topic 8 - Lipids Flashcards
what are lipids?
Lipids are a group of naturally occurring, significantly hydrophobic, molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids, etc.
what are fatty acids made up of?
Two distinct parts:
A long hydrocarbon chain
which is hydrophobic (water insoluble) and not very reactive chemically
A carboxylic acid group
which is ionized in solution (COO-), extremely hydrophilic (water soluble)
and readily reacts with a hydroxyl or an amino group on a second molecule to form esters and amides.
e.g. Palmitic acid
The carboxylic acid group (red) is shown in its ionized form.
A ball-and-stick model (center)
A space-filling model (right) are also shown.
what are fatty acids and where are they found?
Almost all of the fatty acid molecules in a cell are covalently linked to other molecules by their carboxylic acid group.
The many different fatty acids found in cells differ:
- The length of their hydrocarbon chains (18-20 most common)
- The number and position of the carbon-carbon double bonds:
None – saturated
One or more double bond unsaturated
what are phospholipids comprised of?
Phospholipids are comprised of two fatty acids, glycerol, and a phosphodiester head group.
Fatty acid chains vary in length and saturation
what are sphingolipids and what links them together?
Sphingolipids are linked via sphingosine
Fatty acid chains vary – any could be attached
May display complex oligosaccharides
More specific roles than glycerol-based lipids – cell types, signalling etc.
what are steroids?
Steroids are a class of lipid with a polycyclic aliphatic carbon skeleton.
The have fixed shape due to cyclisation.
Stereochemistry greatly influences their shape.
Steroids are important as hormones
Cholesterol has a wider role:
- as an intermediate in biosynthesis of other steroids
- In control of membrane fluidity
what is the most abundant phospholipid?
Phosphatidylcholine is the most abundant phospholipid in cell membranes.
why are fatty acids and phospholipids amphiphilic?
Membrane lipids are amphiphilic/ amphipathic
- Hydrophilic heads (polar) form hydrogen bonds with water
- Hydrophobic tails (non-polar) are excluded by water molecules
what are amphiphiles?
Such as phospholipids are subject to two conflicting forces:
The hydrophilic head is attracted to water
The hydrophobic tail repels water and seeks to aggregate
with other hydrophobic units
Hydrophobic molecules resolve this conflict by aggregating in energetically favourable arrangements in which the head is in a polar environment and the tails come together to create an apolar environment.
why / how do amphiphiles self assemble?
Packing arrangements of lipid molecules in an aqueous environment.
Cone-shaped lipid molecules form micelles, cylinder-shaped lipids form bilayers - membranes.
lipids and membranes?
Biological membranes are composed of a lipid bilayer.
The best arrangements of tails involve close packing which optimises van der Waals interactions but few strongly directing interactions (electrostatics, H-bonding) The best arrangements involve close packing which optimises van der Waals interactions but few strongly directing interactions (electrostatics, H-bonding).
what is the membrane structure?
All biological membranes have a common general structure:
The lipid molecules are arranged as a continuous double layer about 5 nm thick.
This lipid bilayer provides the basic fluid structure of the membrane and serves as a relatively impermeable barrier to the passage of most water-soluble molecules – less polar molecules can pass
Protein molecules that span the lipid bilayer mediate nearly all of the other functions of the membrane, transporting specific molecules across it - including drugs.
These proteins include ion channels, receptors and transporters.
membrane bilayers and 2D fluids
. Lateral diffusion occurs rapidly within the plane of the membrane.
- Individual phospholipids may rotate axially or fatty acids tails flex.
This is because the VdW interactions are very similar for each arrangement, including transitions.
- Flip-flopping from one side to the other is very rare as it is energetically unfavourable – it would require the head group to pass through the non-polar layer.
why is fluidity important?
- Processes such as exocytosis, endocytosis, membrane trafficking and membrane synthesis.
- Allows membranes to fuse and molecules to mix (to form specialised regions on membranes).
- Allows even distribution of membrane molecules between daughter cells following cell division.
what factors influence fluidity?
Length – shorter, reduced interactions of the hydrocarbon tails so more fluid
Saturation – each double bond in an unsaturated tail creates a small kink
Cholesterol controls fluidity
Present in large amounts in the plasma membrane.
These short rigid molecules fill the spaces between the neighbouring phospholipid molecules.
In the kinks left by the unsaturated hydrocarbon tails.
Fluidity decreases with (i) an increase in membrane’s cholesterol content, (ii) with more saturation of the acyl tails.
As packing of phospholipids is closer.
