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A - Biology > Topic 2B - cell membranes DVY * > Flashcards

Topic 2B - cell membranes DVY * Flashcards

cell membrane structure exchange across cell membranes - diffusion exchange across cell membranes - osmosis exchange across cell membranes - active transport

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1
Q

what are cell-surface membranes?

A

they surround cells and divide the cell into different compartments, they are a barrier between the cell and it environment, controlling which substances enter and leave the cell. they’re partially permeable, so let some molecules through but not others

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2
Q

how can substances move across a cell-surface membrane?

A

by diffusion, osmosis or active transport

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3
Q

what are cell membranes composed of?

A

lipids, proteins and carbohydrates

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4
Q

what is the fluid mosaic model?

A

phospholipid molecules form a bilayer which is constantly moving (fluid)
cholesterol molecules and proteins are present in the bilayer
the proteins are like mosaic tiles, some can move through the bilayer, some are stationary
some proteins and lipids have a polysaccharide (carbohydrate) chain attached, these are called glycoproteins and glycolipids

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5
Q

what kind of proteins are there on the cell surface membrane?

A

channel and carrier proteins - allow large molecules and ions to pass through the membrane
receptor proteins - detect chemicals released from other cells, which tell the cell to respond

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6
Q

what do phospholipids do in the cell membrane?

A

with their hydrophilic head and hydrophobic tail they form a bilayer, with the heads facing out towards the water on either side
the centre is hydrophobic so water soluble substances can’t get through forming a barrier to dissolved substances

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7
Q

what does cholesterol do in the cell membrane?

A

they fit between phospholipids and bind to the tails, packing them together, and restricting movement, to make the membrane less fluid
they help maintain the shape of animal cells, especially those without support from surrounding cells

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8
Q

what is cholesterol?

A

its a type of lipid present in all cell membranes (except bacterial)

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9
Q

how to carry out the investigation of how temperature affects beetroot membrane permeability?

A

use a scalpel to cut 5 equal pieces of beetroot and rinse
put each piece in different test tubes with 5cm^3 of water
put each test tube in water baths at different temperatures and wait a set amount of time
remove the beetroot
use a colorimeter to measure the absorbance of light n the mixture, the higher the absorbance the more pigment, and the higher the permeability

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10
Q

what must be done before a colorimeter is used?

A

5 minutes to stabilise and a test tube of water to zero it

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11
Q

what happens to membrane permeability at temperatures below 0*C?

A

phospholipids don’t have much energy, so can’t move much, packed together and rigid
channel and carrier proteins deform, increasingly permeability
ice crystals may form, increasing permeability when they thaw

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12
Q

what happens to membrane permeability at temperatures between 0 and 45 *C?

A

phospholipids an move, and aren’t packed tightly
membrane is partially permeable. as temperature increases phospholipids move more because they have more energy, increasing permeability

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13
Q

what happens to membrane permeability at temperatures above 45*C?

A

phospholipid bilayer starts to melt (break down), membrane becomes permeable. water inside expands, putting pressure on the membrane, channel and carrier proteins deform and can’t control what enters or leaves the cell, increasing permeability of cell

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14
Q

what is diffusion?

A

the passive net movement of particles from an area of higher concentration to an area of lower concentration

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15
Q

how long do particles diffuse for?

A

until they are evenly distributed

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16
Q

what is the concentration gradient?

A

the path from an area of higher concentration to an area of lower concentration. particles diffuse down a concentration gradient

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17
Q

how is diffusion passive?

A

no energy is needed for it to happen

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18
Q

what is simple diffusion?

A

when molecules diffuse directly through a cell membrane

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19
Q

what is facilitated diffusion?

A

the passive net movement of particles from an area of higher concentration to an area of lower concentration through a carrier or channel protein

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20
Q

what particles have to use facilitated diffusion?

A

large molecules - can’t fit between phospholipids well

charged particles - they’re water soluble, the inside of a cell membrane is hydrophobic

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21
Q

what do carrier proteins do?

A

move large molecules across membranes, down a concentration gradient. they’re specific to different molecules

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22
Q

how do carrier proteins work?

A

a large molecule binds to a carrier protein in the membrane
the protein changes shape
this releases the molecule on the opposite side of the membrane

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23
Q

what do channel proteins do?

A

they form pores in the membrane for charged particles to diffuse through, down the concentration gradient.
they’re specific to different charged particles

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24
Q

what factors affect simple diffusion?

A

the concentration gradient
the thickness of the exchange surface
surface area
temperature

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25
Q

how does the concentration gradient affect simple diffusion?

A

the higher, the faster the rate of diffusion
as diffusion takes place the difference in concentration of the 2 sides becomes lower until it reaches equilibrium, this means diffusion slows down over time

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26
Q

how does the thickness of the exchange surface affect simple diffusion?

A

the thinner the exchange surface, the shorter the distance the particles have to travel, the faster the rate of diffusion

27
Q

how does the surface area affect simple diffusion?

A

the larger the surface area, the faster the rate of diffusion

28
Q

how do microvilli increase the rate of diffusion?

A

they are formed by the cell-surface membrane folding up on itself.
they can increase the surface area by about 600 times in human cells.
larger surface area = more particles can be exchanged in the same amount of time, increasing rate of diffusion

29
Q

what factors affect facilitated diffusion?

A

concentration gradient

the number of channel or carrier proteins

30
Q

how does the concentration gradient affect facilitated diffusion?

A

the higher the concentration gradient, the faster the rate of facilitated diffusion, until there aren’t enough proteins.
as equilibrium is reached, the rate of facilitated diffusion will level off

31
Q

how does the number of channel or carrier proteins affect facilitated diffusion?

A

once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster. so the more proteins, the faster the rate of facilitated diffusion

32
Q

what are aquaporins?

A

channel proteins for the facilitated diffusion of water.

some kidney cells are adapted to have lots of aquaporins.

33
Q

what do aquaporins do?

A

they allow cells to reabsorb a lot of the water that would be excreted by the body, about 180 litres need re-absorbing every day

34
Q

what is osmosis?

A

the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential

35
Q

what is water potential?

A

the potential of water molecules to diffuse out of or into a solution.
pure water has the highest water potential

36
Q

what does isotonic mean for water?

A

when 2 solutions have the same water potential

37
Q

what does the rate of osmosis depend on?

A

the water potential gradient
the thickness of the exchange surface
the surface area of the exchange surface

38
Q

how does the water potential gradient affect the rate of osmosis?

A

the higher the water potential gradient, the faster the rate of osmosis. as osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time

39
Q

how does the thickness of the exchange surface affect the rate of osmosis?

A

the thinner the exchange surface, the faster the rate of osmosis

40
Q

how does the surface area of the exchange surface affect the rate of osmosis?

A

the larger the surface area, the faster the rate of osmosis

41
Q

how to make 5 serial dilutions of a sucrose solution, starting with 2M:

A

add 10cm^3 of 2M sucrose solution to a test tube and 5cm^3 of distilled water to 4 others
then add 5cm^3 from the 1st test tube to the second to make 10cm^3 that’s 1M
repeat for each test tube halving the amount in the 1st test tube each time to make 0.5M, 0.25M and 0.125 M

42
Q

how to make solutions of different concentrations from any concentration:

A
  1. start with a solution of known concentration
  2. divide the known concentration by the one you want to make
  3. divide the amount you want to make by this number to find the amount you should take from the known concentration
  4. top up the rest with distilled water
43
Q

how to find the water potential of potato cells experiment:

A

use a cork borer to cut identically sized chips
divide them into groups of 3 and measure the mass of each group with a mass balance
place 1 group into each of the sucrose solutions
leave them for at least 20 mins
remove chips and dry gently
weigh each group again and record results
calculate the % change in mass
make a graph showing % change in mass against sucrose concentration

44
Q

what should the results of the water potential of potato cells show?

A

potato chips gain water in solutions with a higher water potential, and lose water in lower water potential
the point where the curve crosses the x axis (when the mass doesn’t change) is when the water potential of the sucrose solution is the same as the water potential of the potato cells
find the concentration of this solution then look up what the water potential of that solution is

45
Q

what is active transport?

A

the movement of molecules and ions across membranes using energy, usually against a concentration gradient

46
Q

how are carrier proteins used in active transport?

A

a molecule attaches to the carrier protein
ATP is hydrolysed inside the cell
Pi binds the carrier protein
the protein changes shape and this moves the molecule across the membrane, releasing it on the other side
ADP and Pi then recombine to form ATP

47
Q

what are the 2 main differences between active transport and facilitated diffusion?

A

active transport usually moves from low to high concentration, facilitated always moves from high to low
active transport needs energy

48
Q

what is ATP?

A

a common source of energy in the cell, produced in respiration

49
Q

how does ATP release energy?

A

it undergoes a hydrolysis reaction, splitting into ADP and Pi (inorganic phosphate) this releases energy so that the solutes can be transported

50
Q

what do co-transporters do?

A

they bind 2 molecules at the same time

the concentration gradient of one of the molecules is used to move the other against its own concentration gradient

51
Q

what factors affect the rate of active transport?

A

the speed of individual carrier proteins
the number of carrier proteins present
the rate of respiration in the cell and the availability of ATP

52
Q

why can’t glucose diffuse into the bloodstream on its own?

A

concentration of glucose in ileum is too low for it to diffuse into the blood

53
Q

how does glucose enter the blood ?

A

Na+ actively transported out of ileum epithelial cells into blood, via Na+/K+ pump. creating conc. gradient, low conc. in cell
Na+ diffuse from lumen into cell down gradient via sodium-glucose co-transporter protein
co-transporter carries glucose into cell against gradient
glucose diffuses out of cell into blood down gradient via facilitated diffusion through carrier protein

54
Q

how to calculate the rate of diffusion?

A

rate of diffusion(mol/s) = SA X conc. gradient/ diffusion distance

55
Q

what is water potential measured in?

A

kilopascals pure water has a ψ of 0

56
Q

what is solute potential?

A

the contribution that solute makes to water potential, the higher the concentration of a solution the lower the solute potential it is always negative

57
Q

what is pressure potential?

A

the contribution that pressure makes to water potential, the more pressure the higher the tendency of water to move, it is positive

58
Q

what is a hypertonic solution?

A

a solution with a lower water potential than the cells, more solute

59
Q

what is a hypotonic solution?

A

a solution with a higher water potential than the cells, low solute concentration

60
Q

what does turgid mean?

A

when the protoplast pushes against the cell wall, this causes pressure to build up

61
Q

wat does plasmolysed mean?

A

the protoplast shrinks and is completely pulled away from the cell wall

62
Q

why does adding solute to pure water lower its water potential?

A

it gets ‘tied up’ forming a shell around the solute as it dissolves so the water is less free to move

63
Q

how does temperature affect diffusion?

A

increased temperature, increases kinetic energy

more movement so diffusion speeds up

A - Biology (21 decks)

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