Ion Transporters Flashcards
Which type of molecules can pass through the lipid bilayer?
Hydrophobic molecules such Oxygen, C02, N2, Benzene
Small, uncharged polar molecules such as H2O, Urea, Glycerol
Which type of molecules can’t pass through the lipid bilayer?
The large free energy change that would be required for a small hydrophilic molecule or ion to traverse the hydrophobic core of the lipid bilayer make the transverse movement of hydrophilic molecules across an intact biological membrane a rare event. Thus, membranes act as permeability barriers to all charged and hydrophilic molecules.
What is a permeability coefficient?
the ease with which a membrane is permeable to a molecule
What is passive transport dependent on?
Passive Transport is dependent on permeability and concentration gradient
Rate of passive transport increases linearly with increasing concentration gradient
What roles do transport processes have?
- Maintenance of ionic composition
- Maintenance of intracellular pH
- Regulation of cell volume
- Regulating 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
What is active or passive transport dependent on?
- Concentration gradient
- Membrane potential
How much energy do cells spend on active transport?
• Some cells spend up to 30 – 50 % of their ATP
on active transport
Describe the concentration gradient for Sodium
145 out, 12mM in
Describe the concentration gradient for Potassium
155mM in, 4mM out
Describe the concentration gradient for Calcium
1 x 10-7M in, and 1.5mM out
Describe the concentration gradient for chloride ions
123mM out, 4.2mm in
Describe the action of the Na+-K+-ATPase Pump
3 sodium ions extruded, 2 potassium enter
What is the importance of the sodium potassium pump?
Forms Na+ and K+ gradients:
– Necessary for electrical excitability
– (only contributes < - 5 mV to resting membrane potential)
Drives Secondary Active transport:
– Control of pH
– Regulation of cell volume and [Ca2+]
– Absorption of Na+ in epithelia
– Nutrient uptake, e.g. glucose from small intestine
Describe the structure of the Na+-K+-ATPase Pump
-
Describe how Ca2+ concentration in the cell is controlled
- NCX (3 sodium in, for one calcium out. Low affinity, high capacity)
- SERCA
- PMCA (Proton moved in, Calcium ion moved out. ATP phosphorylated. System has a high affinity and low capacity)
- Mitochondrial storage
Describe the roles of the NCX exchanger
Electrogenic – current flows in the direction of the Na+ gradient
Role in expelling intracellular Ca2+ during cell recovery
Possible role in cell toxicity during ischaemia/reperfusion
Depolarised membrane potential reverses mode of operation (during ischaemia, ATP depleted, so sodium accumulates in cell. Causes calcium to move in which can cause toxicity)
Describe the action of PMCA
Describe how mitochondria can buffer intercellular calcium concentration
Describe how calcium is taken up by the endoplasmic reticulum
Describe the action of the Na+-H+ Exchanger
It is an acid extruder. It moves one sodium in, and one proton out. It regulates cell volume.
Amiloride inhibits and it is activated by growth factors
Describe Na+-Glucose Co-Transport
How is cellular pH controlled?
Sodium-Potassium ATPase sets up gradient for all transporters to work.
Na+-H+ Exchanger is an acid extruder
Na+-Cl-HC03-H+ (NBC) cotransporter is an acid extruder and alkali intruder
Na+-HCO3- cotransporter is an alkali intruder
HCO3-Cl- contransporter is an alkali extruder
How is intracellular pH regulation co-ordinated?
pH is held at the set point. Any drift away from this pH is corrected by the increased activity of appropriate transporters
How is cell volume regulated?
Transport of osmotically ‘active’ ions, e.g. Na+, K+, Cl- or organic osmolytes (amino acids). Water follows.
Cell swelling – extrude ions
Cell shrinking – influx ions
