week 4 cell membranes transport

Transport Across Biological Membranes

• One of the main functions of cell membranes is to act as traffic controllers of molecules and charged particles in and out of the cell.

• Cell membranes possess selective or differential permeability to molecules and ions.

Types of Transport

Passive Transport

• Definition: Movement of substances across membranes without energy expenditure.

  • Mechanism: Substances move along their electrochemical gradients, from high to low concentration.

  • Ions are attracted to oppositely charged particles.

• Diffusion: Driven by the kinetic energy of molecules.

  • Example: A drop of ink diffusing in water.

Types of Diffusion

1. Simple Diffusion

   • Definition: Substances diffuse directly through the lipid bilayer.

   • Examples: Nonpolar, lipid-soluble molecules; small polar molecules like water.

2. Facilitated Diffusion

   • Definition: Requires membrane proteins to help biologically important solutes cross.

   • Categories:

     • Channel Proteins: Transport ions and water.

     • Carrier/Transport Proteins: Move solutes like glucose and amino acids.

       • Example: GLUT1 (integral membrane protein for glucose transport).

         • Translocates glucose based on concentration gradient.

Osmosis

• Definition: A specific type of diffusion related to water.

Scenarios Illustrating Osmosis

1. Selective Permeable Membrane with Both Solute and Water

   • Two different solutions with varying osmolarity reach equilibrium.

2. Only Water Permeable Membrane

   • Water molecules move toward a higher solute concentration area until steady state is reached.

   • Terms:

     • Osmolarity: Total concentration of solute particles in a solution.

     • Tonicity: Effect of the solution on cell volume and shape.

       • Hypertonic: Higher solute concentration outside the cell; water moves out, cell shrinks.

       • Hypotonic: Lower solute concentration outside the cell; water moves in, cell swells.

       • Isotonic: Equal concentrations on both sides; no net change in cell size and shape.

Active Transport

• Definition: Movement of substances against their electrochemical gradient requiring metabolic energy (ATP).

Types of Active Transport Mechanisms

1. Uniport System

   • Moves one substance in one direction.

   • Example: Proton pumps that transport hydrogen ions out of the cell.

2. Symporter System

   • Moves two substances in the same direction simultaneously.

   • Example: Transport of glucose against its concentration gradient coupled with sodium ions.

3. Antiporter System

   • Moves two substances in opposite directions.

   • Example: Sodium-potassium pump (three sodium ions out, two potassium ions in); powered by ATP.

Bulk Transport

• Mechanism: Involves exocytosis and endocytosis using vesicles, requiring ATP expenditure.

  • Exocytosis: Release of hormones through the plasma membrane into the bloodstream.

Summary of Membrane Transport Mechanisms

• Review key passive and active transport processes, including examples and definitions for better understanding.