Structure and Function of the Cell Membrane

Question 1

Describe the Membrane Structure

Answer 1

• Is thin and the size is 8nm (8 x10 -9 metre)
• It is a flexible and sturdy
  barrier that surrounds the cytoplasm of a cell
• Fluid mosaic model describes membrane
  structure
  – “sea of lipids in which proteins float like icebergs”
  – membrane is 50 % lipid & 50 % protein
• held together by hydrogen bonds
  – lipid is barrier to entry or exit of polar substances
  – proteins are “gatekeepers” – regulate traffic

Question 2

Phospholipids

Answer 2

Comprises 75% of lipids
* Phospholipid bilayer = 2 parallel layers
of molecules
* Each molecule is amphipathic (has both a
polar & nonpolar region)

Question 3

Membrane fluidity

Answer 3

Membranes are fluid structures and lipids can
move around within the plane of the membrane
leaflet
* Lipids rarely flip flop between membrane
leaflets therefore the lipid composition of the
leaflets can be asymmetric

Question 4

Membrane fluidity is determined by?

Answer 4

• Lipid tail length - the longer the tail, the less fluid the membrane
• Number of double bonds – more double bonds increases fluidity
• Amount of cholesterol – more decreases fluidity

Question 5

Membrane proteins

Answer 5

• Integral proteins: extend into or completely across
  cell membrane (transmembrane protein)
• Peripheral proteins: attached to either inner or outer
  surface of cell membrane and are easily removed from it

Question 6

Integral membrane protein

Answer 6

Integral proteins are amphipathic
* They have hydrophobic regions that span
the hydrophobic core of the lipid bilayer
* These regions usually consists of non
polar amino acids coiled into helices
Hydrophobic region
Hydrophilic region
* Hydrophilic ends of the
proteins interact with the
aqueous solution

Question 7

Functions of membrane proteins

Answer 7

• Receptor Proteins
• Cell Identity Markers
• Linkers
• Enzymes
• Ion Channels
• Transporter Proteins

Question 8

Selective permeability of membrane

Answer 8

The molecular organisation of the membrane results in
selective permeability – the membrane allows some
substances to cross but excludes others
* If we consider just the lipid bilayer, it is:
– Permeable to nonpolar, uncharged molecules - O 2, N2 benzene
– Permeable to lipid soluble molecules – steroids, fatty acids,
some vitamins
– Permeable to small uncharged polar molecules: water, urea,
glycerol, CO 2
– Impermeable to large uncharged polar molecules – glucose,
amino acids
– Impermeable to ions – Na+ , K + , Cl -, Ca2+ , H