Topic 2 B: Cell membranes

Question 1

what are all cells surrounded by?

Answer 1

All cells are surrounded by membranes.
In eukaryotic cells, many of the organelles are surrounded by membranes too.

Question 2

what do cell surface membranes do?

Answer 2

surround cells

Question 3

how are cell surface membranes a barrier?

Answer 3

They are a barrier between the cell and its environment, controlling which substances enter and leave the cell.

Question 4

permeability of a cell surface membrane:

Answer 4

They’re partially permeable — they let some molecules through but not others.

Question 5

how can substances move across membranes?

Answer 5

diffusion, osmosis or active transport

Question 6

other name for cell surface membrane:

Answer 6

plasma membrane

Question 7

what do membranes around organelles do?

Answer 7

divide the cell into different compartments — they act as a barrier between the organelle and the cytoplasm.

Question 8

substances needed for respiration:

Answer 8

The substances needed for respiration (like enzymes) are kept together inside a mitochondrion by the membrane surrounding the mitochondrion.

Question 9

RNA leaving nucleus:

Answer 9

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.

Question 10

basic membrane structure:

Answer 10

composed of lipids (mainly phospholipids) proteins and carbohydrates (attached to proteins or lipids).

Question 11

what is the fluid mosaic model?

Answer 11

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.

Question 12

proteins in fluid mosaic model:

Answer 12

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.

Question 13

what do receptor proteins in the cell surface membrane do?

Answer 13

allow the cell to detect chemicals released from other cells.

Question 14

chemicals on cell surface membranes:

Answer 14

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

Question 15

how do proteins move in the cell surface membrane?

Answer 15

Some proteins are able to move sideways through the bilayer, while others are fixed in position.

Question 16

what are glycoproteins?

Answer 16

when some proteins have a carbohydrate attached

Question 17

what are glycolipids?

Answer 17

Some lipids have a carbohydrate attached

Question 18

what other molecules are present within the bilayer?

Answer 18

cholesterol

Question 19

draw the fluid mosaic model

Question 20

what do phospholipid molecules form?

Answer 20

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

Question 21

what does centre of the bilayer not allow to pass through?

Answer 21

hydrophobic so the membrane doesn’t allow water-soluble substances (like ions and polar molecules) to diffuse through it.

Question 22

what can pass through the bilayer?

Answer 22

Small, non-polar substances (e.g. carbon dioxide) and water can diffuse through the membrane

Question 23

what does cholesterol do?

Answer 23

gives the membrane stability.

Question 24

is cholesterol present in all cells?

Answer 24

present in all cell membranes (except bacterial cell membranes).

Question 25

where is cholesterol in the membrane?

Answer 25

fits between the phospholipids

Question 26

what does cholesterol bind to?

Answer 26

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.

Question 27

what does cholesterol do in animal cells?

Answer 27

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.

Question 28

cholesterol hydrophobic regions:

Answer 28

has hydrophobic regions, so it’s able to create a further barrier to polar substances moving through the membrane.

Question 29

how does temperature affect cell membranes

Answer 29

affects how much the phospholipids in the bilayer can move, which affects membrane structure and permeability.

Question 30

phospholipids with temperature below 0°:

Answer 30

phospholipids don’t have much energy, so they can’t move very much. They’re packed closely together and the membrane is rigid.

Question 31

what happens to channel proteins and carrier proteins in membrane when temperature is below 0°?

Answer 31

denature (lose structure and function), increasing the permeability of the membrane

Question 32

ice in membranes below 0°:

Answer 32

Ice crystals may form and pierce the membrane, making it highly permeable when it thaws.

Question 33

phospholipids with temperatures between 0-45°:

Answer 33

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.

Question 34

phospholipids above 45°:

Answer 34

phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable.i’m

Question 35

water inside cell membrane above 45°

Answer 35

Water inside the cell expands, putting pressure on the membrane.

Question 36

Channel proteins and carrier proteins above 45°

Answer 36

denature so they can’t control what enters or leaves the cell — this increases the permeability of the membrane

Question 37

what variables can you investigate to see the affects of a cell membrane?

Answer 37

temperature, solvent concentration

Question 38

how can you test variables with beetroot?

Answer 38

contain a coloured pigment that leaks out — the higher the permeability of the membrane, the more pigment leaks out of the cell.

Question 39

investigating temperature on beetroot steps:

Answer 39

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.

Question 40

colorimeter steps of beetroot test:

Answer 40

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 (a cuvette is
a small container that fits inside a colorimeter — see Figure 7). 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.

Question 41

drawing graph:

Answer 41

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.

Question 42

investing effect of solvents:

Answer 42

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.

Question 43

diffusion definition:

Answer 43

the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration

Question 44

net movement:

Answer 44

area of lower concentration.

Question 45

concentration gradient definiton:

Answer 45

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

Question 46

what type of process is diffusion?

Answer 46

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.

Question 47

simple diffusion definition:

Answer 47

When molecules diffuse directly through a cell membrane

Question 48

example of simple diffusion:

Answer 48

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.

Question 49

what does the rate of diffusion depend on?

Answer 49

• concentration gradient
• thickness of exchange surface
• surface area

Question 50

how does concentration affect diffusion rate?

Answer 50

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.

Question 51

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

Answer 51

the thinner the exchange surface (i.e. the shorter the distance the particles have to travel), the faster the rate of diffusion.

Question 52

how does surface area affect diffusion rates?

Answer 52

the larger the surface area (e.g. of the cell-surface membrane), the faster the rate of diffusion.

Question 53

specialised cells diffusion:

Answer 53

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.

Question 54

what do carrier proteins do?

Answer 54

move large molecules across the membrane, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules

Question 55

process of carrier proteins:

Answer 55

First, a large molecule attaches to a carrier protein in the membrane.
ƒ Then, the protein changes shape.
ƒ This releases the molecule on the opposite side of the membrane

Question 56

what do channel proteins form?

Answer 56

pores in the membrane for charged particles to diffuse through (down their concentration gradient)

Question 57

what do different channel proteins do?

Answer 57

facilitate the diffusion of different charged particles.

Question 58

factors affecting the rate of facilitated diffusion:

Answer 58

• concentration gradient
• the number of channel or carrier proteins

Question 59

how does concentration gradient affect rate of facilitated diffusion

Answer 59

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.

Question 60

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

Answer 60

once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient.

Question 61

example of channel and carrier proteins affecting facilitated diffusion:

Answer 61

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

Question 62

relationship of rate of facilitated diffusion and number of channel or carrier proteins:

Answer 62

the greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion.

Question 63

aquaporins example:

Answer 63

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.

Question 64

calculating rate of diffusion:

Answer 64

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.

Question 65

osmosis definiton:

Answer 65

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).

Question 66

water potential definition:

Answer 66

the potential (likelihood) of water molecules to diffuse out of or into a solution. the potential (likelihood) of water molecules to diffuse out of or into a solution.

Question 67

water molecules in osmosis:

Answer 67

are small and can diffuse easily
through the cell membrane, but large solute molecules can’t.

Question 68

pure water, water potential

Answer 68

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.

Question 69

example of osmosis:

Answer 69

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.

Question 70

isotonic definition:

Answer 70

If two solutions have the same water potential

Question 71

why don’t cells in an isotonic solution lose of gain water?

Answer 71

there’s no net movement of water molecules because there’s no difference in water potential between the cell and the surrounding solution.

Question 72

what happens when a cell is placed in a solution of higher water potential?

Answer 72

swell as water moves into it by osmosis.

Question 73

what is hypotonic?

Answer 73

Solutions with a higher water potential compared with the inside of the cell

Question 74

what happens if a cell is placed in a solution that has a lower water potential?

Answer 74

it may shrink as water moves out of it by osmosis

Question 75

what is hypertonic?

Answer 75

Solutions with a lower water potential than the cell

Question 76

water movement in isotonic solution:

Answer 76

no net movement of water

Question 77

water movement in hypertonic solution:

Answer 77

no net movement of water out of the cell

Question 78

water movement of a cell in a hypotonic solution:

Answer 78

no net movement of water into the cell

Question 79

factors affecting rate of osmosis:

Answer 79

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

Question 80

how does water potential gradient affect osmosis?

Answer 80

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.

Question 81

how does the thickness of the exchange surface affect osmosis

Answer 81

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

Question 82

how does surface area of the exchange surface affect osmosis?

Answer 82

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

Question 83

making serial dilutions:

Answer 83

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.

Question 84

example of making serial dilutions:

Answer 84

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.

Question 85

how can you make solutions of any concentration?

Answer 85

finding the scale factor

Question 86

finding the scale factor example:

Answer 86

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.

Question 87

what do you do after you have made up a set of serial dilutions?

Answer 87

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..

Question 88

measuring how much mass the potato cells gain or lose in each solution:

Answer 88

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.

Question 89

potato chips changing mass:

Answer 89

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.

Question 90

producing calibration curve:

Answer 90

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

Question 91

example for calibration curve:

Answer 91

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

Question 92

why does active transport use energy?

Answer 92

to move molecules and ions across plasma membranes, usually against a concentration gradient. Carrier proteins and co-transporters are involved in active transport.

Question 93

what is the carrier process?

Answer 93

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

Question 94

differences between active transport and facilitated diffusion:

Answer 94

ƒ 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.
ƒ Active transport requires energy — facilitated diffusion does not.

Question 95

what is ATP?

Answer 95

ATP (a molecule produced by respiration) is a common source of energy in the cell, so it’s important for active transport.

Question 96

what reaction does ATP undergo?

Answer 96

hydrolysis reaction, splitting into ADP and Pi (inorganic phosphate).
This releases energy so that the solutes can be transported.

Question 97

what are co transporters?

Answer 97

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.

Question 98

what is absorbed into the blood stream?

Answer 98

glucose in the small intestine.

Question 99

conc in the mammalian ileum:

Answer 99

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.

Question 100

step 1 of co transport:

Answer 100

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.

Question 101

step 2 of co transport

Answer 101

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.

Question 102

step 3 of active transport:

Answer 102

Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion

Question 103

decreasing concentration in active transport:

Answer 103

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.

Question 104

The rate of active transport is affected by:

Answer 104

ƒ The speed of individual carrier proteins — the faster they work, the faster the rate of active transport.

ƒ The number of carrier proteins present — the more proteins there are, the faster the rate of active transport.

ƒ The rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport can’t take place.

Question 105

diffusion description:

Answer 105

Net movement of particles from an area of higher concentration to an area of lower concentration.
ƒ Passive process — doesn’t require energy.

Question 106

Facilitated diffusion description

Answer 106

ƒ Net movement of particles from an area of higher concentration to an area of lower concentration.
ƒ Uses carrier proteins and channel proteins to aid the diffusion of large molecules and charged particles through the membrane.
ƒ Passive process — doesn’t require energy.

Question 107

Osmosis description

Answer 107

Movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential.
ƒ Passive process — doesn’t require energy

Question 108

Active transport description:

Answer 108

Movement of molecules, usually from an area of lower concentration to an area of higher concentration.
ƒ Uses carrier proteins and co-transporters to transport molecules.
ƒ Active process — requires energy