topic 2 - exchange across cell membrane - diffusion Flashcards
cgp (topic 2B) 40 - 42
what is diffusion
the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration
what way will the molecules and net movement diffuse
molecules will diffuse both ways, but the net movement will be to the area of lower concentration - this continues until particles are evenly distributed throughout the liquid or gas
what is the concentration gradient
the path from an area of higher concentration to an area of lower concentration - particles diffuse down the concentration gradient
is diffusion a passive process, what does this mean
diffusion is a passive process - no energy is needed for it to occur
why can oxygen and carbon dioxide diffuse easily through cell membranes
because they’re small so they can pass through spaces between the phospholipids - they’re also non polar which makes them soluble in lipids, so they can dissolve in the hydrophobic bilayer
what is simple diffusion
a form of diffusion that does not require the assistance of membrane proteins - when molecules diffuse directly through a cell membrane
why would charged particles diffuse slowly
because they’re water soluble and the centre of the bilayer is hydrophobic
what is facilitated diffusion
the passive movement of substances across a plasma membrane by diffusing through carrier proteins or channel proteins located in the plasma membrane
how do carrier proteins affect facilitated diffusion
carrier proteins move large molecules across membranes, down their concentration gradient - different carrier proteins facilitate the diffusion of different molecules
explain the three steps to facilitated diffusion
(1) a large molecule attaches to a carrier protein in the membrane
(2) the protein changes shape
(3) this releases the molecule on the opposite side of the membrane
how do channel proteins affect facilitated diffusion
channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient) - different channel proteins facilitate the diffusion of different particles
what are three factors that simple diffusion depends on
(1) the concentration gradient
(2) the thickness of the exchange surface
(3) the surface area
how does simple diffusion depend on the concentration gradient
the higher it is, the faster the rate of diffusion - as diffusion takes place the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium - meaning diffusion slows down over time
how does simple diffusion depends on the thickness gradient
the thinner the exchange surface, the faster the rate of diffusion
how does simple diffusion depend on the surface area
the larger the surface area, the faster the rate of diffusion
what are microvilli
small finger-like projections that are located on the outermost part of epithelial cells that extend from villi - formed by the cell surface membrane folding up on itself
how do microvilli increase rate of diffusion
they give the cell a larger surface area - meaning that more particles can be exchanged in the same amount of time - increasing rate of diffusion
what are two factors that facilitated diffusion depends on
(1) concentration gradient
(2) number of channel pr carrier proteins
how does facilitated diffusion depend on the concentration gradient
the higher the concentration gradient, the faster the rate of facilitated diffusion - as equilibrium is reached, the rate of facilitated diffusion will level off
how does facilitated diffusion depend on the number of channel or carrier proteins
once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient - so the greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion
what are aquaporins
special channel proteins that allow the facilitated diffusion of water through cell membranes
how does having more channel proteins increases rate of facilitated diffusion
aquaporins allow cells to reabsorb a lot of the water that would otherwise be excreted by the body
