Flashcards in BIOL 0800 Reading- Chapter 4 Deck (85):
What happens as a result of random thermal motion of molecules in a liquid or gas?
They will eventually distribute themselves uniformly throughout a container through diffusion
What is flux?
The amount of material crossing a surface in a unit of time
Which direction does net flux always occur?
From areas of high concentration to areas of low concentration
What four factors does net flux depend on?
Temperature, molecule size, surface area, and medium
How do temperature, molecule size, surface area, and medium affect net flux?
Higher temp leads to greater flux, large molecules have smaller flux, and higher surface area leads to greater flux, and denser mediums lead to slower flux
Diffusion times increase in proportion to what measure of distance?
The square of the distance over which the molecules diffuse
What is bulk flow?
How the circulation system is used to provide local diffusion to speed up the process, since it takes way too long for particles to diffuse over distance larger than a few micrometers
What is the Fick diffusion equation?
J = PA(C0-C1), where P is the permeability coefficient, A is the surface area, and C is the concentration difference
The greater the permeability coefficient, what happens to flux?
The greater the flux
How do polar and nonpolar molecules diffuse through cell membranes?
Polar: very slowly or not at all, and nonpolar: very quickly and easily
Why don't polar molecules diffuse through the cell membrane?
Because they can't dissolve in the nonpolar regions of the membrane occupied by fatty acid chains of phospholipids
What happens when you increasing the lipid solubility of a substance by decreasing the number of polar or ionized groups it contains?
It allows for more molecules to be dissolved in the membrane lipids, which increases the flux of the substance across the membrane
Why are most organic molecules that make up the intermediate stage of metabolic pathways retained within cells and organelles?
Because they're polar and can't cross the cell membrane anyway.
How are ions diffused through cell membranes?
Using protein ion channels that are embedded in the membrane
How big are ion channels?
Very small, to prevent other molecules from passing through
What is more common, a donut ion channel or a multi-protein-walled ion channel?
What is one of the most important qualities of ion channels?
They're selective for certain ions or types of ions, based on the channel diameter, the charged and polar surfaces of the protein, the number of water molecules associated with the ion (waters of hydration)
What is "waters of hydration?"
The number of water molecules associated with an ion
What is a membrane potential?
The separation of electrical charge across plasma membranes of all cells, measure in millivolts
What does the membrane potential do?
Provides an electrical force to influence movement of ions across membranes
What is the electrochemical gradient?
The combination of the driving forces of the concentration difference and the electrical difference that contribute to ion fluxes
What are ligand-gated channels?
Channels that require the binding of specific molecules to their proteins that directly or indirectly produce an allosteric or covalent change in the protein shape
What are voltage-gated channels?
Channels that require changes in the membrane potential to cause movement of charged regions on the protein
What are mechanically-gated channels?
Channels that require physical deformation of the memebrane to affect the conformation of the protein channel
What are the three types of channel protein conformations?
Ligand-gated, voltage-gated, and mechanical-gated
Are ions and their channels matched one-to-one?
No, a single type of ion may be able to pass through multiple types of channels
What is mediated transport?
The movement of substances through a membrane by transporters, for large molecules and nondiffusional movements of ions
How does mediated transport occur?
The solute binds to a specific site on the transporter in the membrane; the transporter undergoes a change in shape; the substance dissociates from the transporter and passes through to the other side of the membrane
What is the main difference between mediated transport and ion channels?
Ion channels typically move several thousand times more ions than do transporters, because they don't usually require a shape change and can be open for a continuous flow
What three factors determine the magnitude of solute flux through a mediated transport system?
Saturation of transport binding sites (depends on solute concentration and affinity of transporters for the solute); the number of transporters available; and how fast the conformation change in the transport protein occurs
How does the flux by mediated transport differ from diffusion?
Flux due to diffusion increases proportionally to the increase in solute, whereas flux due to mediated transport levels off when the transporters are saturated
What are the two types of mediated transport?
Active transport and facilitated diffusion
What is facilitated diffusion?
The net flux of a molecule "downhill" through transporter molecules
What is one of the most important examples of facilitated diffusion, and why is it continuous?
Glucose into the cell; keeps net flux going in because glucose is almost immediately broken down into glucose 6-phosphate
What encodes for each transporter?
A different gene
What is active transport?
Used to move a substance "uphill" against the electrochemical gradient using ATP or an electrochemical gradient
What is the difference between primary and secondary active transport?
Use of ATP vs use of an electrochemical gradient to drive the process
Is the sodium potassium pump present in all cells?
How does a Na/K pump work?
Transporter uses ATP to bind three Na inside itself from inside the cell; this activates the ATPase that phosphorylates the cytosolic surface and releases ADP; this causes a conformation change of the transporter and exposes the Na to the ECF, which also increases affinity for K; two K bind to the transporter, which dephosphorylates the transporter and returns it to its original conformation and repeats the cycle
What are the four most common active transport proteins?
(Na+/K+)ATPase, (Ca2+)ATPase, (H+)ATPase, and (H+/K+)ATPase
Where is (Ca2+)ATPase found?
In the plasma membrane(Ca out) and several organelle membranes including the ER (Ca in)
Where is (H+)ATPase found?
In the plasma membrane (H out, maintains pH) and several organelle membrane, esp. inner mitochondrial and lysosomal
Where is (H+/K+)ATPase found?
In the plasma membranes of acid-secreting cells in stomach/kidneys to pump H out (for digestion in the stomach lumen) and K+ in
How does secondary active transport work?
An ion, typically Na+, binds to the transporter protein because it's going down it's electrochemical gradient; while it does that, it allosterically changes the protein to increase infinity for the solute that's being transported, which attached itself and piggybacks its way through the membrane with the Na+ ion
How are primary and secondary active transport related?
The primary transport through the Na/K pump uses ATP to drive Na+ out of the cell to create a concentration gradient, which is what drives the secondary transport
What does "NaK, NaK" mean for the pump?
Knock knock, let me in: Na--K is the existing gradient, so more Na is out, and more K is in: because it's active transport, this means that Na continues to go out and K continues to go in
What is the movement of Na+ in primary vs secondary transport?
Primary: against gradient, from inside to outside; Seconary: with gradient, from outside to inside
What are cotransport and countertransport?
Also called symport and antiport: when actively transported solute moves into the cell (with Na+) or out of the cell (opposite to Na+ movement)
What kind of molecule facilitates osmosis?
Aquaporins, membrane proteins that allow diffusion of water through the membrane
What is the concentration of pure water?
Since pure water has a molecular weight of 18 g/mol and a liter of water is approximately 1000 g, it's 1000/18 = 55.5 M
What does it mean that one solute molecule will displace one water molecule?
Adding water to a solution to dilute the solution is essentially the same as adding solute to a solution to dilute the water
What does the degree to which the water concentration is decreased depend upon?
The number of particles of solute, not their chemical makeup: so adding 1 mol of urea has the same effect on the water concentration as adding 1 mol of glucose or 1 mol of amino acid or whatever. UNLESS the solute ionizes: then it decreases the water concentration in proportion to the number of ions formed
What is osmolarity?
The total solute concentration of a solution
What is an osmol?
1 mol of solute particles
How do the osmolarities of a 1 M glucose solution differ from a 1 M sodium chloride solution?
The first has 1 OsM, and the second as 2 OsM
How does osmolarity determine water concentration, in addition to referring to the concentration of solute particles?
The higher the osmolarity, the lower the water concentration.
The concentration of water in any two solutions having the same osmolarity is ___ because?
The same, because the total number of solute particles per volume is the same
What happens across the membrane separating A (high osmolarity/low water concentration) and B (low osmolarity/high water concentration) when the membrane is permeable to both vs only to water?
Case 1: both the solute and water will diffuse, settling both sides at equal osmolarity and water concentration; Case 2: since the solute can't diffuse to enable equal osmolarity, the water diffuses, changing the volume, which equalizes both the water concentration and osmolarity (because osmolarity is moles of solute per volume of water)
What is osmotic pressure?
The pressure that must be applied to the solution to prevent the net flow of water into it
What is the relationship between osmolarity and osmotic pressure?
The higher the osmolarity, the higher the osmotic pressure
Does osmotic pressure push water into the solution?
No, it's just the pressure that WOULD be needed to prevent net flow of water into the solution
What is the relationship between water concentration and osmotic pressure?
The lower the water concentration, the higher the osmotic pressure
Why does Na+ act like a nonpenetrating solute?
Because even though it diffuses in, it's pumped right back out
Why does Cl- act like a nonpenetrating solute?
Because of electrical repulsion generate by the membrane potential and secondary transporter action
What are the two main extracellular solute particles?
Sodium and chloride ions
What are the major solute particles inside the cell?
Potassium ions and organic solutes like large polar molecules
What is the approximate osmolarity of extracellular fluid?
How do hyper/iso/hypoosmotic differ from hyper/iso/hypotonic?
Osmotic refers only the moles of solute, so 150 mOsM of Na+ and Cl- and 100 mOsM of urea (readily passes through) has a molarity of 300 mM, isotonic to the inside of the cell, but an osmolarity of 400 mOsM, hyperosmotic to the inside of the cell.
What are the three types of endocytosis?
Pinocytosis (fluids), phagocytosis (solids), and receptor-mediated endocytosis
How does receptor-mediated endocytosis work with clathrins?
Certain molecules bind to specific proteins on the outer surface of the plasma membrane (receptors), and a cytosolic protein called a clathrin is recruited to the plasma membrane to "cage in" more ligand-bound receptors in a clathrin-coated pit that pinches off to form a clathrin-coated vesicle to fuse with an organelle or the other side of the cell to release the contents
What is potocytosis?
An extracellular ligand bins to a plasma membrane receptor, initiating formation of an intracellular vesicle, but usually only for low-molecular weight molecules like vitamins
What is potocytosis usually for?
What is the end result for the vesicles created through potocytosis?
The caveolae deliver the contents right into the cytosol
What are the vesicles created by potocytosis called?
What happens to the vesicles created during clathrin-dependent receptor-mediated endocytosis?
They can 1) fuse with the membrane of an organelle or 2) fuse with the plasma membrane on the other side
What is the advantage of clathrin-dependent, receptor-mediated endocytosis?
Because the clathrin helps group the ligand-receptor complexes together on the plasma membrane, there can be a concentrated intake of ligands without also taking in a lot of ECF
How does the total surface area of plasma membrane volume stay constant if endocytosis is continually pinching off into the cytoplasm?
Exocytosis replaces it at an equal rate
What two functions does exocytosis perform?
1) Replaces plasma membrane, and 2) allows membrane-impermeable molecules like protein hormones to be excreted into the ECF
An increase in what is the most common trigger for exocytosis?
Calcium in the cytosol, which activates proteins that let the vesicles fuse with the plasma membrane to deposit their contents outside the cell
What are the two pathways for transport across and endothelial cell?
Paracellular pathway (between) and transcellular pathway (through)
What limits the rate of paracellular transport across endothelial cells?
The tight junctions that bind the apical side of the epithelium together, because only small ions and water are tiny enough to diffuse through the tight junctions and because there's limited surface area as well
What is the flux direction of Na+ during absorption or secretion?
Na+ from lumen into epithelium during absorption, and Na+ from epithelium into lumen during secretion
What kind of transport is used to transport Na+ from blood/lumen during absorption?
Diffusion down the gradient into the cell on the apical side, and active transport up the gradient into the blood on the basolateral side
How does active absorption of a molecule X across the epithelium occur?
The molecule X is transported via secondary active transport as Na+ goes down its concentration gradient, then by facilitated diffusion across the basolateral membrane into the bloodstream