2B - Cell membrane structure

Question 1

What are the features of the phospholipid bilayer?

Answer 1

Partially permeable barrier regulating entry/exit of particles.

Has binding sites.

Confers antigenic properties to the cell.

Site of certain reactions.

Compartmentalises different cell reactions in organelles.

Question 2

What are the functions of phospholipids in the cell membrane?

Answer 2

Main constituent of the cell membrane.
Similar structure to other lipids.
Allow lipid soluble substances to enter and leave the cell (prevents water soluble substances to move).
Makes membrane flexible and self-sealing.

They form a barrier to dissolved substances. The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances through it.

Question 3

Explain why/how cell membranes have a ‘fluid mosaic’ structure:

Answer 3

Phospholipid bilayer has proteins embedded in it (mosaic).

The phospholipids can move freely through the bilayer (fluid).

Question 4

What is cholesterol?

Answer 4

A fatty substance.

Question 5

Is cholesterol hydrophobic or hydrophilic and what does this mean?

Answer 5

Hydrophobic meaning that it pulls together phosphate tails.

Question 6

What is the function of cholesterol in the membrane?

Answer 6

Adds strength and some rigidity to the membrane (particularly at high temperatures).

It also reduces the movement of other molecules in the membrane.

They fit between the phospholipids, causing them to pack more closely together, restricting the movement of the phospholipids, making the membrane less fluid and more rigid.

Question 7

What is a glycolipid?

Answer 7

A carbohydrate bound to a lipid.

Question 8

Where are glycolipids found in the cell membrane?

Answer 8

Lipid portion embedded in phospholipid bilayer, carbohydrate portion extends outside the cell.

Question 9

What is the function of glycolipids in the cell membrane?

Answer 9

Acts as recognition and binding sites.

Question 10

Which part of a phospholipid is hydrophobic and which bit is hydrophilic?

Answer 10

hydrophobic = tail
hydrophilic = head

Question 11

What are the 2 types of membrane proteins?

Answer 11

Intrinsic and extrinsic

Question 12

Where are intrinsic proteins?

Answer 12

Span the membrane

Question 13

What are the 2 types of intrinsic proteins?

Answer 13

Channel and carrier.

Question 14

What are intrinsic proteins made of?

Answer 14

Made of proteins only.

Question 15

Where are extrinsic proteins?

Answer 15

On the surface of the membrane (internal or external).

Question 16

What do extrinsic proteins do?

Answer 16

Acts as receptors or enzymes.

Question 17

What are extrinsic proteins made of/what might they have bound to them?

Answer 17

May have carbohydrates bound to them.

Question 18

What is the function of the cell membrane?

Answer 18

Barrier between the cell and its environment, controlling which substances enter and leave the cell.

Question 19

Through what processes can substances move across cell-surface membranes?

Answer 19

Diffusion, osmosis and active transport.

Question 20

What do the membranes around organelles do?

Answer 20

Divide the cell into different compartments acting as a barrier between the organelle and the cytoplasm. They control what substances enter and leave the organelle.

Question 21

What is special about the membranes around organelles?

Answer 21

They are partially permeable.

Question 22

What is the name of the model that describes the structure of the cell membrane?

Answer 22

Fluid mosaic model.

Question 23

How can you investigate the permeability of the cell membrane in a lab?

Answer 23

Using beetroot - Beetroot cells contain a coloured pigment that leaks out. The higher the permeability of the membrane, the more pigment leaks out of the cell.

Question 24

Explain the procedure to investigate how temperature affects beetroot membrane permeability

Answer 24

1. Use a scalpel to carefully cut 5 equal sized pieces of beetroot. Rinse the pieces to remove any pigment release during cutting.
2. Add the 5 pieces to 5 different test tubes, each containing 5 cm^3 of water. Use a measuring cylinder or pipette to measure the water.
3. Place each test tube in a water bath at a different temperature for the same length of time measured using a stopwatch.
4. Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
5. Now you need to use a colorimeter (a machine that passes light through the liquid and measures how much of that light is absorbed). The higher the absorbance, the more pigment released, so the higher the permeability of the membrane.
6. You can connect the colorimeter to a computer and use software to collect the data and draw a graph of the results.

Question 25

What does increasing the temperature do to membrane permeability?

Answer 25

Increases it.

Question 26

What happens to membrane permeability at temperatures below 0 degrees C?

Answer 26

The phospholipids don’t have much energy, so they can’t move very much. They’re packed closely together and the membrane is rigid. But channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane. Ice crystals may form and pierce the membrane making it highly permeable when it thaws.

Question 27

What happens to membrane permeability at temperatures between 0 degrees C and 45 degrees C?

Answer 27

The phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable. As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane.

Question 28

What happens to membrane permeability at temperatures above 45 degrees C?

Answer 28

The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell - this increases the permeability of the membrane.