Important Functions of the Cellular Membranes
• thin, typically 5 to 10 nm thick and somewhat fluid
• selective uptake and export of ions and molecules
• cell compartmentalization
• protein sorting
• anchoring of the cytoskeleton
• production of energy intermediates, such as ATP and NADPH
• cell signalling
• Cell and nuclear division
• Adhesion of cells to one another and extracellular matrix
THREE PRIMARY COMPONENTS OF THE PLASMA MEMBRANE: phospholipids, proteins and
Fluid Mosaic Model
• basic framework for a plasma membrane (phospholipid bilayer)
• 1/2 of the bilayer is called a leaflet
• cytosolic leaflet (facing the cytoplasm)
• extracellular leaflet (facing the cells exterior)
• amphipathic molecules
• there are integral and peripheral membrane proteins
• Integral proteins - have transmembrane segments (contain non polar amino acids).
Can be covalently linked to a fatty acid post-translationally to anchor covalently
to a protein to become stably inserted. same move laterally/rotationally, slowly.
If attached tot he cytoskeleton, there can be no movement.
• Peripheral proteins - weaker association with the membrane region. associate with
the with the hydrophilic region, or form ionic interactions to it.
• glycoproteins and glycolipids are covalently bound carbohydrates found in extracellular leaflet
• Sterols (ex. cholesterol) modify fluidity of bilayer (higher temp, membrane is less fluid)
• lipids and proteins can move laterally relative to each other within the membrane.
• biomembranes exhibit properties of fluidity Chapter 6
• individual molecules remain in close association, yet they have the ability to readily
move within the membrane.
• semifluid - can only move in 2 dimensions
• if a membrane is to fluid, at high temp membrane will become 'leaky'. if to solid, functioning
of the membrane proteins will be inhibited
• Movement in 3D (flip-flop movement of lipids from one leaflet to another) does not occur
spontaneously and requires energy to occur
• Lipid raft - a regain of one leaflet of a biological membrane that had a distinctive lipid
composition that typically has straight chain fatty acid tails that all the lipids to pack
more tightly and hence, be more stably associated
• in regions where specific membrane protein are inserted, especially those involved in
• types of movements (3 different types of proteins)
• spontaneous movements - can move laterally but must stay within the bilayer
• Scramblase proteins - method for moving phospholipids between leaflets (along the
• Flippases - move lipid from the outer to thinner leaflet with the energy from ATP
• Floppases - opposite of Flippases, still with the assistance of ATP hydrolysis Chapter 6
1.Transmembrane proteins - proteins that have one or more regions that are physically
embedded in the hydrophobic region of the cells phospholipid bilayer
• stable because of non polar interactions act favourably within the hydrophobic part of the
2. Lipid Anchors - involves the covalent attachment of a lipid to an amino acid side chain within a
- this attachment is done post-translationally after a polypeptide is created
- the fatty acid tail of the lipid anchor keeps the protein firmly bound in the membrane leaflet in
which it is embedded
BOTH TRANSMEMBRANE PROTEINS AND LIPID ANCHORED PROTEINS ARE CLASSIFIED AS
INTEGRAL MEMBRANE PROTEINS - they cannot be released from the membrane unless the
integrity of the membrane is disrupted. Flip flop does not occur.
3. Peripheral membrane proteins
• proteins that do not interact with the hydrophobic interior of the phospholipid bilayer
• noncovalently bound to regions of integral membrane proteins on the extracellular leaflet, or
they are bound to polar head groups of the phospholipids
• typically bound to the membrane by hydrogen or ionic bonds
Glycosylation - the process of covalently attaching a carbohydrate to a protein or lipid, creating
glycoproteins or glycolipids
•The carbohydrates that are attached to proteins and lipids have well-defined structures
that serve in some cases as recognition signals for other cellular proteins.
•have a protective effect (glycocalyx).
•Used to describe the carbohydrate-rich zone on the cell surface that
shields the cell from mechanical and physical damage (cell coat that
shields from damage)
• The glycoproteins/lipids often play a role in cell surface recognition.
Because of their hydrophobic interiors, phospholipid bilayers present a good barrier to the
movement of ions and hydrophilic molecules
Diffusion - a solute moving from a region of high concentration to a region of low concentration Chapter 6
• Passive Diffusion - diffusion occurs without the aid of a
• ex. uncharged polar molecules, gasses
• Larger polar molecules (sugars, amino acids and other
macromolecules) have a rate of diffusion the
is relatively slower
• Macromolecules (proteins and large carbohydrates)
cannot cross the phospholipid bilayer without the
use of a protein channel
Phospholipid bilayer is impermeab