Molecular Movement across cell membranes Flashcards
What is the lipid bilayer ?
Lipid bilayer cell membrane is a selectively permeable barrier to movement of substances.
What is the role of intrinsic proteins ?
Intrinsic proteins in the membrane provide pathways for some substances to cross the membrane.
Uniport
Moving 1 type of molecule in one direction
Co- transport
Moving different types of molecules in different directions
Symport
Different molecules being moved in the same direction
Antiport
Different molecules being moved in different directions
Diffusion
Occurs down a concentration gradient
Through lipid bilayer or involves a protein ‘channel’ or ‘carrier’
No additional energy required
Active transport
Occurs against a concentration gradient
Involves a protein ‘carrier’
Requires energy (ATP)
Describe passive transport
Particles eventually reach an equilibrium
Particles move along the concentration gradient
Net diffusion is equal to outside - inside concentration
What does the rate of diffusion in a cell depend on ?
Thickness and viscosity of membrane and size, shape, polarity and solubility in membrane of substrate.
Delta S
Concentration gradient across a membrane
Partition coefficient (K)
Gives a measure of how well a substance dissolves in lipid or aqueous phase.
X/X
e.g. Oil / Water
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible owing to liquid-liquid phase separation.
What does the rate of transport through lipid phase of membrane depend on ?
Polarity
Less Polar —-> Increase in diffusion rate
Except methanol which is HIGHLY polar, but diffuses quickly.
What transports water ?
Intrinsic protein channels called aquaporins
These were discovered in 1992 and explained the anomaly, as to why water moves more quickly, than in diffusion.
Describe aquaporins
They form tetramers in the membrane - each monomer acts as a water channel.
Each channel is approximately 2.8A at its narrowest point allowing continuous passage of one water molecule at a time.
1 water molecule wide, they are too narrow to permit any of the hydrated ions to pass through.
How many molecules of water can pass through an aquaporin per second ?
Up to 3x10^9 molecules
Describe the membrane arrangement of aquaporins
Secondary structure of aquaporins contains 6 α-helices connected by 3 extracellular and 2 intracellular loops.
All 6 α-helices exist in a closely associated tertiary monomer structure. Water passes through the transmembrane pore formed through the centre of the three-dimensional barrel
AQP monomers homotetermerize and create a five-pore quaternary structure.
Function of the central pore, formed by the space between all four monomers, remains largely unknown.
Describe passive transport of diffusion through aqueous membrane channels
Substance stays in aqueous solution and passes through hydrophilic channels.
Usually highly specific, rates can be very high
Few pores needed to give a big difference in permeability.
How many aquaporins are found ?
13 aquaporins found in different cells/tissues of the human body that differ in number/time/demand thus controlling differential water permeability/flow/function.
Where are aquaporins most commonly found ?
Red Blood cells
Kidneys
Types of Aquaporins in the kidney
AQP1,2,3,4,6,8,11
Aquaporins with a low water permeability
AQP :
0
6
9
10
ADH
Antidiuretic hormone
Regulates AQP-2 in the kidney
Function of AQP-2
ADH regulates AQP-2 in the kidney by stimulating movement of AQP-2 to the luminal side of the renal cell membrane in late distal tubules, collecting tubules and collecting duct
This increases water absorption
There are other AQPs on the basolateral membranes, probably not regulated by ADH
Describe some facts about protein channels
Many are highly specific
May be open/closed by a gate
Gated channels can be voltage/ligand gated
Channels are either open/closed
Channels are open for less than to a few ms
Voltage gated channels
Potential difference inside/outside cell causes a conformational change.
Ligand gated channels
Binding of a chemical ligand (e.g. acetyl choline) causes a conformational change.
Sodium gated voltage channels
They are lined with negatively charged amino acids that pull the sodium ion away from its water shell.
The smaller un-hydrated sodium ion can then diffuse through the channel.
Un-hydrated potassium ions are too large.
K+ uses another channel
Describe the selectivity of the potassium channel
Carbonyl oxygens in the selectivity channel strip water molecules from the potassium molecule (NOT THE SODIUM MOLECULE).
So only potassium ions can permeate.
Similarity between facilitated diffusion and active transport
In both cases, a substance binds onto a specific carrier resulting in a conformational change which transports the substance.
Saturate meaning
Limited binding sites and it takes time for transport to occur.
Vmax
The limit of the carrier protein to operate.
Primary active transport
Energy is directly from an energy source (e.g. ATP)
Secondary active transport
Indirect use of energy
Able to move ‘uphill’ without itself breaking down ATP
Energy which is stored as a concentration difference resulting form a “secondary” process.
Examples of primary active transport
Sodium Potassium pump
Ca2+ ATPase transporter
H+ ATPase transporter
Ca2+ ATPase transporter
Involved in muscle contraction
Present on the cell membrane and the sarcoplasmic reticulum in muscle fibres.
Maintains a low systolic Ca2+ concentration
H+ ATPase transporter
Cells in stomach
Found in parietal cells of gastric glands (HCl secretion) and intercalated cells of renal tubules (controls blood pH)
Concentrates H+ ions up to 1 million fold
Sodium Potassium pump energy source
Na/K ATPase
Obtains energy directly from breakdown of ATP
Transports both sodium and potassium
What determines the direction of the Na K ATPase pump ?
Na, K, ATP, ADP concentrations
What can drive the Na K ATPase pump in reverse ?
ATP being made from ADP
In electrically active cells, how much ATP is used to pump K+ in and Na+ out of cells ?
60-70% of the cells energy
Importance of sodium potassium pump
Controls cell volume
Contributes to electrical potential across the membrane : 3Na out and 2K in - a net loss of ions per pump action
Can be used to drive secondary transport
Coupling of primary and secondary active transport
Primary (direct) active transport
1. H+ is moved from a low to high concentration using energy from ATP
Secondary (indirect) active transport
2. H+ at high concentration outside H+ passes through channel - doesn’t need energy due to the electrochemical gradient.
Process is EXERGONIC
- Energy from 2 is used to transport S against its concentration gradient.
Co transport examples
Sodium and Glucose
Sodium and Amino Acids
Sodium and Calcium (anti port)
Describe secondary active transport with Na+ symporter
Na+ across the membrane is high outside and low inside the cell (maintained using ATP Na/K pump)
Na+ creates electrochemical energy due to pressure to diffuse through the membrane.
Na+ and a second molecule bind to the symporter and both are transported into the cell using Na+ electrochemical energy.
(e.g. glucose, amino acids and 2x HCO3-)
Symporter
Transport substance in the same direction as a ‘driver’ ion. e.g. Na+
Involves the use of an electrochemical gradient (usually from sodium)
Protein co-transporters are classified as symporters or anti-porters.
Na+ Glucose symporter
When both molecules engage conformational changes to allow transport.
Antiport
Transports substance in the opposite direction of a ‘driver’ ion.
Uses Na+ electrochemical energy to transport calcium or hydrogen ions out of the cell while Na+ enters.
Transport is in the opposite direction to the primary ion (e.g. Na+)
