phagocytosis: cellular eating.

  • Pinocytosis: cellular drinking.

    • Uptake of extracellular fluid.

  • Receptor-mediated endocytosis: for specific molecules.

    • Coated pits and coated vesicles.

• Exocytosis: export via vesicles.

  • Release of proteins, neurotransmitters.