Molecular Movement Flashcards
What happens in diffusion?
Down conc. gradient
Through lipid bilayer or protein channel / carrier
No energy required
What happens in active transport?
Against conc. gradient
Protein carrier
Requires energy (ATP)
What happens as extracellular concentration increases in passive transport?
Rate of influx increases in proportion
Linear relationship
How do you calculate rate of diffusion into cell?
Permeability coefficient multiplied by concentration gradient across membrane
Equation for rate of diffusion into cell:
Vinward = PΔS
Vinward = rate of diffusion into cell
P = permeability coefficient
ΔS = conc. gradient across membrane
How do you work out concentration gradient across membrane?
Outside concentration minus inside concentration
ΔS = [S]o - [S]i
What does rate of diffusion in passive transport depend on?
Thickness + viscosity of membrane
Size, shape, polarity + solubility in membrane of substrate
Concentration difference!!
What’s a partition coefficient?
Gives a measure of how well a substance dissolves in lipid or aqueous phase
How do you find partition coefficient?
Shake substance with oil and water mixture
Measure conc. of substance in oil and water after settling
Equation to find partition coefficient:
[X]oil
K = ————
[X]water
Why does water move more quickly than expected?
Intrinsic protein channels transport water (aquaporins)
What do aquaporins form in the membrane?
Tetramers - each monomer acts as a water channel
What happens in diffusion through aqueous membrane channels?
Substance stays in aqueous solution and passes through hydrophilic channels - allow rapid transport
How wide are aquaporins?
1x water molecule wide - too narrow to permit any hydrated ions to pass through
How many aquaporins have been found in the body?
13
What are protein channels like?
Highly specific
Transport ions
May be opened/closed by gate
What are the types of gated protein channels?
Voltage gated
Ligand gated
What happens in a voltage gated channel?
Potential difference inside/outside cell causes confirmation change
What happens in a ligand gated channel?
Binding of chemical ligand causes conformational change
What type of channel is a sodium channel?
Voltage channel
What do sodium channels do?
Lined with negatively charged amino acids
Pull sodium ion away from water shell
Allow smaller sodium ion to diffuse through
Why can’t potassium ions diffuse through sodium channels?
Unhydrated potassium ions too large
What happens in a potassium channel?
Carbonyl oxygens strip water molecules from potassium molecule
What is facilitated diffusion also known as?
Carrier mediated transport
What can carrier mediated transport include?
Active transport - against conc. gradient
Facilitated diffusion - down conc. gradient
What happens in facilitated diffusion?
Substance binds to specific carrier which causes conformational change -> substance transported
What is rate of diffusion in facilitated diffusion limited by?
Vmax of carrier protein
Carrier proteins require time to open and close
What can active transport be inhibited by?
Competitive inhibitors binding to active site
Where can active transport get their energy from?
Directly from energy source - primary AT
Secondarily from energy stored as concentration difference resulting from secondary process - secondary AT
Example of primary AT:
Sodium/potassium pump
Obtains energy directly from breakdown of ATP
What does the sodium/potassium pump do?
Transports Na+ out of cells and K+ into cells
Establishes negative voltage inside cell
Structure of sodium/potassium pump:
2 K+ binding sites on external side
3 Na+ binding sites on internal side
Where does ATPase activity occur on sodium/potassium pump?
Internal side
What is the sodium/potassium pump important for?
Controlling cell volume
Contribution to electrical potential across membrane
Used to drive secondary active transport
What happens in secondary active transport?
Indirect use of energy source
Movement of 2 or 3 substances is linked
What do symporters do?
Transport substances in same direction as ‘driver’ ion
Uses electrochemical gradient
What do antiporters do?
Transport substance in opposite direction of driver ion
