Membrane transport Flashcards
What two properties influence whether a particle can permeate the plasma membrane?
- Solubility of the particle in lipid
* Size of the particle
What two elements are required for any molecule to cross the membrane?
A pathway (assisted or unassisted) and a driving force (passive or active)
What 5 factors make up Fick’s law of diffusion?
- the magnitude of the concentration gradient
- the membrane’s surface area
- the lipid solubility of the substance
- the substance’s molecular weight
- the distance through which the diffusion is taking place.
What two types of gradients can molecule use to cross a membrane unassisted?
- concentration gradient
- electrical gradient
What is osmosis?
The net diffusion of water down its concentration gradient through a selectively permeable membrane.
What structures quicken the process of osmosis?
Aquaporins (water channels)
What does osmolarity mean?
The concentration of osmotically active particles present in a solution. (can be measured or calculated)
Normally around 300mOsm/l in body fluids
What does tonicity mean?
The effect a solution has on cell volume. No units.
What happens to cells in isotonic, hypotonic and hypertonic conditions?
Isotonic conditions- no net movement so no change in cell volume.
Hypotonic conditions- water diffuses into cells, cells swell.
Hypertonic conditions- water diffuses out of cells, cels shrink.
What are the two mechanisms for selective assisted transport?
- Carrier-mediated transport
* Vesicular transport
What are the three important characteristics that determine the kind and amount of material transferred across the membrane?
- Specificity- specialised to transport a certain substance or group of closely related substances.
- Saturation- what the transport maximum (Tm) is.
- Competition- e.g. 1 transporter carries two types of molecules, affecting the rate of transfer for either molecule.
What happens to the carrier in carrier-mediated transport?
When the substance binds, the specific carrier undergoes a conformational change which transports the substance.
What is cystinuria?
A condition in which amino acid cystine cannot be reabsorbed across the membrane in the kidneys. Forms crytals or stones in either the kidney or the urinary tract.
What are the two forms of carrier-mediated transport?
- Facilitated diffusion
* Active transport
Describe facilitated diffusion.
- no energy
- e.g. GLUT 4 transport
Describe active transport.
- requires energy
- works against concentration gradient
What are the two forms of Active transport?
- primary active transport
2. secondary active transport
What is primary active transport?
Energy is directly rewired to move a substance against is concentration gradient. e.g. Na/K pump.
What is secondary active transport?
Energy is required but not used directly to produce uphill movement.
Doesn’t split ATP to move molecule uphill, it uses secondhand energy stored in the form of an ion concentration gradient e.g. Na+ gradient.
What does the Na+/K+ pump transfer (no. of ions)
2K+ into cell
3Na+ out of cell
What two mechanisms can secondary active transport use?
- Symport/co-transport:
The solute moves in same direction as Na+.
2.Antiport (exchange):
The solute moves in an opposite direction to Na+
Describe vesicular transport.
• Requires energy for vesicle formation
• Endocytosis- pinching off of membrane to engulf substance
Exocytosis- intracellular vesicle fuses to membrane and releases its contents to ECF e.g. secrete enzymes and proteins and adds carrier or channels to the membrane like insulin & GLUT4
