As I said in my last post, I'm going to start talking about membranes now. w00t!
As you know, membranes are pretty important for cells. They separate the inside of cells from the outside, and in the case of eukaryotic cells, help to separate the inside of organelles from the cytoplasm. Some organelles have two membranes, such as the nucleus and the mitochondria.
Phospholipids and Sphingolipids
Phospholipids and sphingolipids are important components of the cell membrane. I've already spoken a fair bit about phospholipids in my first post about lipids. If you've forgotten, phospholipids consist of two acyl chains and a phosphate group esterified (that's now my new favourite word because it's kinda fun to type) to a glycerol backbone. The phosphate group, in turn, can be bond to several other head groups. These include choline, ethanolamine, serine and inositol. Inositol has plenty of -OH groups which can be phosphorylated, allowing inositol to play vital roles in signalling processes.
Another important phospholipid is cardiolipin, which is exclusive to mitochondrial membranes and is required for the activity of cytochrome oxidase, part of the electron transport chain that powers the synthesis of ATP. Cardiolipin consists of two phosphatidic acid molecules joined together by another glycerol molecule (yup, it's a large molecule with four acyl chains in total).
Yet another important type of phospholipid are plasmalogens. Plasmalogens have a vinyl ether linkage to the first carbon of glycerol- that is, they have an -O- bond to a C that is double bonded to another C. Their specific function is unknown, but they represent 10% of lipids in the human central nervous system and 50% of phospholipids in cardiac tissue, so they must be at least somewhat important.
Sphingolipids are slightly different. Instead of choline, sphingolipids are based off a long lipid called sphingosine. The third carbon of sphingosine has an -OH group, the second has an -NH2 group and the end carbon has an -OH group, which is where head groups can attach. Sphingosine with an acyl group joined to the -NH2 group is called ceramide.
Sphingomyelin is a type of sphingolipid. In sphingomyelin, the head groups are phosphate joined to choline. Another common type of sphingolipid are glycolipids. These do not have phosphate, but instead are joined directly to glucose, galactose and sometimes other sugars as well. These sugars can be attached in linear chains or in branched chains.
Membrane Fluidity
Lipid bilayers (which I talked about a little bit in my first post) can exist in either a solid gel form or in a more liquid form. The liquid form is more common under biological conditions. As also alluded to in that first post, fluidity can be increased by increasing the temperature, decreasing the length of the acyl chains and increasing the number of cis double bonds. Organisms that live in very cold temperatures tend to have more cis double bonds in their fatty acids in order to make up for the lower temperature.
Another feature of the fluid membranes is that stuff can move around in them. Lipids can undergo lateral diffusion, in which they move side to side within the same layer of the membrane, or transverse diffusion, in which they "flip-flop" from one layer of the membrane to the other. The former movement is very fast- if I remember correctly, a lipid molecule can move from one side of a bacterial cell to the other in a second- while the latter movement is very slow and can take hours to days (though there are special enzymes called flippases that can increase the rate of "flip-flop"). Lipids can also rotate on the spot quite rapidly. Proteins within the membrane can also move laterally and rotate, but they cannot "flip-flop." Some proteins, however, may be less mobile if they are anchored to the cytoskeleton by proteins such as ankyrin. Some lipids can also interact with membrane proteins, and lipids next to proteins tend to be less mobile- probably because there's a hulking big protein in the way.
Membrane Permeability
Small uncharged molecules are very permeable in the lipid bilayer, particularly if they are also highly lipid-soluble (due to the high lipid content in the bilayer). Other hydrophobic, lipid-soluble molecules such as steroid hormones can also diffuse through the membrane. Charged molecules, however, are impermeable, as losing the "shell" of water that surrounds them in the outside solution in order to try and move through a lipid bilayer is energetically unfavourable. Hence water (a small uncharged molecule) can diffuse through the membrane 10^9 times faster than Na+!
To make up for the inability of charged particles to pass through the membrane easily, there are proteins that serve as transporters through the membrane. ATP-powered pumps utilise energy from ATP to pump molecules between the outside and the inside of the cell. Ion channels can open and close to allow ions to move from an area of high concentration to an area of low concentration. Finally, transporter proteins can bind a molecule on one side and then change conformation to release the molecule on the other. There are different types of transporter proteins: uniporters move one molecule at a type, symporters move multiple molecules in the same direction, and antiporters move multiple molecules in opposite directions. Some cells also have a special transporter for water, which is called an aquaporin.
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