Role Of Membranes As Permeability Barriers Flashcards
What type of molecules can passively diffuse across a lipid bilayer?
- HYDROPHOBIC MOLECULES such as O2, CO2, benzene and steroid hormones
- SMALL UNCHARGED POLAR MOLECULES such as water, urea and glycerol
Name 2 types of molecules that cannot passively cross a lipid bilayer and give examples of each
- LARGE UNCHARGED POLAR MOLECULES e.g. Glucose, sucrose
- INORGANIC IONS (carry a full charge) e.g. Na+, Ca2+, K+, Cl-
Define passive transport and state what it is dependant on
- Transport of substances across membrane without the use of protein transporters
- Dependant on PERMEABILITY COEFFICIENT and CONCENTRATION GRADIENT
What equation would you use to calculate the net rate of transport across a membrane, J?
- J = P (C1-C2)
- P is the permeability coefficient
- C1 and C2 are the concentration gradients on each side
What is the relationship between the the permeability coefficient (P) and the net rate of transport (J)?
- The higher the permeability coefficient, the more permeable a substance is
- Therefore the net rate of transport is greater (J is directly proportional to P)
Name 3 roles of transport processes in the membrane
- Maintenance of intracellular ion concentrations and pH
- Regulation of cell volume
- Extrusion of waste products
- Generation of action potentials through ion exchange
Give two examples of ligand-gated ion channels and how they can be activated
- NICOTINIC ACH RECEPTORS on the post-synaptic membrane (binding opens voltage gated Na+ channels)
- ATP SENSITIVE K+ RECEPTOR in pancreatic β cells (when intracellular ATP is high it binds and closes K+ channels)
Describe an experiment to test the permeability of a lipid bilayer to certain substances
- BLACK FILM
- Container of water with a septum that has a pinhole
- Paint phospholipids over pinhole (form bilayer) so can investigate the relative permeability of substances
Define active transport
- Transport of ions or molecules against an unfavourable concentration and/or electrical gradient
- Requires ENERGY directly or indirectly by ATP hydrolysis
What is facilitated diffusion?
- Diffusion of ions through SELECTIVE CHANNELS in the bilayer
- Each channel is selective for the ion it transports and depends on a concentration gradient
Describe the process of membrane transport at a gated pore
- PING PONG TRANSPORT
- Binding of substrate to receptor causes the membrane protein to undergo a CONFORMATIONAL CHANGE
- Substrate binds on outside and change in conformation allows the substrate to be released on the other side, so it can enter the cell
Describe how a ligand gated ion channel works and give an example
- Ion channel is CLOSED AT REST
- Binding of a transmitter/hormone opens the gated channel and allows the import of cations into cell
- e.g. Nicotinic ACh receptor
Describe how voltage gated ion channels are stimulated
- Channels do not interact with other molecules
- Stimulated to open by change in electric potential across the membrane e.g. Na+ channel
- Causes DEPOLARISATION or HYPERPOLARISATION of the membrane
Why is passive transport spontaneous?
- Passive transport relies on a concentration gradient (C1 -> C2)
- Energy is given off so -ΔG and it is spontaneous
Why does active transport require energy?
- Active transport is AGAINST A CONCENTRATION GRADIENT
- +ΔG so requires an energy input via ATP HYDROLYSIS
How much of a cell’s ATP is used in membrane transport?
30-50%
What are the normal intracellular concentrations of Na+,K+, Ca2+ and Cl-?
- Na+ = 12mM
- K+ = 155mM
- Ca2+ = 10^-7 M
- Cl- = 4.2mM
Is the extracellular concentration of K+ greater inside or outside the cell?
- GREATER INSIDE (155mM inside and 4mM outside)
- Net movement of K+ is OUT OF CELL
What are the normal concentrations of Cl- inside and outside of the cell?
- 4.2mM INSIDE
- 123mM OUTSIDE
What is the net movement of Na+ across the bilayer?
OUTSIDE cell —> INSIDE cell
Why is Ca2+ kept at a very low concentration inside the cell? (10^-7 M)
- Increase in intracellular Ca2+ can activate proteins and enzymes within the cell
- In some cells this is advantageous (stimulates muscle contraction) however in most cells this can lead to tissue necrosis
Where in the cell would you find an example of active transport down a concentration gradient (in the opposite direction)?
- ATP SYNTHASE enzyme in the inner mitochondrial membrane
- Transports H+ down a concentration gradient
- ACTIVE as it uses potential energy of the PMF
Give an example of a primary active transporter and describe its function
- PMCA (plasma membrane Ca2+ ATPase)
- Transports Ca2+ OUT OF CELL against the concentration gradient using ATP hydrolysis (requires Mg2+!!)
Co-transport of ions/molecules can occur by which two mechanisms of transport?
- SYMPORT (same direction)
- ANTIPORT (opposite direction)
What is meant by co-transport?
More than one type of ion or molecule is transported on a membrane transporter PER REACTION CYCLE
Describe the function of the Na+/K+ ATPase transporter and state which mechanism of transport it uses
- ANTIPORT
- Actively transports 3Na+ out of cell and 2K+ into cell using ATP hydrolysis
- NET transport of +1 into cell; uses 25% of BMR to maintain ion gradients
What is meant by ‘P-type ATPase’?
ATP phosphorylates ASPARTATE producing a phosphoenzyme intermediate
Why do some membrane proteins have a β subunit?
β subunit glycoprotein that directs the protein transporter to the plasma membrane from the ER membrane
Describe the action of the drug OUABAIN
- Cardiac glycoside drug
- INHIBITS Na+/K+ ATPase
Which pump is the most important when maintaining the resting membrane potential?
- K+ pump
- Outflow of K+ leaves a NEGATIVE MEMBRANE POTENTIAL of -70-80mV
What role does the Na+/K+ ATPase pump have in maintaining the resting membrane potential?
- Na+/K+ ATPase sets up a high intracellular concentration of K+
- This allows K+ to be transported out of the cell which maintains the -ve membrane potential
- ONLY contributes - 5-10mV to the resting membrane potential through electrogenic pump activity
Which two membrane transporters are involved in maintaining a low intracellular Ca2+ concentration?
NCX and PMCA
What is the role of the Na+/Ca2+ exchanger, NCX?
- ANTIPORT transport of 3Na+ into cell and 1Ca2+ out of cell (NET +1 into cell)
- SECONDARY ACTION of cell to maintain low intracellular Ca2+
What is the relationship between PMCA and NCX with respect to affinity and capacity for Ca2+?
- PMCA has a high affinity but low capacity
- NCX has a low affinity but high capacity
Which transporter is present in the gut cells (enterocytes) to aid glucose absorption?
Na+/Glucose co-transporter
Name the 4 types of co-transporter systems that can occur in membranes
- Na+/K+ ATPase
- NCX
- Na+/H+ exchanger
- Na+/Glucose co-transporter
Describe how the action of the Cl- membrane transporter in cystic fibrosis relates to its pathophysiology
- DEFECTIVE Cl- transport due to mutation of CTFR gene
- Decreased Na+ transport out of cell to maintain resting membrane potential
- Less Na+ so LESS WATER so results in viscous dehydrated mucus
Explain the function of the Cl- transporter is affected by Cholera
- Cholera causes activation of transporter via phosphorylation
- Cl- transporter is UPREGULATED so more Cl- transported into lumen of large bowel
- Results in more Na+ transported out, water follows so causes DIARRHOEA
Explain why the permeability for Cl- in an erythrocyte membrane is ~10^7 fold greater compared to a phosphotidylserine bilayer
- Band 3 protein facilitates the transport of Cl- across membrane in exchange for HCO3-
- This is essential for the function of the cell
Why are co-transporters considered as secondary active transport?
- Transport of one substance is linked to the concentration gradient of another
- Primary energy source e.g. ATP hydrolysis is used INDIRECTLY
What is the action of a UNIPORT transporter?
Transports ONE solute molecule across membrane from one side to the other
Describe the action of the Na+/Glucose co-transporter and state which mechanism of transport this uses
- SYMPORT
- Transports Na+ into enterocytes down its concentration gradient
- Entry of Na+ provides energy for entry of glucose
Give 2 examples of ANTIPORT co-transport systems in cells
- Na+/Ca2+ exchange (NCX) which transports 3Na+ in and 1Ca2+ out
- Na+/H+ exchange (NHX) which transports 1Na+ in and 1 H+ out
