Topic 2 B: Cell membranes Flashcards
(108 cards)
1
Q
what are all cells surrounded by?
A
All cells are surrounded by membranes.
In eukaryotic cells, many of the organelles are surrounded by membranes too.
2
Q
what do cell surface membranes do?
A
surround cells
3
Q
how are cell surface membranes a barrier?
A
They are a barrier between the cell and its environment, controlling which substances enter and leave the cell.
4
Q
permeability of a cell surface membrane:
A
They’re partially permeable — they let some molecules through but not others.
5
Q
how can substances move across membranes?
A
diffusion, osmosis or active transport
6
Q
other name for cell surface membrane:
A
plasma membrane
7
Q
what do membranes around organelles do?
A
divide the cell into different compartments — they act as a barrier between the organelle and the cytoplasm.
8
Q
substances needed for respiration:
A
The substances needed for respiration (like enzymes) are kept together inside a mitochondrion by the membrane surrounding the mitochondrion.
9
Q
RNA leaving nucleus:
A
leaves the nucleus via the nuclear membrane (also called the nuclear envelope). DNA is too large to pass through the partially permeable membrane, so it remains in the nucleus.
10
Q
basic membrane structure:
A
composed of lipids (mainly phospholipids) proteins and carbohydrates (attached to proteins or lipids).
11
Q
what is the fluid mosaic model?
A
1972 the suggested to describe the arrangement of molecules in the membrane
In the model, phospholipid molecules form a continuous, double layer (called a bilayer). This bilayer is ‘fluid’ because the phospholipids are constantly moving.
12
Q
proteins in fluid mosaic model:
A
Proteins are scattered through the bilayer, like tiles in a mosaic. These include channel proteins and carrier proteins, which allow large molecules and ions to pass through the membrane.
13
Q
what do receptor proteins in the cell surface membrane do?
A
allow the cell to detect chemicals released from other cells.
14
Q
chemicals on cell surface membranes:
A
The chemicals signal to the cell to respond in some way, e.g. the hormone insulin binds to receptor proteins on liver cells, which tells the cells to absorb glucose
15
Q
how do proteins move in the cell surface membrane?
A
Some proteins are able to move sideways through the bilayer, while others are fixed in position.
16
Q
what are glycoproteins?
A
when some proteins have a carbohydrate attached
17
Q
what are glycolipids?
A
Some lipids have a carbohydrate attached
18
Q
what other molecules are present within the bilayer?
A
cholesterol
19
Q
draw the fluid mosaic model
A
20
Q
what do phospholipid molecules form?
A
form a barrier to dissolved (water-soluble) substances. Phospholipids have a ‘head’ and a ‘tail’. The head is hydrophilic —
it attracts water. The tail is hydrophobic — it repels water.
The molecules automatically arrange
themselves into a bilayer — the heads face out towards the water on either side of the membrane
21
Q
what does centre of the bilayer not allow to pass through?
A
hydrophobic so the membrane doesn’t allow water-soluble substances (like ions and polar molecules) to diffuse through it.
22
Q
what can pass through the bilayer?
A
Small, non-polar substances (e.g. carbon dioxide) and water can diffuse through the membrane
23
Q
what does cholesterol do?
A
gives the membrane stability.
24
Q
is cholesterol present in all cells?
A
present in all cell membranes (except bacterial cell membranes).
25
where is cholesterol in the membrane?
fits between the phospholipids
26
what does cholesterol bind to?
binds to the hydrophobic
tails of the phospholipids, causing them to pack
more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
27
what does cholesterol do in animal cells?
helps to maintain the shape of animal cells (which don’t have cell walls). This is particularly important for cells that aren’t supported by other cells, e.g. red blood cells, which float free in the blood.
28
cholesterol hydrophobic regions:
has hydrophobic regions, so it’s able to create a further barrier to polar substances moving through the membrane.
29
how does temperature affect cell membranes
affects how much the phospholipids in the bilayer can move, which affects membrane structure and permeability.
30
phospholipids with temperature below 0°:
phospholipids don’t have much energy, so they can’t move very much. They’re packed closely together and the membrane is rigid.
31
what happens to channel proteins and carrier proteins in membrane when temperature is below 0°?
denature (lose structure and function), increasing the permeability of the membrane
32
ice in membranes below 0°:
Ice crystals may form and pierce the membrane, making it highly permeable when it thaws.
33
phospholipids with temperatures between 0-45°:
phospholipids can move around
and aren’t packed as tightly together
— the membrane is partially permeable.
As temperature increases phospholipids move more as they have more energy — this increases the permeability of the membrane.
34
phospholipids above 45°:
phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable.
35
water inside cell membrane above 45°
Water inside the cell expands, putting pressure on the membrane.
36
Channel proteins and carrier proteins above 45°
denature so they can’t control what enters or leaves the cell — this increases the permeability of the membrane
37
what variables can you investigate to see the affects of a cell membrane?
temperature, solvent concentration
38
how can you test variables with beetroot?
contain a coloured pigment that leaks out — the higher the permeability of the membrane, the more pigment leaks out of the cell.
39
investigating temperature on beetroot steps:
1. Use scalpel to cut five equal sized pieces of beetroot on a cutting board.) Rinse pieces to remove pigment released during cutting.
2. Add five pieces to different test tubes, containing 5 cm3 of water. Use a measuring cylinder/ pipette to measure the water.
3. Place each test tube in a water bath at a different temperature, e.g. 10 °C, 20 °C, 30 °C, 40 °C, 50 °C, for the same length of time (measured using a stopwatch).
4. Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
40
colorimeter steps of beetroot test:
use a colorimeter — a machine that passes light of a specific wavelength through a liquid and measures how much of that light is absorbed. Many colorimeters use filters to make sure the light passing through the liquid is at the desired wavelength.
6. Firstly, switch the colorimeter on and allow five minutes for it to stabilise. Then set up the colorimeter so you’re using a blue filter (or a wavelength of about 470 nm).
7. Add distilled water to a cuvette so it is three quarters full
Put the cuvette into the colorimeter. Two of the cuvette’s sides may be ridged or frosted — you need to make sure you put the cuvette into the colorimeter the correct way, so that the light will be passing through the clear sides. Calibrate the machine to zero.
8. Next, use a pipette to transfer a sample of the liquid from the first test tube to a clean cuvette — again it should be about three quarters full.
9. Put the cuvette in the colorimeter and read and record the absorbance of the solution.
10. Repeat steps 8-9 for the liquids in the remaining four test tubes (using a clean pipette and cuvette each time).
11. You’re now ready to analyse your results — bear in mind, the higher the absorbance reading, the more pigment released, so the higher the permeability of the membrane.
41
drawing graph:
you may be able to connect the colorimeter to a computer and use software to collect the data and draw a graph of the results.
42
investing effect of solvents:
could do a similar experiment with beetroot to investigate the effect of solvents on the permeability of cell membranes, i.e. by placing the beetroot cubes in different concentrations of a particular solvent (e.g alcohol/acetone). Surrounding cells in an increasing concentration of a solvent increases membrane permeability as solvent dissolves the lipids in the cell membrane, causing it to lose its structure.
43
diffusion definition:
the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration
44
net movement:
area of lower concentration.
45
concentration gradient definiton:
the path from an area of higher concentration to an area of lower concentration. particles diffuse down the concentration gradient
46
what type of process is diffusion?
a passive process — no energy is needed for it to happen. Particles can diffuse across cell membranes, as long as they can move freely through the membrane.
47
simple diffusion definition:
When molecules diffuse directly through a cell membrane
48
example of simple diffusion:
oxygen and carbon dioxide can diffuse easily through cell membranes because they’re small, so they can pass through spaces between the phospholipids. They’re also non-polar, which makes them soluble
in lipids, so they can dissolve in the hydrophobic bilayer.
49
what does the rate of diffusion depend on?
- concentration gradient
- thickness of exchange surface
- surface area
50
how does concentration affect diffusion rate?
the higher it is, the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium (i.e. the concentration on both sides is equal). This means that diffusion slows down over time.
51
how does the thickness of exchange surface affect rate of diffusion?
the thinner the exchange surface (i.e. the shorter the distance the particles have to travel), the faster the rate of diffusion.
52
how does surface area affect diffusion rates?
the larger the surface area (e.g. of the cell-surface membrane), the faster the rate of diffusion.
53
specialised cells diffusion:
Some specialised cells (e.g. epithelial cells in the small intestine) have microvilli — projections formed by the cell-surface membrane folding up on itself. Microvilli give the cell a larger surface area — in human cells microvilli can increase the surface area by about 600 times. A larger surface area means that more particles can be exchanged in the same amount of time — increasing the rate of diffusion.
54
what do carrier proteins do?
move large molecules across the membrane, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules
55
process of carrier proteins:
First, a large molecule attaches to a carrier protein in the membrane.
Then, the protein changes shape.
This releases the molecule on the opposite side of the membrane
56
what do channel proteins form?
pores in the membrane for charged particles to diffuse through (down their concentration gradient)
57
what do different channel proteins do?
facilitate the diffusion of different charged particles.
58
factors affecting the rate of facilitated diffusion:
- concentration gradient
- the number of channel or carrier proteins
59
how does concentration gradient affect rate of facilitated diffusion
the higher the concentration gradient, the faster the rate of facilitated diffusion (up to a point, see below).
As equilibrium is reached, the rate of facilitated diffusion will level off.
60
how does the number of channel or carrier proteins affect rate of facilitated diffusion?
once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient.
61
example of channel and carrier proteins affecting facilitated diffusion:
Glucose is absorbed from blood plasma into red blood cells via facilitated diffusion, using GLUT 1 carrier proteins. As the red line on the graph shows, the rate of uptake increases as the extracellular glucose concentration increases. The
rate of uptake levels off as equilibrium
is reached. After this, the rate of
facilitated diffusion increases only
slightly even at much greater glucose
concentrations, as many of the
GLUT 1 proteins are already in use
62
relationship of rate of facilitated diffusion and number of channel or carrier proteins:
the greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion.
63
aquaporins example:
Aquaporins are special channel proteins that allow the facilitated diffusion of water through cell membranes. Some kidney cells are adapted to have lots of aquaporins. The aquaporins allow the cells to reabsorb a lot of the water that would otherwise be excreted by the body.
64
calculating rate of diffusion:
For a straight line graph, this means finding the gradient of the line. For a curved graph, it means drawing a tangent and finding the gradient of the tangent.
65
osmosis definiton:
the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential (i.e. higher concentration of water molecules) to an area of lower water potential (i.e. lower concentration of water molecules).
66
water potential definition:
the potential (likelihood) of water molecules to diffuse out of or into a solution.
67
water molecules in osmosis:
are small and can diffuse easily
through the cell membrane, but large solute molecules can’t.
68
pure water, water potential
zero.
0 water lowers its water potential — so the water potential of any solution is always negative. The more negative the water potential, the stronger the concentration of solutes in the solution.
69
example of osmosis:
Glass A contains pure water — it’s got a water potential of zero.
Glass B contains a solution of orange squash. The orange squash molecules are a solute. They lower the concentration of the water molecules.
This means that the water potential of the orange squash is lower than the water potential of pure water.
70
isotonic definition:
If two solutions have the same water potential
71
why don’t cells in an isotonic solution lose of gain water?
there’s no net movement of water molecules because there’s no difference in water potential between the cell and the surrounding solution.
72
what happens when a cell is placed in a solution of higher water potential?
swell as water moves into it by osmosis.
73
what is hypotonic?
Solutions with a higher water potential compared with the inside of the cell
74
what happens if a cell is placed in a solution that has a lower water potential?
it may shrink as water moves out of it by osmosis
75
what is hypertonic?
Solutions with a lower water potential than the cell
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water movement in isotonic solution:
no net movement of water
77
water movement in hypertonic solution:
no net movement of water out of the cell
78
water movement of a cell in a hypotonic solution:
no net movement of water into the cell
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factors affecting rate of osmosis:
- water potential gradient
- thickness of exchange surface
- surface area of exchange surface
80
how does water potential gradient affect osmosis?
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.
81
how does the thickness of the exchange surface affect osmosis
the thinner the exchange surface, the faster the rate of osmosis.
82
how does surface area of the exchange surface affect osmosis?
the larger the surface area, the faster the rate of osmosis.
83
making serial dilutions:
make up several solutions of different, known concentrations to test the cylinders in. You use a serial dilution technique. A serial dilution is when you create a set of solutions that decrease in concentration by the same factor each time.
- useful technique for creating a very weak solution, as it means you don’t have to measure out very small volumes of liquid.
84
example of making serial dilutions:
Line up five test tubes in a rack.
2. Add 10 cm3 of the initial 2 M sucrose solution to the first test tube and 5 cm3 of distilled water to the other four test tubes (see Figure 2).
3. Then, using a pipette, draw 5 cm3 of the solution from the first
test tube, add it to the distilled water in the second test tube and mix the solution thoroughly. You now have 10 cm3 of solution that’s half as concentrated as the solution in the first test tube (it’s 1 M).
4. Repeat this process three more times to create solutions of 0.5 M, 0.25 M and 0.125 M.
85
how can you make solutions of any concentration?
finding the scale factor
86
finding the scale factor example:
1. Start with a solution of a known concentration, e.g. 1 M.
2. Find the scale factor by dividing the concentration of this solution by the concentration of the solution you want to make. So in this case the scale factor = 1 M ÷ 0.4 M = 2.5.
3. This means that the solution you want to make is 2.5 times weaker than the one you have. To make the solution 2.5 times weaker, use 2.5 times less of it, i.e. 15 cm3 ÷ 2.5 = 6 cm3. Transfer this amount to a clean test tube.
4. Top up the test tube with distilled water to get the volume you want to make. In this case you want to make 15 cm3 of solution, so you need to add: 15 – 6 = 9 cm3 of distilled water.
87
what do you do after you have made up a set of serial dilutions?
use them to find the water potential of potato cells. First you need to measure how much mass the potato cells gain or lose in each solution..
88
measuring how much mass the potato cells gain or lose in each solution:
1. Use a cork borer to cut potatoes into identically sized chips,
about 1 cm in diameter. Divide the chips into groups of three and measure the mass of each group using a mass balance.
2. Place one group into each of your sucrose solutions and leave the chips in the solutions for at least 20 minutes (making sure that they all get the same amount of time).
3. Remove the chips and pat dry gently with a paper towel. Weigh each group again and record your results. Calculate the percentage change in mass for each group.
89
potato chips changing mass:
will gain water (and therefore mass) in solutions with a higher water potential than the chips, and lose water in solutions with a lower water potential.
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producing calibration curve:
plot percentage change in mass against the concentration of sucrose solution. You can then use your calibration curve to determine the water potential of the potato cells:
Topic 2 — B: Cell Membranes
91
example for calibration curve:
the point at which your calibration curve crosses the x axis is the point at which the water potential of the sucrose olution is the same as the water potential of the potato cells. find conc at this point, then lookup the water potential for that concentration of sucrose solution
92
why does active transport use energy?
to move molecules and ions across plasma membranes, usually against a concentration gradient. Carrier proteins and co-transporters are involved in active transport.
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what is the carrier process?
a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side
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differences between active transport and facilitated diffusion:
Active transport usually moves solutes from a low to a high concentration — in facilitated diffusion, they always move from a high to a low concentration.
Active transport requires energy — facilitated diffusion does not.
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what is ATP?
ATP (a molecule produced by respiration) is a common source of energy in the cell, so it’s important for active transport.
96
what reaction does ATP undergo?
hydrolysis reaction, splitting into ADP and Pi (inorganic phosphate).
This releases energy so that the solutes can be transported.
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what are co transporters?
a type of carrier protein. They bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient.
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what is absorbed into the blood stream?
glucose in the small intestine.
99
conc in the mammalian ileum:
the final part of a mammal’s small intestine) the concentration of glucose is too low for glucose to diffuse out into the blood. So glucose is absorbed from the lumen (middle) of the ileum by co-transport.
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step 1 of co transport:
Sodium ions are actively transported out of the
epithelial cells in the ileum into the blood, by the sodium-potassium pump.
- creates a concentration gradient — there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cell.
101
step 2 of co transport
sodium ions to diffuse from the lumen of the ileum into the epithelial cell, down their concentration gradient. They do this via the sodium-glucose co-transporter proteins. The co-transporter carries
glucose into the cell with the
sodium. As a result the concentration of glucose inside the cell increases.
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step 3 of active transport:
Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion
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decreasing concentration in active transport:
Factors affecting the rate of active transport
When active transport moves molecules and ions against their concentration gradient, a decreasing concentration gradient doesn’t affect the rate of active transport.
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The rate of active transport is affected by:
The speed of individual carrier proteins — the faster they work, the faster the rate of active transport.
The number of carrier proteins present — the more proteins there are, the faster the rate of active transport.
The rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport can’t take place.
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diffusion description:
Net movement of particles from an area of higher concentration to an area of lower concentration.
Passive process — doesn’t require energy.
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Facilitated diffusion description
Net movement of particles from an area of higher concentration to an area of lower concentration.
Uses carrier proteins and channel proteins to aid the diffusion of large molecules and charged particles through the membrane.
Passive process — doesn’t require energy.
107
Osmosis description
Movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential.
Passive process — doesn’t require energy
108
Active transport description:
Movement of molecules, usually from an area of lower concentration to an area of higher concentration.
Uses carrier proteins and co-transporters to transport molecules.
Active process — requires energy
