The level and complexity of compartmentalization varies among organisms and among mammalian cells. Some cells also change in size and organelle complexity after biological stimulation. The versatility of biological membranes is dependent on their structures and biophysical properties, which are dictated by the types of lipids and proteins that compose the membranes.
The functions of membranes require a fluid plasticity that is accomplished through alteration in lipid composition. Lipid composition is diverse, not only among different organisms, but also among different compartments within the same cells and between the two leaflets of the same membrane. Lipid composition is determined through regulation of de novo synthesis at designated cellular sites, selective distribution or trafficking to new sites, and by localized remodeling reactions.
Understanding the relationships between the dynamic changes in membrane lipid composition and specific cellular events is our current challenge. This review is focused on membrane phospholipid biogenesis in mammalian cells with a particular emphasis on the role played by the endoplasmic reticulum ER.
The ER, together with the Golgi apparatus, is a major site of de novo bulk membrane lipid synthesis, and recent experiments demonstrate a link between phospholipid synthesis and secretion from this compartment.
Major classes of phospholipids are made by more than one pathway. Acyl chain composition of phospholipids can be modified by deacylation. Food technology[ edit ] Phospholipids can act as emulsifiers , enabling oils to form a colloid with water. Phospholipids are one of the components of lecithin which is found in egg-yolks, as well as being extracted from soybeans , and is used as a food additive in many products, and can be purchased as a dietary supplement.
Lysolecithins are typically used for water-oil emulsions like margarine, due to their higher HLB ratio. The next one is our glycerol backbone, and glycerol is a pretty basic structure. It looks like this. It has three carbons attached to three hydroxyl groups-- three alcohol groups. And there's only one glycerol in each phospholipid. And just like you would think, there's a phosphorus in a phosphate group, and there are four oxygens attached to it.
Now, what does this actually look like all put together? Just for the sake of time, I've pre-drawn a picture of all this put together. So you can see that we have our two fatty acid chains attached through an ester bond with our glycerol attached through another ester bond with our phosphate group. Now, you'll notice that one of the negative oxygens is missing, and it's been replaced with a hydroxyl group-- an alcohol group.
And that's what this is in the orange. Well, that's because in our cell, a phospholipid actually looks like this. The negative oxygen actually picks up the hydrogen and becomes an alcohol group. Now, a phospholipid molecule that looks like this is actually pretty rare in our cell membrane, and the reason why is because phospholipids can occur.
And the reason why is because this molecule could actually bond with several different molecules, giving a really diverse set of phospholipids.The plastid membranes are mainly composed of galactolipids, while those of extrachloroplast membranes consist of phospholipids as in the animal cell [ 4 ]. All the enzymes identified to date that are involved in unusual fatty acid biosynthesis are structurally related to enzymes of primary lipid metabolism. In order to obtain these fatty acids regularly and in large quantities for industrial use, it will either be necessary to domesticate the plant or introduce the specific gene of the nonconventional fatty acid into an oleaginous plant grown to obtain sufficient yields for industrial uses.
The multisubunit MS complex ACCase, present in plastids of all plants, except Poaceae and Geraniaceae, is involved in de novo fatty acid synthesis [ 13 ].
Understanding the relationships between the dynamic changes in membrane lipid composition and specific cellular events is our current challenge.
The ER region in close proximity with the mitochondrium is the mitochondrium-associated membrane. For plants, acetyl CoA carboxylase ACCase directs the flow of carbon from photosynthesis to primary and secondary metabolites. There are four sequential reactions involved in two-carbon addition Figure 2.
In all plants, MF ACCase is involved in very long-chain fatty acid and flavonoid biosynthesis in the cytosol [ 13 ]. Plant lipids have a substantial impact on the world economy and human nutrition. And just like you would think, there's a phosphorus in a phosphate group, and there are four oxygens attached to it.
The structure of this fatty acid is unusual; in plants, all the double bonds of the fatty acids of the membrane lipids are of cis type with the exception of this fatty acid. These desaturases have no specificity with respect to the length of the fatty acid chain or its position on glycerol. Possible desaturation scheme of prokaryotic MGDG in plastids. The fatty acid synthase type I which is characterized by a large, multifunctional proteins typical of yeast and mammals and the fatty acid synthase type II, found in prokaryotes which is composed of four dissociable proteins that catalyze individual reactions. Understanding the relationships between the dynamic changes in membrane lipid composition and specific cellular events is our current challenge. This desaturation scheme is similar to that proposed for the desaturation of lipid acyls in the blue seaweed Anabaena variabilis [ 22 ].
And so our carboxylic acid is like this. Although plant cells are eukaryotic, the fatty acid synthase found in plastids is of type II [ 15 ]. These storage lipids represent the main source of carbon and energy mobilized during germination. The sensitivity of plastidial ACCase to sethoxydim and the presence of a kDa biotinylated polypeptide in soybean plastids provide a biochemical indication for the possible presence of two ACCase isoforms, one resistant MS and one sensitive MF , in soybean leaf chloroplasts [ 14 ]. All components of fatty acid synthase occur in plastids, although they are encoded in the nuclear genome and synthesized on cytosolic ribosomes. Well, that's because in our cell, a phospholipid actually looks like this.
This enzyme plays a key role in determining the ratio of saturated to unsaturated fatty acids [ 17 ].