Topic 2B: Cell Membranes

Question 1

What is the structure of the cell surface membrane?

Answer 1

Following the fluid mosaic model, it is a phospholipid bilayer with:
1) proteins
2) protein channels
3) receptors
4) glycoproteins
5) glycolipids
5) cholesterol 
scattered throughout it or on its surface.

Question 2

What is a glycoprotein?

Answer 2

A protein found on the cell surface membrane which has a carbohydrate attached.

Question 3

What is a glycolipid?

Answer 3

A lipid in the phospholipid bilayer of the cell’s surface membrane which has a carbohydrate attached.

Question 4

What is the function of receptor proteins in the cell surface membrane?

Answer 4

Detect chemicals released from other cells, and signal for the cell to respond.

Question 5

What is cholesterol and what is it’s function in the cell surface membrane?

Answer 5

Cholesterol is a lipid which binds to the hydrophobic tails of the phospholipids. This restricts movement of the phospholipids making the membrane less fluid to help maintain a cell’s shape.

Question 6

What is the function of the phospholipids in the cell surface membrane?

Answer 6

Phospholipids have hydrophobic tails meaning they arrange themselves into a bilayer. Therefore, water-soluble substances are unable to pass through it.

Question 7

Which molecules can pass directly through the phospholipids bilayer?

Answer 7

• Very small
• No charge
• Non polar
• Lipid soluble

Question 8

How does temperature affect the cell surface membrane?

Answer 8

< 0°c:
Channel/carrier proteins denature so the membrane is very permeable.

0°c to 45°c:
The phospholipids have little energy so move around slightly meaning the membrane is only partially permeable. As temperature increases, phospholipids gain energy and move around more, increasing permeability.

+ 45°c:
Channel/carrier proteins denature so the membrane becomes very permeable once again.

Question 9

What is diffusion?

Answer 9

A passive process where there is a net movement of molecules from an area of higher concentration to an area of lower concentration, down it’s concentration gradient.

Question 10

What are the factors affecting the rate of diffusion?

Answer 10

• Surface area.
• Concentration gradient.
• Length of diffusion pathway.

Question 11

What is facilitated diffusion?

Answer 11

A passive process where there is a net movement of larger/charged molecules from an area of higher concentration to an area of lower concentration, down it’s concentration gradient, through a carrier/channel proteins.

Question 12

How do carrier proteins enable facilitated diffusion to occur?

Answer 12

1) A molecule attaches to the carrier protein.
2) The protein changes shape.
3) This releases the molecule on the opposite side of the membrane.

Question 13

How do channel proteins enable facilitated diffusion to occur?

Answer 13

Channel proteins form pores which allow charged particles to diffuse through.

Question 14

What are the factors affecting the rate of facilitated diffusion?

Answer 14

• Concentration gradient.

* Number of channel/carrier proteins.

Question 15

What is osmosis?

Answer 15

A passive process where there is a net movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential, down it’s water potential gradient.

Question 16

What is water potential?

Answer 16

The likelihood of water molecules diffusing out of or into a solution.

Question 17

What is the water potential of pure water?

Answer 17

Zero.

Question 18

What does isotonic mean?

Answer 18

If two solutions have the same water potential, meaning there is no net movement of water molecules as there is no water potential gradient.

Question 19

What is a cell in hypertonic solution?

Answer 19

When a cell is placed in a solution with a lower water potential, meaning water moves out of the cell via osmosis, causing the cell to shrink.

Question 20

What is a cell in hypotonic solution?

Answer 20

When a cell is placed in a solution with a higher water potential, meaning water moves into the cell via osmosis, causing the cell to swell.

Question 21

What are the factors affecting the rate of osmosis?

Answer 21

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

Question 22

How can you investigate water potential?

Answer 22

• Make serial dilutions.
• Measure the change in mass.
• Produce a calibration curve.

Question 23

How do you use a calibration curve?

Answer 23

1) Find the point where the calibration curve crosses the x axis.
2) Find the concentration of sucrose solution at this point.
3) Look up the water potential for that concentration of sucrose solution in an external source.

Question 24

What is the calculation used to work out how to make a solution with a specific concentration?

Answer 24

(Example: How would I make a 20cm³ solution of 0.5M from 1M?)

1) Work out the dilution factor:
Divide concentration of known solution by the concentration of the solution you want to make.
(1M / 0.5M)

2) Find the volume of the solution needed:
Divide the overall volume you want by the dilution factor.
(20cm³ / 2 = 10cm³)

3) Find the volume of distilled water needed:
Minus the volume of solution needed from the overall volume you want.
(20cm³ - 10cm³ = 10cm³)

4) Write a conclusion stating how much solution and distilled water you need:
(Therefore, you would need 10cm³ of solution and 10cm³ of distilled water)

Question 25

How do carrier proteins enable active transport to work?

Answer 25

1) A molecule attaches to the carrier protein.
2) Hydrolysis splits ATP into ADP + P, releasing energy at the same time.
3) This allows the protein to change shape.
4) This releases the molecule on the opposite side of the membrane.

Question 26

How do co-transporters enable active transport to work?

Answer 26

1) Two molecules bind to the co-transporter at the same time.
2) One of the molecules moves down it’s concentration gradient and the co-transporter changes shape.
3) This allows the other molecule to move through the co-transporter against its concentration gradient.

Question 27

What is active transport?

Answer 27

An active process where there is a net movement of molecules from an area of lower concentration to an area of higher concentration, up its concentration gradient, through carrier proteins / co-transporters using ATP.

Question 28

How is glucose absorbed?

Answer 28

1) Sodium ions are actively transported out of the epithelial cell’s of the ileum into the blood, by a sodium-potassium pump.
2) This produces a concentration gradient, causing sodium ions to diffuse back into the epithelial cells of the ileum, down their concentration gradient via sodium-glucose co-transporters.
3) The co-transporter also carries glucose into the cell with the sodium, causing the concentration of the glucose in the cell to increase.
4) Glucose diffuses out of the cell, into the blood, down it’s concentration gradient through a protein channel via facilitated diffusion.