Lecture 2 Flashcards
What is a semi-permeable membrane?
A layer through which only allowed substances can pass
What is diffusion?
Movement from high to low concentration
• Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances
Passve transport - depends on what two things?
Permeability of a membrane
Concentration gradients
Don’t need to learn the following equation, just need to understand what it means:
J = P(C1 - C2)
Net rate of transport (J) given by
J = P (C1 – C2 )
Where P = permeability coefficient
C1 and C2 = concentration gradients on side 1 and 2
Rate of passive transport increases linearly with increasing concentration gradient
How can black film be used?
To test the permeability of phospholipids
What is the permeability of the following molecules:
- Hydrophobic molecules
- Small uncharged, polar molecules
- Large uncharged, polar molecules
- Ions
What does a high permeability coefficient mean?
What does a low permeability coefficient mean?
High - more permeable
Low - less permeable
Permeability coefficients - (DO NOT NEED TO LEARN!)
- different for each ion in different types of membranes
Transport processes have important roles including…
- Maintenance of ionic composition
- Maintenance of intracellular pH
- Regulation of cell volume
- Concentration of metabolic fuels and building blocks
- The extrusion of waste products of metabolism and toxic substances
- The generation of ion gradients necessary for the electrical excitability of nerve and muscle
Permeability of the membrane for certain substances can be increased in the presence of certain proteins
Models of membrane transport proteins - which are themodynamically unlikely?
- Reciprocating carrier (flip-flop)
- Rotating carrier
Facilitated diffusion by…
Gated pore -
‘Ping-pong transport’
- Substrate binds to transport protein
- This causes the protein to change it’s conformation
- The protein can then be released to the other side of the membrane
Faciliated diffusion by…
Ion channels
Channels open to the presence of the specific ion
Facilitated diffusion by…
Ligand-gated ion channels
Binding of the neurotransmitter e.g. acetylcholine, the channel opns allowing the specific ion e.g. Na+ to pass through
Facilitated diffusion by…
Voltage-gated ion channels
- Voltage sensor is present in the channel (sensor is located within the plane of the bilayer in the protein of the channel)
- A change in the membrane potential, e.g. it becomes depolarised, the protein sensors would move away from the positive charge, so would move upwards (in e.g. in pic below)
- This movement will result in a conformational change in the protein
Left of pic - membrane is polarised
Right of pic - membrane is depolarised
Don’t need to learn this just understand it…
What does the following graph show -
This graph shows that the proteins in the membrane, greatly increase the amounts of substances like glucose that can be transported across it (enhancing the permeability of the membrane for this substance)
What does -ΔG mean for diffusion?
Along electrical gradient.
What does +ΔG mean?
Against concentration gradient
Movement across a membrane is dependent on…
Concentration gradient
Membrane potential
Pic showing active and passive transport affected by concentration ratio and membrane potential…
- concentration ratio
- membrane potential
What is active transport? Where is energy from?
- Active transport allows the transport of ions or molecules against an unfavourable concentration and/or electrical gradient
- Energy directly or indirectly from ATP hydrolysis
- Some cells spend up to 30 – 50 % of their ATP on active transport
Summary of active and passive transport
Summary - checkpoint
• What is a semi-permeable membrane?
• What types of molecules can permeate a lipid bilayer?
• How do proteins contribute to membrane permeability?
– Transporters
– Channels
• Where does the energy come from for membrane transport?
Movement of water across membranes - can it move across without a transporter protein?
- ‘Surprisingly’, water can move relatively readily across the hydrophobic membrane bilayer
- Small and uncharged − despite the dipole created by the electron distribution
- Although it is polar, it is still able to move by diffusion through the dynamic bilayer environment (the phospholipids are moving and water is able to move between them)
- Driven by osmotic gradient of solutes on either side of the membrane
Distribution of body water - values in each ‘section’
Ionic distribution between ICF and ECF - (don’t learn values from this slide - learn values from flashcard 26)
Intracellular
Intravascular and interstitial water (very similar)
Free ion distribution across the cell membrane - values of the 4 major ions…
- intracellular
- extracellular (included intravascular/plasma and interstitial)
These values will vary a little from tissue to tissue, but if I have an idea of the value, it will really help clinically, e.g. if K+ is 6mM extracellularly - requires clinical interventions
Tonicity of solution - when comparing two solutions together:
What is a isotonic solution?
Equal solute, equal water (no net movement of water)
Tonicity of solution - when comparing two solutions together:
Hypotonic?
Less solute, more water
Tonicity of solution - when comparing two solutions together
- Hypertonic?
More solute, less water
How is the direction of osmosis determined?
Direction of osmosis is determined by comparing total solute concentrations
Arrows show water movement:
Hypotonic -> Isotonic -> Hypertonic
Net movement of water
Risk of hypotonic solution and hypertonic solution?
What is osmality?
The concentration of a solution expressed as the total number of solute particles, per kg
Under normal conditions the osmolality of:
plasma = interstitial fluid = intracellular fluid
280-310mOsm/kg
(or 280/310mmol/L)
this is the osmotically effective concentration
How is the osmottically effective concentration worked out?
Remember ‘pic’
How to calculate osmolality?
- In clinical practice, serum osmolality can be estimated by doubling serum sodium… (this is because also need to take the Cl- into consideration, as it dissociates from Na1+
e. g. what is the osmolality of a 154 mM NaCl solution, where osmotic coefficient = 0.93
154 x 2 x 0.93 = 286.4 mOsm/L
Osmosis definition
The movement of water from an area of low conc to an area of higher conc of solute
(from high water potential to low water potential)
Solute definition
Atoms, ions, or molecules dissolved in a liquid. The higher the conc. of solute, the faster the osmosis. If a membrane is present, the water will flow to the high conc. of solute
Osmotic pressure
The pressure created by water moving across a membrane due to osmosis. The more water moving across the membrane, the higher the osmotic pressure.
Osmole definition
• Measure of solution’s ability to create osmotic pressure and thus affect the movement of water
• Proportional to the number of osmotic particles in solution
• 1 mole of non-ionizble substance = 1 osmole
− 1 mole of glucose forms a 1 osmolar solution in 1L water
− 1 mole of NaCl forms a 2 osmolar solution in 1L water
− 1 mole of CaCl2 forms a 3 osmolar solution in 1L water
Osmolality definition
The concentration of a solution expressed as a total number of solute per kg
- When the concentration of a solution is expressed in osmoles per kilogram of water, the osmolar concentration fo a solution is referred to as its osmolality
- 1 osmole/kg H2O – 1 osmole/L = 1000 milliosmoles/L = 1000 mOSM = 1000 mmol/L
Osmosis - diffusion of water
- Diffusion of water from a hypotonic solution to hypertonic solution
- across a semi-permeable membrane
Aquaporin channels - Another way water can move through a semi-permeable membrane
(channels for…, allows…, isoforms…)
• Integral membrane proteins – tetrameric (4 subunit
• 13 or more isoforms (AQP0-AQP10 ……) - 13 different forms of these channels
• NOT ion channels
• Channels for water (some cases, small solutes), but think of them as water channels
• Permit rapid water diffusion above the rate of passive diffusion through the lipid bilayer - allows osmosis to occur more rapidly
• Distinct isoform distribution suggests important and specific roles in different cells/organs -> they have important and different toles in different cells/organs
Aquaporin channels - more about the channel i.e. charge…, what can’t travel through…
• Hydrophilic pore
• Single file passage
• Positively charged residues in pore
− prevent the movement of charge ions e.g. protons (H+)
− therefore H+ ion gradients not disrupted (therefore pH is not disrupted)
Don’t learn!
Aquaporin family members are widely distributed..
Just understand that there are a large number of aquaporin types and they are distributed widely throughout the body
Checkpoint slide - need to know…
• How does water permeate a lipid bilayer?
