Topic 1.3 - Membrane structure

Question 1

What is the main component of the cell membrane ?

Answer 1

The phospholipid bilayer

Question 2

How is the phospholipid bilayer structured ?

Answer 2

Contain a polar (hydrophilic) head composed of phosphate (+ glycerol)
Contain two non-polar (hydrophobic) tails, each composed of a fatty acid chain
Hence, phospholipids are amphipathic (have hydrophilic and hydrophobic parts)

Question 3

Knowing that the bilayer is bipolar, which part of the structure is inside the cell and which one is outside ?

Answer 3

The hydrophilic head is outside as it likes water and the hydrophobic fatty acid tail is on the inside as it hates water.

Question 4

What are the three properties of the phospholipid bilayer ?

Answer 4

The bilayer is held together by weak hydrophobic interactions between the tails
Individual phospholipids can move within the bilayer (fluidity and flexibility)
Amphipathic properties restrict passage of certain substances (semi-permeable)

Question 5

In which cell type membrane is cholesterol present ?

Answer 5

In animal cell membranes

Question 6

What does cholesterol reduce in membrane structure ?

Answer 6

It reduces fluidity and permeability to some solutes

Question 7

What are the effects of having cholesterol in a membrane ?

Answer 7

The anchoring of certain peripheral proteins and the prevention of crystallisation.

Question 8

What functions do membrane proteins serve

Answer 8

Junctions
Enzymes
Transport
Recognition
Anchorage
Transduction

Question 9

Why are cell membranes presented as the fluid mosaic model ?

Answer 9

Fluid – membrane components can move position
Mosaic – phospholipid bilayer is embedded with protein

Question 10

Who proposed the fluid mosaic model and which model did it falsify ?

Answer 10

This model was proposed by Singer-Nicolson in 1972,
following the falsification of the Davson-Danielli model

Question 11

What does trilaminar mean ?

Answer 11

trilaminar = three distinct layers

Question 12

What model did Davson-Danielli propose ?

Answer 12

Davson-Danielli proposed a model whereby a phospholipid bilayer was flanked by two protein layers (sandwich model)

Question 13

Why was the Davson-Danielli model falsified ?

Answer 13

Fluorescent tagging showed the proteins are mobile
Not all membranes have a constant lipid : protein ratio
Freeze fracturing identified transmembrane proteins