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Lipid and Protein Composition of Cell Membrane

    In this Lipid and Protein Composition of Cell Membrane post we have briefly explained about lipid and protein chemical composition of cell membrane and function in cell membrane.

    Lipid and Protein Composition of Cell Membrane

    A cell’s life is totally dependent on planar sandwiches known as membranes, which shield and separate the cells from the outside world. These membranes are typically impenetrable to macromolecules, but they can allow some molecules to pass through and help to maintain homeostasis at a higher level.

    Membranes are made up of two layers of lipids with proteins inserted in between. They function as a two-dimensional entity, with a polar group facing the outside of the cell and hydrocarbon tails towards the inside.

    While the membranes that encircle the entire cell function as barriers against the outside world, the membranes that surround each organelle create a distinct internal area for the organelle’s particular biochemical processes, ultimately contributing to the cell’s life.

    The present understanding of membranes stems from E.coli discoveries in 1855, which said that a membrane is made up of a lipid bilayer. Biochemical and X-ray specialists proposed the membrane as a surface covering with proteins dispersed in place of membrane, floating or spanning, and lipid anchors in the next century.

    Chemical Composition of Cell Membrane

    Lipid

    From the opening remarks, it is clear that lipids, in conjunction with anchoring proteins, can create the framework of a membrane. Aliphatic/aromatic hydrocarbons make up the lipid molecules, which are smaller than 1000 Dalton in size. The bilayer structure of a membrane is made up of several kinds of lipids.

    Composition of Cell Membrane

    Chemical Composition of Cell Membrane

    Phosphoglycerides

    Because they include phosphate, phosphoglycerides are sometimes referred to as glycerolphospholipids. They are the main components of the membrane’s lipid bilayer. 

    They’re amphipathic because they have hydrophobic (water-hating) and hydrophobic (water-loving) portions. Phosphoglycerides in membranes have an approximately rectangular form. Aliphatic fatty acids can have one or more double bonds (as in C1), or they can have two or more double bonds (as in C2) (as in C2).

    Glycolipids

    Glycolipids as their name indicates are sugar containing lipids. The sugar residues are always present on the external side of the membrane. There are three types of glycolipids (i) sphingolipids (ii) glycerolglycolipids in which sugars are attached to hydroxyl group on C3 of triglycerides (iii) glycosylphosphatidyl inositol (GPI).

    GPI anchors proteins to the outer leaflet of the plasma membrane. The C-terminal residue of a protein is attached through an amide linkage to the amino group of an ethanolamine which is connected by a sugar backbone (Mannose and Glucosamine) to the phosphaditylinositol.

    Shingolipids

    Composition of Cell Membrane

    Chemical Composition of Cell Membrane: Shingolipids

    The hydrophobic region of sphingolipids is made up of a long chain sphingoid base with around 18 carbons that is connected to the acyl group of a fatty acid through an amide bond. In the parent molecule, ceramide, the polar head group (X) linked to C1 is X=H.

    Glycolipids are sphingolipid components that include a sugar molecule and are found in biological membranes. Sphingosine with a polar head group linked to C1 gives these lipids their name. Membranes are densely packed with sphingolipids. Sphingolipids are classified based on (I) the fatty acid linked to C2 through an amide bond or (ii) the type of the polar head group esterified to the hydroxyl group at C1.

    Composition of Cell Membrane

    Chemical Composition of Cell Membrane: Shingolipids

    Glycophospholipids are sphingolipids that include one or more sugars and can be neutral or negatively charged. Sphingomyelin is a phosphate ester that may connect a base (choline or ethanolamine, which are comparable to phosphoglycerides) to C1 of sphingolipids in some cases. Gangliosides are complex glycolipids with oligosaccharides as the polar head group and one or more sialic acid residues as the terminal sugar.

    Chemical Composition of Cell Membrane

    Sterols

    Chemical Composition of Cell Membrane: Sterols

    Sterols are the third most abundant lipid in the lipid bilayer. Cholesterol is the most common form of sterol found in animal membranes, whereas flat sterols are found in bacteria, lower eukaryotes, and plants. Cholesterol’s cyclopentanoperhydrophenanthrene ring is apolar, entering into the lipid bilayer’s core with the hydroxyl group at C3 near the surface. Isoprenol, geranyl, and farnesylisoprenols, the precursor moieties of cholesterol, function as hydrocarbon anchors for a variety of membrane proteins.

    Triglycerides

    Triglycerides are composed of glycerol with fatty acids esterified to all the ‘C’s. They do lack polar head group but they are not present in membrane. Lipoproteins are denoted as proteins that are chemically attached to membrane lipids, and they are present on either sides of a membrane. Water, the universal solvent is also a component of a membrane.

    The cell membranes include water molecules, either bound to the polar groups visible on the polar side of the membrane molecules (structured water) or unbound (bulk water) within pores and certain ion channels crossing the membranes .

    Ions are associated with membranes by simple adsorption to the two surfaces of the membrane or can just pass through ionic channels (membrane proteins) or ion pumps (membrane proteins with enzymatic character). The ions that contribute to the structure and functions of membrane are: H+, Na+, K+, Cl- , Ca+, and HCO3.

    Proteins

    Membrane proteins

    Most of the functions of a membrane depend on the protein present in it. You can recall that it was mentioned elsewhere that there are two classes of proteins. Both integral and peripheral membrane proteins which form the structure of the membrane. Integral proteins both have hydrophobic and hydrophilic interaction surface with hydrocarbons core and exterior of lipid cage.

    Integral Protein

    Some of the membrane proteins are tightly embedded in the membrane and they cannot be isolated unless, the membrane is disintegrated. They are called as Integral or Intrinsic membrane proteins.

    They are again classified into two. (a). Transmembrane proteins, which traverse (pass through) or span the membrane. These proteins will have domains on either side of the membrane. Many cell surface receptors belong to this class. (b). Lipid anchored proteins that are present either on the cytosolic side or on the extracytosolic side.

    They insert themselves in the membrane by a lipid (acyl chain) attached to the N terminal end. Transmembrane proteins are of two types. Single pass transmembrane proteins that traverse the membrane only once. Multipasstransmembrane proteins that traverse the membrane more than once.

    Peripheral Protein

    Those proteins that are present on the surface of the membrane are called as peripheral proteins. They can be easily isolated from the membrane. eg. spectrin present in the RBC membrane.

    Further Readings

    Reference