Topic 13: Lipids & Membranes Flashcards
What are the 5 functions of the cell membrane
Compartmentalization of cellular components and processes, transport of cellular metabolites and waste into and out of cell, communication between the cell and other cells and its environment, intercellular interactions and energy transduction using chemical or light energy
What gives rise to the properties of a fatty acid?
The length and degree of unsaturation of the fatty acid.
What are the most common types of membrane lipid?
Phospholipids, glycolipids and cholesterols
What are the 4 components of a phospholipid
One or more fatty acids, a platform to which the fatty acid chains are attached, a phosphate and an alcohol attached to the phosphate
Describe the structure of phosphoglycerides
In phosphoglyerides, the hydroxyl groups at C1 and C2 of the glycerol are esterified to the carbboxyl groups of the two fatty acid chains. The C3 hydroxyl group is esterified to phosphoric acid
When no further additions are made, the resulting compound is phosphatidate (diacylglycerol 3-phosphate), the simplest phosphoglyceride
In most glycerphospholipids however, the inorganic phosphate, Pi, is esterified to the OH of a polar head group (X), where X can be: cholinem serinem ethanolamine, glycerol or inositol
The 2 fatty acids tend to be non-identical. They may differ in length and/or the presence/abscence of double bonds
Describe the structure of sphingolipids
Phospholipids built on a sphingosine backbone are called sphingolipids. Sphingosine is an amino alcohol containing a long, unsaturated hydrocarbon chain
When the amino group of sphingosine can form an amide bond with a fatty acid as sphingomyelin does, it is called a ceramide, which usually include a polar head group, esterified to the terminal OH of the sphingosine, as seen in the phosphorylcholine in sphingomyelin
Describe the structure of glycolipids
Like sphingomyelin, the glycolipids in animal cells are derived from sphingosine. The amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin.
Glycolipids differ from sphingomyelin in the identity of the unit that is linked to the primary hydroxyl group of the sphingosine backbone. In glycolipids, one or more sugars (rather than phosphorylcholine) are attached to this group. The simplest glycolipid, called a cerebroside, contains a single sugar residue, either glucose or galactose.
Describe the fluid mosaic model
On the basis of the mobility of proteins in membranes, the overall organization of biological membranes can be described as a fluid mosaic model. The essence of this model is that membranes are two-dimensional solutions of oriented lipids and globular proteins.
The term “fluid” emphasizes that the lipid bilayer of the membrane is not rigid but rather has a dynamic and flexible nature. The individual lipid molecules in the membrane can move laterally within the plane of the membrane. This fluidity is crucial for various cellular processes, such as the movement of molecules across the membrane and the flexibility of the membrane structure
The term “mosaic” reflects the diverse and heterogeneous nature of the membrane’s composition. The membrane is composed of a variety of molecules, including lipids, proteins, and carbohydrates, arranged in a mosaic pattern. These components are not randomly distributed but are rather organized in a way that supports the membrane’s functions.
How does cholesterol lower the phase transition temperature?
The use of cholesterol in membranes, like plasma membranes, that include many lipids with lon-chain saturated fatty acids, lowers the phase transition temperature. In the absence of cholesterol, such membranes would crystallize at physiological temperatures
At lower temperatures, cholesterol prevents the lipids from packing too closely, reducing the likelihood of the membrane entering a gel (crystalline) phase.
By maintaining some degree of disorder in the lipid bilayer, cholesterol helps prevent the membrane from becoming too rigid and losing its fluidity at colder temperatures
What are the two classifications of membrane proteins
Membrane proteins can be classified based on their interaction with the hydrophobic interior of the membrane.
Integral membrane proteins are embedded in the hydrocarbon chains of membrane lipids, and they can only be released when the membrane is physically disrupted. In fact, most integral membrane proteins span the lipid bilayer.
Because they are embedded in the hydrocarbon chains, they cannot be removed with salt, and must be solubilized with detergent
Intramembrane domains of the protein have largely hydrophobic surfaces that interact with membrane lipids
Peripheral membrane proteins are bound to the head groups of lipids or the exposed portions of integral membrane proteins by electrostatic and hydrogen-bond interactions, hence they are water soluble. These interactions may occur on either the cytoplasmic or the extracellular side of the membrane
They can be dislodged by conditions that disrupt ionic and H-bond interactions, like extraction with solutions containing high salt, a change of pH and/or chelators that bind divalent cations
Other proteins are anchored to the lipid bilayer by a covalently attached hydrophobic chain, such as a fatty acid
