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Sugars can be defined as monosaccharide or disaccharide molecules particularly used by organisms to store energy. The disaccharides have generally sweet taste thus it is also used as sweetener or preservative.
What is Sugar?
Any disaccharide or monosaccharide is commonly known as sugar. They serve as a structural component for living cells and also provide energy to many organisms. The classification of sugar depends upon the number of monomeric units present in the structure. For example, monosaccharides are termed simple sugar that consists of single monomeric units.
The term table sugar is used for sucrose, which is made up of two monosaccharides: glucose and fructose and thus named disaccharide. Many people are familiar with the sucrose form of sugar. It is the most common form used as a sweetener in cakes, pastries, and desserts.
There are four major groups of biomolecules including proteins, amino acids, nucleic acids, and carbohydrates. The carbohydrate is an organic molecule composed of oxygen, hydrogen, and carbon in the ratio of 1:2:1.
The general formula of carbohydrates is Cn (H2O) n. They are considered as one of the most abundant biomolecules and major nutrients that provide energy or fuel for various metabolic processes. The classification of carbohydrates is based upon their structural complexity.
They are mainly categorized as simple carbohydrates and complex carbohydrates. Simple carbohydrates or simple sugar can be easily digested thus used as a source of energy whereas complex carbohydrates cannot be digested and metabolized easily.
Characteristics of Sugar
Carbohydrate or sugar is an organic molecule, containing covalently bonded carbon atoms. The bonds are especially C-C and C-H. The structure of carbohydrates is mainly composed of carbon, hydrogen, oxygen, and nitrogen. Sugar has the general formula Cn (H2O) n, in which the value of n ranges from 3 to 7 and the hydrogen and oxygen atoms present in a ratio of 2:1. Therefore the carbohydrates are also known as hydrates of carbon.
The nomenclature of sugars includes –ose suffix that may contain aldehyde or ketone group. The monomeric unit of sugar is a saccharide that may join with other monosaccharides to form a long chain. These monosaccharides are joined with a special type of covalent bond known as a glycosidic bond or glycosidic linkage.
Classification of Sugars
Based on the number of saccharides that unites the carbohydrates are classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides or simple sugars are the most common type of sugars. Examples of monosaccharides include fructose, galactose, and glucose.
Fructose naturally occurs in fruits, cane sugar, and honey thus it is also referred to as fruit sugar. The most common sugar is glucose that is essential for various cellular activities. Plants produce glucose by the process of photosynthesis. These monosaccharides are monomeric units that join together to form disaccharides, oligosaccharides, and polysaccharides.
The carbohydrates consisting of two monomeric units are called disaccharides. Some common disaccharides are maltose, lactose, and sucrose. Sucrose or common sugar is also a disaccharide that is commercially used as a sweetener. Sucrose is harvested from sugarcane and sugar beet plants.
These sugars are mainly obtained from plants such as sugarcane and sugar beet. Some fruits such as bananas, pears, peaches, pineapples, plums, and strawberries, and vegetables such as potatoes, carrots, yams, and sweet potatoes are also common sources of sugars. Sugarcane and sugar beet are major sources of sugar being sold in the market. The commercially sold sugar is mainly sucrose.
Brown sugar contains about 97% of carbohydrates including molasses and is also rich flavored than white sugar.
White sugar is the most common sweetener used at home that consists of 99.9% of carbohydrates. Some diseases such as diabetes, obesity, tooth decay, and cardiovascular diseases are linked with excessive consumption of sugar.
Biological Importance of Sugars
i. Polymer Formation
Natural polymers were particularly created by monosaccharides. Polymers consisting of more than ten simple sugars are referred to as oligosaccharides such as raffinose, maltotriose, and maltotetraose. However starch, cellulose, and glycogen are examples of polysaccharides that are made up of several saccharide units.
ii. Structural Component
Various sugar molecules are the structural component of biological material. The examples are nucleic acids, such as DNA and RNA that consist of sugar component deoxyribose and ribose sugar respectively. Except this, glycoproteins, glycolipids, proteoglycans, etc. also include sugars in their structure.
iii. Source of Nutrition and Energy
Sugar is the major source of nutrients for many organisms. Many organisms require sugars because they provide chemical energy to the body. Simple sugars are easily digestible whereas complex sugars in contrast need a longer time to be digested and metabolized.
The sugars are converted into ATP by the process of cellular respiration. It includes a series of reactions that convert glucose molecules into pyruvate. Then the pyruvate is converted into acetyl coenzyme A by an oxidation reaction.
The acetyl CoA takes part in the citric acid cycle and finally, the electron transport chain leads to the production of ATPs. The dietary carbohydrates consist of complex sugars that are broken down by digestion.
iv. Energy Storage
The energy stored in our body is in the form of polysaccharides such as starch and glycogen in plants and animals respectively. The monosaccharides are converted into polysaccharides and stored in amyloplasts inside the cells of various organs such as fruits, seeds, tubers, etc. In animals, it is stored in the liver and the muscle cells.
Common Biological Reactions
Glucose is mainly produced by plants and other photoautotrophs. The plants use carbon dioxide, water, inorganic salts, and light energy to perform photosynthesis. The sunlight is captured by chlorophyll, which is used to produce carbohydrates, water, and oxygen.
The polysaccharides are formed by joining monomeric units via glycosidic bonds that release water as a byproduct, thus the reaction is called hydration reaction.
Saccharification is the process of degradation of carbohydrates into their simpler forms. It entails hydrolysis. The higher animals including humans convert carbohydrates into simpler forms through the action of salivary amylase. In the small intestine, the enzymes maltase, lactase, and sucrase break down disaccharides into monosaccharides.
The epithelial cells of the small intestine absorb the monosaccharides from the intestinal lumen through the sodium ion- glucose symport system. The GluT proteins facilitate the transport of monosaccharides, which are then released into capillaries by facilitated diffusion.
The glucose is converted into glucose-6-phosphate inside the cell and then take participate in one of the metabolic pathways: (1) glycolysis (2) glycogenesis and (3) pentose phosphate pathway. The glycolysis occurs in the cytosol of the cell. It is a series of reactions that convert a molecule of glucose into two molecules of pyruvate and produce a small amount of energy in the form of ATP and NADH.
The process is followed by the Krebs cycle and oxidative phosphorylation. Anaerobic respiration takes place in the absence of oxygen. In gluconeogenesis, the pyruvate is converted into glucose, thus it is referred to as reverse glycolysis. Here, non-carbohydrate precursors are used to forming glucose such as pyruvate, lactate, glycerol, etc. the process is shuttled from the endoplasmic reticulum into the cytoplasm.
The metabolic process of producing glycogen from glucose is called glycogenesis. The process is mainly used for the storage of sugar.
Glycogenesis primarily occurs in liver and muscle cells because they are the responsible organs for carbohydrate storage. The glycogen is broken down into glucose when the body needs energy. The process is called glycogenolysis. Thus, glycogenolysis is referred to as the reverse reaction of glycogenesis.
The pentoses and NADPH are synthesized in the cytosol by the glucose metabolic pathway. The process completes in the liver, adrenal cortex, adipose tissues, testis of animals. The pentose-phosphate pathway is an alternative metabolic route in the breakdown of glucose.
A part of the pathway functions in the formation of hexoses from CO2 in plants. Here fructose takes part in glycolysis. Fructose is metabolized in the adipose tissues, and in the kidney in animals. The milk sugar is comprised of galactose that is derived from lactose.
The phosphorylation of galactose completes via the enzyme galactokinase and then it enters glycolysis. To ensure proper metabolism of sugar, there should be appropriate assimilation of it. Their levels of glucose in the body should be regulated.
In humans, pancreatic cells secrete hormones such as insulin and glucagon that regulate blood sugar levels. The glucagon is released in case of low glucose levels in the blood whereas insulin is produced in a high level of glucose in the body. Glucogen stimulates the production of sugar and also stimulates the conversion of stored glycogen into glucose.
On the other hand, insulin enhances the rate of uptake of glucose from the bloodstream by muscle cells and adipose tissues. Thus the excess glucose is converted into glycogen through the process of glycogenolysis.
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