Topic 2B - Cell Membranes

Question 1

What is the role of the cell-surface membrane?

Answer 1

They are a partially-permeable barrier between a cell and it’s environment, controlling which substances enter and leave the cell.

Question 2

What are cell membranes composed of?

Answer 2

Lipids (phospholipids), proteins, and carbohydrates.

Question 3

Give three properties of the cell membrane.

Answer 3

Bilayer, fluid because phospholipids constantly moving. Proteins scattered through the bilayer like tiles in a mosaic. Some can move sideways, others are fixed in position. Cholesterol.

Question 4

Describe proteins in the membrane and their features.

Answer 4

Channel proteins and carrier proteins allow large molecules and ions to pass through the membrane. Receptor proteins on the membrane allow the cell to detect chemicals released from other cells.

Question 5

What can’t enter through the membrane and why?

Answer 5

Water-soluble substances like ions can’t enter because the centre of the bilayer is hydrophobic, so it acts like a barrier.

Question 6

What is cholesterol? Describe its purpose in the cell membrane.

Answer 6

A type of lipid. It is present in all membranes except bacterial cell membranes. They fit between the phospholipids, binding to the tails and causing them to pack closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid. They also help to maintain the shape of animal cells as they don’t have cell walls.

Question 7

How do you calibrate a colorimeter?

Answer 7

Using pure water.

Question 8

Name some control variables in the experiment about the permeability of membranes.

Answer 8

Equally sized beetroot/same mass.
Same volume of water.
Same length of time for beetroot in water.

Question 9

What is the permeability of a cell membrane like below 0° and why?

Answer 9

The phospholipids don’t have much energy and can’t move, so they’re packed close together and make the membrane rigid. However, channel proteins and carrier proteins deform, increasing the permeability. Ice crystals form, and pierce the membrane.

Question 10

What is the cell membrane permeability like between 0 and 45° and why?

Answer 10

Phospholipids have more energy and can move, so they aren’t as packed together. Membrane is partially permeable. As temp increases, phospholipids move more due to more energy, so increases permeability.

Question 11

What is the permeability of the cell membrane like above 45° and why?

Answer 11

Phospholipid bilayer melts. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins deform, increasing permeability.

Question 12

What is diffusion?

Answer 12

Net movement of particles from an area of higher concentration to an area of lower concentration.

Question 13

Name two molecules that can easily diffuse through cell membranes and why.

Answer 13

Oxygen and carbon dioxide. They’re small, so they can pass through spaces between the phospholipids. They’re also non-polar, so they are soluble in the hydrophobic lipid bilayer.

Question 14

Name two large molecules and two charged particles.

Answer 14

Amino acids and glucose are large. Ions and polar molecules are charged.

Question 15

Why can’t charged particles diffuse quickly through the cell membrane?

Answer 15

They’re water soluble, and the centre of the bilayer is hydrophobic.

Question 16

Describe how carrier proteins work.

Answer 16

They move large molecules across membranes, down their conc gradients. A large molecule attached to the binding site of the protein, and the protein changes shape. This releases the molecule on the opposite side of the membrane.

Question 17

Why do we say that carrier proteins and channel proteins are specific.

Answer 17

Different carrier proteins facilitate the diffusion of different molecules. Same with channel proteins.

Question 18

What does simple diffusion depend on?

Answer 18

Concentration gradient. Difference in concentration between the two sides of the membrane decreases until equilibrium is reached.
Thickness of exchange surface. Shorter distance travelled the thinner the surface is.
Surface area.

Question 19

What are microvilli?

Answer 19

Projections formed by the cell-surface membrane folding up on itself. Increases surface area.

Question 20

What does facilitated diffusion depend on?

Answer 20

Conc gradient.
Number of channel and carrier proteins. They become the limiting factor.

Question 21

What are aquaporins?

Answer 21

Special channel proteins that allow the facilitated diffusion of water through cell membranes. Some kidney cells are adapted to have a lot of aquaporins.

Question 22

What is osmosis?

Answer 22

The diffusion of water molecules across a partially permeable membrane, from an area of high water potential to an area of lower water potential.

Question 23

What is isotonic?

Answer 23

Two solutions with the same water potential.

Question 24

What is the rate of osmosis dependent on?

Answer 24

Water potential gradient.
Thickness of exchange surface.
Surface area of exchange surface.

Question 25

Describe an example of a serial dilution technique.

Answer 25

Add 10cm^3 of 2M sucrose to the first test tube, and 5cm^3 of distilled water to the other four. With a pipette, draw 5cm^3 of the solution from the first test tube, add it to the second and mix. Now you have 10cm^3 of 1M sucrose. Repeat until 0.5M, 0.25M, and 0.125M.

Question 26

In a calibration curve, what does the point where the curve meets the x-axis signify? Y-axis is change in mass and x-axis is conc of sucrose.

Answer 26

Where the water potential of the sucrose solution is the same as the water potential of the potato cells.

Question 27

What is the definition of active transport.

Answer 27

Using energy to move molecules and ions across membranes against a concentration gradient.

Question 28

What is ATP and what is the reaction that releases energy?

Answer 28

It is a source of energy in the cell produced by respiration. ATP undergoes a hydrolysis reaction, forming ADP and Pi.

Question 29

Which protein in the cell membrane can use active transport and which doesn’t?

Answer 29

Carrier proteins are involved in active transport. Channel proteins are not.

Question 30

What are Co-transporters and how do they work?

Answer 30

They are a carrier protein. They bind two molecules at a time, and the concentration gradient of one of the molecules is use to move the other molecules against its concentration gradient.

Question 31

What are the 3 factors affecting the rate of active transport? What doesn’t affect active transport?

Answer 31

A decreasing concentration gradient doesn’t affect rate of active transport.
Speed of individual carrier proteins, number of carrier proteins, and rate of respiration in the cell and availability of ATP affect rate of active transport.

Question 32

Describe the processes occurring in the mammalian ileum.

Answer 32

Sodium ions are actively transported out of the ileum epithelium cells into the blood by the sodium-potassium pump, creating a concentration gradient where there is a higher concentration of sodium ions in the lumen of the ileum than inside the cell. Sodium ions diffuse from the lumen of the ileum into the epithelial cell, down their conc gradient via sodium-glucose Co-transporter proteins. Glucose is carried too, which increases the concentration of glucose inside the cell, so it can then diffuse out of the cell into the blood down its concentration gradient through a channel protein.

Question 33

Why might both CO2 and H2O be able to enter an aquaporin channel protein?

Answer 33

They are both small so fit channel. They both have similar shapes so bind to channel.

Question 34

Why does surrounding cells in alcohol increase membrane permeability?

Answer 34

The solvent dissolves the lipids in the cell membrane, causing it to lose its structure.

Question 35

Explain the function of ATP hydrolase in reference to the sodium potassium pump.

Answer 35

Breaks down ATP into ADP and Pi to release energy to allow the active transport of ions.

Question 36

The movement of substances across cell membranes is affected by membrane structure. Describe how (5)

Answer 36

Phospholipid bilayer allows diffusion of non-polar/lipid-soluble substances.
Phospholipid bilayer prevents diffusion of polar/lipid-insoluble/water-soluble substances.
Carrier proteins allow active transport.
Channel/carrier proteins allow facilitated diffusion/co-transport
Shape/charge of channel/carrier determines which substances move.
Number of channels/carriers determines how much movement.
Cholesterol affects fluidity.

Question 37

Why was it important that the same volume of water is in each tube in the beetroot experiment?

Answer 37

Too much water would make the concentration of water lower.
So results from different temperatures are comparable.

Question 38

Suggest how the increase in temperature of the water causes the release of the red pigment (2)

Answer 38

Damage to the cell surface membrane. Proteins denature. Increased fluidity.

Question 39

Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients (2)

Answer 39

Lots of carrier and channel proteins for for active transport/facilitated diffusion/ co-transport (all 3 reasons give marks).
Membrane folded so large surface area.

Question 40

NHE3 actively transports one sodium ion into the cell in exchange for one proton out of the cell. How? (3)

Answer 40

Co-transport.
Uses hydrolysis of ATP.
Sodium ion and proton bind to the protein.
Proteins changes shape.

Question 41

The mean mass of contents in the gut in the group with tenapanor (which reduces gut absorption) is 4.1g and the group without is 2.0g. The p value is less than 0.05. Explain how they reached this conclusion (4)

Answer 41

There is a significant different between A and B
There is a less than 0.05 probability that the difference is due to chance. Water potential in gut is reduced. Water moves into the gut by osmosis.

Question 42

High absorption of salt from the diet can result in a higher than normal concentration of salt in the blood plasma entering capillaries, which can lead to a build-up of tissue fluid. Explain how (2)

Answer 42

Higher salt results in lower water potential of tissue fluid, so less water returns to capillary by osmosis at the venule end.

Question 43

Compare and contrast the processes by which water and inorganic ions enter cells. (3)

Answer 43

Both move down concentration gradient. Both move through channels in membrane. Ions can move against a concentration gradient by active transport but water can’t.

Question 44

Group A had a substance that inhibited respiration and group B did not have that substance added. Explain the graph (the total uptake of chloride ions graph for B is greater than A) (4)

Answer 44

Group A is diffusion only so initial uptake slower. Group A levels off because same concentrations inside cells and outside cells. Group B has faster uptake because by diffusion plus active transport.
Group B fails to level off because uptake against gradient.
Rate slows for group B because fewer chloride ions in external solution.