Difference Between Alkaloid and Glycoside

Main Difference – Alkaloid vs Glycoside

Alkaloids are naturally occurring, nitrogen-bearing organic compounds that are basic. These compounds have a wide range of biological activities and physiological effects on humans and animals. Plants are the oldest known sources of alkaloids. Glycosides are naturally occurring substances in which a sugar is bound to another functional group via a glycosidic bond. The glycosidic bond joining these two components is usually through an oxygen (O), sulfur (S) or a nitrogen atom (N). The main difference between alkaloid and glycoside is that alkaloids react with acids forming a salt whereas glycosides show no reaction with acids.

Key Areas Covered

1. What is Alkaloid
      – Definition, Occurrence, Properties
2. What is Glycoside
     – Definition, Components, Occurrence
3. What is the Difference Between Alkaloid and Glycoside
     – Comparison of Key Differences

Key Terms: Alkaloids, Amphoteric, Glycosides, Glycosidic Bond, Nitrogen, Sugar  

What is Alkaloid

An alkaloid is a class of nitrogenous organic compounds of plant origin, which have pronounced physiological actions on humans. Alkaloids are made of mainly carbon, hydrogen and nitrogen atoms, sometimes along with oxygen and sulfur. Most alkaloids contain oxygen.

Oxygen-containing alkaloids are usually colorless crystals. But oxygen free alkaloids are typically volatile, colorless liquids that are very oily. (But there are some colored alkaloids as well (Ex: Berberine). Most alkaloids are weakly basic compounds. But there are also some amphoteric alkaloids (ex: Theophylline). Since alkaloids are organic compounds, they dissolve poorly in water but are highly soluble in organic solvents.

Most alkaloids have a bitter taste. Sometimes these compounds can be poisonous when ingested. Alkaloids are primarily found in plants and are common in some flowering plants. The first individual alkaloid was obtained from the opium poppy. This alkaloid was morphine. Alkaloids are present in plant tissues as water-soluble salts of organic acids, esters, or combined with tannins or sugars, rather than as free bases.

Figure 1: Opium Poppy

Alkaloids have a wide variety of chemical structures. But each and every alkaloid contains at least one nitrogen atom that is in amine form. This means the chemical structure of alkaloids is similar to that of ammonia whose hydrogen atoms are replaced by hydrocarbon groups.

The nitrogen atoms present in alkaloids cause the basicity of alkaloids. Therefore, these compounds undergo acid-base reactions. Similar to alkali compounds, alkaloids can react with acids to form salts. Most alkaloids have ring structures. These ring structures contain nitrogen atoms.

What is Glycoside

Glycosides are naturally occurring substances composed of a sugar that is linked with another functional group via a glycosidic bond. The glycosidic bond joining these two components is usually through an oxygen (O), sulfur (S) or a nitrogen atom (N). For example, a glycoside with a sulfur-containing glycosidic bond is known as a thioglycoside and a glycoside with a nitrogen-containing glycosidic bond is known as a glycosamine.  

Figure 2: Glycosidic Bond Formation

Most glycosides can be found in plants as pigments in flowers and fruits (Ex: Anthocyanin). Many plants use inactive forms of glycosides to store chemicals. These inactive forms can be activated by enzyme hydrolysis. These enzyme hydrolysis reactions can break the sugar part from the glycoside, making the other component reactive.

Difference Between Alkaloid and Glycoside

Definition

Alkaloid: An alkaloid is any of a class of nitrogenous organic compounds of plant origin which have pronounced physiological actions on humans.

Glycoside: Glycosides are naturally occurring substances composed of a sugar that is linked to another functional group via a glycosidic bond.

Presence of Nitrogen

Alkaloid:  Alkaloids are essentially composed of nitrogen atoms.

Glycoside: Glycosides may or may not contain nitrogen atoms.

Occurrence

Alkaloid:  Alkaloids are primarily found in plants and are common in some flowering plants.

Glycoside: Most glycosides can be found in plants, as pigments in flowers and fruits.

Nature

Alkaloid:  Alkaloids are basic nitrogenous compounds.

Glycoside: Glycosides are condensation products of a sugar portion and a non-sugar portion.

Reaction with Acids

Alkaloid: Alkaloids react with acids to form a salt.

Glycoside: Glycosides do not react with acids.

Solubility

Alkaloid:  Alkaloids are poorly soluble in water and are well soluble in organic solvents.

Glycoside: The solubility of glycosides depends on the sugar present in the glycoside.

Conclusion

Alkaloids and glycosides are chemical components that can be found in plants. Alkaloid essentially contains nitrogen atoms, along with carbon and hydrogen. Glycosides may or may not contain nitrogen. The main difference between alkaloid and glycoside is that alkaloids react with acids forming a salt whereas glycosides show no reaction with acids.

Reference:

1. “Glycoside.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 15 Dec. 2014, Available here.
2. “Alkaloid.” Alkaloid – an overview | ScienceDirect Topics, Available here.
3. “Alkaloid.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 29 Nov. 2017, Available here.

Image Courtesy:

1. “388988” (CC0) via Pixabay
2. “Ethyl-glucoside” By AxelBoldt at English Wikipedia – Transferred from en.wikipedia to Commons by Common Good using CommonsHelper (Public Domain) via Commons Wikimedia