Cell Physiology Flashcards
What is diffusion?
The spontaneous movement of individual molecules in a solution - from a region of high concentration to a region of low concentration
What is Brownians Motion?
The theory that solute molecules move independently of solvent molecules and other solute molecules
What is flux (J)?
Describes the magnitude and direction of solute movement.
Flux refers to the movement of substances across a membrane in a specific direction, taking into account both the influx and efflux of molecules. It is an important process that helps maintain the balance between the internal and external environments of cells.
What percentage of male and females bodies are water?
Men 66%
Women 55%
What percentage of the bodies water total is ICF?
66% (and 33% ECF)
What doe ECF consist of?
ISF
Plasma (solution in blood)
Transcellular (inside epithelial lined spaces)
What percentage of ECF is ISF?
75%
What percentage of ECF is plasma?
20%
What percentage of ECF is transcellular?
5%
How many mol L -1 of Na+, K+ and Cl- is in plasma ECF?
150
5
110
How many mol L -1 of Na+, K+ and Cl- is in muscle ICF?
10
150
8
What is the plasma membrane?
Physical and chemical barrier separating the inside of a cell from the outside of a cell.
What is the composition of a plasma cell membrane?
A lipid bilayer with embedded proteins that are 5-10 mm thick.
It has phosphate-rich heads on the outside and hydrophobic lipid tails on the inside.
It is soft and flexible
Lipid bilayer is impermeable to water-soluble molecules.
What does Fick’s law describe?
It describes the net diffusion flux
What are the units for Ficks law?
cm2 or m2/sec
What three factors influence the diffusion coefficient (the amount of a particular substance that diffuses across a unit area in 1 s under the influence of a gradient of one unit)?
Temperature
Viscosity of the solution
Difference in size of solute molecules
What is “P” in Ficks Law?
Permeability coefficient
it is the combination of D, A and x constants.
What does the permeability coefficient tell us?
Measures how fast solute crosses membrane
gives an indication of the rate of movement - quantifies the ease with which solutes cross the cell membrane
BUT makes no assumption about the mechanisms involved for the solutes crossing the membrane
How is the permeability coefficient worked out?
Experimentally determined
Obtained from the modification of Ficks Law
Jnet = P x change in C (which is calculated by concentration outside - concentration inside)
The steeper the concentration gradient, the faster the rate of diffusion BUT what?
But only if the cell membrane is permeable (P) to that solute (and this varies a lot).
What does solubility diffusion explain?
the movement of most non-electrolytes (uncharged solutes)
What does permeability correlate with?
Lipid solubility and size of solute
E.g.,
The more lipid soluble the solutes, the more permeable they are
The smaller the molecule (molecular weight) the faster the rate of diffusion
On a graph showing the relationship between lipid solubility and permeability what is on the Y axis and what is on the X axis (include units)?
Y axis = permeability x 10 ^7 (cm/s^-1)
X axis = solubility in lipids (oil water partition coefficient - includes methanol, ethanol, butanol, propanol and acetamide).
Will smaller solutes always have greater permeability?
Normally but not if the thing that makes them bigger make it move faster.
What does solubility diffusion not account for?
The permeability of electrolytes (Na+, K+, Cl-, Ca2+ etc) - often the permeability is much greater than predicted from their lipid solubility.
E.g., does not account for charged particles.
Is the permeability of two of the same ion always the same?
No - it can vary between cell to cell and within a cell.
What is osmosis?
The diffusional movement of water from a region of greater water concentration/activity to an area of lower water concentration/activity
What is the change in water concentration due to?
A difference in the solute concentrations - when you add solute molecules water molecules are displaced and so the concentration of water is reduced.
So high solute means low water.
What is the equation for concentration?
C = n / v
Concentration is equal to the number of molecules in relation to the specific volume
What is osmotic pressure?
The pulling force acting on water
What is hydrostatic force?
The amount of applied force required to stop osmosis
means quantifying the driving force for water movement by osmosis
impractical for cell so we generally can’t work this out
What equation describes the osmotic pressure of a solution?
The Van’t Hoff Equation
What does osmotically active mean?
free floating
What are the essential points about osmotic pressure?
The chemical nature of the solute is irrelevant (all that is important is the total number of osmotically active solutes)
And therefore the salts (electrolytes) and non-electrolytes are treated the same.
Is it electrolytes or non electrolytes that dissociate?
Salts (electrolytes)
How do you work out osmolarity?
Using the Van’t Hoff equation
osmolarity = Ci
The Van Hoff Equation tells us that osmotic pressure in atmospheres is equal to the solute concertation times the number of solute molecules formed by dissociation.
E.g., if you has 150mmol of NaCl you would times 150 by 2 to get the osmolarity of 300 mOsmol L-1.
What is the difference between Osmolarity and Osmolality?
Osmolarity Oslo L -1 solution
Osmolality is Oslo Kg-1 solution
In physiological terms this difference is insignificant
What is the critical requirement for osmotic pressure to occur?
It can only exist when osmosis can occur. Therefore when (1) there is a difference in the osmolarity between two compartments and (2) water can freely movement compartments
Why can cells not move water against a gradient?
There if no water pump - the movement of water is passive only when we move solutes and there is a osmotic pressure across the membrane.
Can water diffuse through the bilayer?
Yes but it is slow so not efficient
What type of conductivity does the bilayer have?
high hydraulic conductivity - but varies between different membranes
What increases the hydraulic conductivity of membranes?
Aquaporins
How can hydraulic conductivity be regulated?
By vasopressin or antidiuretic (ADH) hormones
What is an aquaporin?
A small protein in the membrane that creates pores that increases the water permeability (100 fold)
Does water movement through aquaporins require a driving force?
Yes they still use the concentration gradient as a driving force for movement (no pump)
Compare what osmolarity describes vs what tonicity describes?
Osmolarity describes the contents of the solution and the cell whereas, tonality describes only the contents of the solution and the effect that has on the cell.
What is tonicity?
Toncity is about the effect a solution has on cells - e.g., whether the solution causes the cell to swell or to shrink.
What is our physiological reference point for working out osmolarity?
Approx 285 mOsmol L-1
Is osmolarity or tonicity dependant on what the specific molecules in the solution are?
Toncity
What is osmolarity?
The number of osmotically active molecules dissolved in a solution
What does it mean by water has FOMO?
That water always wants to move from a area of high water content to low water content (in other words from low salt to high salt - think of it as water get jealous that is has less salt so moves to where there is higher salt content).
What does isosmotic mean?
Same osmolarity
What does hyperosmotic mean?
Greater osmolarity
What does hypo-osmotic mean?
Lower osmolarity
If the cell has 285 mOmolL-1 and the solution has 310 mOmolL-1 is the solution isosmotic, hypo-osmotic or hyperosmotic?
Hyperosmotic because the solution has more osmotically active molecules than the cell.
(Cell is hypo-osmotic compared to the solution).
What does isotonic mean?
Cell volume stays the same - solution has no effect on the cell
What effect does a hypotonic solution have on a cell?
Solution causes the cells to swell
What effect does a hypertonic solution have on a cell?
Solution causes the cells to shrink
What is meant by protein mediated membrane transport?
Means that translocation across the membrane is not via the lipid bilayer, but via membrane transport proteins, that is, pores or channels (passive transport) or carriers, or ATP pumps (active transport).
What is passive transport?
The movement of molecules down their concentration gradient - therefore without energy (ATP).
E.g., diffusion, facilitated diffusion and osmosis through pores and channels.
What is active transport?
The movement of molecules against their concentration gradient through the use of energy (ATP).
E.g., Primary active transport via ATP dependant pumps and Secondary active transport via Carriers and Transporters
What is channel competition?
Channel competition refers to the scenario where different molecules or ions compete to pass through the same transport channel or carrier protein embedded in a cell membrane.
It can occur in the same direction or opposite direction.
What is channel inhibition?
Occurs when there are undesirable chemical interactions resulting in interference with channel activity or ability for substrates to bind.
Is channel inhibition always a bad thing?
No - it can be taken advantage of for example through inhibitory pharmaceutical drugs
What is the Na+/K+ ATPase?
Pump that actively transports Na+ out of the cell and K+ into the cell - essential for maintaining the ion gradient across the cell membrane (RMP).
What are pores?
Small tunnel through which molecules can move through rapidly
How are pores regulated?
By inserting / removing protein from membrane
What molecules are transported through pores?
Water (Aquaporins)
Small solutes
What is the key difference between pores and channels?
Pores are always open whereas channels exist in an open or closed state (a channel is a gated pore).
Are channels able to grade the extent of their flux?
No - when they open, they open to the same extend every time.
As a result the rate of ion flux through a channel is either nothing (when it is closed), or maximum (when it is open).
What causes channels to open and close?
Nothing - they open and close spontaneously - propensity of opening can also be modified by other agents
What are the two types of Na+ channels?
Voltage Gated Na Channels
ENaC (Epithelial Na Channel)
What determines permeability (P) for solutes crossing the membrane through channels?
The open probability (Po) = the amount of time the channel is open.
AND The number of channels (N) in the membrane for that solute.
What are examples of mechanically gated ion channels?
touch receptors and hair cells
What are carriers/transporters defined by?
Defined in terms of their substrate
What are examples of secondary active transporters?
SGLT (sodium-glucose linked transporter)
NHE (Na+ - H+ exchanger)
AE (anion exchanger)
What are examples of primary active transporters?
Na+-K+ ATPase
H+-K+ ATPase
What is a carrier?
A carrier is membrane spanning protein with a binding site.
Are carriers open from both sides at the same time?
No - carriers are conduits that are gated by two doors that are never open at the same time
How does a Na+ coupled solute cotransporter work?
The movement of Na+ down its electrochemical gradient drives the movement of another solute up its electrochemical gradient in the same direction
What are examples of Na+ coupled solute cotransporter?
Na+ glucose cotransporter (SGLT)
Na+ amino acid cotransporter (SNAT)
Na+H+ anti porter exchanger (NHE)
Are gated channels or carrier/transporter’s faster transporters?
Gated channels
Where are gated channels accessible from?
Both sides of the membrane when the channel is open
What happens on the other side of the membrane to where a ligand binds to a carrier/transporter?
Substrate is released
What is the relative movement of Na+ and K+ by the Na+/K+-ATPase?
Moves 3 Na+ for every 2 K+
What is the use of ATP blocked by?
Ouabain
What is the range of the mV inside a cell?
-60 to -90 mV
What cells have a static RMP?
Epithelial cells
What cells have a dynamic RMP?
Muscle cells
Dynamic means changes from negative to positive
What does a voltmeter measure?
Allows measurement of any voltage that develops across the membrane
What determines movement of uncharged solutes?
Concentration gradient
What is the electrochemical gradient a measure of?
The driving force on a charge solute
What does the Nernst potential calculate?
Calculates the diffusion potential for a given ion and thus the direction is will move
What two things can occur if there is an ion gradient across a membrane and the membrane is permeable to that ion?
1) The membrane potential will move towards the equilibrium potential for that ion (if the membrane is more permeable to that ion than others)
2) If the membrane potential is held at the equilibrium for that ion there will be no net movement of the ion across the membrane.
Do cell membranes have more K+ channels or Na+ channels?
More K+ channels
What is created by the fact that cell membranes have a lot more K+ channels?
A K+ leak current
What ion are cell membranes most permeable to?
K+ (because most channels)
If the voltage across the membrane is held at the equilibrium potential for that ion what will the net movement be?
there will be no net movement of the ion
What does the membrane potential (vm) and the extracellular concentration of Cl- determine?
The intracellular concentration of Cl-
Does Cl- determine the membrane potential?
No
In what circumstance can Cl drive the membrane potential?
Only if it had a higher permeability than K+ or Na+
What ion drives the membrane potential towards -90mV?
K+
What ion drives the membrane potential towards +72mV?
Na+
What sort of potential is Vm?
A steady-state potential
Is it easier to measure absolute permeability or relative permeability?
Relative permeability
What is permeability?
The capability of ions to flow across the membrane
What does conductance measure?
The movement of charge across the membrane
What is Ohm’s law?
V = IR
Where V is voltage (Volts), I is current (Amps) and R is resistance (Ohms)
What is the relationship between current and voltage in Ohms law?
proportional (if I is large so is V)
What is the relationship between current and resistance in Ohms law?
Inversely proportional (if I is small, R will be big)
What is the units of conductance?
Siemens
Finish the sentence - “the greater the conductance of an ion, the more…”
it will influence the membrane potential
What is the relationship between resistance and conductance?
Inversely proportional
= If R is small, conductance is high
= If R is large, conductance will be low
Channels open and close spontaneously but propensity of opening can also be modified by other agents. What are these agents?
Membrane potential
Mechanical Stress
Ligand Binding
Phosphorylation/Dephosphorylation
What factors influence the specificity of a membrane channel?
Solute size
Charge
AA interaction
