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AQA A level Biology (2022) > Transport across cell membranes > Flashcards

Transport across cell membranes Flashcards

1
Q

Describe the fluid mosaic model of membranes

A
  • Fluid: phospholipid bilayer in which individual phospholipids can move (membrane has flexible shape)
  • Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded
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2
Q

Explain the role of cholesterol & glycolipids in membranes

A
  • Cholesterol: steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable
  • Glycolipids: cell signalling & cell recognition
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3
Q

Explain the functions of extrinsic proteins in membranes

A
  • Binding sites/receptors e.g. for hormones
  • Antigens (glycoproteins)
  • Bind cells together
  • Involved in cell signalling
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4
Q

Explain the functions of intrinsic proteins in membranes

A
  • Electron carries (respiration/photosynthesis)
  • Channel proteins (facilitated diffusion)
  • Carrier proteins (facilitated diffusion/active transport)
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5
Q

Explain the functions of membranes within cells

A
  • 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
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6
Q

Explain the functions of cell-surface membrane

A
  • Isolates cytoplasm from extracellular environment
  • Selectively permeable to regulate transport of substances
  • Involved in cell signalling/cell recognition
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7
Q

Name and explain 3 factors that affect membrane permeability

A
  • 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
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8
Q

Outline how colorimetry could be used to investigate membrane permeability

A
  1. Use plant tissue with soluble pigment in vacuole, tonoplast & cell-surface membrane disrupted = increased permeability = pigment diffuses into solution
  2. Select colorimeter filter with complementary colour
  3. Use distilled water to set colorimeter to 0, measure absorbance/& transmission value of solution
  4. High absorbance/low transmission = more pigment in solution
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9
Q

Define osmosis

A

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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10
Q

What is water potential (Ψ)?

A
  • Pressure created by water molecules measured in kPa
  • Ψ of pure water at 25°C & 100 kPa: 0
  • More solute = Ψ more negative
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11
Q

How does osmosis affect plant and animal cells?

A
  • Osmosis INTO cell:
    plant: protoplast swells = cell turgid
    animal: lysis
  • Osmosis OUT of cell:
    plant: protoplast shrieks = cell flaccid
    animal: crenation
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12
Q

Suggest how a student could produce a desired concentration of solution from a stock solution

A
  • 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
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13
Q

Define simple diffusion

A
  • Passive process required 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)
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14
Q

Define facilitated diffusion

A
  • 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
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15
Q

Explain how channel proteins work

A
  • Channel: hydrophilic channels bind to specific ions = one side of the protein closes & the other opens
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16
Q

Explain how carrier proteins work

A
  • 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
17
Q

Name 5 factors that affect the rate of diffusion

A
  • Temperature
  • Diffusion distance
  • Surface area
  • Size of molecule
  • Difference in concentration (how steep the concentration gradient is)
18
Q

State Fick’s law

A

(Surface area x concentration difference) / thickness of membrane

19
Q

How are cells adapted to maximise the rate of transport across their membranes

A
  • Many carrier/channel proteins

- Folded membrane increases surface area

20
Q

Explain the difference between the shape of a graph of concentration (x-axis) against rate (y-axis) for simple vs facilitated diffusion

A
  • 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
21
Q

Define active transport

A
  • 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)
22
Q

Compare and contrast active transport and facilitated diffusion

A
  • 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
23
Q

Define co-transport

A

Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration/electrochemical gradient

24
Q

What is meant by symport and antiport?

A

Substances bind to complementary intrinsic protein:

  • Symport: transports substances in same direction
  • Antiport: transports substances in opposite direction e.g. sodium-potassium pump
25
Q

Explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine

A
  1. Na+ actively transported out of epithelial cells & into bloodstream
  2. Na+ concentration lower in epithelial cells than lumen of gut
  3. Transport of glucose/amino acids from lumen to epithelial cells is ‘coupled’ to facilitated diffusion of Na+ down electrochemical gradient
26
Q

Explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine

A
  1. Na+ actively transported out of epithelial cells & into bloodstream
  2. Na+ concentration lower in epithelial cells than lumen of gut
  3. Transport of glucose/amino acids from lumen to epithelial cells is ‘coupled’ to facilitated diffusion of Na+ down electrochemical gradient

AQA A level Biology (2022) (8 decks)

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