Topic 2: Cell membrane evidence

Question 1

We study membranes because the ______ ________ is found in the ______ __________ of mucus producing cells.

Answer 1

CFTR protein

cell membrane

Question 2

In 1915, red blood cell membranes were analysed.

What 2 things were they found to be composed of?

Answer 2

1. Proteins

2. Lipids (the lipids in the cell membrane are now known to be phospholipids)

Question 3

Describe the method to show that the cell membrane is a bilayer (using red blood cells)
(Gorter and Grendel, 1924)
(4 steps)

Answer 3

1. Measured the surface area of red blood cells
2. Removed the contents of the red blood cells to leave only red blood cell membrane sac
3. Used detergents to dissolve the membrane sacs to release the phospholipids
4. Spread the phospholipids on the surface of water, forming a MONOLAYER, and measured the surface area that the layer covered

Question 4

In the red blood cell membrane experiment to show that the cell membrane is a bilayer, what were the results?
(2 points)

Answer 4

1. There is enough phospholipids in the monolayer to cover the surface of a red blood cell TWICE
2. The phospholipids must present as a double layer (BILAYER)

Question 5

Describe the 3 layer protein-lipid sandwich model

2 points

Answer 5

1. Phospholipid bilayer in the centre (light layer)

2. Layer of protein on both sides (dark layers)

Question 6

What were the problems with the 3 layer protein-lipid sandwich model?
(5 problems)

Answer 6

1. {POLAR MOLECULES / IONS} CANNOT PASS THROUGH THE MEMBRANE
2. molecules would not be able to diffuse through the 2 protein layers
3. no channel proteins or carrier proteins for facilitated diffusion, so no active transport or osmosis
4. LIMITS FLUIDITY of membrane and {fusion/formation} of vesicles
5. Does not allow the HYDROPHILIC PHOSPHATE HEADS to be in contact with the water

Question 7

Frozen membranes were fractured between the two lipid layers and the inner surface coated with a heavy metal. The inner surface was viewed using scanning electron microscopy.

Inside the membrane was a smooth surface with bumps in it.

Give the explanation for this (2 points)

Answer 7

1. Bumps seen on the inside of the membranes are PROTEINS
2. which are EMBEDDED IN THE MEMBRANE rather than present as a single layer on the outside

(this is the evidence of integral proteins - ones that are embedded in the membrane)

Question 8

In the 3 layer protein-lipid sandwich model, the centre was light and it had a dark layer on the outer side and a dark layer on the inner side.

The 3 layer protein-lipid sandwich model suggests the light layer is the phospholipid bilayer, and the dark layers were layers of protein.

Give an explanation for this which rejected this model.
(2 points)

Answer 8

1. Phospholipid tails in the centre (light layers)

2. Phospholipid heads on both sides (dark layers)

Question 9

(Frye and Edidin 1970)
In an experiment, a cell labelled with green fluorescent protein and a cell labelled with red fluorescent protein were fused.
The fused cell contained both green and red fluorescent proteins.

What is the evidence in this experiment that supports the fluid mosaic model? (3 points)

What is the scientific explanation for this evidence? (1 point)

Answer 9

1. Both types of protein are in the fused cell
2. Proteins mixed
3. Same original number of proteins
4. Phospholipids and proteins are free to move (supports membrane ‘fluidity’)

Question 10

(Unwin and Henderson 1984)
Placed membranes in solution and increased the ionic strength of the solution.
Then repeated the experiment, but added detergents.

What are the results for this experiment? (2 points)

Answer 10

1. Some proteins dissociated (became separated) from the membrane easily, when ionic strength was increased.
2. Other proteins only dissociated from the membrane when detergents were used (harder to release).

Question 11

(Unwin and Henderson 1984)
Placed membranes in solution and increased the ionic strength of the solution.
Then repeated the experiment, but added detergents.

Some proteins dissociated (became separated) from the membrane easily, when ionic strength was increased.

Other proteins only dissociated from the membrane when detergents were used (harder to release).

What is the scientific explanation for this? (2 points)

Answer 11

1. Peripheral/extrinsic proteins are only loosely attached to the membrane – removed easily when membrane stays intact
2. Integral/intrinsic proteins are fully embedded in membrane – only removed when membrane is disrupted/broken down by detergents

Question 12

Lectins are molecules that only bind to CARBOHYDRATE.

They were labelled with FERRITIN, which makes them visible under the electron microscope.

The ferritin-labelled lectins were mixed with membranes.

What were the results of this experiment? (1 point)

Answer 12

1. Fluoresecence (ferritin) seen only on OUTSIDE of the membrane (tissue fluid side), so lectins must bind here.

Question 13

Lectins are molecules that only bind to carbohydrate.

They were labelled with ferritin, which makes them visible under the electron microscope.

The ferritin-labelled lectins were mixed
with membranes.

Fluoresecence seen only on outside of the membrane (tissue fluid side), so lectins must bind here.

What is the scientific explanation for this? (1 point)

Answer 13

1. Carbohydrate is present only on the OUTSIDE of cells (in contact with tissue fluid).

Question 14

Carbohydrates are only on the __________.

Proteins are _________ in the bilayer.

There are __________ and __________ proteins.

Answer 14

outside

embedded

intrinsic

extrinsic