Saccharide or sugar is a very important organic compound in cells. It contains carbon (C), hydrogen (H), and oxygen (O) in a general ratio of 1:2:1. Therefore, it also be written as Cn(H₂O)n, and referred to as a carbohydrate sometimes. The deoxyribose is one of the exceptions where an oxygen atom is missing and its chemical formula is C₅H₁₀O₄ instead. Carbohydrates not only provide most energy for living things, but also serve as building blocks of biomacromolecules like proteins as well as nucleic acids among others. They are categorized into monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides contain only one unit that cannot be hydrolyzed. Glucose, fructose and galactose are common monosaccharide. They are all composed of six carbon atoms and are isomers of each other.
| Monosaccharide | ||
|---|---|---|
| Glucose | C₆H₁₂O₆ | It is the main energy source for life, and all plant and animal contain glucose. |
| Fructose | C₆H₁₂O₆ | Sweet fruit and honey |
| Galactose | C₆H₁₂O₆ | mammal milk, plant gum, nerve system |
Glucose Structure, Function, Physical, Chemical Properties
Glucose was isolated from raisins or grapes originally, and was named as grape sugar. It’s the simplest monosaccharide that occurs widely in nature. It exists mainly as a white crystalline powder melting at 146°C under normal conditions. Hydrogen bonds are formed with water easily due to the multi-hydroxyl structure, so that it dissolves readily in aqueous solutions. At room temperature about 91g glucose will be dissolved in 100 milliliters water. While not very sweet, the 70% sweetness of the sucrose is still very noticeable on your tongue.
A molecule of glucose has six carbon atoms (hexose), five hydroxyl groups (-OH) and one aldehyde group (-CHO). It is classified as aldose. The fifth carbon’s hydroxyl group attacks aldehyde group to form a more stable six-membered ring. Thus, in aqueous solution, a few linear molecules and many ring molecules are in a dynamic equilibrium. There are two forms of cyclic glucose: α and β. They differ in orientation of hydroxyl group (-OH) attached to the first and fourth carbon atoms. Their reducing nature comes from the aldehyde group that provides electrons to silver ions (Tollens' reagent) and copper ions (Fehling's reagent).
It is one of the energy sources for cellular aerobic respiration in mitochondria. Some energy is stored in ATP for life activities during this process, while remaining part lost as heat into environment. Glucose also serves as precursor molecule for starch, glycogen and cellulose that are cell structures or energy reserves. Moreover, it derivatives are involved in the synthesis of nucleic acids and certain proteins.
Fructose Structure, Function, Physical, Chemical Properties
Its melting point is 103°С and it is a white crystalline powder at room temperature. Its polyhydroxy structure endows it with greater hydrophilicity than glucose. This makes it highly soluble in water and hygroscopic in air. Thus, higher tightness is required for storage. Fructose dissolves much more easily in water than glucose or sucrose. At the room temperature, its solubility is 370g per 100 milliliter water.
Although molecular formula is the same as glucose, the carbonyl group implies it is ketose and very different from aldose. In aqueous solution, the stable five or six membered rings tend to form rather than linear chains. The hydroxyl group on its fifth carbon attacks the carbonyl group to form a five-membered ring called furanose fructose. If the sixth carbon’s hydroxyl group attacks the carbonyl group, a more stable six-membered ring, called pyranose fructose, appears. Dynamic chemical equilibrium exists between few linear molecules and many cyclic ones in solution. There are about 70% pyranose and 30% furanose in water. Its crystal is mostly made up of pyranoses.
Fructose solution undergoes isomerization in an alkaline environment. Some linear fructose with carbonyl turns into an intermediate enediol glucose that then transforms into linear chain with aldehyde group. The weakly basic media containing fructose would cause Tollens' reagent to show a silver mirror reaction or Fehling's reagent to appear red precipitate despite lacking an aldehyde group. However, only alkaline solutions exhibit this reducing property since monosaccharides are quite stable in dilute acids.
It is sweeter than sucrose and all other monosaccharides. Sweet natural foods such as lychee, honey and mangoes contain this compound in large quantities. Some fructose is converted into fructose-phosphate in liver cells or muscle cells for aerobic respiration process via glycolytic pathway. Some also used to synthesize fatty acids in liver cells. It breaks down faster and more readily depletes ATP since it bypasses the rate limiting enzyme of glycolysis. The liver may get overloaded and damaged, if fructose is consumed in large amounts over a short time. Long-term intake of processed foods and beverages will result in non-alcoholic fatty liver disease.
Galactose, its structure, functions and physical-chemical properties
Galactose is a common monosaccharide with a melting point of 168–170°C. It is a white crystalline powder at room temperature. It is soluble in water but not as much as glucose or fructose. The solubility is only about 65 grams per 100 milliliters. It is not very sweet and sweetness is about 65% of sucrose.
Galactose and glucose share same molecular formula, but their structures are different from each other slightly. The five hydroxyl groups and one aldehyde group classify it as an aldose. Linear molecules prefer to form rings in aqueous solutions. The water is dominated by six-membered ring pyranose with a few five-membered rings. In the six membered rings, the hydroxyls on fourth and sixth carbons are in the identical orientation. These hydroxyl groups are oriented in different directions in glucose. The reducing nature of galactose promote a silver mirror reaction in Tollens' reagent and turns Fehling's reagent red.
It is rare for galactose to be found alone since it usually occurs together with glucose to form lactose, or polymerized to oligogalactoses. Both of them are abundant in milk. They promote beneficial bacteria within intestines and enhance the absorption of calcium and magnesium. The cerebrosides composed of galactose is abundant in the nerve cells and beneficial for neural growth and repair. In plants, galactose is often found as polysaccharides, such as gum arabic and carrageenan.
Galactose can’t directly serve as an energy provider rather converted into glucose at the liver before entering into metabolic pathways. Only few babies are born without relevant enzyme for galactose conversion, so the accumulation of galactose damages their liver and brain.