Transport Across Cell Membranes Flashcards
Describe the fluid mosaic model of membranes
Fluid: phospholipid bilayer in which individual phospholipids can move
Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded
Explain the role of cholesterol in membranes
Steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable
Explain the role of glycolipids in membranes
Cell signalling & cell recognition
Explain the functions of extrinsic proteins in membranes
- binding sites/receptors
- antigens
- bind cells together
- involved in cell signalling
Explain the functions of transmembrane proteins in membranes
- electron carriers
- channel proteins
- carrier proteins
Explain the 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 the functions of the 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 that affect membrane permeability
- temperature: high = denatured membrane proteins/phospholipid molecules
- pH: changes tertiary structure of membrane proteins
- use of 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 so higher permeability so pigment diffuses into solution
- Select colorimeter filter with complementary colour
- Use distilled water to set colorimeter to 0 + measure absorbance
- High absorbance = more pigment in solution
Define osmosis
Water diffuses across semi-permeable membranes from an area of higher wp to an area of lower wp until a dynamic equilibrium is established
What is water potential?
- pressure created by water molecules measure in kPa
- wp of pure water at 25 degrees & 100 kPa:0
- more solute = wp more negative
How does osmosis affect plant and animal cells?
- osmosis INTO cell:
- plant - protoplasm swells/ cell turgid
- animal - lysis
- osmosis OUT of cell:
- plant - protoplasm 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] x final volume needed / [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 down a concentration gradient
Define facilitated diffusion
- passive process
- specific channel or carrier proteins with complementary binding sites transport large, polar molecules down concentration gradient
Explain how channel and carrier proteins work
Channel: hydrophilic channels bind to specific ions = one side of the proteins closes & the other opens
Carrier: binds to complementary molecule = conformational change releases molecule on other side of membrane
Name 5 factors that affect the rate of diffusion
- temperature
- diffusion distance
- SA
- size of molecule
- difference in concentration
State Fick’s law
SA x difference in concentration / diffusion distance
How are cells adapted to maximise the rate of transport across their membranes?
- many carrier/channel proteins
- folded membrane increases surface area
Explain the difference between the shape of a graph of concentration (x-axis) against rate (y-axis) for simple vs facilitated diffusion
Simple diffusion: straight diagonal line; rate of diffusion increases proportionally as concentration increases
Facilitated diffusion: straight diagonal line later levels off when all channel/carrier proteins are saturated
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 against concentration gradient
Compare and contrast active transport and facilitated diffusion
- both may involve carrier proteins
- AT requires energy from ATP hydrolysis; FD is passive
- FD may also involve channel proteins
Define co-transport
- movement of a substance against its concentration gradient is coupled with movement of another substance down its electrochemical gradient
- substances bind to complementary intrinsic protein:
- symport: transports substances in the same direction
- antiport: transports substances in opposite direction e.g NaK pump
Explain how co-transport is involved in the absorption of glucose / amino acids in the small intestine
- Na+ actively transported out of epithelial cells & into bloodstream
- [Na+] lower in epithelial cells than gut lumen
- Transport of glucose/amino acids from lumen to epithelial cells is ‘coupled to FD of Na+ down electrochemical gradient
