Unassisted Membrane Transport

Permeability of cells differs depending on the arrangement of lipids and proteins in their plasma membrane. Passage across the membrane can be either unassisted (passive) or assisted (active; requiring energy/ATP) Unassisted Membrane Transport: -diffusion is the movement of a solute molecule from an area of greater solute concentration to lower solute concentration; “downhill” along a concentration gradient. Diffusion Filtration -the plasma membrane selectively restricts diffusion as not all substances can easily penetrate the membrane. To diffuse across a membrane, molecules must pass through the lipid portion of the membrane (simple diffusion) or pass through a membrane channel. -filtration is the movement of water and solutes through a membrane because of a higher hydrostatic pressure on one side of a membrane than the other. -refers to the net diffusion of water across the membrane. Osmosis -in the presence of non-penetrating solutes, water will move along the concentration gradient by osmosis to reach a steady state. Water is able to readily permeate the plasma membrane (via water channels called aquaporins), with the movement of water being driven by the concentration gradient. -osmosis is related to both hydrostatic pressure and solute concentration. -osmotic pressure is the force of osmotic water movement; the force with which water moves into a solution as a result of solute concentration. Osmotic vs Hydrostatic Pressure -hydrostatic pressure is the mechanical force of water pressing against the cellular membrane; the pressure exerted by fluid on an object. -essentially, hydrostatic pressure pushes out while osmotic pressure pulls in. Tonicity -concentration of non-penetrating solutes in ECF. While osmolarity describes the solute concentration of a solution, tonicity describes how that solution will impact the volume and shape of cells. -review the difference between isotonic, hypotonic and hypertonic solutions and the effect that they will have on cells (refer to Figure 3-17 on p94) Assisted Membrane Transport: - Active transport allows ions to move across a membrane, regardless of the concentration gradient. - Two types Primary Active Transport Uses the energy from ATP to pump ions against a concentration gradient. Active Transport e.g. Sodium-potassium exchange pump (exchanges intracellular sodium for extracellular potassium ions.) Secondary Active Transport Moves a substrate down its concentration gradient - doesn’t directly involve ATP, but the cell must expend energy later to restore homeostasis. Carrier proteins can give a “free ride” to another substrate at the same time. e.g. Sodium linked cotransport (Glucose absorption - carrier protein will only bind with glucose if the carrier is also bound to two sodium ions.) - The movement of materials in/out of the cell in vesicles (aka bulk transport) - Requires the breakdown of ATP for energy. - Two types Endocytosis Movement of extracellular materials into the cell with Vesicular Transport vesicles called endosomes. 3 types Receptor-mediated endocytosis (see figure 3-21 p. 98) Pinocytosis (“cell drinking”) Phagocytosis (“cell eating”) Exocytosis Secretory vesicles formed by the Golgi apparatus fuse with the plasma membrane to release their contents into the extracellular fluid.