2.3 Transport Across Membranes Flashcards
(22 cards)
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.
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Glycolipids: cell signalling & cell recognition.
Explain the 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 (facilitated diffusion/ active transport)
Explain the functions of membranes within cells.
Provide internal transport system.
Selectively permeable to regulate passage
of molecules into / out of organelles.
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Provide reaction surface.
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Isolate organelles from cytoplasm for
specific metabolic reactions.
Explain the functions of the cell-surface membrane.
Isolates cytoplasm from extracellular environment.
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Selectively permeable to regulate transport of substances.
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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.
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pH: changes tertiary structure of membrane proteins.
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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.
What is water potential (ψ)?
•pressure created by water molecules measured in kPa
•Ψ of pure water at 25℃ & 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.
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 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?
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many carrier/ channel proteins
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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.
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Both may involve carrier proteins.
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Active transport requires energy from ATP
hydrolysis; facilitated diffusion is a passive process.
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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.
