Cell membrane transport Flashcards
What are the types of transport across cell membranes?
Active
Passive
What are the types of passive transport?
Simple
Facilitated
No energy input (ATP) is required for these
What is simple diffusion?
Movement down a concentration gradient.
No membrane proteins are involved.
What is simple diffusion dependent on?
Concentration gradient
Hydrophobicity / charge
Size
What is the relative ease of compounds simply diffusing across membranes?
Easy: Hydrophobic molecules - O2, CO2, N2 steroid homones
Small uncharged polar molecules - H2O, urea, glycerol
Large uncharged polar molecules - glucose, sucrose
Impossible: Ions - H+, Na+, HCO3-, K+ etc
What is transport of inorganic ions required for?
Regulation of intracellular ion concentrations.
Uptake of nutrients - e.g. glucose and amino acids.
Excretion of metabolic waste products.
What are the classes of facilitated diffusion?
Channels - discriminates on size and charge
Uniporter carrier proteins - involves a binding site for solutes.
What is facilitated diffusion?
Transports inorganic ions or small molecules across the membrane passively along their concentration / electrochemical gradients.
What is an electrochemical gradient?
Combines the concentration gradient and membrane potential.
This force drives a charged solute across a membrane.
What are ion channels?
Membranes are impermeable to ions so ion channels are used to transport ions across membranes.
What are the features of ion channels?
Exhibit ion selectivity.
Driven by concentration/electrochemical gradient.
Fast - transport 10^7 molecules per sec
May be regulated (open in response to stimulus)
How are ion channels regulated?
Voltage gated
Ligand-gated - extracellular ligand
Ligand-gated - intracellular ligand
Mechanically gated
What are K+ channels?
Most common ion channels.
Continuously open
Selective.
Quickly moves K+ out of the cell.
What are uniporter carrier proteins?
E.g. the Glucose transporter (Glut2) in gut epithelia.
Highly selective - transported molecule is bound to carrier
Relatively slow - <1000 molecules per second - because it requires conformational change.
What does a uniporter carrier protein look like?
What are glucose transporters?
They only transport glucose.
Expressed by most cell types
12 pass membrane spanning proteins
Alternate between 2 conformations.
How is glucose transported into erythrocytes by Glut1?
Glucose concentration is higher in blood than in erythrocyte.
Transported into cell by Glut1 along concentration gradient.
Glut1 works in both directions so gradient needs maintaining - glucose is converted to glucose-6-phosphate.
Glucose-6-phosphate not recognised by Glut1 so one-directional transport.
Why do cells maintain electrochemical gradients?
To drive transport across membranes.
To maintain osmotic balance
Electrical forces inside and outside of the cell must be balanced.
What happens if there is no active transport?
Without active transport to maintain electrochemical gradients, ions would flow down their gradients through channels, disturbing osmotic balance.
What is active transport?
It moves solutes against their electrochemical gradients.
This requires energy.
How do cells carry out active transport?
ATP-driven pumps
Coupled transporters
Light-driven pumps
What are ATP-driven pumps?
Couple the transport of a solute against its gradient to the hydrolysis of ATP - primary active transport.
What are coupled transporters?
Couple the transport of one solute with the gradient to another solute against the gradient - secondary active transport.
What are light-driven pumps?
Couple the transport of a solute against its gradient to the input of energy from light.
