L3 Ion Channels and transporters Flashcards
a) What type of molecules can get through the lipid bilayer?
b) What type of molecules can’t?
a) hydrophobic molecules such as O2, CO2, N2 and benzene as well as small uncharged polar molecules such as h20, urea and glycerol
b) Large uncharged polar molecules such as glucose and sucrose are too big and charged molecules (ions) such as H+, Na+, K+, Ca2+, Mg2+, Cl- and HCO3-
What is a permeability constant?
Calculated values that determine the speed at which a substance crosses the lipid bilayer (the higher the value, the faster the speed and hence the more permeable)
a) Difference between passive and active transport?
b) Types of Passive transport
a) Passive- no energy required and movement is down a concentration gradient (-∆G)
Active- energy is required (atp) and movement is against a concentration gradient (+∆G)
b)
- Diffusion
- Facilitated diffusion: through a protein channel eg. ion or voltage gated channels (open due to a stimulus)
- Osmosis
What is the ionic distribution between the ECF and ICF in a 70kg man?
TBW = 60/100 of 70 = 42 Litres
UPGRADE
What is flux?
Describes how fast a solute moves- the number of mole crossing a unit area of membrane per unit time (moles/cm2/s)
What happens with the diffusion of 2 solutes?
Each substance diffuses down its own conc gradient independent of the conc gradients of other substances.
What is osmosis?
Diffusion of water from a hypotonic solution (high WP) to hypertonic (low WP) across a semi-permeable membrane.
What is a semi-permeable membrane?
Is a membrane that regulates the the transport of materials by acting as a filter allowing some particles to pass or not according to size, solubility, electrical charge, or other chemical or physical property.
What would happen if a cell was placed in a:
a) Hypertonic solution
b) Isotonic solution
c) Hypotonic solution
a) The concentration of solutes is higher outside the cell and so water will flow out of the cell, by osmosis, causing the shell to shrivel.
b) the concentration of solutions is equal inside and outside the cell and so is in equlibrium.
c) The concentration of solutes is lower outside the cell (higher inside the cell) and so water will enter the cell by osmosis, causing the cell to swell and burst (lysis)
a) What is an osmole?
b) What is osmolality?
a) Measure of a solutions ability to create osmotic pressure and affect movement of water. It is proportional to the no. of osmotic particles formed in solution
b) It is a function of the concentration of particles in solution (expressed in mOsm/kg)
a) What is the normal osmolality of plasma, interstitial fluid and the intracellular fluid?
b) How can you estimate a patients serum osmolality and why?
a) 280-310 mOsm/kg or mmol/L
b) doubling serum sodium because Na+ always has Cl- attached and each is independently osmotically active
What would the patients osmolality be if their plasma concentrations were the following:
- Sodium = 140mmol/L
- Potassium= 5mmol/L
- Urea = 5mmol/L
- Glucose= 5mmol/L
300mOsm/kg
How can you explain the rapid diffusion of water possible in some cells which is faster than diffusion across the membrane should allow?
- Aquaporins (AQP)- Integral membrane proteins
- They are a channel for transfer of h20 and urea
- contain 6 transmembrane alpha helix proteins
- inner cavity lined with hydrophilic AA
- at the centre there is + charged residues which prevent movement of charged ions so they don’t distrupt ion gradients
- still uses osmosis as depends on conc gradient
What are the types of faciliated diffusion methods?
- Ligand gated ion channels:
e. g. Ach binding to ach receptor so that sodium ions enter the channel (Nicotinic ach receptor)
e. g. ATP sensitive channel, Atp binding closes channel - Voltage gated ion channels- voltage sensor inside structure and membrane depolarisation affects it
Active or passive transport- what is it dependent on?
Dependent on the electrochemical gradient:
a) Chemical gradient: difference in solute concentration across a membrane - will be passive if down concentration gradient and active if against concentration gradient
b) Electrical gradient: difference in charge across a membrane membrane potential- passive if down electrogradient and active if against - have to look at charge of particles and charge on side of membrane
What are the roles of transport processes?
- Maintenance of intracellular pH
- Maintenance of ionic composition
- Regulation of cell volume
- Generation of ion gradients for electrical excitability of nerves and muscles
- Extrusion of waste products of metabolism and toxic substnaces
- Concentration of metabolic fuels and building blocks
Difference between Primary active transporters and secondary active transporters?
Primary- directly uses a source of chemical energy e.g. ATP to move molecules across membrane
Secondary- uses electrochemical gradient set up by Primary AT store energy to move substances across membrane
Define the following terms:
a) Uniport
b) Symport
c) Antiport
d) Co-transport
a) Transport of only one ion or molecule on a membrane transporter in one direction
b) Transport of 2 or more ion or molecules on a membrane transporter in one direction- uses co-transport
c) Transport of 2 or more ions or molecules on a membrane transporter in opposite directions - uses co-transport
d) Cotransport is the name of a process in which two substances are simultaneously transported across a membrane by one protein- Symport and Antiport
Examples of Primary active transporters
- PMCA- Plasma membrane calcium ATPase
- ATP synthetase
- Na+ pump (Na+-K+-ATPase)
What is the Na+/K+- ATPase (Na+ pump)?
- Antiport: 3 Na+ ions expelled for every 2K+ ions that enter
- Uses ATP- Primary AT
- maintains cellular concentrations of Na+ and K+
- Main function is to generate ion gradients that are used to allow secondary active transport and action potentials
Why is the control of intracellular Calcium ion concentration important?
A high intracellular Ca2+ concentration is toxic to cells
Examples of secondary active transporters?
- Na+-H+ exchanger
- Na+-Ca2+ exchanger
- Na+-glucose co-transport
What is the Na+-Ca2+ exchange? (NCX)
- 3 Na+ in the cell, 1 Ca2+ outside the cell
- Antiport
- Low affinity, high capacity
- secondary
What is the Na+-H+ exchange?
- Na+ inward flow down its concentration gradient leads to cell alkalinisation by removing H+
- antiport
- secondary
