Membrane Transport Flashcards
Phospholipid bilayer is permeable to:
small molecules and gases (O2, CO2, N2, H2O, glycerol, ethanol, benzene)
Phospholipid bilayer is impermeable to:
Large, uncharged or polar molecules (amino acids, glucose, nucleosides) and ions (Na+, K+, H+, Cl-, etc)
Overcoming the permeability barrier of the cell
It is crucial to proper functioning of the cell
Key components of transport:
Energy requirements
Selectivity/specificity
Specialized proteins involved
Size of transported molecule
Energy requirements
No energy required for transport - passive transport
Energy required for transport - active transport
Solutes cross membranes by:
Simple diffusion, facilitated diffusion, or active transport
Two categories of Passive Transport:
Simple diffusion and facilitated diffusion
Simple Diffusion
no specialized proteins required; rate of diffusion depends on concentration gradient, size of molecule, and hydrophobicity; limiting factor is hydrophobicity of the molecule; non-selective
flow of solute = concentration gradient x permeability coefficient
Unassisted diffusion down concentration gradient; typically only for gases and nonpolar molecules
Facilitated Diffusion
Transporters and channels; no energy required; solutes move down their concentration gradients; specialized proteins required to provide a path through the lipid bilayer; highly specific
Transporters
Carrier proteins; substrates binds to a specific binding site; protein and solute for an intermediate; conformational change occurs and solute is released on other side of membrane; regulated by external factors; saturation can occur
Alternating Conformation Model
Carrier proteins are constantly alternating their conformations regardless of the presence of the solute; increased concentration gradient drives solute transport
Types of Carrier Proteins
uniport, symport, or antiport
Glucose Transporter
Transports glucose across the membrane; uniport; transports via alternating conformation mechanism; GLUT1 (10 different glucose transporters in humans); glucose phosphorylated inside cell to prevent glucose being taken out of cell
Channels
Proteins which form a hydrophilic transmembrane channel that allows for specific solutes to cross the membrane; no energy required - solutes travel down concentration gradient
Types of Channels
Ion channels, porins, and aquaporins
Porins
Transmembrane proteins that allow rapid passage of various solutes (large); nonspecific; beta sheets form porins
Aquaporins
Transmembrane channels that allow rapid passage of water; tetramers formed from alpha helices; hydrophilic amino acid side chains form channel
Ion Channels
Transmembrane proteins that allow the rapid passage of specific ions; tiny pores; high selective; ion gradients important in cell function, therefore gradients are maintained through gates - voltage, ligand, mechanical;
Electrochemical gradient
Chemical/potential gradient; ions are charged, therefore ion concentration gradients are both chemical and electrical/potential;
Gated Ion Channels
Most ion channels are gated and open in response to some stimulus; opening and closing is an “all-or-none” response
Ion Channel Specificity
The size of the pore and the groups that interact with the solute determines what ions can cross through; specific amino acid side chains determine what ions can cross through pore
Active Transport
Protein-mediated movement of a solute up its concentration gradient; requires energy input; unidirectional; primary (direct) and secondary (indirect)
Three ways cells drive active transport:
Coupled transporter, ATP-driven pump, light-driven pump
Indirect Active Transport
Depends on the simultaneous transport of two solutes; favorable movement of one solute down its gradient drives the unfavorable movement of the other solute up its gradient; symport or antiport
Direct Active Transport
Direct hydrolysis of ATP drives the movement of solutes up their gradient;
Three types: V-type, F-type, P-type
V-type ATPase
Pump protons into organelles (vacuoles, vesicles, lysosomes, Golgi, etc)
F-type ATPase
Two components: transmembrane pore and peripheral membrane component which contains ATP binding site
Reverse process: ATP synthase
Present in bacteria, mitochondria, chloroplasts
P-type ATPase
Transport ions or phospholipids; reversibly phosphorylated by ATP on aspartate residue
Na+/K+ pump
Na+/K+ Pump
Maintains electrochemical ion gradients; allosteric protein; two alternating conformations with different affinities for Na+ and K+
ABC Transporters
Hydrolyze 2 ATPs to move larger, hydrophobic molecules; large class of proteins; two domains; important medically because some are used by cells to pump antibiotics or drugs out of the cells rendering the cell resistent to the drug
Na+ Symport
Drives the uptake of glucose by the cell