cell membrane experiment- lqc 2b

Question 1

Explain the problems with (limitations of) Davson and Danielli’s model.

Answer 1

• Polar molecules are not able to diffuse through the two protein layers
• Limits fluidity of membrane and fusion/formation of vesicles
• Does not allow the hydrophilic phosphate heads to be in contact with the water

Question 2

What did we learn about cell membrane structure from freeze-fracture electron microscopy?

Answer 2

• Freeze-fracture electron microscopy. Frozen membranes were fractured between the 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.

Question 3

what was the conclusion from the freeze-fracture electron microscopy

Answer 3

The bumps are proteins. Proteins are embedded IN the lipid bilayer rather than present in layers on the outside of bilayer.

Question 4

How does the work of Frye and Eddin provide further evidence for the fluid mosaic model?

Answer 4

• Fused a mouse cell with proteins given a green fluorescent label with a human cell with proteins given a red fluorescent label. Observed fused cell under the light microscope.
• Immediately after fusion the proteins were in their separate halves, but after 40 mins at 37c there were the same number of proteins but they had fully mixed.

Question 5

conclusion of the Frye and Eddin experiment

Answer 5

The cell membrane must be fluid, allowing proteins to move.

Question 6

How does the work of Unwin and Henderson provide further evidence about the proteins in the fluid mosaic model?

Answer 6

• Placed membranes in solution and increased the ionic strength of the solution. Repeated the experiment but added detergents.
• Some proteins dissociated from membrane. Some proteins only dissociated from membrane when detergents used.

Question 7

what is the conclusion of the Unwin and Henderson experiment

Answer 7

Some proteins are intrinsic – embedded in the membrane. Other proteins are extrinsic – loosely associated with the membrane.

Question 8

How does the work with Lectins provide further evidence about the carbohydrate in the fluid mosaic model?

Answer 8

• Lectins (molecules that bind to carbohydrate) were labelled with ferritin, which is visible under the electron microscope. The labelled lectins were mixed with membranes.
• Lectins only bound to the outside of the membrane (not the inner).

Question 9

conclusion of the experiment with Lectins

Answer 9

The outer surface of the membrane is different to the inner surface – carbohydrate is only on the outside.