Membrane Transport Mechanisms Flash Cards

Simple Diffusion

Directly through the plasma membrane (diffusion

Ion Channels - Selective Pores

Ion channels have 4 functional components: 1. A pore region 2. A selectivity filter 3. A gating mechanism 4. An inactivation particle

Important channels and ions

Na+ channels 1. Voltage: action potential (depolarization) 2. Non-voltage: Na+ transport - epithelia K+ channels 1. Membrane potential 2. Nerve & muscle excitability (repolarization) 3. Homeostasis of K+ (GI tract, kidney) Ca2+ channels 1. entry to cells 2.Signaling mechanisms Cl- channels 1. Excitability (CNS neurons) 2. Cl- out of epithelia 3. Control cell volume

Selectively Permeable Plasma Membrane

Permeability through lipid bilayers 1. Smaller molecules are more permeable than larger molecules 2. Hydrophobic molecules are more permeable than polar molecules 3. Uncharged molecules are more permeable than charged molecules

Facilitated Diffusion

diffusion aided by specific transporter or carrier

Kinetics of facilitated diffusion

facilitated transporters have a Jmax and with transport rate INCREASES vs simple diffusion With ↑ [substrate] = ↑ J until all carriers are loaded: Jmax = max rate

Pump Leaked Coupling

maintenance of all transmembrane gradients across plasma membranes secondary active transport of calcium drives it out of the cell and sodium enters the cell 3 sodium now exit and obligatory coupling means 2 potassium now enter potassium leaking and this generates a negative membrane potential

Passive movement of solutes

move from region of high concentration to low concentration (chemical gradient) How fast a molecule moves across the membrane depends on the concentration difference

Kinetic characteristics of Simple Diffusion

rate of diffusion is influenced by the lipid solubility of the solute, so Lipophilic (likes lipid) substances will show much higher diffusion rates than hydrophilic due to their composition

Fick's Law of Diffusion

rate of movement of a solute (X) across a barrier (e.g. a cell membrane) of a given surface area is called FLUX (J) and is determined by Fick's law.

Facilitated diffusion via carriers/transporters

uniporters are carrier proteins that bind a specific solute (the substrate), and cause conformational change of the uniporter protein, results in movement of the substrate across the membrane e.g. facilitated diffusion of glucose through "GLUT" (GLUcose Transporter) family. 15

Endocytosis and Exocytosis

use of membrane vesicles to enter and exit the cell membrane

Ion Channels - GATING MECHANISM

(A) Membrane voltage • Voltage-gated channels (B) Chemicals • Ligand-gated channels (C) Mechanical force • Stretch-activated channels

Pore Diffusion

1. Aquaporin: aids in water recovery in kidney. Permeability regulated by adding/removing AQP. 2. Gap junctions: Form pores between cells to allow for exchange of small signaling molecules and electrical signals. 3. a-hemolysin: toxin from staphylococcus aureus forms pores in blood cells, causing lysis & tissue damage ("staph" infection).

Fluids Outside and Inside a Cell

1. ECF is rich in Na+ and Cl- , but low in K+ 2. ICF is rich in K+ , but low in Na+ and Cl- 3. There is a massive difference in Ca2+ levels in ECF and ICF 4. There is more protein in ICF than ECF 5. ICF is more acidic (lower pH) than ECF

Secondary Active Transport

1. Exchangers (antiporters) - opposite direction e.g. use of inwardly directed Na+ or Cl- gradient to pump H+ , Ca2+ or HCO3 - out of cells. 2. Cotransporters (symporters) - same direction e.g. use of inwardly directed Na+ gradient to recover glucose/amino acids from intestinal lumen & renal tubule 18

Primary Active Transport Use of ATP Against Gradient

1. Na+/K+ ATPase (sodium-potassium pump): • 3 Na+ out & 2 K+ in - 2. Ca2+/H+ ATPase: • Exchange 1 Ca2+ for 2H+ ions for intracellular calcium 3. . H+/K+ ATPase: • Pump 2 H+ ions out of the cell in exchange for 2 K+ ions in stomach and kidney cells