1.3 Membrane structure

Question 1

Phospholipid Bilayer

Structure of Phospholipids

Answer 1

• 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 2

Phospholipid Bilayer

Arrangement in Membranes

Answer 2

• Phospholipids spontaneously arrange into a bilayer
• The hydrophilic phosphate heads face out into the surrounding solution, while
  the hydrophobic fatty acid tails face inwards and are shielded from the polar fluids

Question 3

Phospholipid Bilayer

Properties of the Phospholipid Bilayer

Answer 3

• 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 4

Cholesterol

Cholesterol

Answer 4

Cholesterol is a fundamental component of animal cell membranes
* It is not present in plant cell membranes (as they have a rigid cell wall)
Cholesterol reduces membrane fluidity and permeability to some solutes
* It also anchors certain peripheral proteins and prevents crystallization

Question 5

Membrane Proteins

What are the functions of membrane proteins?

Answer 5

• Junctions
• Enzymes
• Transport
• Recognition
• Anchorage
• Transduction

Question 6

Fluid Mosaic Model

Cell membranes are represented as a fluid-mosaic model

Answer 6

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

Question 7

Membrane Models

Sandwitch model

Answer 7

A phospholipid bilayer was flanked by two protein layers.
This model was falsified based on the following findings:
* Fluorescent tagging showed the proteins are mobile
* Not all membranes have a constant lipid : protein ratio
* Freeze fracturing identified transmembrane proteins