Cell membrane

      The lipid layer is an amazing thing. If you press it, or tear it, it will spontaneously reassemble into a prefect layer. The main function of this layer is to separate cell innards from the outside environment. A secondary, but very important role for this layer, is to act as a gatekeeper to many different molecules. Things like O2, CO2, or benzene may pass through with ease. These are non-polar, hydrophobic molecules. A third role is structural. It holds in place the proteins on the surface used for communication of the cell. 

      The lipids are formed in the Endoplasmic reticulum, where a glycerol is added to two fatty acid tails. A phosphate group is also added, and a polar head group. Some of these groups are serine, choline, and ethanol. The largest lipid present in human cell membrane is phosphatidylcholine. It is also numerous in nerve cells. The lipids are only added to one side of the bilayer, making it more dense on one side, causing the layer to deform and curve. An enzyme named flippase flips proteins to the right side.

      There are many proteins that live in the lipid bilayer of cellls. There are integral proteins(inside the layer) and transmembrane proteins(goes completely through the layer). Other proteins associate through functional groups and may inside or outside attached by one functional group. These are peripheral proteins. The amino acids in a protein channel are just a set of multipass amino acid helixes. Some form beta barrels with a set of 10 or more beta sheets. If an amino acid wants to pass through a membrane it must form an alpha helix with hydrophilic side chains inwards and hydrophobic outwards. The functions of these proteins vary. Some act as tranporters. These will be transmembrane, multipass proteins. Some act as receptors to receive information (things like turning on/off functions of a cell or to produce specific proteins). There are enzymic proteins(transmembrane), which also tell the cell to perform functions. We use freeze-fracture to freeze cells and split them along the layer to view which proteins are in which cell. Peripheral proteins may be plucked out fairly easily. But some transmembrane and covalently linked proteins require detergents to unlink them from the membrane.

      Lipids in the lipid bilayer can move about in a transverse manner, freely. They can also rotate up to 30,00 RPM. They can also flip(energetically unfavorable) although it does happen less than once a month naturally. There are enzymes called flippase and floppase to catalyze this function. Lipids, proteins, enzymes can clump together and form “rafts” the move about freely in the layer. Cholesterol also fits in between the layers, especially where the fatty acids are unsaturated(kinked which allow more space between fatty acids). The cholesterol regulates the fluidity of the layer.

    The entire outer surface of animal cells are coated with carbohydrates in the form of oligosaccharides. These react with water to provide a slimy surface to cells which aid it as protection from toxins and physical damage. It also keeps cells from sticking to each other because the similar chemical makeup and proteins would interact together. These oligosaccharides also help the cell with cell recognition. Most have sensors for certain oligosaccharides, since these sugars can structurize in hundreds of different ways.