2.3 Flashcards
Describe the 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 the 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 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 temperature denatures membrane proteins / phospholipid molecules have more 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 = ↑ permeability = pigment diffuses into solution.
- Select colorimeter filter with complementary colour.
- Use distilled water to set colorimeter to 0. Measure
absorbance/ % transmission value of solution. - high absorbance/ low transmission = more pigment in
solution.
Define osmosis.
Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established.
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What is water potential (ψ)?
● pressure created by water molecules measured in kPa
● Ψ of pure water at 25°C & 100 kPa: 0 ● more solute = ψ 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 (i.e. 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 hydrogen
Name 5 factors that affect the rate of diffusion
● Temperature ● Diffusion distance ● Surface area ● Size of molecule ● Difference in concentration (how steep the concentration gradient is)
State Fick’s law.
surface area 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 from area of low concentration to area of higher concentration (i.e. 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 coupled with the movement of another substance down its concentration/ electrochemical gradient.
Substances bind to complementary intrinsic protein: symport: transports substances in same direction antiport: transports substances in opposite direction e.g. sodium-potassium 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+ 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 electrochemical gradient.
