Carbohydrates are macronutrients made by 3 important elements carbon, hydrogen, and oxygen with the hydrogen-oxygen ratio of 1:2. They are macromolecules and are called carbs, sugars, or calories in layman terms. The general formula of carbs is written as Cn (H2O) n. Carbs are the most essential nutrients as they supplement the body`s energy in the form of simple sugars. About 65-70% of our total dietary intake is primarily of carbohydrates because thus making them the priority nutrients than any other nutrients. A gram of carbohydrate consumed through food gives 3.87 kilocalories of energy while it is 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods. Energy plays a vital role in our day-to-day activities such as walking, sitting, running, working etc. Even when we are at rest, our body uses at least 30% of the total calorie intake in the for basal metabolic energy. However, we need energy even when we are at rest and it is called BMR or basal metabolic rate. BMR is the collective energy demands from all major organs, systems and each cell of the body for their metabolic processes. 20% of the available energy goes to brain functions while another 30% is consumed by rest of the vital organs so, in total, only 50% of the energy is available for our day-to-day chores. The term carbohydrate is used in different contexts under different domains. Most biologists simply infer them as they are the output of photosynthesis while biochemist uses it to describe various cycles including, glycolysis and glycogenesis.

Functions of Carbohydrates

  1. Carbs are the key ingredients that supply energy for metabolic activities and activities of daily living.

  2. They add the bulk into the diet as most of the foods we eat contain carbs.

  3. They help to prevent ketosis through healthy metabolism of fats. Ketosis is the formation of ketone bodies which are usually released during a state of hunger.

  4. Sometimes, our body can utilize proteins to release the energy required for the body and is generally harmful. Right intake of carbs suppresses the breakdown of proteins.

  5. Diets containing carbs have rich fibre that prevents constipation and indigestion. So, the optimal intake of fibres helps in improving the health of the digestive system. Cellulose is one example of a dietary component found in the cell walls of all plants. It is one of the main components of insoluble dietary fibre. 

  6. Calories to add taste and fell into the diet to make it palatable, and fragrant. While artificial colours and fortified agents are also made up of sugars.

  7. A healthy intake of carbs can help in weight management, nevertheless, not all carbs do this for us. Therefore, it is good to know what fits right for an individual.

  8. Polysaccharides serve as the energy reserve in our body.
  9. Some co-enzymes and enzymes help in the metabolic activities are made up of saccharides.

Sources of Carbohydrates

Almost every food contains some proportion of carbs while many of them are pure carbohydrates as they make up 75% of their composition with the help of carbs alone.  A good carbohydrate should be easily digestible, low in its sodium and unsaturated fats, and must contain low trans-fats. The most preferred carbohydrates foods include cereals,  potatoes, processed cheese, pasta and milk products. Some fruits are rich in simple sugars in the form of fructose hence consuming them on a daily basis is a great advantage. Some processed milk products contain galactose and lactose essential for the body in many ways.    Sucrose is obtained from raw sugar, savouries, ice cream, sweets, and honey. Additionally, white flour, some cereals, sweets, fresh fruits, vegetables, corn and potatoes also contain some carbohydrates. Rice is the major supplier of carbs in the Indian subcontinent.

Classification of Carbohydrates

Carbohydrates are generally complex. They are used as polyhydroxy aldehydes, acids, ketones, alcohols, their simple derivatives and their polymers. They may be classified according to their degree of polymerization into three principal groups: sugars, oligosaccharides and polysaccharides. Polymerization helps to make complex sugars by joining many simpler units or a complex one can be broken down back to constituent units. The 3 main classes; sugars, oligosaccharides and polysaccharides are practically divided into many other subtypes based on whether a given molecule is simple or complex. The chart below helps to understand the classification of carbohydrates.


Simple carbohydrates

Simple carbohydrates are those that can be easily digested and absorbed which is one of the reasons why we need to consume them in larger proportions. They contain a limited number of (one or two) constituent sugar molecules rather than many. Some foodstuffs that contain simple carbs(but doesn’t contain fibre)are beer, fruits, refined sugars, ice creams, candies, etc. Chlorophyll found in the plants synthesizes glucose (C6H12O6) with the help of carbon dioxide and water by absorbing sunlight. Some simple carbs are explained as under.




1. Monosaccharides

Monosaccharides are classified based on 3 major criteria: the placement of its carbonyl group, the total number of constituent carbon atoms, and its chiral handedness which is rarely used. The monosaccharide is an aldose when its carbonyl group is an aldehyde, while a monosaccharide is a ketose if the carbonyl group is a ketone. On the other hand, monosaccharides are classified into trioses, pentoses and hexoses depending upon are a number of units of sugars. Trioses have 3 carbon atoms per molecule while tetroses contain 4 carbon atoms,  pentoses are made up of 5 carbon atoms and hexoses have 6 carbon atoms. Monosaccharides are the essential fuel molecules in addition to being the building blocks for nucleic acids. 

1.1 Glucose 

Glucose is otherwise called aldohexose and dextrose and is abundant on earth.  Glucose is of the most common and essential monosaccharides is .it can be prepared from sucrose and starch using 2 different methods. When the sucrose is boiled with dilute acid in an alcoholic solution, the 2 products we can get are glucose and fructose.  We can also prepare this by hydrolysis of starch and by boiling it with dilute H2SO4 at high pressure and the temperature of 393K. Glucose has isomeric properties as it can assume cyclic and branched structure. The ring structure of glucose can show up a number of properties of glucose which cannot be figured by open-chain structure. 

1.2 Fructose

Fructose is a ketohexose having a  molecular formula C6H12O6 and contains ketonic functional group at carbon number 2. It is either a  straight or ring-chain and has six carbon atoms. The cyclic structure of fructose is shown below:


2. Disaccharides

Disaccharides are formed when 2 monosaccharides joined by the glycosidic linkage with the common formula C12H22O. Most common disaccharides are sucrose, lactose, and maltose where Sucrose is one of the most common disaccharides. Sucrose is formed when glucose and fructose join while lactose is the galactose- glucose mixture maltose is formed by glucose-glucose linkage. Looking at these combinations, it is clear that the hydrolysis of disaccharides yields 2 molecules of either the same or different monosaccharides. Maltose and lactose are also called milk sugars.

3. Oligosaccharides

An Oligosaccharide is formed when 2-9 monomers are condensed. Some off the most common oligosaccharides are fructose and galactose. So, a polysaccharide is the long unit formed when many monosaccharide units joined together by glycosidic linkage. Many polysaccharides provide a great opportunity to store carbohydrate reserves especially starch. Starch is a polymer of α glucose made up of 2 components-Amylose and Amylopectin. Cellulose is another common polysaccharide found in plants. It has many β-D- glucose units joined by a glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.

Complex Carbohydrates

They are otherwise known as polysaccharides and it takes more time for the complex carbs to get digested and absorbed by the body.  The term poly means many, indicating that these are made up of 2 or more sugar molecules along with some fibres. They are generally called starchy foods. Foodstuffs that contain complex carbs are peanuts, potatoes, peas, lentils, beans, corn, whole-grain bread, cereals, etc. Polysaccharides are classified into starch, glycogen, and cellulose. Starch is made up of 2 components- amylose and amylopectin. Glycogen or animal starch resembles starch but is more branched than the other. Cellulose is a complex carbohydrate found in the plant cell wall. Cellulose is a very tough and fibrous structure help in forming a linear polymer.

 Digestion and absorption of carbs

Oud GI system contains a lot of enzymes that help in breaking the complex carbohydrates into simple sugars. Some of the most important enzymes that aid in the digestion of carbohydrates are Salivary amylase, sucrose, and galactose etc. The food being consumed is cut, tared, and chewed(masticated) into much simpler units with the help of teeth. Saliva present in the mouth gives the food a semi-solid consistency. Saliva found in the mouth contains ptyalin, an enzyme which helps to digest some simple carbohydrates however, the bulk of the carbohydrates are digested and absorbed in the intestine.  The digested food particles reach villi (finger-like projections) of the small intestine. From the villi, they are absorbed into the tiny capillaries which carry them to the circulatory system. The circulatory system pushes these nutrients to reach the cell membranes of each cell of the body. Blood plasma plays an important role in circulating the nutrients across different cells of our body.

Transportation of digested carbs

The Active membrane transport systems of cell membranes help in the absorption of most of the simple sugars. The cellular organelles take responsibility for cell-level metabolism of carbohydrates with the help of many pathways as explained above. Some common pathways include Gluconeogenesis, Glycogenolysis, Glycogenesis etc. Some of these pathways break the glucose in the presence of oxygen and water to release ATP which is later stored inside the mitochondrion of each cell. The stored ATPs are released as and when required by our body.

Read More

  1. Biomolecules in our body

  2. Amino Acids

  3. Recommended dietary intake

  4. Biotechnology -principles and processes

 Check your understanding

  1. What is the primary function of carbohydrates?

  2. Name any 5 foods that contain carbohydrates.

  3. What is the difference between a monosaccharide and an oligosaccharide?

  4. Briefly explain the digestion and metabolism of carbohydrates.




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