Studied by 1 person
Diffusion definition
Net movement of particles (molecules/ions) from area of higher conc. to area of lower conc.
Concentration gradient
Path from area of higher conc. to area of lower conc.
Particles diffuse down conc. gradient
Diffusion is a ____ process
passive
→ no energy required
Simple diffusion
When molecules diffuse directly through cell membrane
Why is facilitated diffusion needed?
Some larger molecules (e.g. amino acids, glucose) would diffuse very slowly through phospholipid bilayer because they’re big
Charged particles, e.g. ions + polar molecules, would diffuse slowly because they’re water soluble, and centre of bilayer is hydrophobic
Facilitated diffusion
To speed things up, large/charged particles diffuse through carrier/channel proteins in membrane
Particles move down conc. gradient
Facilitated diffusion is a ____ process
passive - no energy
How do carrier proteins work?
Move large molecules across membranes
Diff carrier proteins facilitate diffusion of diff molecules
Large molecule attaches to carrier protein in membrane
Protein changes shape
This releases molecule on opposite side of membrane
How do channel proteins work?
Form pores in membrane for charged particles to diffuse through
Diff channel proteins facilitate diffusion of diff charged particles
How does concentration gradient affect rate of diffusion?
Bigger gradient = faster diffusion
→ as diffusion occurs, difference in conc. between two sides of membrane decreases until it reaches equilibrium
→ diffusion slows down over time
How does thickness of exchange surface affect rate of diffusion?
Thinner exchange surface = faster diffusion
→ shorter distance for particles to travel
How does surface area affect rate of diffusion?
Larger SA = faster diffusion
Example of adaptation to increase rate of simple diffusion
Some cells (e.g. epithelial cells in small intestine) have microvilli - projections formed by cell-surface membrane folding up on itself
Microvilli give cell larger SA
Larger SA → more particles exchanged in same time → faster diffusion
How does number of carrier/channel proteins affect rate of facilitated diffusion?
Once all proteins are in use, facilitated diffusion can’t happen any faster
→ greater number of channel/carrier proteins = faster diffusion
Example of adaptation to increase rate of facilitated diffusion
Aquaporins are special channel proteins that allow facilitated diffusion of water through cell membranes
Some kidney cells are adapted to have lots of aquaporins
Aquaporins allow cells to reabsorb a lot of water that would otherwise be excreted by body
