Summary The Plasma Membrane - Cell Biology

The Plasma Membrane

Learning outcomes:
 How are membranes used by cells?
 What are cell membranes made of?
 What is an amphipathic lipid and how does its structure influence the properties of cell
membranes?
 What are some roles that water plays in cell membrane function?
 Are membrane lipids static? How can they move?
 How can the lipid composition of cell membranes change with temperature?
 How can a cell use membrane lipid asymmetry?
 How can membrane proteins associate with the plasma membrane?
 What is the significance of hydrophobic stretches in transmembrane proteins?
 What is glycosylation and what are some functions of membrane protein glycosylation?



 How are membranes used by cells?


Defines the boundaries around and within the cell – separates ICF and ECF
e.g. lipid bilayer prevents free movement of charged ions into the cell – movement
of ions regulated by hydrophilic protein channels

Maintains essential differences between cytosol and ECF – concentration gradients
e.g. sodium-potassium pump creates EC gradient – needed for depolarisation

Maintains essential differences between cytosol and environment inside organelles
e.g. proton gradient between mitochondrial membranes – needed for ATP synthesis


Involved in cell adhesion, cell signalling


Used as an attachment surface for cytoskeleton and ECM



 What are cell membranes made of?


All biological membranes have a common general structure: lipid bilayer with membrane
proteins

The lipid bilayer: Basic structure for all cell membranes
Membrane proteins: determine cell membrane characteristics, perform specific tasks, vary
in structure and in association with the lipid bilayer

,  What is an amphipathic lipid and how does its structure influence the properties of cell
membranes?

Amphipathic: a molecule which contains both a hydrophilic part and a hydrophobic part
Amphiphilic: a molecule which contains both a hydrophilic part and a hydrophobic part
(Amphipathic and amphiphilic are interchangeable)

All the lipid molecules in cell membranes are amphiphilic:
e.g. phospholipids (most abundant membrane lipid) have:
Hydrophilic head group containing a phosphate group
2 hydrophobic hydrocarbon tails

Hydrocarbon tails (usually fatty acids in animal, plant & bacterial cells):
Differ in length (normally 14-24 C atoms)
One tail typically has 1+ CIS double bond – kink in chain


Structure Properties Why?
Amphipathic nature Phospholipid bilayer Hydrophobic tails aggregate together to shield
spontaneously forms from water, hydrophilic heads exposed to water.

Bilayer > dispersed because dispersed
phospholipids force adjacent water molecules to
reorganize into ice-like cage structures around
the molecule. More ordered, entropy decreases,
free energy increases ∴ not feasible. If
Hydrophobic tails cluster together, free-energy
cost minimized because number of water
molecules that become more ordered decreases.

Sealed compartment Free edges in bilayer energetically unfavorable
spontaneously forms (results in ordering of water molecules). Bilayer
closes in on itself to avoid having free edges
forming sealed compartment

2 hydrocarbon tails Bilayer > micelle Micelle forms when amphiphilic molecule is cone-
shaped (i.e. head attached to one HC tail), bilayer
forms when amphiphilic molecule is cylindrical
(i.e. “ “ 2 HC tails).
Variable Longer tail = less fluid Longer tails (more C atoms in chain) form
length/saturation of membrane and VV stronger VDW interactions because there is a
hydrocarbon tails greater surface area for lipid tails to interact.

More C=C bonds = More double bonds (more kinked chain) means
more fluid membrane lipids don’t pack together as closely, weaker VDW
and VV interactions form.