By Robert Hazen, George Mason University
Carbohydrates are composed of carbon, hydrogen, and oxygen. They’re the most abundant biomolecules on earth. This is because carbohydrates form most of the physical structures as well as the support structures in plants. These molecules are synthesized by plants from water and CO2, and they play various roles in living things.
Monosaccharides
The most common carbohydrates are sugars, which typically have a simple ring-like structure with 5, 6, or 7 carbons. They’re composed of hydrogen, carbon, and oxygen, and these molecules contain lots of energy.
The simplest sugar molecules are called monosaccharides; that’s a word meaning “one sugar”. You may have heard the word “saccharine”, which implies sweetness. It has the same Greek root in both the word “saccharine” and, of course, “monosaccharide”.
The most abundant of these sugars in nature is the 6-carbon sugar called glucose. Like most sugars, it has a ratio of atoms of 1:2:1 of carbon to hydrogen to oxygen; so twice as many hydrogens as carbons and oxygens.
Photosynthesis
Plants synthesize glucose by the process of photosynthesis, which uses the Sun’s radiant energy to transform water, plus CO2, into glucose, plus a little bit of oxygen. Both plants and animals use the energy of glucose to make adenosine triphosphate (ATP); the energy of glucose is released by oxidizing the sugar molecules to release CO2 and water in the process of respiration.
There are dozens of other monosaccharides. They’re found in all kinds of living things, both as isolated molecules, and also linked together to form larger structures.
“Two Sugars”
You often have two sugars linked together to form a disaccharide, or “two sugars”. Maltose is formed from two glucose molecules linked together. Sucrose, or common cane sugar, is formed from the disaccharide with one glucose molecule and another 6-carbon molecule, fructose; two different kinds of sugars linked together to form cane sugar.
Disaccharides form by a process of condensation polymerization. When an OH group from one sugar reacts with an H of another sugar, an H2O molecule is formed; the H2O molecule goes off and a carbon-oxygen-carbon (C-O-C) bond forms between the two sugars. The result of this reaction is a molecule with two joined rings, often each with six sugars.
This article comes directly from content in the video series The Joy of Science. Watch it now, on Wondrium.
What If There Are Chains of Sugars?
Then we have polysaccharides. Polysaccharides are made of many sugars. They are long polymer chains that are formed from hundreds, or sometimes thousands, of sugar molecules. They can be formed from one, two, or many different kinds of sugars being linked together. There are lots of common polysaccharides, but the two most important ones are called cellulose and starch.
You have probably heard of these two terms. They’re both important parts of diets, and they’re both synthesized by plants from glucose. Both of these polymers serve structural roles in plants, but they use very different kinds of sugar-sugar linkages that bond together differently, and so starch and cellulose have different physical properties.
A Glucose Network
Starch includes an extensive glucose network of chains and branches, with single C‑O‑C bonds that link each of these units to each other. C‑O‑C bonds are somewhat flexible, so starch is not a really strong structural material. In fact, it’s rather soft; it forms larger, bulky structures in many plants.
Because the C‑O‑C bond is not very strong, and we have digestive enzymes in our own stomach, humans and other animals can break down starch into individual glucose molecules. So starch, a form of carbohydrates, is a good energy source.
When you talk about eating pasta, or “carbing up”, you’re talking about basically eating a supply of glucose molecules. The glucose molecules can thus be used for food.
One of the real classic examples of a starch is the potato. Potatoes are bulky masses, somewhat soft; a great source of energy which can be released over time, because you gradually split glucose molecules off the ends of these long polymers that form the solid mass of the potato.
Somehow Similar yet Different
Cellulose is also a polysaccharide of glucose, typically with 10,000 to 15,000 individual sugar molecules linked end-to-end. The bonding and its physical properties, however, are very different from starch.
In cellulose, each pair of glucose molecules forms a three-way bond. Not just a single C‑O‑C bond, you also have two strong hydrogen bonds that link up. It’s a three-way linkage; there are three bonds together that form cellulose.
This means that the cellulose bond can’t bend or twist. If you have a single bond, it’s fairly easy to twist; but that three-way bond, with two hydrogens and one C‑O‑C bond together, forms a strong structure. As a result, cellulose provides a rigid structural material for all kinds of plants: tree trunks, plant stems, grass, leaves, and other vegetable material.
Thus, cellulose finds many applications in our daily lives. Paper, wood, straw, and cotton are all forms of cellulose that have structural strength, and therefore can be used in various ways in our environment.
We can also eat cellulose. Celery, for example, is a very pure form of cellulose, and it has the kinds of fibers and stems that you’d associate if you break cellulose. You can hear that breaking, because you’re breaking lots of those sugar-sugar bonds.
Different Bonding Results in Different Properties
We see a major structural difference here between the glucose that links together as cellulose in celery and the glucose that links together with single bonds in starch, such as a potato. If you slice a potato, it’s very easy to slice through, and you see that soft structure.
It would be hard to make a tree trunk out of something like a potato. Celery has a much stronger linkage, and you see those fibers that extend up through the celery; fibers which show you the very strong bonds in these polymers of cellulose.
Common Questions about Carbohydrates
As the most abundant biomolecules on earth, carbohydrates are composed of carbon, hydrogen, and oxygen. They are what the support structures of plants are composed of.
There are different types of carbohydrates, including monosaccharides, disaccharides, polysaccharides, starch, and cellulose.
Each type of carbohydrate is different in its molecular bonding, thus they all have different chemical characteristics.
