Carbohydrates are compounds that contain carbon (C), hydrogen (H), and oxygen (O). They are more familiar to us as a variety of sugars, starch and cellulose (fiber). Sugars are the simplest carbohydrates. Starch and cellulose are called complex carbohydrates, but they are really just long chains of simple carbohydrates (sugars).

The name "carbohydrate" was originally chosen because it was first thought that carbohydrates were hydrated (meaning water) carbon compounds based on their empirical formula C_n(H₂O)_n. However, if we examine the molecular structure of glucose, for example, we find that it isn't H₂O that is bonded directly to carbon, but instead it is OH and H.

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____Glucose (C₆H₁₂O₆)

French scientists involved in the early days of sugar chemistry, used the term 'hydrate de carbone' for carbohydrate.

Plants use carbon dioxide (CO₂), water (H₂O) and energy from sunlight in a process called photosynthesis to make carbohydrates.

Glucose, Galactose and Fructose are all examples of Simple Sugars. These sugars are also called Monosaccharides because they have only one sugar unit. Although the three sugars have the same molecular formula (C₆H₁₂O₆) they differ in molecular structure (the positions of the individual atoms). All three sugars are known as "hexose" sugars because they all have six carbon atoms.

Sucrose, more commonly known as Table Sugar, is a Disaccharide because

it has two sugar units: a Glucose ring and a Fructose ring.

Lactose (also known as milk sugar) is another example of a disaccharide. It is made from one glucose molecule and one galactose molecule. Sugars made from 2 or 3 sugar units are called oligosaccharides.

The three-dimensional molecular structures of some Simple Sugars are shown below. Fructose or "Fruit Sugar," Sucrose (Table Sugar) and Glucose (also known as Dextose and "Grape Sugar"). The black-colored spheres are carbon atoms, the red ones are oxygen and the smaller white-colored spheres represent hydrogen atoms.

In 1747, the German pharmacist Andreas Marggraf, extracted glucose from grapes (raisins) and sucrose from sugar beets. Soon after, it was discovered that the sugar found in grapes was the same as the sugar found in honey. In 1838, the French chemist Jean Baptiste Andre Dumas, coined the name glucose from the Greek word "glycos," meaning sugar or sweet. At the start of the 20th century, the molecular structures of glucose and a number of other simple sugars (fructose, galactose and mannose) were determined by the German chemist Emil Fischer. Fischer is considered to be the "father of carbohydrate chemistry."

Starch and Cellulose are both polymers (long-chain molecules) of glucose. Scientists refer to starch and cellulose as polysaccharides (having many glucose units), whereas the general public calls starch and cellulose, "complex carbohydrates."

There are two basic types of starch molecules: amylose, which consists of linear, unbranched chains of hundreds of glucose units and amylopectin, which differs from amylose in that it is highly branched.

Plants store glucose, both as amylose and as amylopectin.

Grains like wheat, rice and corn are also good sources of starch as are beans and legumes and various fruits and vegetables.

Cellulose is a linear polysaccharide with many glucose units and is the major component in the cell walls of plants. Wood is constructed largely of cellulose. Both cotton and paper are nearly pure cellulose.

Humans are unable to digest cellulose because we lack the necessary enzymes to break the sugar linkages in cellulose. As a consequence, cellulose serves as fiber in our diet.

Cows, for example, have cellulose-digesting enzymes. This allows them to digest grass and other plant matter. Unfortunately, termites also have these enzymes, which is why they are able to eat wooden structures, like people's homes.

{The numbers given in red specify the positions of the carbon atoms in the 6-carbon glucose molecule (a hexose sugar). The figure also illustrates the two different ways that glucose rings are linked together in glycogen. Linkages are designated 1-4 and 1-6, meaning that carbon atom 1 in one glucose ring is connected to either carbon atom 4 or carbon atom 6 in the other glucose ring, respectively. It should be noted that the 1-4 linkages are found in the straight chains and the 1-6 linkages, form the branches in glycogen, which is a highly branched molecule.}

Humans and animals can store excess glucose in a molecular form called glycogen. Glycogen has a structure similar to that of amylopectin, except that the branches in glycogen tend to be shorter and more numerous.

Glycogen is stored in the body predominantly in the liver and the skeletal muscles. Muscle and liver glycogen stores can be used up in less than a day when carbohydrates are not being consumed.

Metabolizing Glycogen

Breaking Down Glycogen

Splitting glycogen into glucose molecules is a catabolic process known as glycogenolysis or the "splitting of glycogen." In glycogenolysis, glycogen is converted first to glucose-1-phosphate by the enzyme glycogen phosphorylase, which uses inorganic phosphate in the form of HPO₄^2- to remove glucose-1-phosphate molecules from the non-reducing ends of glycogen molecules.

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Glucose-1-phosphate is then converted into glucose-6-phosphate by the enzyme phosphoglucomutase. Glucose-6-phosphate may enter into glycolysis or it can lose its phosphate group in a process called dephosphorylation and be released into the blood as glucose.

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The enzyme glucose-6-phosphatase catalyzes this dephosphorylation reaction, which takes place mainly in the liver since most other tissues of the body (with the exception of the small intestine and the kidneys) lack this enzyme. For example, glucose released from muscle glycogen, which is in the form of glucose-6-phosphate, can only be used in glycolysis and not to maintain blood glucose levels because muscles lack glucose-6-phosphatase. It is the liver that is essential in maintaining adequate blood glucose levels by converting excess glucose to glycogen (a process known as glycogenesis) or by converting glycogen to glucose (glycogenolysis) when glucose levels become too low.

In glycogenolysis, two enzymes (glycogen phosphorylase and amylo-1,6-glucosidase) are needed to cleave the glycosidic linkages in glycogen. The figure given above for glycogen shows two sets of linkages between glucose units in glycogen, alpha 1-4 linkages and alpha 1-6 linkages. Glycogen phosphorylase catalyzes the phosphorolytic cleavage of alpha (1-4) glycosidic linkages in glycogen, while amylo-1,6-glucosidase, which acts as a debranching enzyme, hydrolyzes the alpha 1-6 linkages.

The figure below shows how both alpha 1-4 and beta 1-4 linkages are constructed. The alpha 1-4 linkage in glycogen, for example, designates that two glucose rings are linked together starting from carbon atom number 1 on one glucose ring to carbon atom number 4 on the other glucose ring. In this figure, you can also see how an alpha 1-4 bond connects the two glucose rings from below the respective planes of the glucose molecules, whereas the beta 1-4 bond connects the two glucose rings from across their respective planes.

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Phosphorolytic cleavage of a chemical bond is achieved by the addition of phosphoric acid across that bond. This is what glycogen phosphorylase does in splitting glycogen into glucose units. The hydrolytic cleavage of a chemical bond is the addition of water (H₂O) across the bond. This is known as the hydrolysis of a bond.

Making Glycogen

Glycogenesis is the process of converting excess blood glucose into glycogen for storage. The diagram shown below is a flow chart illustrating the various steps taken (along with the enzymes needed) in synthesizing glycogen. The first step in the journey to glycogen requires the energy of one ATP molecule for every glucose molecule bonded into the structure of glycogen. This is an anabolic process since energy is required for synthesis.

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Uridine Triphosphate (UTP) is an interesting molecule. It is actually a nucleotide and a substrate for the enzyme RNA polymerase, which is used in the synthesis of ribonucleic acid (RNA).

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In glycogenesis, UTP serves as a coenzyme used to "activate" glucose and form a complex called UDP-glucose, which is subsequently used by the enzyme glycogen synthase to install glucose into the ever-growing glycogen chain. Glycogen synthase catalyzes the formation of alpha 1-4 glycosidic bonds between carbon 4 of the growing glycogen chain and carbon 1 of glucose from UDP-glucose, releasing UDP (Uridine Diphosphate) in the process. The enzyme amylo-(1,4 to 1,6)-transglycosylase functions as a "branching" enzyme in glycogen synthesis and is responsible for catalyzing the formation of the many alpha 1-6 branches in glycogen.

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If necessary, glucose can be made from protein. This is a process called gluconeogenesis and uses non-carbohydrate sources to synthesize glucose. However, unless you are eating a diet rich in protein, this generally means breaking down muscle protein. In other words, you will be sacrificing muscle tissue for energy in the form of glucose. Unfortunately, your body cannot use fats effectively to make glucose.

Because the human brain runs on glucose, it needs a steady supply throughout the day. Your best bet is to eat a diet rich in complex carbohydrates.