Glucose

Glucose is a carbohydrate, and is the most important simple sugar in human metabolism. Glucose is called a simple sugar or a monosaccharide because it is one of the smallest units which has the characteristics of this class of carbohydrates. Glucose is also sometimes called dextrose. Corn syrup is primarily glucose. Glucose is one of the primary molecules which serve as energy sources for plants and animals. It is found in the sap of plants, and is found in the human bloodstream where it is referred to as "blood sugar". The normal concentration of glucose in the blood is about 0.1%, but it becomes much higher in persons suffering from diabetes.

When oxidized in the body in the process called metabolism, glucose produces carbon dioxide, water, and some nitrogen compounds and in the process provides energy which can be used by the cells. The energy yield is about 686 kilocalories (2870 kilojoules) per mole which can be used to do work or help keep the body warm. This energy figure is the change in Gibbs free energy ΔG in the reaction, the measure of the maximum amount of work obtainable from the reaction. As a primary energy source in the body, it requires no digestion and is often provided intravenously to persons in hospitals as a nutrient.

Energy from glucose is obtained from the oxidation reaction

where a mole of glucose (about 180 grams) reacts with six moles of O₂ with an energy yield ΔG = 2870 kJ. The six moles of oxygen at STP would occupy 6 x 22.4L = 134 liters. The energy yield from glucose is often stated as the yield per liter of oxygen, which would be 5.1 kcal per liter or 21.4 kJ per liter. This energy yield could be measured by actually burning the glucose and measuring the energy liberated in a calorimeter.

But in living organisms, the oxidation of glucose contributes to a series of complex biochemical reactions which provides the energy needed by cells. The first step in the breakdown of glucose in all cells is glycolysis, producing pyruvate which is the starting point for all other processes in cellular respiration. In cells where oxygen is present (aerobic respiration) these processes have been modeled in the TCA or Krebs cycle. A major part of the use of the energy from glucose oxidation is the conversion of ADP to ATP, with the energy-rich molecule ATP being subsequently used as the energy currency of the cell.

Glucose is manufactured by plants with the aid of energy from the sun in the process called photosynthesis. This synthesis is carried out in the small energy factories called chloroplasts in plant leaves. The chloroplasts capture the energy from light and fabricate glucose molecules from carbon dioxide from the air and water from the soil.

Glucose can also be found in a linear form. The right end of this molecule shows the form of an aldehyde.

Fructose

Fructose is a carbohydrate, and is a simple sugar found in fruits. It is often called just "fruit sugar". Fructose is called a simple sugar or a monosaccharide because it is one of the smallest units which has the characteristics of this class of carbohydrates.

Fructose is also found in honey. It is classified as the sweetest of all the sugars.

Fructose can also be found in a linear form. The right end of this molecule shows the form of an ketone.

Comparison of Glucose and Fructose

Glucose and fructose are the two most important simple sugars for human consumption. They have the same molecular formula, C₆H₁₂O₆, but have different structures. Each is a carbohydrate which can be considered to be the combination of 6 water molecules and 6 carbon dioxide molecules with the yield of 6 oxygen molecules. They are classified differently as hydrocarbon derivatives, glucose being classified as an aldehyde and fructose as a ketone.

The sugars differ in the bond environment of the oxygen atom in the sugar. The differences in the functional groups can be seen more clearly in the linear forms below.

Cellulose

Cellulose is a form of carbohydrate in which some 1500 glucose rings chain together. It is the chief constituent of cell walls in living organisms. Wood is mostly cellulose, making cellulose the most abundant type of organic compound on the Earth.

Cellulose molecules tend to be straight chains, and the fibers which result from collections of cellulose molecules have the strength to form the supporting structures of plants. Even though human digestion cannot break down cellulose for use as a food, animals such as cattle and termites rely on the energy content of cellulose. They have protozoa and bacteria with the necessary enzymes in their digestive systems. Cellulose in the human diet is needed for fiber.

Comparison of Starch and Cellulose

Both starches and cellulose are carbohydrates which are classified as polysaccharides since they are composed of chains of glucose molecules. While they are similar, starches can be used as energy sources by the human body while cellulose cannot. Enzymes are important in the metabolism of foods, and these enzymes are very specific. They are somewhat like keys which will fit the geometry of the starch bonds, but not those of the cellulose bonds.