B2.1 - membrane transport (3h) Flashcards
what is simple diffusion?
simple diffusion is passive movement of molecules down a concentration gradient (from an area of high concentration to an area of low concentration)
- when equilibrium is reached, random movement continues, but there is no net movement
what molecules are transported in simple diffusion?
this occurs because small, non-polar or uncharged molecules can move between phospholipids
- oxygen
- carbon dioxide
- steroid hormones
what is osmosis?
osmosis is the random, passive movement of water from an area low solute concentration (hypotonic solution) to high solute concentration (hypertonic solution), until equilibrium is reached
- via diffusion across a partially permeable membrane (which is impermeabe to solute molecules) and requires differences in solute concentration
- it is a passive process that doesn’t require ATP
what happens to osmosis in an isotonic solution?
if isotonic solutions occur on either side of the membrane, there is no net movement because equilibrium is achieved
what molecules are transported during osmosis?
water molecules move more readily across membranes than might be expected by their polar property
what is the role of aquaporins?
most membranes have channel proteins, called aquaporins, that allow water molecules to move through a pore in their structure, facilitating water molecule movement across the membrane
what is facilitated diffusion?
facilitated diffusion is a type of passive transport; a particular type of diffusion that occurs through a channel protein
what is the role of channel proteins?
channel proteins have pores through which specific molecules can pass, dependent on their size and charge, which makes membranes selectively permeable
how can channel proteins be controlled?
some channel proteins can be open, allowing movement of molecules through the pore, but have their shape changed so that no molecules can pass through
- (eg) ligand-gated channels - open when a specific signaling molecule (ligand) binds to part of the channel protein
- (eg) voltage-gated channels - open and closed as the charge difference across a membrane changes
what is required for facilitated diffusion to occur?
a concentration gradient is required, with net movement down a concentration gradient occurring until the concentrations in and out of the cell are equal and equilibrium is reached
what does the maximum rate of facilitated diffusion depend on?
what occurs at low concentrations?
the maximum rate of facilitated diffusion depends on the number of channel proteins; where there is a large concentration gradient the channels may become saturated
- at low concentrations, increasing concentration of ions outside the cell increases the concentration gradient and the rate of facilitated diffusion
what is active transport?
active transport is the movement of specific molecules against the concentration gradient through pump proteins, which use ATP to release energy
how is energy released for active transport?
energy is released through hydrolysis of ATP –> ADP + Pi for the conformational change
what is the role of pump proteins?
pump proteins are specific to one type of molecule, based on the shape and chemical properties
- (eg) Sodium-potassium pump to establish the resting membrane potential in neurons
what are the benefits of active transport?
- active transport allows uptake of molecules even when the extracellular concentrations are very low
- active transport allows nearly all of a substance to be absorbed, building up a concentration gradient or maintaining a specific concentration within a cell
