5.1 Overview of the structure of membranes
Fluid mosaic membrane: membranes consist of fluid lipid molecules in which proteins are
embedded and float freely rather than being rigid and molecules locked in a space.
- The mosaic part refers to the fact that membranes contain a wide variety of different types of
proteins and lipids, each with specific function.
- Proteins move more slowly in the fluid environment of the membrane because they they are
larger than lipid molecules.
- Glycolipids and glycoproteins are formed as a result of the linkage of lipid and protein
component of membrane to carbohydrates.
Lipid molecules of all biological membranes exist in a double layer, called a bilayer.
- The molecules feely move around, side by side, and back and forth in the bilayer half. Integral
Myelin: Functions to insulate nerve fibres, it is composed mostly of lipids and a little bit proteins.
Depending upon the type of the membrane, the amount of proteins and lipids within the
Membrane asymmetry: The two halves of the protein bilayer and its components differ from each
other. Aslo, it reflects that diffrences in functions performed in each side.
Receptor proteins: found on the external surface of plasma membrane, hormones and growth
factors bind to them, the binding riggers changes to distinctly different protein components found
on the inner surface of the membrane which lead to passing(transduction) of the signals.
Experimental evidence in support of the fluid mosaic model:
Membranes are Fluid: Frye-Eddin Experiment
red dye dye
Conclusion: Mouse and human proteins had moved around in the fused membrane. The membrane
bilayer was as fluid as light machine oil.
Both colours fused,
after 1 h/ completely
- A block of cell is rapidly frozen by dipping in into liquid nitrogen.
- Than the block is fractured by hitting it with a microscopically sharp knife edge.
- Most of the times the fracture splits bilayers into inner and outer halves. - In the electron microscope: split layers appear as smooth layers in which individual particles the
size of protein are embedded.
- Conclusion: the images clearly show that particles on either side of the membrane differ in size,
number, and shape, providing evidence that the two sides are distinctly different.
- The visible particles are integral protein membranes (interior).
5.2: the lipid fabric of a membrane
Phospholipids: an important property of phospholipids which is crucial to the structure and function
of membranes is amphipathic. Meaning that, each phospholipid contains a region that is
hydrophobic (tail-Nonpolar) and a region that is hydrophilic (head-polar).
- Detergents are excellent in removing oil stains because they are also amphipathic molecules.
(They solubilize lipids and proteins in the membrane by inserting into phospholipid bilayers).
When added into an aqueous solution phospholipids associate with each other and resemble to
form a bilayer.
- (The bilayers from spontaneously because of the tendency to the hydrophobic fatty acid to
combine together while the polar head group associate with water-lowest energy state).
Membrane fluidity: fluidity depends on how densely the lipid molecules can pack.
- Influenced by two factors: composition of the lipid molecules that make up the membrane and
- Fatty acids with saturated hydrocarbon: tend to be straight. This allows the protein to pack
- Lipid molecules with unsaturated fatty acids: less straight as the double bonds introduces kinks
or bends in its structure. As a result they pack together less closely.
- Increase in temp: more unsaturated : Decrease in temp: more saturated
- Normal fluid state is reached by mixing population of unsaturated and saturated fatty acids.
- More C=C within fatty acid tails = more space between neighbouring lipids.
Low Temp High Temp
Electron transport stops operating if the Membrane breakage due to high fluidity/
membrane solidifies. liquidity cause by an increase in the molecular
Membrane permeability in inhibited due to the Ions such as K, Na, Ca begin to freely diffuse
viscosity of the membrane caused by low temp. across the membrane causing irreversible
disruptions of cellular ions which can lead to cell
Unsaturated fatty acids produced by removing two H atoms from a fatty acid, creating C=C, during
fatty acid synthesis through the action of a group of enzymes called Disaturases.
Changes in the transcription of the gene cause changes in abundance f its transcript (mRNA) and
resulting protein abundance.
Transcript abundance of Disaturases gene increase as temperature lowers.
Higher amount of desaturates increase abundance of unsaturated fatty acids.
For some organisms such as cyanobacteria a room temperature is a very cold temperature. Sterols also effect the membrane fluidity
- Cholesterol is an example of sterol which is found only in membranes of animal cells.
- Sterols at high temperature: help restrain the movement of lipid molecules, thus reducing the
fluidity of the membrane.
- Sterols at lower temperatures: disrupt fatty associating by occupying the space between fat
molecules and therefore slowing the transition to non-fluid gel state.
5.3: membrane proteins: Key functions of membrane proteins
Transport: proteins provide hydrophilic channels that allow the movement of specific compound.
Membrane proteins can change shape and move from one side to another in a membrane.
Signal transduction: receptor proteins on the outer surface of the membrane that bind to specific
chemicals such as hormones. These signals trigger changes on the inside surface of the membrane
that lead to transduction of the signal through cell.
Attachment and recognition: all of the proteins on either side of the membrane act as attachment
point for cytoskeletal elements and also other components involved on cell to cell recognition.
5.3b/ membrane proteins can be classified into two parts: integral and peripheral
Integral membrane proteins: are proteins that are embedded in phospholipid bilayer.
- All of them possess at least one region that interacts with the hydrophobic core of the
- Most integrals are transmembrane protein meaning that it goes from one side of a membrane
through to the other side of the membrane. Many act as “loading docks” meaning that they
permit the transport of specific substances through the membrane