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1. Saccharide

Definitions, structures, and functions of monosaccharides, disaccharides, and polysaccharides with some examples.

Organic macromolecule

An organic molecule is a molecule that primarily contains carbon atoms bonded to hydrogen atoms, and may also include oxygen, nitrogen, phosphorus, sulfur, and other elements. They are the building blocks of life and are found in all living things, as well as in many non-living things such as fossil fuels, coal, and petroleum.

In living things, the organic molecules are combined into larger molecules which are called organic macromolecules. There are four main types of organic macromolecules, all essential for life.

From now on, we will see them one by one.

Carbohydrates

Carbohydrates are defined as a group of organic compounds that consist of carbon, hydrogen, and oxygen atoms in a ratio of 1:2:1.

The unit structure of carbohydrates is called monosaccharide. Monosaccharides can bind to each other by glycosidic linkage, a type of covalent bond. When two monosaccharides are bound together, the molecule is called a disaccharide, and when lots of monosaccharides are put together by glycosidic linkage, then the molecule is called a polysaccharide. 

In our body, saccharides are an important energy source. At the same time, we use them as a form of energy storage.

Monosaccharides

Monosaccharides are the unit structure of saccharides, meaning if we break down any saccharide into the smallest pieces, we will surely obtain monosaccharides. In other words, monosaccharides cannot be hydrolyzed into simpler substances.

They are composed of 3 to 7 carbon atoms, and characterized by a carbonyl group and lots of hydroxyl groups. We can classify monosaccharides according to the number of carbons included and the type of carbonyl group.

Number of carbon atoms

Type of carbonyl group

Combining them, we can call a monosaccharide with 5 carbon atoms and an aldehyde group an aldopentose. Here, I give you some examples of monosaccharides with their classification.

Disaccharides

A disaccharide comprises two monosaccharides joined by a glycosidic linkage. Each type of disaccharide is composed of a specific combination of monosaccharides. Below are examples of disaccharides with their composition.

They are energy sources for our bodies. For example, sucrose (commonly known as table sugar) and lactose (known as milk sugar) can be broken down into monosaccharides by our digestive system. These monosaccharides are then absorbed into the bloodstream and become a readily available source of energy for our cells.

Polysaccharides

They are repeating units of monosaccharides. There are several types in this category, which have different functions in the cell.

Starch is a type of polysaccharide that is formed from numerous α-glucose connected by glycosidic linkages. Plants use starch as their energy storage in their cell, which means we eat plants to get energy by digesting starch stored in them. There are two classes of starch, amylose and amylopectin. Both of them are composed of α-glucose, but they are different in composition. Amylose is an unbranched chain of α-glucose, meaning glucose is connected only in one direction. On the other hand, amylopectin is a branched chain of α-glucose, meaning it consists of not only a single chain but multiple branches that arise from one chain.

Glycogen is also composed of branched α-glucose, but the branching is more numerous.  On top of that, the function of glycogen is energy storage in animals, not plants. In most cases, glycogens are stored in the liver and muscles, because they have to manage large amounts of metabolisms.

Cellulose is also a type of polysaccharide, but its function is not energy storage. They can be found in the cell wall (a structure surrounding the cell) and give a structural framework for the cell. Additionally, they are different in terms of composition, they consist of β-glucose. 

These three examples are pure saccharides, which are composed of carbon, oxygen, and hydrogen atoms in a ratio of 1:2:1. However, in some cases, this molecular composition is modified and those kinds of molecules no longer have the genuine composition of saccharides. Here, let’s call them “chemically modified saccharides”. There are several types of chemically modified saccharides, I listed some examples below.

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