Importance of Sugars

Sugars represent 50% of the biomass of the Earth


Glucose - a type of sugar We are now ready to turn our attention to the world of carbohydrates. Carbohydrate is a pretty big word, but what does it really mean? It is just a fancy word for sugar. You already know something about sugar – it is what makes many of the things we eat taste sweet. Sugars are typically composed of carbon, oxygen, and hydrogen atoms. The breakdown of sugars in our bodies serves as a major source of energy for us. And sugars are everywhere in Nature. It has been estimated that approximately 50% of the Earth’s biomass is composed of carbohydrates. Sometimes sugars are also referred to as “saccharides.” Glucose has a chemical formula of C6H12O6, and a representation of it is given to the upper right. It forms a six-sided ring structure. Take a look at the bonding around glucose’s hydrogen, oxygen, and carbon atoms. Does the bonding pattern make sense with respect about what we learned from the octet rule?

Glucose


Percent of people with diabetes

One very important sugar is glucose. The breakdown of glucose serves as our body’s major source of energy. Too much glucose in the bloodstream can result in diabetes, which can then lead to heart disease, stroke, kidney disease, nervous system damage, blindness, high blood pressure, and amputation of the lower limbs. Diabetes is a serious health concern in this country. Indeed, the incidence of diabetes among those 40 years or older is scary. Recently there has been an attempt to educated the public about the rise in cases of diabetes in this U.S.

Fructose


Fructose - the sweetest sugar

Another important sugar in our diets is fructose whose structure is presented to the right. It too has a chemical formula of C6H12O6, but its three-dimensional structure is very different from that of glucose. Note that it forms a five-sided ring structure. As a consequence, it has different chemical properties. It is the sweetest tasting of all naturally occurring sugars, and that is why it is used commercially in foods and beverages. While the research regarding fructose does not point to it as harmful, many experts agree that consuming too much food containing fructose can have a negative result to ones overall health.

Sucrose


Sucrose is also referred to as table sugar

Both glucose and fructose are calledmonosaccharides. Another sugar you are most likely aware of is sucrose or common table sugar. It is referred to as a disaccharide because it is composed of a molecule of glucose and a molecule of fructose linked together as shown to the right.

 

Plants use the energy in light to make sugars out of carbon dioxide and water


The photosynthetic process

The sun is the main source of energy for life on earth.  Using the sun's light, plants convert carbon dioxide and water to sugars (glucose) and oxygen. This process is known as photosynthesis. Also important in this process is the byproduct of oxygen. The Calvin Cycle is composed of enzymes that allow this to happen. We will talk more about enzymes in the protein section.

As mentioned above, the breakdown of glucose is the body's major source of energy. Further, the structure of sugars are extremely important and relevant to several aspects of life, including the photosynthetic process, the structure of DNA and RNA, energy storage and consumption, and ABO blood typing, to name a few.

 

Mono-, di-, and polysaccharides and how they relate to everyday life


Lactose is a disaccharide

In the discussion above we talked about glucose and fructose, which are important monosaccharides. We also talked about sucrose, which is a disaccharide because it consists of two sugar units linked together. Another important disaccharide is lactose. In mammals, lactose is broken down in the small intestine by a protein called lactase. When this protein is absent, lactose passes through the large intestine undigested resulting in abdominal pain, diarrhea, and flatulence. It has been estimated that 1 out of every 10 Americans is lactose intolerant due to a complete absence or a lower than normal concentration of lactase. The rates vary among ethnic groups, however. Approximately ~90% of Asian Americans and Native Americans are lactose intolerant, but the rate among those of Northern European descent is considerably less, sometimes in the 5% range. Lactose is found in lots of foods including milk, yogurt, cheese, ice cream, instant potatoes, bakery products, cereals, salad dressings, pancake mixes, and soups. In fact, the list goes on and on. So be careful if you are “lactose-intolerant.” And don’t go feeding milk products to your adult dog Fido or your cat Fluffy because over time they lose their lactase and thus their ability to digest lactose. If you feed them milk, you will inevitably be cleaning up a mess!

There are two other terms that are frequently used in describing carbohydrates: oligosaccharide and polysaccharide. An oligosaccharide contains three to ten sugar units linked together, whereas a polysaccharide consists of more than ten sugars.

 

 


Raffinose - a trisaccharide

Two biologically important oligosaccharides include raffinose and stachyose. These two oligosaccharides are found in vegetables such as green beans, soybeans, brussel sprouts, broccoli, and asparagus. Some bacteria found in the large intestines of humans possess a protein, alpha-galactosidase, which breaks down these oligosaccharides into smaller pieces. Humans, however, do not possess alpha-galactosidase activity, and as a result, raffinose and stachyose pass through to the large intestine in an undigested state, ready to serve as fuel for gas-producing bacteria.


Stachyose - a trisaccharide

The flatulence associated with eating some vegetables such as beans is a direct result of our inability to degrade these complex sugars. Commercially available Beano® (image), sold as a natural dietary supplement to prevent bloating and other intestinal symptoms, contains alpha-galactosidase.

For the most part, there are only a few important oligosaccharides, but the situation is quite different for the polysaccharides.

Cellulose


Cellulose - a polysaccharide

Cellulose is one such polysaccharide that functions as a structural component of plant cell walls. It is a linear chain of thousands of glucose molecules. Due to its extreme abundance on Earth, it has become a recent target for the biofuel industries. Chitin, a polysaccharide familiar to those who enjoy crabs, lobsters, and shrimp, is found on the exoskeletons of numerous arthropods and insects and elsewhere. Two other important polysaccharides are starch, which is synthesized by green plants and used to store energy and glycogen, which is used for energy storage in animals rather than in plants.

A medically important polysaccharide is hyaluronate (image). It is widely distributed in the human body and is found, for example, in the fluid that lubricates joints. Commercially, hyaluronate has attracted significant attention because it has been touted as “the key to the fountain of youth.” (image) As an anti-aging remedy, it is often found in anti-wrinkle face creams and dietary supplements for joint repair.

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