Session 2 - Membrane Permeability Flashcards
Define facilitated transport
Diffusion of a molecule across a membrane by utilising specific proteins to increase permeability. It does not require energy expenditure
Define active transport
Movement of a molecule across a membrane against an electrochemical gradient. It requires energy which is obtained by coupling to a thermodynamically favourable reaction
What is secondary active transport?
Active transport where the energy for the movement is obtained by moving another molecule across the membrane down a concentration gradient through a co-tansporter
What is uniport?
Transport of one solute molecule species from one side of a membrane to the other
What is symport?
Transport of two or more molecules across a membrane where all of the molecules travel in the same direction across the membrane
What is antiport?
Transport of two or more molecules across a membrane where the molecules move in opposite directions across the membrane
What are cotransporters?
Transporters in cell membranes which transport more than one molecule across the membrane
What is the Gibbs free energy change in passive transport?
Negative as it is spontaneous
What is the Gibbs free energy change in active transport?
Positive, the transport requires energy input
How can protein channels used in facilitated diffusion be gated?
Ligand gated, voltage gated or gap junctions
How do carrier molecules operate in facilitated diffusion?
Substrate binding indices a conformational change which causes substrate movement across the membrane
What is the normal concentration of potassium inside a cell?
155mM
What is the normal concentration of potassium outside a cell?
4mM
What is the normal concentration of calcium inside a cell?
10^-7M
What is he normal concentration of calcium outside a cell?
1.5mM
What is the normal concentration of chloride ions inside a cell?
4.2mM
What is the normal concentration of chloride ions outside a cell?
123mM
What is the normal concentration of sodium ions inside a cell?
12mM
What is the normal concentration of sodium ions outside a cell?
145mM
What are the main functions of Na-K-ATPase?
Forms sodium and potassium gradients
Drives many secondary active transport processes
Which primary active transporters are involved in control of resting calcium levels?
PMCA, SERCA
What are the characteristics of the PMCA transporter?
Expels one molecule of calcium for every molecule of hydrogen brought into the cell
High affinity and low capacity so removes residual Ca
What are the main features of the SERCA pump?
Accumulates calcium into the SR/ER
One molecule of calcium brought into the SR/ER for every molecule of hydrogen that is removed from the SR/ER
High affinity and low capacity so removes residual calcium
What secondary active transporters are involved in control of resting calcium levels?
Na-Ca-exchanger (NCX), mitochondrial calcium uniports
What are the features of the NCX transporter?
Removes one calcium ion from the cell for every 3 sodium ions moved into the cell
Activity is membrane potential dependent: depolarisation reverses the mode of operation
Low affinity and high capacity so removes most calcium from cell
What are the features of mitochondrial calcium uniports?
Operate at high calcium concentrations
Act to buffer damaging calcium concentrations
How can the NCX transporter contribute to ischaemia?
- ATP depletion inhibits the Na-K pump
- Na accumulates inside the cell so the cell depolarises
- Depolarisation reverses the NCX
- Calcium is moved into the cell
- High calcium levels are toxic to the cell
How is acidification opposed in cells?
Expelling hydrogen ions or inward movement of bicarbonate ions
How can alkalisation be opposed in cells?
Expelling bicarbonate ions via the anion exchanger
What mechanisms are commonly employed by cells to regulate pH?
Na-K-ATPase, Na-H exchange, Band 3 anion exchange
What is the function of Na-K-ATPase in cellular pH regulation?
To provide a gradient to provide energy for all transporters except the anion exchange
How do cells resist cell swelling?
Osmotically active ions are extruded from the cell which causes water to leave via osmosis
E.g. Conductive systems extruding potassium and chloride via uniport
Extruded ions are overall electroneutral to avoid depolarising the cell
How do cells resist shrinking?
Via influx of osmotically active, electroneutral ions to cause water to move into the cell via osmosis
E.g. Conductive system where sodium and calcium are transported into the cell via symport
How is sodium taken up in the proximal tubules of the kidney?
- NkCC2 moves 1 sodium, 1 potassium and 2 chloride ions into the cells from the tubule
ROMK moves potassium from the cells into the tubule
Na-K-ATPase moves sodium into the blood and potassium from the blood into the cells
CIC-Kb moves chloride from the cells into the blood
KCICT moves chloride and potassium from the cells into the blood via symport
How can oedema or hypertension be treated?
By blocking one or more of the sodium reabsorption mechanisms using diuretics
Na excretion is increased so hyperosmotic urine is produced and more water is excreted
Define passive diffusion
Spontaneous diffusion of a molecule across a membrane along an electrochemical gradient without expending any energy
