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 saccharin 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.
Here is more of a 3-D representation of glucose. The gray areas are the two lone pairs of electrons present on each oxygen atom, just like they would be in water. They lend any sugar a great deal of polar character near each oxygen, thus a high degree of solubility in water.
This configuration is often referred to as the "chair" configuration (because the carbon backbone resembles a chaise lounge chair, I think). The structure is really rather floppy, though.
The primary energy source for most cells is glucose. Many cells can derive their needed energy from other sources, including fatty acids, and the Archea – thermophilic bacteria – can derive energy from ammonia (NH3) and hydrogen gas (H2), but the vast majority require glucose to maintain cellular functions.
The chemical energy stored in the bonds of glucose is transferred to adenosine triphosphate (ATP) molecules in most cells via the citric acid cycle (also called the Krebs cycle), and in higher organisms also via the electron transport chain. The whole process is called cellular respiration.
Excess glucose – that not needed for immediate use – can be stored in some tissues of organisms, primarily muscle, as glycogen. Glycogen is a multiply-branched polymer of as many as 50,000 glucose molecules that is stored in muscle and liver cells.
The five-carbon sugars ribose and deoxyribose, shown here with their labeling schemes, are key components of RNA and DNA molecules, respectively.
Along with alternating phosphate (OPO4-) groups, they form the backbone of the long nucleotide chains and the four DNA and RNA bases, adenine, quanine, cytosine and thymine (uracil in RNA).
The figure below shows a sample four-nucleotide sequence of DNA. Note where the ribose sugars are.
About half of all proteins in eukaryotes (higher organisms) are glycoproteins, proteins "decorated" (glycosylated) with short, linked chains of carbohydrates called oligosaccharides. The role of glycosylation varies from protein to protein, but proper glycosylation is necessary in most cases in order for proteins to fulfill their functions.
Carbohydrates play a balancing role in the metabolism of fats and proteins. When there is plenty of glucose present in the blood, cellular respiration proceeds as normal. When the levels drop, cells turn to converting fatty acids to glucose for energy. When those are depleted, proteins can serve the same role, but that's a last resort, the organism is essetially "eating itself" at that point.
Proteins can be glycosylated a an infinite variety of ways as chains of carbohydrates can be linked and branched via two distinct kinds of bonding between sugars.
Here are just a couple of other examples of carbohydrate molecules. The first, below, is fructose, a simple sugar found in fruits, corn syrup and in other foods. Fructose is a "simple sugar," a five-carbon sugar that is easily metabolized.
Sucrose, common table sugar, is a disaccharide, composed of two linked simpler sugars, fructose and glucose. Sucrose is also
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