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Topic 1 - Cell Membranes
Membrane Composition and Features
Membrane Composition - The membrane is composed of lipids, proteins and carbohydrates.
The lipids hydrophobic and hydrophilic areas lead to the formation of closed sheets.
The proteins act as pumps, channels, receptors and enzymes.
Whilst the carbohydrates are linked to the lipids and proteins.
Membrane Features - Membranes contain many features, and these include sheet-like structures, with a thickness of about 6-10nm. They tend to form closed boundaries and are made up of an asymmetric, twodimensional fluid that is partially electrically polarised.
Membrane Fluidity - Membrane fluidity is generally influenced by temperature and composition, and its mobility generally increases towards the bilayer centre.
Membrane Asymmetry - This is important for membrane function, signalling, synthesis and lipid transport.
Lipids and Structure
A lipid is any of the large group of fats which occur in living proteins and are characteristically soluble in certain organic solvents but only sparingly in water. The cell membrane contains Glycerol-backed phospholipids, Sphingosine-backed lipids and Sterols.
Most Phospholipids are based on glycerol with 2x fatty acid (acyl) chains with one head group (phosphate X).
Hydrophilic + Polar Solutes - These can organise in water without affecting the water structure.
Hydrophobic + Non-Polar solutes - These force the adjacent water molecules to reorganise into a more organised lattice with a decreased entropy and a higher free energy in the hydrophobic effect.
There is a large variation in membrane lipids and these influence the surface biochemistry, membrane dynamics, fluidity and shape.
Phospholipids are amphipathic and contain both hydrophilic heads and hydrophobic tails.
Phospholipid head groups and backbones vary through different glycerol backbones and amino alcohol
Glycolipids vary from other lipids as instead of a phosphate they contain a sugar head group.
Phospholipases are enzymes which can cleave phospholipids at various points.
Phospholipase D cleaves the phospholipid at its head, leaving Phosphatidate.
Phospholipase C cleaves DAG + IP3 which can act as second messengers.
Phospholipase A cleaves Lysophoslipid.
Acyl Chains (Fatty Acid Tails)
Acyl chains have a range of headgroup structures.
The acyl chains are built from acetyl-coA units (this explains why natural lipids occur always in even numbers)
and the shorter the chain the more fluid the lipid. It is 18 carbons on average.
The acyl chains can be saturated (with no double bonds), allowing free rotation, and making them longer.
The chains can also be unsaturated (double bonds) with a kink at each bond, disallowing free rotation from occurring. The more unsaturated the lipid the more fluid it is.
Sphingolipid acyl chains are both unsaturated and generally longer (22-24 Carbons).
Organisms that live at different temperatures can change the lipid composition of their membrane to adapt to changes in the temperature (eg: Thermophiles, Psychrophiles).
In animals the Sterols are Cholesterol, whilst in yeasts it is Ergosterol and in Plants it is Stigmasterol.
The effects Sterols have on membranes in Eukaryotes is to sit between the acyl chains and reduce the fluidity and mobility of the membrane.
The rigid steroid rings interact with and partially immobilise the outer region of the hydrophobic core, causing them to become less fluid.
Cholesterol hence moderates the membrane fluidity as well as the membrane thickness, as more ordered acyl chains are longer, causing the membrane to become thicker.
Sphingolipid synthesis begins in the cytosolic leaflet of the Endoplasmic Reticulum, and it finishes in the luminal leaflet of the Golgi.
Sterols are synthesised in the luminal leaflet of the Endoplasmic Reticulum membrane.
Glycerophospholipids are inserted into the cytosolic leaflet of the ER membrane. They are synthesised in the cytoplasmic leaflet.
Glycerol-backed Phospholipid synthesis 1) Glycerol-backed Phospholipid synthesis begins with a Glycerol backbone, which is then activated by the addition of a phosphate, making Glycerol 3-Phosphate.
2) Then acyl chains made in the cytoplasmic leaflet are then added to the Glycerol 3-Phosphate from Acetyl
Coenzyme A, generating the intermediate Phosphatidate.
3) The head groups are then added to the Phosphatidate in different pathways. (The Phosphatidate has a high energy bond, so a high energy precursor is required to generate its energy).
Phosphatidylserine (PS) and Phosphatidylinositol (PI) - For Phosphatidylserine and Phosphatidylinositol,
the Phosphatidate is charged [-] by adding cytodine triphosphate to create the intermediate CDP -
Diacylglycerol. This will then react with Serine or Inositol to generate either Phosphatidylserine or
Phosphatidylcholine (PC) + Phosphatidylethanolamine (PE) - For Phosphatidylcholine or
Phosphatidylethanolamine there is a different route and the headgroup is charged. The first step is the release of phosphate to form Diglycerol (DAG) and the addition of a charged headgroup (either CDP-choline or CDPethanolamine) to generate Phosphatidylcholine or Phosphatidylethanolamine.
Diffusion of Phospholipids
The lateral diffusion of lipids is very fast, and phospholipids can diffuse 2µm a second.
The transverse diffusion of lipids (flip flop) is very slow and occurs only once every several hours, as it has a very high free energy barrier.
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