Understanding Carbohydrates: Types, Functions, and Importance

Posted on Mar 22, 2025 in Biology

Carbohydrates: An Introduction

Carbohydrates: These biomolecules are composed of carbon, hydrogen, and oxygen (CHO). They always contain a carbonyl group (a group attached to an oxygen atom by a double bond), which can be either an aldehyde group (-CHO) or a keto group (-CO-). They are defined as polyhydroxyaldehydes or polyhydroxyketones.

Classification of Carbohydrates

Carbohydrates are classified into three main categories:

  1. Monosaccharides: These are single-unit carbohydrates, consisting of a single polyhydroxyaldehyde or polyhydroxyketone.
  2. Oligosaccharides: These carbohydrates are formed by the union of 2 to 10 monosaccharides. Disaccharides are the most important type of oligosaccharides.
  3. Polysaccharides: These incorporate more than 10 monosaccharides.

Monosaccharides: Detailed Properties

Monosaccharides cannot be broken down further by hydrolysis. They are named by adding the suffix ‘-ose’ to the number of carbon atoms they contain.

Physical Properties

  1. Monosaccharides are crystalline solids, white in color, and soluble in water.
  2. They have a sweet taste and are commonly referred to as sugars.

Chemical Properties

Monosaccharides possess a reductive nature due to the presence of aldehyde and ketone groups, which can be oxidized to carboxyl groups. They can also associate with amino groups.

Monosaccharides contain 3 to 7 carbon atoms in their linear form. Examples include:

  • Trioses (C3H6O3): There are two types: glyceraldehyde and dihydroxyacetone. These are involved in glucose degradation. Glyceraldehyde has an asymmetric carbon atom (its four valences are saturated with different radicals) and exhibits optical activity, while dihydroxyacetone does not.
  • Tetroses (C4H8O4): Erythrose is an important tetrose with two asymmetric carbon atoms.
  • Pentoses (C5H10O5): There are three types: Ribose (found in RNA), deoxyribose (found in DNA), and ribulose (involved in the dark phase of photosynthesis).
  • Hexoses (C6H12O6): There are three types: Glucose, galactose (aldoses), and fructose (ketose). (glucose + glucose = maltose, glucose + galactose = lactose, glucose + fructose = sucrose)

Glycosidic Bonds

There are two main types of glycosidic bonds:

  • N-glycosidic bonds: Formed between an α-OH group and an amino compound.
  • O-glycosidic bonds: Formed between two hydroxyl groups of different monosaccharides, releasing a water molecule. If the bond involves the anomeric carbon hydroxyl of one monosaccharide and the alcohol group of the second, it is a monocarbonyl glycosidic bond. If the hydroxyl groups involved are from the anomeric carbons of both monosaccharides, it is a dicarbonyl glycosidic bond.

Disaccharides: Two-Unit Sugars

Disaccharides: These are formed by joining two monosaccharides via a monocarbonyl or dicarbonyl glycosidic bond. The most abundant disaccharides are maltose, sucrose, and lactose.

Polysaccharides: Complex Carbohydrates

Polysaccharides: These are formed by the union of many monosaccharides through O-glycosidic bonds. They can play structural roles (β-glycosidic bonds), such as cellulose and chitin, or serve as energy reserves (α-glycosidic bonds), such as starch and glycogen.

  • Starch: The main carbohydrate reserve synthesized by plants and the primary source of glucose for animals. It consists of amylose and amylopectin and is easily hydrolyzed by α-amylase (specialized proteins that cleave α-glycosidic bonds).
  • Glycogen: The glucose reserve polysaccharide in animals, equivalent to starch in plants. It is found abundantly in muscle and liver and is readily hydrolyzed like starch.
  • Cellulose: A polysaccharide with a skeletal function, serving as the primary structural component of plant cell walls.
  • Chitin: The structural component of the exoskeletons of invertebrates.

Physiological Functions of Carbohydrates

Glucose provides energy to the body. Polysaccharides and fats act as energy reserves that must be processed before being utilized. Disaccharides, such as fructose and sucrose, are responsible for the sweet taste of many fruits, making them more attractive for seed dispersal. Oligosaccharides are part of glycoproteins that act as regulators or in molecular recognition. Starch and glycogen serve as energy reserves, while other polysaccharides have structural functions.