Membrane Structure (1.3)

Question 1

The Davson-Danielli Model

Answer 1

Proposed in 1930 and nicknamed the Sandwich.

The Davson-Danielli described the cell membrane as a bilayer of phospholids sandwiched between a layer of protein on each side.

Question 2

Evidence for The Davson-Danielli Model

Answer 2

• Chemical analysis which showed the membrane was composed of phospholipid and protein
• Electron micrograph images showed membranes as dark lines separated by a lighter area
• Membranes are semi-permeable so proteins could be acting as a barrier to some substances

Question 3

Evidence that falsified the Davson-Danielli model

Answer 3

• Electron micrographs showed globular proteins were in the centre of the phospholipid layer
• Red/green fluroscent tagging
• Analysis of protein showed that some were hydrophobic. So proteins extended all the way through the phospholipid layer and within

Question 4

How did fluorescent antibody tagging show that membrane proteins were free to move in a membrane?

Answer 4

Membrane proteins of mouse cells were tagged with red fluorescent markers and membrane proteins of human cells were tagged with green fluorescent markers. The cells were fused together and the red and green markers mixed throughout the membrane. This showed that membrane proteins were free to move and were not fixed in a peripheral layer.

Question 5

Phospholipids

Answer 5

Phospholipids are amphipathic meaning part of the molecule is hydrophilic (attracted to water) and part of the molecule is hydrophobic (not attracted to water).

O Hydrophilic, polar phosphate head.
| | Hydrophobic, non-polar, fatty acid tails.

Question 6

Phospholipids arrangement in membranes

Answer 6

Phospholipids self-organise in the membrane to keep their heads wet and their tails dry. This makes the membrane very stable but also flexible.

Question 7

Cholesterol

Answer 7

Cholesterol is a component of animal cell membranes (approx. 20%). Cholesterol molecules are amphipathic and are found embedded in the phospholipid layer. They reduce the fluidity and permeability of the membrane.

Question 8

Membrane Proteins

Answer 8

• Integral (transmembrane) proteins are embedded in the phospholipid bilayer.
• Peripheral proteins are attached to the outer surface and are involved in cell to cell communication.
• Channel proteins act as pumps eg. potassium channels in nerve cells.
• Glycoproteins have sugar units which are attached to the outer surface of the membrane. They are important for cell recognition by the immune system and as hormone receptors.

Question 9

Membrane Proteins Function

Answer 9

TRACIE
Transport
Receptors
Anchorage
Cell recognition
Intercellular joinings
Enzymatic activity