During our introduction to carbohydrates, we reviewed their basic
structure. Why are some carbohydrates different that others?
If we take a single carbohydrate molecule of any type and link it together with other identical molecules we now have a larger molecule (see figure below).
Because of the different types of elements and possible arrangements on a carbohydrate "skeleton" or "backbone", there is the chance of making many different types of carbohydrates, as you may imagine. And this is how it is.
We have identified many different types of carbohydrates, with slightly different chemical properties, physical properties, and function. Remember earlier, when we said that by combining two different atoms, the resulting molecule now has different properties. Carbohydrates are no exception. And this is why different carbohydrates have different colours, weights, tastes, etc. There are even certain carbohydrates that humans cannot digest because we do not synthesize the enzymes to metabolise them. However, there is always some type of creature out there that can! Cows have the enzymes to metabolize certain carbohydrates that humans can't. This is why cows and other ruminents can eat grass, and we can't. If we eat grass, it will pass through our GI tract as fiber (see below) !
Now we are starting to see what makes carbohydrates different. We have just learned that sugar molecules can link together to form a polysaccharide (don't forget, poly, meaning "many", and saccharide, meaning "sugar". We have names to describe different types of saccharides as well.
A single sugar unit is called a monosaccharide (mono=1, and saccharide as you know is sugar). A disaccharide is a polysachharide with two sugars linked together (di=2). What else can make a carbohydrate different?
The structure of the molecule is another differentiating factor in determining carbohydrate class. Earlier we said that by combining two atoms together, they acquire new physical and chemical properties as a molecule? (similar to harmless table salt, which is composed of poisonous chlorine gas and explosive sodium!). Carbohydrates are no different. As long as the molecule has the same carbon, hydrogen and oxygen ratios we discussed earlier, it will be classed as carbohydrate. We can conform to that formula and still add extra elements and functional groups. This gives the molecule different physical and chemical characteristics, just like any other molecule. These differences gives each carbohydrate its characteristic taste, colour and other qualities.
Structures of some of the common sugar molecules are illustrated below.
The structures of glucose, fructose, galactose and mannose. These are a type of carbohydrate that can exist as independent units known as simple sugars. Notice that glucose, galactose and mannose have the same amounts of carbon, hydrogen and oxygen, yet have different names. By examination, we can see that some of the atoms occupy different positions. This difference in position gives new physical and chemical qualities to the molecule, such as a different taste, weight, etc. And you can test this for yourself, by tasting different types of sugars. This difference in sweetness is a consequence of structural difference. Also note that sugars such as glucose are single sugar (saccharide) molecules and thus can also be called monosaccharides.
To gain a thorough understanding of carbohydrates, their functions and types, we will briefly discuss chemical bonding between atoms.
A bond is what holds two or more atoms together. Discussing sodium chloride (salt), we learned that an atom of sodium combines with an atom of chlorine. The two atoms accomplish this connection via a bond. This bond, enables them to stay together and function as a unit. Creating a molecule by bonding two atoms together requires energy as you may imagine. Energy has to be invested in making the bond. Since energy is invested to make this molecule, we can imagine what happens when we disassemble the molecule back into its original atoms. The invested energy is released. This is the importance of consuming carbohydrates for energy! The energy locked up in the chemical bond of the carbohydrate, is released while we metabolize them. This bond energy is used for many processes in the human body, and thus makes the carbohydrate essential for survival.
Although the chemical bonds of the carbohydrate are important for energy, humans cannot metabolize all carbohydrates. The object of digesting complex carbohydrates (sugars that are in chains and branches) is to break apart these chains and branches, so that free sugars are liberated for energy. Enzymes produced in the salivary glands and pancreas help to break the bonds between these sugars. Different types of carbohydrates have different types of bonds. Humans do not synthesize enzymes for every type of bond. Therefore, certain types of carbs that we eat, are not able to be fully digested and pass through the GI tract, functioning as fiber. Fiber is beneficial, since it attracts water and causes waste products to swell, aiding in the peristaltic stretch reflex (see GI physio review if needed). to the rule.
The bonds that we cannot digest are called alpha-glycosidic bonds. In a sense, it is beneficial that we don't have every enzyme for digestion. Otherwise, we wouldn't be able to benefit from fiber. As discussed above, cows and other grazing animals do have these enzymes.
To this point, we've learned some of the important chemical features which create diversity amongst carbohydrates.
1) Composition-the type of atoms in the molecule.
2) Structure-The position of the atoms.
3) Shape and length-Polysaccharides being a chain (with a length) of many
saccharides (sugars). This chain has a shape and can also have branches. Branches
help in maximising space for storage as we will see.
Dietary carbohydrates are those
that we not only consume, but derive some form of nutritional benefit.
The term dietary helps us keep in mind the significance of
what we've discussed above: we cannot derive benefits from all carbohydrates.
You've done well. You've learned about the structure of carbohydrates, and how they are grouped. Now we can sit back and choose the carbohydrates we wish to learn about. Click on the link below to view the list of carbohydrates and their groupings. The list is by no means exhaustive and any rarely encountered carbohydrate can be searched for in the database.
Carbohydrate Guide - A well-stocked list of common carbohydrates
Carbs - Sleep and Energy Levels
introduction to Carbohydrates