Membranes and Membrane Transport sl

Membranes and Membrane Transport

  • Key Questions:
    • How do lipid and protein molecules assemble into biological membranes?
    • What determines whether a substance can pass through a biological membrane?

Lipid Bilayers as the Basis of Cell Membranes

  • Phospholipids:
    • Amphipathic lipids that form sheet-like bilayers in aqueous environments.

Lipid Bilayers as Barriers

  • Hydrophobic Core:
    • Hydrocarbon chains provide low permeability for large molecules, hydrophilic particles, ions, and polar molecules, thus serving as barriers between aqueous solutions.

Simple Diffusion Across Membranes

  • Example: Movement of oxygen (O₂) and carbon dioxide (CO₂) between phospholipids.
  • Mechanism: Particles move from areas of high concentration to low concentration until equilibrium is reached.

Integral and Peripheral Proteins in Membranes

  • Types of Proteins:
    • Integral Proteins: Embedded in one or both lipid bilayers.
    • Peripheral Proteins: Attached to the surface of the bilayer.
  • Function: Diverse structures allow for various functions like cell adhesion, enzymatic activity, and transport.

Movement of Water Molecules Across Membranes

  • Osmosis:
    • Passive movement of water from low solute concentration to high solute concentration across a semi-permeable membrane.
    • Role of Aquaporins: Channel proteins facilitating water transport.

Channel Proteins for Facilitated Diffusion

  • Function: Selectively permit specific ions to diffuse through when channels are open.
  • Mechanism: When closed, ions cannot pass; opening and closing regulated by various factors (gated channels).

Pump Proteins for Active Transport

  • Description: Pumps utilize ATP to transport specific particles against their concentration gradient.
  • Types of Transport:
    • Primary: Direct use of ATP.
    • Secondary: Depend on ion gradients established by primary active transport.

Selectivity in Membrane Permeability

  • Facilitated Diffusion vs. Active Transport:
    • Facilitated diffusion is passive and selective based on size and polarity; active transport requires energy to move against a gradient.

Glycoproteins and Glycolipids Structure and Function

  • Location: Carbohydrate structures linked to proteins or lipids, located on the extracellular side of membranes.
  • Roles: Cell adhesion and cell recognition mechanisms.

Fluid Mosaic Model of Membrane Structure

  • Components:
    • Peripheral proteins, integral proteins, glycoproteins, phospholipids, and cholesterol.
  • Characteristic: Fluidity due to lateral mobility of components within the bilayer.
  • Diagram Representation: Drawing includes distinct hydrophobic and hydrophilic regions.

Concentration Gradient and Diffusion

  • Diffusion Definition: Passive movement from high to low concentration until dynamic equilibrium.
  • Factors Affecting Rate: Concentration gradient steepness, surface area, size/type of molecules, and moisture.

Types of Cellular Transport

  1. Passive Transport (No ATP required):

    • Facilitated diffusion (through channel/protein carriers)
    • Simple diffusion (directly through membranes)
    • Osmosis

  2. Active Transport (Requires ATP):

    • Protein pumps transport specific substances—uniporters (one direction), symporters (two different in same direction), and antiporters (two different in opposite directions).
    • Vesicular transport (exocytosis and endocytosis).

Summary of Membrane Transport

  • Membranes form a selective barrier, vital for cellular homeostasis.
  • Different molecules diffuse based on size, charge, and polarity requirements:
    • Small hydrophobic molecules (e.g., O2, CO2) pass freely.
    • Charged or large polar molecules require facilitated or active transport.

Importance of Membrane Structure

  • Membranes provide compartmentalization, protection from environments, and serve as surfaces for cellular interactions. Both integral and peripheral proteins are critical in maintaining membrane functionality.