Carbohydrates or sugars are a class of organic molecules containing carbon, oxygen and hydrogen. They are found as monomers (single molecules), as dimers (two linked molecules) and as long-chain polymers. Carbohydrates are the primary source of energy and energy storage for living things, and they also play a structural role in many cells and organisms.
Most living things on Earth derive energy to remain alive from various sources of the simple carbohydrate glucose, one of the simplest sugars. Carbohydrates are, in turn, derived from plants that convert the energy of the Sun into the useful work of assembling sugars from carbon dioxide (CO2) and water (H2O) from the atmosphere and the soil.
In this section, we'll look at the basic structures of carbohydrates, and explore their uses.
Most carbohydrate names contain the suffix "ose." We've already seen glucose, and we'll discuss sugars like sucrose, fructose, lactose and others later.
The name carbohydrate means hydrated (watered) carbon. It suggests the basic ratio of C to H to O of most carbohydrates, Cn(H2O)n In other words, carbohydrates contain about as many oxygens as carbons, and about twice as many hydrogens, though this is just an average.
Carbohydrates are also commonly called saccharides, and you'll encounter monosaccharides, disaccharides and polysaccharides. The artificial sweetener saccarin is named for the sugar it is supposed to taste like.
There are many ways to write the structures of carbohydrates, each with its own usefulness. Let's get used to them by looking at one of the most frequently-encountered carbohydrate molecules, glucose (C6H12O6). The first is a more-or-less structurally faithful version of the linear form of glucose (there's a cyclic form, too, but more about that later):
Recall that in such a structure, every vertex and every unlabeled end of a line is a carbon atom, unless otherwise specified. Dark wedges indicate that a moeity (like OH) sticks out above the plane of the page, and that a dashed wedge indicates that it points back behind the page. Lines are in the plane of the page. See chemical notation for more.
By convention, the carbons of a saccharide are labeled beginning with the one double-bonded to an oxygen atom, as shown in red. One of the main differences between saccharides is the number of carbons.
For simplicity, we often draw a more simplified stick drawing of linear saccharides, a so-called Fisher projection. The glucose structure is written like this:
The Fisher projection is a little easier to draw, and we just assume the structural details as we would for any other stick drawing of an organic molecule.
Moeity is a word for a generic constituent part of a whole, in context. If we're talking about polymers, a moeity is one of the building blocks that make up the larger polymer. In DNA, each nucleotide-phosphate is a moeity. If we're talking about condominium units, a moeity might be one condo in the complex, and so on.
Most of the time glucose is not found in its linear form. It forms a heterocyclic ring composed of 5 carbon atoms and the carbonyl oxygen. Cyclization occurs when, in a concerted reaction, the double bond between the C1 and the oxygen weakens, and that O bonds to C5. simultaneously, the C5 hydroxyl group (-OH) transfers to the C1 carbon. The resulting carbon numbering system stays intact.
There is more to this reaction than I'll go over here, but for most purposes, this is the important part. Most simple sugars exist mostly as heterocycles like glucose.
In organic chemistry, when we refer to a cyclic molecule, we mean that the backbone of the cycle is composed of carbon atoms only, such as benzene, C6H6. Heterocyclic structures are composed of carbon atoms and one or more others, usually nitrogen (N) or oxygen (O). Many important biological molecules are heterocyclic.
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