Basics of Carbohydrates

Carbohydrates are one of the 3 macronutrients, along with fats and proteins, serving as a source of energy to the body. These biomolecules store energy in the form of glycogen and starch, and play a role in defining the cellular structure (e.g., cellulose). Carbohydrates can be broadly classified into simple and complex carbohydrates based on the number of sugar units and chemical bonds. Carbohydrates are the most abundant organic molecules in nature and are the structural basis of many organisms. These biomolecules have a carbon:water ratio of 1:1 and a general molecular formula that can be represented as Cn(H2O)n.

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Nomenclature and Structure


Carbohydrates are composed of carbon, hydrogen, and oxygen; they are defined as aldehydes or ketones of polyalcohols and have a general molecular formula of Cn(H2O)n. Carbohydrates are named based on the number of polymers (number of units) present in the molecule: 

  • Sugars: composed of 1 or 2 units
    • Monosaccharides: molecules containing only 1 actual or potential sugar group; cannot be further hydrolyzed into smaller units
    • Disaccharides: combination of 2 monosaccharides with the elimination of a water molecule
  • Oligosaccharides: combination of 3–10 sugar units
  • Polysaccharides:
    • Combination of > 10 sugar units
    • Homopolysaccharides: polysaccharides with only 1 type of monosaccharide unit
    • Heteropolysaccharides: polysaccharides with different monosaccharide units
    • Mucopolysaccharides:
      • Also known as glycosaminoglycans (GAGs)
      • Repeating disaccharide units containing uronic acid and an amino sugar
    • Proteoglycan:
      • Carbohydrate chain attached to a polypeptide chain
      • Glycoprotein: if the carbohydrate content is < 4% 
      • Mucoprotein: if the carbohydrate content is > 4%


  • Carbohydrates consist of carbon, hydrogen, and oxygen.
  • The basis of all carbohydrates is a monosaccharide unit. 
  • A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose).
  • The general empirical structure is Cn(H2O)n.
  • Carbohydrates are organic compounds organized in the form of aldehydes or ketones with multiple hydroxyl groups attached to a carbon chain.
  • Carbohydrates can be structurally represented in any of the 3 forms:
    • Open-chain structure: a long, straight-chain form of carbohydrates
    • Hemiacetal structure: The 1st carbon of glucose condenses with the hydroxyl group of the 5th carbon to form a ring structure.
    • Haworth structure: representation using the pyranose ring structure
Haworth Structure

Haworth projection (glucose being the reference molecule)

Image by Lecturio. License: CC BY-NC-SA 4.0


Carbohydrates are classified into simple and complex carbohydrates based on their polymerization.


  • A simple sugar that forms the most basic carbohydrate unit
  • Cannot be further hydrolyzed into smaller units
  • Glucose is the most well-known monosaccharide: 
    • Most abundant monosaccharide
    • Plays a pivotal role in metabolism as a source of energy to the body
    • Often stored as starch in plants and glycogen in animals
  • Monosaccharides are further classified depending on the number of carbon atoms: triose (C3), tetrose (C4), pentose (C5), hexose (C6), heptose (C8), and so on


  • 2 monosaccharides combined by a glycosidic linkage
  • Disaccharides are water-soluble simple sugars.
  • The glycosidic linkage is formed by a condensation reaction (i.e., by the elimination of a water molecule).
  • Disaccharides can be hydrolyzed further by enzymes known as disaccharidases.
  • There are 2 functionally different classes of disaccharides:
    • Reducing disaccharides
    • Nonreducing disaccharides

Sucrose, lactose, and maltose are common disaccharides. The image highlights the composition of these carbohydrates.

Image by Lecturio. License: CC BY-NC-SA 4.0


  • Saccharide polymer containing 3–10 monosaccharide units
  • Commonly present as glycans, i.e., linked to lipids or compatible amino acid chains in proteins by N- or O-glycosidic bonds
  • The main functions of oligosaccharides include cell recognition and cell adhesion.


  • Polymerized products containing > 10 monosaccharide units
  • Further categorized into homopolysaccharides and heteropolysaccharides
  • Homopolysaccharides: 
    • Composed of a single type of monosaccharide
    • Starch and glycogen are 2 major homopolysaccharides that are carbohydrate reserves in plants and animals, respectively.
  • Heteropolysaccharides: 
    • Polysaccharides containing > 1 type of sugar unit
    • Peptidoglycan is a representative example and consists of linear polysaccharides connected to short peptide chains.


  • Also known as GAGs
  • Present in animals and bacteria
  • GAGs are long, linear polysaccharides consisting of repeating disaccharide units. 
  • The repeating disaccharide units consist of a uronic sugar and an amino sugar.
  • Exception: keratan sulfate, which consists of galactose instead of uronic sugar
  • GAGs are highly polar and hydrophilic; therefore, they function as lubricants or shock absorbers.


  • Heavily glycosylated proteins
  • The basic proteoglycan unit consists of a core protein with 1 or more covalently attached GAGs.
  • Proteoglycans are further classified into glycoproteins and mucoproteins.
  • Glycoproteins:
    • Proteoglycan with a carbohydrate content < 4%
    • Glycoproteins are often integral membrane proteins and play a role in cell-to-cell interactions.
  • Mucoproteins:
    • Proteoglycan with a carbohydrate content > 4%
    • Mucoproteins are found in mucous secretions, GI tract, and synovial fluid.


Table: Types of carbohydrates
Type of carbohydrateExamples
  • Glucose
  • Fructose
  • Galactose
  • Sucrose
  • Lactose
  • Maltose
  • Raffinose
  • Stachyose
  • Starch
  • Glycogen
  • Cellulose
  • Pectin
  • Agar
  • Agarose
  • Galactose
  • Hyaluronic acid
  • Heparin
  • Chondroitin sulfate
  • Keratan sulfate
  • Immunoglobulins
  • Hormones
  • Histocompatibility antigens
  • Found in saliva
  • Gastric juice
  • Mucous secretions
  • Synovial fluid


Physical properties


Compounds having the same structural formula but differing in spatial configuration are called stereoisomers:

  • The spatial arrangements of H and OH groups are important, as they contain asymmetric carbon atoms.
  • The reference molecule is glyceraldehyde.
  • The number of possible stereoisomers depends on the number of asymmetric carbon atoms and can be determined using the formula 2n, where n = number of asymmetric carbon atoms.
  • The 2 mirror forms are denoted as D and L.

Stereoisomers with glyceraldehyde as the reference molecule

Image by Lecturio. License: CC BY-NC-SA 4.0

Optical activity: 

Optical activity is the ability of stereoisomers to rotate plane-polarized light. The molecules are designated dextrorotatory (+) or levorotatory (-) depending on the direction of light rotation.

  • D and L notations are unrelated to optical activity.
  • An equimolecular mixture of optical isomers has no net rotation and is known as a racemic mixture.


Diastereoisomers are nonsuperimposable stereoisomers that are not mirror images of each other.

  • Configurational changes with regard to C2, C3, and C4
  • Results in 8 different monosaccharides, of which 3 are found in the human body: glucose, galactose, and mannose.
  • An epimer is a diastereoisomer that differs by only 1 stereocenter.


Image by Lecturio. License: CC BY-NC-SA 4.0


Anomerism is the spatial configuration with respect to the 1st carbon atom in aldoses and 2nd carbon atom in ketoses.


Epimers are compounds that differ from each other only in configuration with regard to a single carbon atom, other than the reference carbon atom.


Image demonstrating epimers: compounds that differ in configuration around only 1 stereocenter

Image by Lecturio. License: CC BY-NC-SA 4.0

Chemical properties

  • Osazone formation:
    • Osazones are carbohydrate derivatives that form when sugars are boiled with an excess of phenylhydrazine.
    • Each sugar will have a characteristic crystal form of the osazone.
    • Osazones are highly crystalline and colored.
  • Benedict’s reaction:
    • Benedict’s reagent is commonly used to detect the presence of glucose in urine.
    • Principle: Sugars form enediols in an alkaline medium, wherein cupric ions are reduced and sugar is oxidized.
    • Sugars with a free aldehyde/keto group are called reducing sugars.
  • Oxidation:
    • Monosaccharides are reducing sugars if their carbonyl groups can be oxidized to yield carboxylic acid.
    • Under mild oxidation conditions (hypobromous acid), the aldehyde group is oxidized to a carboxylic group to yield an aldonic acid.
    • Under strong oxidation conditions (nitric acid + heat), the 1st and last carbon atoms are simultaneously oxidized to form dicarboxylic acids, known as saccharic acids.
  • Reduction to alcohols:
    • The C=O groups in the open-chain forms of carbohydrates can be reduced to alcohols using sodium borohydride or catalytic hydrogenation.
    • The end products are called alditols.

Nutrition and Functions

Carbohydrates play many roles in the sustenance of life. Primarily, carbohydrates serve as a source of nutrition. Additionally, they are structural components of cells, and partake in immune defense and intracellular communication.


  • Carbohydrates are the primary source of energy in the body:
    • Brain cells and RBCs are almost completely dependent on carbohydrates as a source of energy.
    • Carbohydrates serve as an instant source of energy (e.g., breakdown of glucose).
    • The recommended carbohydrate intake per day in a 2000-calorie diet is 275 grams.
    • Carbohydrates contribute to 45%–65% of the total calories.
    • 1 gram of carbohydrate provides 4 calories.
  • Sources:
    • Vegetables, fruits, whole grains, and milk products are major sources of carbohydrates.
    • Grains and certain vegetables, including corn and potatoes, are rich in starch. Sweet potatoes mainly contain sucrose and not starch.
    • Fruits and dark-green vegetables contain little or no starch but provide sugars and dietary fibers.
  • Storage:
    • Energy storage: glycogen in animals and starch in plants 
    • Stored carbohydrates can be used as an energy source instead of proteins.

Communication and structure

  • Serve as intermediates in the biosynthesis of fats and proteins
  • Carbohydrates associate with lipids and proteins to form surface antigens, receptor molecules, vitamins, and antibiotics.
  • Carbohydrates linked to proteins and lipids are important for cell-to-cell communication and in interactions between cells and other elements in the cellular environment.
  • Important constituent of connective tissues in animals
  • Fiber-rich carbohydrates help prevent constipation. 
  • Help form the structural framework of RNA and DNA


Carbohydrates are central to many metabolic pathways in the body and include the entire biochemical processes responsible for the formation, breakdown, and interconversion of carbohydrates in living organisms.

Key metabolic pathways:

  • Glycolysis: conversion of glucose to pyruvate in aerobic conditions and to lactate in anaerobic conditions
  • Gluconeogenesis: conversion of noncarbohydrate carbon substrates to glucose
  • Glycogenolysis: breakdown of glycogen in liver, muscles, and kidneys
  • Glycogenesis: conversion of glucose to glycogen
  • Pentose phosphate pathway:
    • Alternative pathway to oxidize glucose
    • Anabolic pathway 
    • Mainly occurs in erythrocytes 
    • Generates pentose and nicotinamide adenine dinucleotide phosphate
  • Fructose metabolism: metabolism of fructose to enter the glycolysis pathway
  • Galactose metabolism: phosphorylation of galactose by galactokinase to enter the glycolysis pathway
Glucose metabolism

Diagram highlighting the pathways of carbohydrate metabolism

Image by Lecturio. License: CC BY-NC-SA 4.0

Clinical Relevance

Several conditions are associated with disorders in carbohydrate metabolism:

  • Glycogen-storage disorders: there are multiple types of glycogen-storage disorders that all are related to an inability to process glycogen. The most commonly tested disorders from this family of conditions are von Gierke’s disease, Pompe’s disease, and McArdle’s disease. Treatment depends upon the condition and may include dietary changes or treatment with medications including allopurinol.
  • Galactosemia: an autosomal recessive condition that causes defective galactose metabolism. Galactosemia presents in infants with symptoms such as lethargy, nausea, vomiting, diarrhea, and jaundice. Dietary modification to eliminate lactose and galactose is a treatment option. Serious neurologic complications include speech and motor deficits.
  • Disorders in fructose metabolism: 3 major conditions (essential fructosuria, hereditary fructose intolerance, and intestinal fructose intolerance) are considered important disorders of fructose metabolism. Depending on the condition, patients may be asymptomatic or exhibit symptoms of vomiting, bloating, flatulence, and diarrhea. The fundamental treatment is dietary modification.
  • Lactose intolerance: a condition caused by a lack of the enzyme lactase in the small intestine. There are 4 classifications of this disease: primary, secondary, developmental, and congenital. Lactose intolerance leads to a decreased ability to digest lactose. The symptoms include abdominal pain, bloating, diarrhoea, flatulence, and nausea.


  1. The Medical Biochemistry Page. (2020). Biochemical Properties of Carbohydrates. Retrieved June 24, 2021, from
  2. Eufic. (2020). The functions of carbohydrates in the body. Retrieved June 24, 2021, from
  3. Slavin, J., Carlson, J. (2014). Carbohydrates. Advances in Nutrition. 5(6), 760-761.
  4. Aryal, S. (2018). Carbohydrates – definition, function, types, examples, functions. Retrieved June 24, 2021, from
  5. Murray, R.K., Granner, D.K., Rodwell, V.W. (2006). Harper’s Illustrated Biochemistry 27th edition (pp. 112–120).

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