2.3- Transport across membranes Flashcards
Describe fluid mosaic model of membranes
Fluid: phospholipid bilayer in which individual phospholipids can move= membrane has flexible shape
Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded
Explain role of cholesterol & glycolipids in membranes
Cholesterol: steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable
Glycolipids: cell signalling & cell recognition
Explain functions of extrinsic and intrinsic proteins in membranes
Extrinsic:
- binding sites/ receptors e.g. for hormones
- antigens (glycoproteins)
- bind cells together
- involved in cell signalling
Intrinsic:
- electron carriers (respiration/ photosynthesis)
- channel proteins (facilitated diffusion)
- carrier proteins (active transport/ facilitated diffusion)
Explain functions of membranes within cells
- provide internal transport system
- selectively permeable to regulate passage of molecules into/ out of organelles
- provide reaction surface
- isolate organelles from cytoplasm for specific metabolic reactions
Explain functions of cell-surface membrane
- isolates cytoplasm from extracellular environment
- selectively permeable to regulate transport of substances
- involved in cell signalling/ cell recognition
Name and explain 3 factors affecting membrane permeability
- Temperature: high temperature denature membrane proteins/ phospholipid molecules have move kinetic energy & move further apart
- pH: changes tertiary structure of membrane proteins
- use of a solvent: may dissolve membrane
Outline how colorimetry could be used to investigate membrane permeability
- Use plant tissue with soluble pigment in vacuole. Tonoplast & cell-surface membrane disrupted= increased permeability = pigment diffuses into solution
- Select colorimeter filter with complementary colour
- Use distilled water to set colorimeter to 0. Measure absorbance of solution
- High absorbance/ low transmission= more pigment in water
Define osmosis
Water diffuses across semi-permeable membrane from an area of high water potential to an area of lower water potential until a dynamic equilibrium is reached
What is water potential?
- pressure created by water moleculels in kPa
- water potential of pure water at 25 degrees c &100kPa: 0
- more solute= water potential more negative
How does osmosis affect plant and animal cells?
- Osmosis INTO cell:
plant- protoplast swells= cell turgid
animal- lysis - Osmosis OUT of cell:
plant- protoplast shrinks= cell flaccid
animal- crenation
Suggest how a student could produce a desired concentration of solution from a stock solution
- volume of stock solution= required concentration x final volume needed/ concentration of stock solution
- volume of distilled water= final volume needed - volume of stock solution
Define simple diffusion
- Passive process requires no energy from ATP hydrolysis
- Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration (down a concentration gradient)
Define facilitated diffusion
- Passive process
- Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules/ ions (not soluble in hydrophobic phospholipid tail) down concentration gradient
Explain how channel and carrier proteins work
- Channel: hydrophilic channels bind to specific ions= one side of the protein closes & the other opens
- Carrier: binds to complementary molecule= conformational change releases molecule on other side of membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis
Name 5 factors that affect rate of diffusion
- Temperature
- Diffusion distance
- Surface area
- Size of molecule
- Difference in concentration (how steep the gradient is)
State Fick’s law
surface area x difference in concentration/ diffusion distance
How are cells adapted to maximise rate of transport across membranes?
- many channel/ carrier proteins
- folded membrane increases surface area
Define active transport
- Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape
- Specific carrier protein transports molecules/ ions from area of low concentration to area of higher concentration (against concentration gradient)
Compare and contrast active transport and facilitated diffusion
- Both may involve carrier proteins
- Active transport requires energy from ATP hydrolysis; facilitated diffusion is a passive process
- Facilitated diffusion may also involve channel proteins
Define co-transport
- Movement of a substance against its concentration gradient is couple with the movement of another substance down its concentration/ electrochemical gradient
- Substances bind to complementary intrinsic protein:
- symport- transport substances in same direction
- antiport- transports substances in opposite direction e.g sodium-potassium pump
Explain how co-transport is involved in absorption of glucose/ amino acids in small intestine
- Na+ actively transported out of epithelial cells & into bloodstream
- Na+ concentration lower in epithelial cells than lumen of gut
- Transport of glucose/ amino acids from lumen to epithelial cells is ‘coupled’ to facilitated diffusion of Na+ down gradient
