Carbohydrates: Structure, Classification, and Biological Roles
Carbohydrates are biomolecules composed of one or more chains formed by carbon, hydrogen, and oxygen. Many sugars are sweet.
Classification of Carbohydrates
Monosaccharides
Monosaccharides are carbohydrates made of a single chain that typically contain between 3 and 7 carbon atoms. They are named by adding the suffix ‘-ose’, such as:
- Trioses
- Pentoses
- Hexoses
- Heptoses
Physical Properties
- Solid, white, crystalline.
- Water-soluble and sweet.
- Their solubility is due to the high polarity of their hydrogen and hydroxyl groups.
Chemical Properties
- Monosaccharides can be oxidized, meaning they are able to lose electrons.
- They are the basic source of energy for cells.
- They also have the ability to aminate, associating with an amino group (-NH₂), or can react with acids.
Reducing Fehling’s Reagent
Their reducing capacity is used in the laboratory to identify their presence and to assess their concentration through qualitative and quantitative analysis.
Optical Activity
The presence of an asymmetric carbon atom provides optical activity, leading to dextrorotatory (right-handed) and levorotatory (left-handed) forms.
Types of Monosaccharides
Trioses
Trioses are carbohydrates formed by a carbon chain of 3 carbon atoms, with an empirical formula of C₃H₆O₃. There are two types of trioses: aldotrioses (with an aldehyde group) and ketotrioses (with a ketone group).
Tetroses
Tetroses are carbohydrates formed by 4 carbon atoms. We can distinguish two aldotetroses: erythrose and threose, and one ketotetrose: erythrulose.
Pentoses
Pentoses are monosaccharides with 5 carbon atoms. Their most stable structure is cyclic, not linear.
Hexoses
Hexoses are monosaccharides with 6 carbon atoms. The most important aldohexose is glucose, and a significant ketohexose is fructose.
Aldohexoses have 4 asymmetric carbons.
Glucose
Glucose provides most of the energy needs for cells due to its ability to cross the plasma membrane readily.
Galactose
Galactose can be found in animal urine in the form of β-D-Galactose.
Mannose
Mannose is found in the form of D-mannose in certain plant tissues.
Ketohexoses (Fructose)
One of the most important ketohexoses is D-fructofuranose, which has a pentagonal cyclic form. It is left-handed (levorotatory) and is also called levulose.
Disaccharides
Disaccharides result from the union of two monosaccharides through an O-glycosidic linkage.
Maltose
- Formed by two molecules of D-glucopyranose linked by an α(1→4) linkage.
- It is found in germinating barley.
Cellobiose
- A disaccharide formed by two molecules of D-glucopyranose linked by a β(1→4) bond.
- It is yielded by cellulose hydrolysis.
Lactose
- Is a D-galactopyranose molecule attached to a D-glucopyranose by a β(1→4) linkage.
- It is found in mammalian milk.
Sucrose
- A molecule consisting of D-glucopyranose and a D-fructofuranose molecule linked by an α(1→2) linkage.
- It is found in cane sugar.
- It is dextrorotatory, but upon hydrolysis, the resultant mixture of D-glucose and D-fructose is levorotatory.
Polysaccharides
Polysaccharides are carbohydrates formed by the union of many monosaccharides through O-glycosidic bonds.
Homopolysaccharides
Homopolysaccharides are polymers of a single type of monosaccharide, including starch, glycogen, cellulose, and chitin.
Starch
The reserve polysaccharide of plants. It consists of two main components:
- Amylose: A polymer formed by maltose units attached via α(1→4) linkages, forming helical structures of glucose molecules.
- Amylopectin: A polymer composed of maltose units attached via α(1→4) linkages, with branches at α(1→6).
Glycogen
- A polymer with an energy reserve function in animals.
- It constitutes a polymer of glucose units attached via α(1→4) linkages, with numerous ramifications at α(1→6).
Cellulose
- A polymer with a structural function in the plant cell wall.
- It is composed of β-glucose units attached via β(1→4) linkages.
Chitin
- The main component of the exoskeleton of animals.
- It is a polymer of N-acetyl-glucosamine attached via β(1→4) linkages.
Heteropolysaccharides
Heteropolysaccharides are formed by more than one type of different monosaccharide, such as agar, pectin, and gum arabic.