1: Models of Membrane Structure

Question 1

State 2 models of membrane structure? Which one superseded the other (and is therefore the one we use today)?

Answer 1

1. Davson-Danielli Model
2. Singer-Nicolson Model

Singer-Nicolson superseded Davson-Danielli and is therefore the one we use today.

Question 2

Outline the membrane structure in the Davson-Danielli Model.

Answer 2

• bilayer of phospholipids in centre of membrane
• protein on either side

(like phospholipid sandwich)

Question 3

When was the Davson-Danielli Model developed?

Answer 3

1930s

Question 4

Sketch the Davson-Danielli Model.

Answer 4

p.8

• bilayer of phospholipids in centre of membrane
• protein on either side

Question 5

Give three reasons for the Davson-Danielli Model.

Answer 5

1. chemical analysis of membranes showed that they were composed of phospholipids and protein
2. evidence suggested plasma membrane of red blood cells has enough phospholipids to form an area twice as large as the area of the plasma membrane, suggesting a phospholipid bilayer
3. experiments showed that membranes for a barrier to the passage of some substances, despite being very thin, and layers of protein could act as the barrier.

Question 6

What was first produced in the 1950s that could be used to test the Davson-Danielli Model?

Answer 6

high magnification electron micrographs

Question 7

How did membranes appear when looked at in high magnification electron micrographs? How did this fit with the Davson-Danielli Model? Why?

Answer 7

• as two two dark lines separated by lighter band

seemed to fit model:
- proteins usually appear darker than phospholipids in electron micrographs

Question 8

When did evidence that did not fit with the Davson-Danielli Model begin to accumulate?

Answer 8

1950s and 1960s

Question 9

Give three examples of evidence that did not fit with the Davson-Danielli Model.

Answer 9

1. freeze-fracture electron micrographs
2. analysis of membrane protein
3. fusion of cells with membrane proteins tagged with different coloured flourescent markers

Question 10

Explain ‘freeze-fracture electron micrographs’ as an examples of evidence that did not fit with the Davson-Danielli Model.

Answer 10

freeze-fracture electron micrographs showed:
- globular proteins were present in the centre of the phospholipid bilayer

(whereas D-D believed they were on either side of the phospholipid bilayer)

Question 11

Explain ‘analysis of membrane protein’ as an examples of evidence that did not fit with the Davson-Danielli Model.

Answer 11

analysis of membrane proteins showed that:

• parts of membrane proteins were hydrophobic
• therefore, they would be positioned in the bilayer
• in some cases the proteins would extend from one side to the other

(whereas D-D believed they were on either side of the phospholipid bilayer)

Question 12

Understand, perhaps even know how to draw the membrane protein diagram (p8).

Answer 12

-

sorry, got lazy

Question 13

Explain ‘fusion of cells with membrane proteins tagged with different coloured flourescent markers’ as an examples of evidence that did not fit with the Davson-Danielli Model.

Answer 13

fusion of cells with membrane proteins tagged with different coloured flourescent markers showed that:

• these proteins can move within the membrane
• colours became mixed within a few minutes of cell fusion

e.g.

(cells with red membrane protein) fused with (cells with green membrane protein) = red and green mixed

Question 14

What did the 3 examples of evidence that did not fit with the Davson-Danielli Model achieve?

Answer 14

falsified (showed it to be incorrect) the Davson-Danielli Model

Question 15

Who was the new model (after Davson-Danielli) proposed by? When was it proposed? What are the two names of the new model?

Answer 15

• Singer and Nicolson
• 1966
• Singer-Nicolson Model OR fluid mosaic model