Which of the following statements about monosaccharide structure is true?

A) Aldoses and ketoses differ in the position of their hydroxyl groups. B) Monosaccharides can be classified according to the spatial arrangement of their atoms. C) All monosaccharides contain carbon, hydrogen, oxygen, and nitrogen atoms. D) A six-carbon sugar is called a pentose.

The correct answer is B) Monosaccharides can be classified according to the spatial arrangement of their atoms.

Diagram of glucose (left) and fructose (right) showing the arrangement of the atoms
Diagram of glucose (left) and fructose (right) showing the arrangement of the atoms (Jü [CC0])

The monosaccharides are the simplest form of carbohydrate macromolecules. These are the monomers which can link together by glycosidic bonds to form larger sugar molecules.

Different monosaccharides differ in not only the number of carbons present but also in the arrangement of atoms. They can often be classified based on how many carbons they have, for instance, a five-carbon sugar is a pentose while a six-carbon sugar is known as a hexose.

There are isomers that exist such as glucose and fructose, which are both monosaccharides that have the exact same molecular formula but a different atomic arrangement.

Disaccharide sugars are formed when two monomers, monosaccharides link together by the process of dehydration synthesis, in which a molecule of water is removed and glycosidic bonds are formed. Examples of disaccharides include sucrose, maltose, and lactose.

Polysaccharides are carbohydrates that are large and often complex. They are formed when more than two monomers of sugar link together to form a linear or branched chain.

Starch and glycogen are both examples of storage polysaccharides made of glucose monomers. Glycogen is how sugar is stored in animals, while starch is how it is stored in plant cells.

There are also polysaccharides that are good for providing structural support to cells. For instance, chitin is a substance that is an important part of the exoskeleton of arthropods. It provides strength and flexibility to the skeleton of these animals.

Cellulose and hemicellulose are carbohydrates that together form the tough cell wall of plant cells.

Monosaccharides

The monosaccharide is the simplest unit or monomer of carbohydrates. All carbohydrates contain carbon, hydrogen, and oxygen that occur in varying proportions and with different arrangements of atoms.

The most common and well-known monosaccharide is probably glucose which has the formula C6H12O6 and is the sugar that is formed in photosynthesis and is broken down in cellular respiration to release energy.

The molecular structure of a monosaccharide includes a carbonyl group and a hydroxyl group, and it is classified based on the number of carbon atoms that are present. In other words, a sugar with five carbons is a pentose, while a sugar with six carbons is known as a hexose.

Glucose and fructose are both examples of hexose sugars. These two monosaccharides, as well as galactose, have the exact same formula but the atoms making up each molecule are arranged differently. They are therefore known as isomers which are slightly different in terms of the properties of the sugar.

Two examples of pentose sugars are deoxyribose C5H10O4 found in DNA, and ribose C5H10O5, which is found in RNA molecules. These sugars bond to the phosphate groups and make up the backbone of these two nucleic acids.

How polymers are made

Monosaccharides bond together to form larger sugar molecules that are known as disaccharides (2 monosaccharides), or polysaccharides (more than 2 sugars linked together).

The process in which these monomers link together is known as dehydration synthesis, and usually, a water molecule is removed in the process.

The monosaccharides are linked together by glycosidic bonds which are formed between the monomers. The breaking down of polymers is the reverse of dehydration synthesis and is known as hydrolysis. It often involves the addition of a water molecule to split the polymer into various monomers.

Disaccharides

Disaccharides such as maltose, sucrose, and lactose are polymers that are made up of two monosaccharides that are bonded together at the hydroxyl groups. Maltose is formed when two alpha-D-glucose molecules link together.

Lactose is the sugar found in milk and it consists of the monosaccharides of galactose and glucose which are bonded together. Fructose and glucose combine to form the carbohydrate known as sucrose.

Artificial sweeteners have been created by chemical modification of carbohydrates. It is the chemical arrangement and type of atoms present that often dictates how sweet a sugar tastes.

For instance, sucralose was formed by modifying the sucrose molecular structure so that a very sweet substance which is much sweeter than conventional sugar was formed.

Polysaccharides

Polysaccharides are formed when more than two subunits of monosaccharides link together. These can be very large complex molecules that have functions such as energy storage and structural support.

Glycogen and starch are made of several alpha-D-glucose monomers that are bonded together. These are both polysaccharides that function as energy storage, however, glycogen is found in animal cells while starch is found in plant cells.

The starch and glycogen molecules can be broken down by hydrolysis reactions to provide energy in living organisms when needed. Chitin and cellulose are two examples of polysaccharides that are important in providing structural support.

Chitin forms the exoskeleton of invertebrate animals, and it is made of glucosamine sugar molecules that are bonded together. The chitin is a major component of the insect cuticle and enables the cuticle to be both strong and flexible.

Cellulose is comprised of numerous monosaccharides that are known as beta-D-glucose which are linked together to form a structure that is strong and has great tensile strength.

This polysaccharide also combines with other substances such as pectin and hemicellulose to form the tough outer cell wall of plant cells.

The hemicellulose contains different types of monomers linked together such as for instance, mannose, xylitol, arabinose, and galactose among others. The chains that are formed in hemicellulose are also shorter and branched which is unlike those formed in cellulose.

References

  1. Editors of Encyclopedia Britannica (2019). Monosaccharide. Retrieved from Encyclopedia Britannica.
  2. S Eggling (2003). Disaccharides and polysaccharides. Retrieved from clackamas.edu.
  3. EA Davidson (2019). Carbohydrate. Retrieved from Encyclopedia Britannica.
  4. Editors of Encyclopedia Britannica (2019). Cellulose. Retrieved from Encyclopedia Britannica.
  5. D Elieh-Ali-Komi, MR Hamblin (2016). Chitin and chitosan: production and application of versatile biomedical nanomaterials. International Journal of Advanced Research.

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