The breakdown and utilization of carbohydrates are vital as they form Glucose that provides energy to our body. The latter can be easily taken by the bloodstream. Carbohydrates or just called ‘sugars’ is one amongst the macronutrients required by the body. Carbohydrates are present in plant and bacterial cell walls, and also found in broad categories of healthy and unhealthy foods- fruits, vegetables, milk, whole grains, corn, cookies, bread, pasta, and other processed foods.
Classification and Structure
Carbohydrate, a biomolecule, is made up of carbon (C), oxygen (O), and hydrogen (H) with empirical formula Cm(H2O)n, where positive integers are denoted by m and n. The unit structure of carbohydrates is saccharides. This formula does not apply to polysaccharides. Carbohydrates can be classified in a few ways.
Firstly, depending on the number of sugar units, the carbohydrates are classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides cannot be hydrolyzed further. Sugar that gives two or more (less than 15) monosaccharides on hydrolyzing the glycosidic bonds are disaccharides and oligosaccharides, respectively.
Polysaccharides are the long-chain polymer of carbohydrate, which may range from linear to highly branched. Secondly, classification depends on the nature of the functional group present. If the functional group is an aldehyde, then the monosaccharide is called an aldose while if ketone, then a ketose.
That is how the monosaccharide gains one more definition, which is they are the aldehydes and ketones with two or more functional groups. Monosaccharides can also be named according to the number of carbon atoms present. For example, monosaccharides with three, four, and five carbon atoms are named trioses, tetroses, and pentoses, respectively.
Glucose is an aldose with six carbon atoms, hence called an aldohexose. Fructose is also a hexose sugar but with the functional group ketose and therefore named as ketohexose.
Monosaccharides with a minimum of five carbon atoms can form a closed or ring structure. It does have an open or linear arrangement. The nucleophilic reaction of carbonyl and hydroxyl group of the same monosaccharide creates a ring-like structure and forms a hemiacetal or hemiketal entity depending on the functional group.
Disaccharides are the category of sugars with two monosaccharides joined through glycosidic linkage. Lactose, maltose, sucrose are some of the disaccharides found. Lactose is formed by the joining of galactose and glucose subunits. Sucrose is composed of glucose and fructose.
Polysaccharides are either homo-polysaccharides, made of single type monosaccharides or hetero-polysaccharides, made of two or more types of monosaccharides. Homopolysaccharides include starch, glycogen, and cellulose, while hyaluronic acid, chondroitin sulfate, heparin, keratin sulfate are examples of hetero-polysaccharides.
Table 1. Examples of carbohydrates based on different classes
| CLASS | EXAMPLES |
| Monosaccharides | Glucose, Fructose, Ribose, Galactose, Ribulose |
| Disaccharides | Maltose, Sucrose, Lactose, Cellobiose, Chitobiose |
| Oligosaccharides | Raffinose, Fructooligosaccharides, Malto- oligosaccharides |
| Polysaccharides | Starch, Glycogen, Cellulose, Pectin, Gums, Insulin |
| Carbohydrate derivatives | Hyaluronic acid, Chondroitin sulfate, Heparin, Keratin sulfate |
Glycogen Utilization
In animals, energy is stored in the form of glycogen and has a significant role in muscle function. In humans, the glycogen is synthesized and stored in hepatic cells and skeletal cells. Kidneys, glial cells, white blood cells, red blood cells also have a small quantity of glycogen stored. Glycogen is also stored in the uterus, during pregnancy.
We consume a majority of carbs; when digested, it increases the blood glucose level. This activates the secretion of insulin. Glucose is transported to hepatic cells where insulin acts on it. Insulin tells the cell to open up so that glucose can enter.
Breakdown of Glycogen
It is branched homo-polysaccharide with glucose moieties. Glucose molecules are linked via α-1,4-glycosidic bonds, meaning the first carbon of one glucose is attached to the fourth hydroxyl group of next glucose. Branches occur at every tenth residue through α-1,6-glycosidic linkage.
Glycogen degradation is biochemically simple and contains three steps catalyzed by enzymes. During the breakdown, terminal glucose residue removes as glucose-1-phosphate from non-reducing end. This reaction is also called as phosphorolytic cleavage catalyzed by glycogen phosphorylase.
The act of glycogen phosphorylase repeats until it encounters four glucose residues away from the branch point. Phosphorylase is not susceptible to cleave branch points, and hence for further degradation, another enzyme comes into action.
A debranching enzyme, glucantransferase, remove three glucose moieties from the end and transfers to the linear chain. Now this chain can be broken down by glycogen phosphorylase to glucose-1-phosphates. One glucose is still attached to the linear chain in a branch that is removed by glucosidase. It is essential for the Glucose-1-Phosphate to be converted to Glucose-6-Phosphate or else, it will not be able to enter the metabolic mainstream.
The glucose-6-phosphate has three possible fate pathways, (i) Glycolysis, (ii) Pentose phosphate pathway, and (iii) release as free glucose in the bloodstream.
Glycolysis
Glycolysis is also known as the Embden-Meyerhof pathway. It is the oxidative splitting of one mole of glucose, a six-carbon molecule into two moles of pyruvate, a three-carbon molecule. It is a cascade of enzyme-catalyzed reactions.
Phosphate is added to the isomer of Glucose-6-phosphate, Fructose-6-phosphate forming, fructose-1,6-bisphosphate. This six-carbon compound is then cleaved to two three-carbon molecules. These are glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate are isomers of each other, and hence, dihydroxyacetone phosphate is isomerized to Glyceraldehyde-3-phosphate. Two molecules of Glyceraldehyde-3-phosphate undergo oxidation, substrate-level phosphorylation, and hydrolysis to yield pyruvate and ATP.
Pyruvate is not the end product formed when a monosaccharide is digested, but it is the product of glycolysis alone. The fate of pyruvate is determined by at least two factors- Presence or absence of oxygen and whether mitochondria are present.
In the aerobic condition, two molecules of pyruvate are reduced to two molecules of carbon dioxide and a molecule of NADH through the TCA cycle inside the matrix of mitochondria. These reactions are universal and occur in plants, animals, and even in microbial cells.
When talking about the anaerobic condition and in the absence of mitochondria, pyruvate is fermented to result in ethanol or lactic acid. This happens in yeast cells as they are facultative anaerobic organisms. During vigorous exercise, a similar anaerobic condition is created in human skeletal muscles, which creates a shortage of oxygen in these cells where they undergo fermentation.
Pyruvate is fermented to lactate or lactic acid. Lactic acid is toxic to cells since there is a pH drop and leads to muscle cramps. Lactate is sometimes oxidized to ATP, CO2, and water by aerobic metabolism, or it is converted back to glucose when transported to the liver.
Pentose Phosphate pathway
Alternative names of this pathway are hexose monophosphate shunt (HMP) and phosphogluconate pathway. The main functions of the pathway include:
It provides NADPH as the source of electrons for the reduction of molecules during biosynthesis of proteins, amino acids, nucleic acids, and fatty acids. Erythrose, a four-carbon molecule synthesized during the pathway, is used to synthesize amino acids and vitamin B6.
Ribose-5-phosphate is also synthesized in this pathway, which is the significant component of nucleic acids, and ribulose-5-phosphate is a CO2 acceptor in photosynthesis.
Glyceraldehyde-3-phosphate is one of the intermediates formed in this pathway, which can return to glycolysis.
Functions of Carbohydrates in Plants
Photosynthesis
Plants are considered autotrophs, as they can make their food. Plants produce carbohydrates (glucose) through a well-known process, photosynthesis. This compound is essential in providing energy. Plants have special machinery to harvest light energy and reduce carbon dioxide to glucose. Plant defense and cell signaling
Oligosaccharides like raffinose or trehalose have ROS scavenging properties and can be considered as potent antioxidants. Carbs also have their roles in plant immunity, plant defense, or inducing resistance against pathogens and in signaling molecules. All these are possible due to the complex structure.
Plant and the fungal cell wall are made of a ubiquitous carb, β-glucan, cellulose, pectin, and chitin. The cell wall type varies in different plants. The cell wall in red and brown algae is composed of sulfated polysaccharides, fucans, and carrageenans, whereas the cell wall of green algae is composed of ulvans, glycoproteins or agar.
Arabidopsis thaliana, a weed, is popular as a model in genetics research. Researchers showed that extensive cell signaling occurs in plants using this model.
Source of food
According to the food chain, plants comprise the first trophic level and serve themselves as food for animals. Only plants have the bility to use the Sun’s energy. They trap the sun’s energy and pass it until the last trophic level.
Functions of Carbohydrates in Animals
Energy production and energy storage
Carbs serve as a significant energy reservoir. When excess carbs are present, they are converted into fat and also stored as glycogen. These are energy reserve materials in the liver and muscles of animals. Glycogen is broken down into glucose and energy when the situation demands it.
Macromolecule manufacture
Macromolecules such as DNA, RNA, ATP contains a carbohydrate residue in their structure. Sugar present in DNA and RNA is deoxyribose and ribose, respectively.
Health Risks
Carbs and Obesity
Though it is debated that excessive intake of carbohydrates leads to obesity, other factors contribute to this condition. These include a sedentary lifestyle, consumption of junk foods, lack of sleep, and stress.
Diabetes or Hyperglycemia
Carbs are digested to glucose by the body. It enters the bloodstream and increases the sugar level. When the pancreas cannot produce insulin or the production is minimal, the glucose level in the blood continues to rise and cause the condition of diabetes. High glycemic index is the characteristic of carbs that raise blood sugar.
Hypoglycemia
When blood sugar level falls from the normal range, the condition is called hypoglycemia. Possible causes that lead to this condition are starving, insulin overproduction, medication, excessive drinking, or hormonal imbalance.
Glycogen storage diseases
Glycogen storage diseases are caused due to the genetic deficiency of enzymes that catalyze glycogen metabolism. They are Von Gierke’s disease, Pompe’s disease, Hers’ disease, Andersen’s disease, etc.
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Featured Image: Breakdown and Utilization of Carbohydrates @Couleur
Kerala, India
M.Sc Microbiology
