Transport across cell membranes Flashcards
Movements across cell membrane occurs how?
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
- co-transport
What is diffusion?
- Net movement of particles down conc gradient
- random movement, particles possess KE & passive process
Properties of molecules that can pass through phospholipid bilayer by simple diffusion?
- lipid soluble
- small structures
- non charged (e.g. no ions)
- non polar
Fick’s law
Rate of diffusion propertional to Conc grad X SA / DD
What’s microvilli?
tiny hair projections that increase SA and therefore efficient exchange
What’s facilitated diffusion?
- transport of substance across biological membrane down conc gradient by transport protein ( carrier or channel)
What kind of process is facilitated diffusion?
passive
Why are transport proteins needed?
- ions & polar molecules diffuse slowly
- transport proteins speed things up
Describe how carrier proteins work?
- have binding sites present where molecules to be transported bind
- proteins undergo CONFORMATIONAL CHANGE
- eventually, opening up on other side of cell membrane, solute released to other side of membrane
Describe how channel proteins work?
- interact weakly with material to be transported
- provides a hydrophilic passageway
- if open, specific solutes can freely be transported & pass through them
Factors affecting facilitated diffusion?
- conc gradient
- Number of transport proteins! (more in use, faster rate of diffusion)
Explain why the phospholipid bilayer is described as the ‘fluid mosaic’ model
FLUID: individual phospholipid molecules can move relative to each other. Gives membrane a flexible structure that is constantly changing shape
MOSAIC: proteins embedded in phospholipid bilayer vary in shape, size & pattern the same way stones & tiles do in a mosaic
What is osmosis?
- movement of water molecules down water potential gradient through selectively permeable membrane
Water potential measured in what?
kPa
Pure water has a water potential of what?
0
What is water potential?
- tendency of water to diffuse to another area by osmosis
What’s a hypertonic solution?
- higher conc of solutes outside of cell
What’s an isotonic solution?
- equal conc of solutes
What’s a hypotonic solution?
- lower conc of solutes outside cell
Rate of osmosis depends on what?
- water potential gradient
- thickness of exchange surface
- SA of exchange surface
What is active transport?
- movement of molecules or ions in & out of cell against conc gradient using ATP & carrier proteins
compare facilitated diffusion & active transport
1 active transport requires ATP but FD is passive
2 active transport moves up conc gradient, FD moves down
3 FD uses carrier & channel proteins but active transport only uses carrier proteins
Describe the process of direct active process
1 molecules bind to receptor sites on protein, on inside cell ATP binds to protein, causing it to split into ADP and a phosphate molecule
2 as a result, protein changes shape & opens to other side of membrane
3 molecules released to other side of membrane
4 phosphate molecule released from protein which causes protein to revert back to original shape
5 phosphate molecule recombines with ADP to form ATP during respiration
What’s co-transport?
- type of transport across cell membrane that requires transport proteins embedded into membrane of cell
Describe co-transport process
1 Na+/K+ pump uses active tramsport to move K+ ions into epithelial cells & Na+ (sodium ions) out of cell 2 establishes conc gradient between epithelial cell & ileum 3 sodium ions move into epithelial cell by facilitated diffusion (down conc gradient) 4 glucose (or AA) is co-transported along w/ sodium into cell 5 glucose (or AA) pass into blood plasma by facilitated diffusiom
Describe the phospholipid bilayer
two layers of phospholipids with hydrophillic heads & hydrophobic tails
Structure of plasma membrane
- fluid mosaic model
- made from phospholipids, cholesterol (provides rigid structure), proteins and carbohydrates
Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane [5 Marks]
- simple/facilitated diffusion down conc gradient
- small/non-polar/lipid soluble molcules pass via phospholipid bilayer
- water moves by osmosis
- active transport against conc gradient
- active site and FD involves carrier proteins
- active transport requires ATP
- ref. to Na+/glucose co-transport
