Cell Membrane Transport Flashcards
What are the types of passive transport?
Simple diffusion
Facilitated diffusion
What are the types of active transport?
Primary (direct)
Secondary (indirect)
What is an electrochemical gradient?
Drives passive transport
Depends on concentration gradient of solute
For charged molecules also depends on difference in voltage between ICF and ECF
What is simple diffusion?
Movement of uncharged hydrophobic solutes (e.g. CO2) through the phospholipid bilayer down their electrochemical gradient
What is flux (Jx)?
How fast the solute X moves
What does flux depend on?
Permeability coefficient of X (Px)
Difference in [X] between ECF and ICF
What are transmembrane proteins?
Integral membrane proteins
Composed of membrane-spanning alpha-helical domains
Can be single or multi pass
What are the types of transmembrane proteins?
Pore
Channel
Carrier
Pump
All have multiple transmembrane segments surrounding a hydrophilic solute permeation pathway through membrane
What is a pore?
Protein that helps to form a pathway a solute can move through
What is a channel?
Gated pore
What is a carrier?
Protein that a solute binds to in order to move through
What is a pump?
Protein that requires ATP in order to change its configuration and move the solute through
What are amphipathic helices?
Proteins that have alternating hydrophobic and hydrophilic AA
Hydrophobic surfaces face lipid membrane
Hydrophilic surfaces create central pore
What is facilitated diffusion?
Movement across a membrane where the driving force is the electrochemical gradient
Allowed by pores (always open) and channels
What do all channels have?
A moveable gate
A sensor - voltage, ligand, mechanical
A selectivity feature
An open channel pore once sensor activated
What are the different types of channels?
Voltage-gated - senses changes in voltage
Ligand-gated - intracellular or extracellular ligands bind -> involved in cell to cell communication
Mechanical-gated - opens when the membrane is stretched
What is carrier mediated facilitated diffusion?
- Driving force is electrochemical gradient
- Never has a continuous transmembrane path
- Much slower than channel or pore - carriers have two gates that open and close at different times
- Can become saturated - Jx is limited by no of carriers in membrane and speed (stays occupied for longer)
What is active transport?
- Moves a solute against its electrochemical gradient
- Used by cells when they require a higher concentration of solute than is in ECF
- Requires metabolic energy through ATP
- Mediated by carriers - pumps, cotransporters, exchangers
What is primary active transport?
- The driving force is a chemical reaction e.g. ATP hydrolysis - releases energy used to move solutes against elec chem grad
- e.g. Na+/K+ ATPase
- Pumps can become saturated
What is secondary active transport?
- Uses cotransporters and exchangers
- Driving force is using the energy of a solute moving along its electrochemical gradient to move another against its gradient
What are cotransporters?
- Move both solutes in the same direction
- Requires a driving solute whose elec chem grad provides the energy to drive movement of something else out of cell
- Often the inward Na+ gradient
