Transport Across Cell Membranes Flashcards
How does glucose enter the bloodstream?
Leave intestine, enters blood, crosses 4 lipid membranes and interstitial space
What determines whether a molecule/ion/gas will cross the cell membrane?
Chemical gradient
Electrical gradient
Permeability
What are the forms of passive transport?
Simple diffusion
Simple diffusion through protein channels
Facilitated diffusion
What passes through simple diffusion?
Nonpolar molecules
What passes through simple diffusion through protein channels?
Inorganic ions and water
What passes through facilitated diffusion?
Small organic molecules
Glucose
Explain Brownian Motion.
Molecules of a solution are in a constant state of motion as a result of their thermal energy
How do gated channels open when closed?
Chemical, mechanical and electrical signals
According to Fick’s Law of Diffusion, what is diffusion rate proportional to?
Concentration gradientmembrane permeabilitysurface area/membrane thickness
What is membrane permeability proportional to?
Lipid solubility/molecular size
What factors affect the rate of diffusion through cell membranes?
Lipid solubility
Molecular size
Cell membrane thickness
Concentration gradient
Membrane surface area
Composition of lipid layer
How do ions passively move through channels?
Channels form pores in membranes
Specific for single substance
Flux of ions is passive and is dictated by concentration gradient
How do uniporters work?
Bind and transport molecules of one type down concentration gradient
Regulated by insertion/removal of uniporter from membrane
Why does glucose use a transporter rather than a channel?
Charge, size and shape
Glucose channel would allow movement of other ions and water
Loss of selectively permeable nature of cell membrane
What is primary active transport?
Transport by ion pumps and is ATP dependent
What is secondary active transport?
Symporters and antiporters
Uses energy stored in concentration gradients by primary active transport
What is the osmolarity of plasma?
150 mM Na+
5mM K+
2mM Ca++
130mM other solutes
How do you calculate osmolarity in biological fluids?
Measure depression of freezing point - affected by total concentration of solution
One mole of solute depresses the freezing points of water by 1.86oC
Depression in freezing point/-1.86 = x Osm = 1000x mOsm
What is the solute composition of ECF?
150mM Na+
5mM K+
2mM Ca++
What is the solute composition of ICF?
10mM Na+
150mM K+
0.0004 mM Ca++
Is tonicity or osmolarity more physiologically relevant and why?
Tonicity as it accounts for cell membrane permeability
What is osmolality?
The number of solute particles in 1kg of solvent
In the case of dehydration, how is blood osmolality regulated?
Blood volume decrease and plasma osmolality increase
Osmoreceptors are activated by this increase
Thirst is stimulated, releasing vasopressin
Water retention promoted
How does glucose cross epithelial cells and enter interstitial space?
ICF [Na+] is reduced by the Na+ pump on the basolateral membrane
Low [Na+] creates a gradient for Na+ to enter, but there are no suitable channels on apical membrane
Na+/Glu symporters harness the energy from Na+ gradient to allow Na+ and glucose to enter
Glucose leaves the basolateral membrane using a glucose uniporter
What must be in place on epithelia to make glucose enter blood?
Sodium pump must be on basolateral membrane
Na/Glu symporter must be on apical membrane
Glucose uniporter must be on basolateral membrane
How do antiporters and symporters move molecules through the plasma membrane?
Uses energy stored in concentration gradients established by primary active transport
What does the route of water in transport through epithelia depend on?
Type of epithelia
Location within the body
What is the paracellular pathway of water transport?
Leaky tight junctions
Requires osmotic gradient, but essentially a constitutive process
How does absorption work?
Na+ pump extrudes Na+ via basolateral membrane
Na+/Glu symporter allows Na+ to cross apical membrane
Na+ is pumped out of basolateral membrane and glucose leaves via uniporter
How is water absorbed into a cell?
Uses the Na+/Glu movement which creates a large osmotic gradient across the cell
Water moves paracellularly from lumen to interstitial space
How does oral rehydration therapy work?
Increased interstitial fluid [Na+] is required for reabsorption of water
Glucose accelerates absorption of solute and water
How is secretion of H2O work?
Opposite of absorption
Export Cl-
H2O and Na+ will follow
