Membrane transport Flashcards
What is meant by “selectively permeable” when applied to the plasma membrane?
The membrane allows some molecules to pass through but not others.
What properties influence whether a molecule can permeate the plasma membrane unassisted? (2)
- Solubility of the particles in lipid
- Size of the particle
What are the two methods of unassisted transport?
-Diffusion down a concentration gradient
And/Or
-Movement along an electrical gradient
When does diffusion cease?
When dynamic equilibrium (no net movement of molecules) is reached.
What is Fick’s law used to determine?
The rate of net diffusion across a membrane.
What are the 4 factors that influence the rate of net diffusion used in Fick’s law?
- Magnitude of the concentration gradient
- Surface area of the membrane where diffusion is taking place
- Lipid solubility of the substance
- Molecular wight of the substance (Smaller easier to diffuse)
- Distance through which diffusion must take place.
What is an electrical gradient?
A difference in electrical charge between two areas.
What is a cation?
A positively charged ion.
What is osmosis?
The net diffusion of water down it’s own concentration gradient.
What feature of a cell membrane allows water (which should have poor permeability of the membrane due to it’s low solubility in water) to pass through readily?
Aquaporins, protein channels that allow water to pass in and out.
What is osmolarity?
The concentration of osmotically active particles present in a solution.
What is tonicity?
The effect that a solution has on cell volume
What is meant by isotonic?
A solution that contains an equal solute concentration to the cell, therefore does not affect cell volume.
What is meant by hypertonic?
A solution that contains a solute concentration greater than that of the cell, hence causes cell to shrink.
What is meant by hypotonic?
A solution that contains a solute concentration less than that of inside the cell, causing the cells to swell (and eventually burst)
Describe carrier mediated transport.
A substance binds onto a specific carrier which undergoes a conformational change to transport the substance.
What 3 characteristics determine the kind and amount of material transferred across the membrane? Describe these characteristics.
- Specificity- Each carrier is specialised to carry a specific substance or a few closely related compounds.
- Saturation- When all carrier proteins are occupied by substance, the transport rate can no longer increase.
- Competition- When the presence of one substance diminished the rate of transport of another.
What two forms does carrier mediated transport take?
- Facilitated diffusion (Not requiring energy)
- Active transport (requires energy)
Describe facilitated diffusion.
A carrier is used to assist the transfer of a substance across the membrane down the concentration gradient.
Describe active transport.
A carrier expends energy to transfer a substance against a concentration gradient.
What are the two forms of active transport?
- Primary
- Secondary
Describe primary active transport.
Energy is directly required to move a substance against it’s concentration gradient.
Describe secondary active transport.
- Energy is required but not used directly to produce “uphill” movement.
- The carrier does not split ATP, moves molecule uphill using a second hand energy stored in the form of an ion concentration gradient.
What is another name for the NA+K+ ATPase?
Sodium potassium pump
What is the ratio of transport in the sodium potassium pump?
-3 Na+ out for every 2 K+ into the cell.
What are the three important roles of sodium potassium pumps?
- Establish Na+ and K+ concentration gradients across plasma membranes.
- Regulates cell volume by controlling solute concentration inside cells.
- The energy used to drive the pump indirectly serves as the energy source for secondary active transport.
What are the two mechanisms that secondary active transport occurs by?
Symport
Antiport
Describe symport.
The solute and Na+ move in the same direction.
Describe antiport.
The solute and Na+ move in the opposite direction to each other
What is endocytosis?
The pinching off of membrane to engulf a substance from the extracellular matrix.
What is exocytosis?
The release of contents from the intracellular matrix into the extracellular matrix.
