Biomolecules and the Chemistry of Life
Topics in Depth
The Theme of Lipids in Biomolecules and the Chemistry of Life
Lipids are an eclectic bunch of biomolecules, but all are hydrophobic and therefore do not dissolve in water. This is due to the fact that long stretches of lipid molecules have only carbons and hydrogens. Carbon and hydrogen share electrons equally, and both carbon-carbon bonds (C-C) and carbon-hydrogen bonds (C-H) are nonpolar or hydrophobic or water-phobic or...well, you get the idea. Water, remember, is a polar snob. Since the feeling is mutual, long stretches of nonpolar lipid remain undissolved in solution.
There are three kinds of lipids, and each one has a different function:
1. Fats, Oils, and Waxes
The first group of lipids includes fats, oils, and waxes. Slippery. They are composed exclusively of carbon, hydrogen, and oxygen, or CHO, and usually do not roll into little rings like the monosaccharides we talked about earlier. The monomer, so to speak, is a fatty acid, or a long string of carbons and hydrogens with a carboxyl group at the end. A carboxyl group is a chemical group—everyone loves those—with one carbon doubly bound to one oxygen, C=O, singly bound to a hydroxyl group, C–OH, and bound to one atom friend of choice. As you may have noticed, this "friend" is usually called A or R so he can remain anonymous. Carboxyl groups are usually written as –COOH or –CO2H. Carboxyl groups are also notorious for kicking out one of their hydrogens depending on the pH of the solution. This makes them carboxylic acids (they're back…) with the much cooler notation of –COO-.
Anyway, when three of these fatty acids come in contact with a molecule called glycerol (C3H8O3), dehydration synthesis (that old chum of ours) occurs, and poof! You now have a triglyceride. You didn't even see that coming, did you? Bam. Triglycerided.
Fatty acids can be saturated or unsaturated. Remember, carbons are friendly; they can bind to up to four other atom friends. If every carbon in the fatty acid is singly bound to four friends, whether they be carbons or hydrogens, it has totally maximized its bonding potential. Score. We say that it is saturated. No space for any more friends. Facebook page closed.
If, on the other hand, a carbon double bonds to another carbon, then it can only form single bonds with two other atom friends. Therefore, carbon now has only three friends in total. We call this unsaturated because carbon has not maximized its friending – er, bonding – potential. Notice that when two carbons form a double bond, it causes a little bend in the fatty acid chain. Serious relationships always have a few kinks, eh?
Take a look at carbon in his two different friending situations: unsaturated and saturated. (Psst. When there are zigzag lines with no atom in sight [/\/\/\], this means that carbon is the central atom, and hydrogens are "assumed" to be there. Yes, chemists are lazy.)
What’s the difference between a fat and an oil? You might say, "Why the random questions, Shmoop?" Bear with us.
Basically, the difference between a fat and an oil all comes down to the amount of unsaturation in the fatty acids. Saturated fatty acids form straight chains; therefore, triglycerides with saturated fatty acids can pack really close together. A bit like screaming tweens at a Miley Cyrus concert. These fats tend to be solid at room temperature. When there are some unsaturated fatty acids, the chains get all kinky and bent out of shape, and triglycerides can’t pack in as tightly. This looks more like the concert of a washed-up, one-hit wonder. Mmm…Vanilla Ice, anyone? Oils are the result of living in unsaturated fat country, like olive, canola, or sesame. Note that all of these are liquids at room temperature.
Fats and oils store a ton of energy. At 9.3 Calories per gram (capital C), they store over twice as much energy as carbs or proteins, both of which only yield about 4.1 Calories per gram. Per unit weight, fats are clearly a more efficient way to keep energy for long-term use.
Waxes are like fats in that they are solid at room temperature. This solidity is due to the high degree of saturation, but waxes tend to be used for functions other than energy storage, like waterproofing leaves and fur (baby seal time again!), or making floors shiny.
The second kind of lipid is a phospholipid. Similar to fats, oils, and waxes, phospholipids have a glycerol backbone, which is bound to fatty acids. The only catch is that, this time, there are only two fatty acids instead of three. Sneaky phospholipids. The place that would otherwise be occupied by the third fatty acid is replaced by a phosphate group, written as –H2PO4R, give or take three hydrogens, depending on the pH of the solution it's in. (Hence the name "phospholipid." Biologists aren’t really that creative when it comes to naming things.)
Time out: Please meet phosphorus. He has 15 protons, 15 electrons, and 16 neutrons. He loves to be a negatively charged ion and spends his days hanging out in a group called CHNOPS, or the six elements of life:
Yes, like the rat pack of elements.
The phosphate "head" of this big complex is polar, and you already know that fatty acids, or the "tails" are nonpolar. This creates quite the dual personality for the phospholipid. Does it hate water, or does it love it? Both! Cell membranes take advantage of these split tendencies by lining up two layers of phospholipids, with the nonpolar parts facing toward each other. The result is a barrier between the inside and outside of the cell. How convenient for a little blob of human machinery that needs some protection. This is called the phospholipid bilayer.
Here is the fascinating personality of the phospholipid exemplified, followed by the bilayer that cells and other biological structures take full advantage of. Remember, no atoms + zigzags means carbon is the central atom, and hydrogens are "assumed."
Here are phospholipids in action. Phospholipid bilayers form the protective layer around cells.
The last group of lipids is the steroids. You have probably heard a lot about these since Major League Baseball players seem to be incapable of keeping their paws off them. But, steroids that are abused by athletes are only one kind of steroid; there are lots. All steroids are composed of carbon rings stuck together—four of them, to be exact. They may have various other chains of atoms hanging off one or more rings, but at heart, they all share the four carbon rings. Cholesterol is one kind of steroid, which you may have heard good and bad (but mostly bad) things about, and many hormones, including testosterone, estradiol, and stress hormones are steroids, too. Who knew?
Because you were so curious, we provided you with the typical steroid chemical structure. Again, no central atom means carbon is the man, and hydrogens are "assumed."
Your cell membranes are made of lipids, and some viruses actually hijack those lipids so they can hide from your immune system and invade other cells—one of these nasty viruses is HIV.
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