1.3 Membrane Structure

Question 1

What is the Davson Danielli Model?

Answer 1

1930

a bilayer of phospholipids sandwiched between a layer of protein on either side

Question 2

Evidence for the Davson Danielli Model

Answer 2

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

Question 3

Evidence for falsification of Davson Danielli

Answer 3

in 1950s/60s

• electron micrographs showed globular proteins were in the centre of the phospholipid layer
• analysis of protein showed some were hydrophobic, hence some extended all the way through the phospholipid layer and some within.

Question 4

Fluorescent tagging experiment falsification of DD model

Answer 4

• fluorescent markers were attached to antibodies which would bind to membrane proteins
• within 40 mins the markers were mixed throughout the membrane of the fused cell
• this showed that the membrane proteins are free to move within the membrane rather than being fixed in a peripheral layer

Question 5

What is the Singer-Nicholson/Fluid Mosaic Model of the cell membrane?

Answer 5

double layer of phospholipids with proteins and cholesterol embedded in it
the membrane is semi-permeable

Question 6

What is a phospholipid?

Answer 6

make up the cell membrane
are amphipathic (part hydrophobic, part hydrophilic)
polar phosphate head and non-polar fatty acid tails
head is hydrophilic, tails are hydrophobic
head behaves like a solid and the tails like a liquid hence ‘fluid’
tails are attracted to one another and hence self arrange into a bilayer

Question 7

What is Cholesterol’s role in cell membrane?

Answer 7

Cholesterol is only in animal cells (20% of the cell)
embedded in the phospholipid
hydrophobic and hydrophilic ends
cholesterol reduces the fluidity and the permeability of the membrane to ions such as sodium and hydrogen

Question 8

What are the 4 types of membrane proteins?

Answer 8

Integral (transmembrane): proteins embedded in bilayer
Peripheral: proteins attached to the outer surface involved in cell communications
Channel proteins: which act as pumps
Glycoproteins: have sugar units attached to the outer surface of the membrane and are important for cell recognition by the immune system and as hormone receptors

Question 9

Membrane protein functions

Answer 9

TRACIE

• transport: protein channels and protein pumps
• receptors: e.g. for hormones
• anchorage: cytoskeleton and cell wall attachments
• cell recognition: immune system proteins
• intercellular joining: cells attached to each other
• enzymatic activity: movement of ions back and forth across the membrane