CARBOHYDRATES
Carbohydrates are the most abundant class of biomolecules in nature. They are widely distributed in plants and animals. Carbohydrates are the compounds of carbon, hydrogen and oxygen with empirical formula Cn(H2O)n, where n represents number of carbon atoms. However, not all the carbohydrates have this empirical formula; some also contain nitrogen, phosphorus and Sulphur. They are also called hydrates of carbon because in them hydrogen and oxygen occur in the radio of 2:1, similar to that found in water. Carbohydrates are broadly defined as polyhydroxy aldehydes or ketones and their derivatives or substances that yield one of these compounds on hydrolysis or in other words they are defined as the compounds having either an aldehyde (-CHO) or a ketone (=CO) group or a modified aldehyde or ketone group and other carbon atoms with alcoholic (-OH) groups.
CLASSIFICATION OF CARBOHYDRATES
Carbohydrates are classified into three main classes: -
1). Monosaccharides: These are the simplest carbohydrates, cannot be hydrolyzed into small molecules.
2). Oligosaccharides: This class includes, the carbohydrates comprise of short chains of monosaccharide units, most common are disaccharides comprise of two monosaccharide units.
3). Polysaccharides: This class includes the carbohydrates consists of long chains of monosaccharide units generally hundreds or thousands.
MONOSACCHARIDES: -
Monosaccharides are simple carbohydrates having single or one saccharide (Sugar) molecule. These cannot be hydrolyzed further into simpler molecules. They are colourless crystalline, sweet tasting. substances soluble in water, sparingly soluble in alcohol and insoluble in ether. They are reducing agent as they reduce the oxidizing agent and get oxidized at the carboxyl group. They are called monomers and form structural units of oligo and polysaccharides. The number of carbon atoms in monosaccharides varies from 3 to 7. The carbon atoms form an unbranched straight chain, joined together by single covalent bonds. On the basis of number of carbon atoms monosaccharides are classified into 5 classes: -
Trioses: with three carbon atoms (C3H6O3)
Tetroses: with four carbon atoms (C4H8O4)
Pentoses: with five carbon atoms (C5H10O5)
Hexoses: with six carbon atoms (C6H12O6)
Heptoses: with seven carbon atoms (C7H14O7)
A monosaccharide molecule has two functional groups: A carbonyl (-C=O) group in which oxygen is attached to carbon atom by a double bond. Hydroxyl (-OH) groups which are attached to all other carbon atoms of the chain. On the basis of the position of the carbonyl (-C=O) group at the chain, monosaccharides are divided into two families:
A). Aldoses
B). Ketoses
A). Aldoses: If the carbonyl group is present at the end of the carbon chain, aldehyde (-CH=O) group is formed and the sugar is called as an aldose sugar.
B). Ketoses: If the carbonyl group is present at any other position, ketone (-C=O) group is formed and the sugar is called as ketose sugar.
Glyceraldehyde and dihydroxyacetone are the simplest monosaccharides with three carbon atoms. Here glyceraldehyde is an aldose as it contains an aldehyde group while, dihydroxyacetone is a ketose because it contains a keto group. Glucose is a hexose sugar with six carbon atoms (C6H12O6). It is an aldose as it contains an aldehyde group, while fructose is a keto hexose sugar because it contains a keto group.
OLIGOSACCHARIDES: -
Oligosaccharides are composed of few monosaccharide units (2-20). During union of monosaccharide units, water molecule is eliminated and the units are linked through an oxygen bridge. This chemical bond that joins the two monosaccharide units is called a glycosidic bond. Depending upon the steric configuration at C1, which is involved in the formation of glycosidic linkage of monosaccharide unit, the bond is called α- and β- bond. The natural source of oligosaccharides is green plants. They are crystalline substances and readily soluble in water, most of them are obtained as colourless solids. Oligosaccharides are classified according to the number of their constituent monosaccharide units. In the modern system of nomenclature these are named according to the name of their constituent monosaccharide units and the position of glycosidic bond such as:
1). Disaccharides (2 sugar units),
2). Tri-saccharides (3 sugar units),
3). Tetra-saccharides (4 sugar units) etc.
Out of these most common are disaccharides.
1). Disaccharides: - Disaccharides consists of two monosaccharide units covalently bound to each other with a glycosidic bond. Their empirical formula is C12H22O11. These can be hydrolyzed to yield their free monosaccharide compounds by boiling with dilute acid. Following are the examples of disaccharides.
✔ Maltose: -It is the simplest disaccharide, composed of two units of D-glucose joined together through their 1 and 4 carbon atoms. It does not occur in nature and also called as malt sugar. It is the product of hydrolysis of starch by the enzyme amylase. Maltose is hydrolyzed to two molecules of glucose by the intestinal enzyme maltase.
✔ Sucrose: -It is the most abundant disaccharide, made up of glucose and fructose also known as cane sugar. It is obtained commercially from cane or beet. It is very sweet, crystalline and freely soluble in water. It can be cleaved into its components by the enzyme sucrase.
✔ Lactose: It occurs naturally only in milk thus also known as milk sugar. It is made up of glucose and galactose units joined together through carbon 1 of galactose and carbon 4 of glucose. It is not much sweet to taste and hydrolyzed to its monosaccharides by enzyme lactase in human beings.
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| Fig: - example of disaccharides |
2). Trisaccharides: -These are composed of three molecules of monosaccharide units. Examples of trisaccharides are:
✔ Raffinose: - It is made up of glucose, galactose and fructose units. It is found in sugar beet and cotton seed meal and in some fungi.
✔ Gentianose: -It is composed of one unit of fructose and two units of glucose. It is found in rhizomes of several gentian species.
3). Tetra-saccharides: - These are composed of four molecules of monosaccharide units. Example of tetra saccharide is-
✔ Stachyose: - It is composed of two units of galactose, one unit of glucose and one unit of fructose. It is found in the hair cot beans.
POLYSACCHARIDES: -
Polysaccharides are composed of several monosaccharide units linked together by glycosidic bond. Polysaccharides are tasteless and colourless amorphous powder which are little soluble in water, although some may form colloidal solutions. The most common monosaccharide units in polysaccharides are glucose, although fructose, galactose and other hexoses also occur. If all the monosaccharide units are the same in a polysaccharide, it is called a homopolysaccharide. For example, starch, cellulose and glycogen, they all are polymers of glucose. If the monomeric units are different in a polysaccharide, it is called a heteropolysaccharide. For example, Hyaluronic acid, made up of alternating residues of uranic acid and N-acetyl glucosamine. On the basis of their function, polysaccharides are classified into two classes: -
1). Storage polysaccharides
2). Structural polysaccharides.
1). Storage Polysaccharides: Polysaccharides are stored as reserve products in many tissues. They occur as starch in plant cells and as glycogen in animal cells.
✔ Starch: Starch is a storage polysaccharide as it is the reserve food material in the plants. It is a homopolysaccharide made up of glucose units. It is especially abundant in tubers such as potatoes, in seeds especially corn, but most plant cells are able to produce starch. It contains two types of glucose polymers- α-amylose and amylopectin. Α-amylose consists of long chain of D-glucose units linked by α (1→4) linkage while amylopectin is a branched chain of D-glucose units. The glycosidic linkages in linear chain are α (1→4) but at the branch points these are α (1→6) occurring every 24 to 30 residues.
✔ Glycogen: Glycogen is a storage polysaccharide as it is the major reserve food material in animals, the counterpart of starch in plant cells. It is a homopolysaccharide made up of glucose units. It is especially abundant in the liver, where it may constitute as much as 7% of the wet weight; it is also present in the skeletal muscle. Glycogen is a branched polymer of glucose in which glucose residues are linked by α(1→4) and α(1→6) glycosidic linkages just like in amylopectin, but glycogen is more extensively branched (on average every 8 to 12 residues) and more compact than starch.
2). Structural Polysaccharides: Polysaccharides are involved in the structural organization of many tissues in microorganisms, higher plants and animals. They occur as cellulose in plant cells and as chitin in animal cells.
✔ Cellulose: Cellulose is a major structural polysaccharide in plant cells and also present in some microorganisms. It is a fibrous, tough and water insoluble substance. Cotton is almost pure cellulose while wood is largely made up of cellulose. Cellulose is a linear, unbranched, homopolysaccharide of 10,000 or more D-glucose residues, connected by β (1→4) glycosidic linkage. However, cellulose resembles with amylase and the main chain of amylopectin of glycogen but in those, the linkage is α (1→4) glycosidic linkage.
✔ Chitin: Chitin is the structural component of exoskeleton of invertebrates such as insects and crustaceans. It is also present in the cell-walls of fungi and many algae. Chitin is a homopolysaccharide made up of N-acetyl-D-glucosamine linked by β (1→4) glycosidic linkage.
METABOLISM OF CARBOHYDRATES: -
Carbohydrates are the major source of energy in the organisms. Dietary carbohydrates mainly consist of polysaccharides (starch) and disaccharides (sucrose, lactose and maltose). These are hydrolyzed into monosaccharides and are absorbed in the blood. Glucose is an important monosaccharide which served as the major metabolic fuel of the cell and tissues. Non-glucose monosaccharides are also converted into glucose. Glucose is metabolized in different ways to produce energy in the body. The main pathways of carbohydrates metabolism are –
I. Glycolysis
II. Citric acid cycle
III. Glycogenesis
IV. Glycogenolysis
V. Gluconeogenesis
VI. Pentose Phosphate Pathway
I). Glycolysis: -
Glycolysis is a process in which one molecule of glucose is converted into two molecules of pyruvate. The pathway is also known as Embden-Meyerhof Parnas (EMP) Pathway after two pioneer investigators Gustav. Embden and Otto Meyerhof. Glycolysis occurs in the cytoplasm of the cell and virtually in all tissues. The process is completed in 10 steps and each step is catalyzed by a specific enzyme. Glycolysis is divided into two phases. In the first phase, one molecule of glucose is converted into two molecules of glyceraldehydes-3-phosphate. The process requires 2 molecules of ATP. In the second phase the molecules of glyceraldehydes-3-phosphate are converted into pyruvate with the release of 4 ATP and 2 NADH molecules. Steps of glycolysis are as follows: -
End products of glycolysis
✒From one molecule of glucose two molecules of pyruvate are formed.
✒2 ATP molecules are used while 4 ATP molecules are formed. Therefore, there is net gain of 2ATP molecules.
✒2H+ atoms are formed. These are accepted by NAD, which changes into NADH+H+. Hydrogen ions are transferred to electron transport system.
Fate of pyruvate: -
The pyruvate formed, as a result of glycolysis, can be further metabolized via any of the three catabolic pathways: -
I. Under aerobic conditions, pyruvate is oxidized completely to yield CO2 and H2O by citric acid cycle.
II. Under anaerobic conditions, in some microorganisms and under hypoxia in skeletal muscles, pyruvate is reduced to lactate via lactic acid fermentation.
III. In some plant tissues invertebrates and microorganisms under hypoxic or anaerobic conditions pyruvate is converted into ethanol and CO2 via ethanol (alcohol) fermentation.
Citric Acid Cycle: -
Under aerobic conditions pyruvate is completely oxidized to yield CO2 and H2O. pyruvate is first converted to acetyl Co-A which is then oxidized to produce CO2 and H2O. The whole process occurs in a cyclic manner and is called citric acid cycle. The cycle is also known as TCA cycle (Tricarboxylic acid cycle) or Krebs cycle after its discoverer Hans Krebs. The enzymes for TCA cycle located in the mitochondrial matrix.
During glycolysis each glucose molecule yields 2 molecules of pyruvate, and 8 ATP are gained. Hence when these two molecules of pyruvate undergo complete oxidation by TCA cycle 24 ATP are formed. In addition, 2NADH are formed during the formation of acetyl Co-A by pyruvate through dehydrogenase complex which yields 6 ATP molecules. Thus, complete oxidation of one molecule of glucose will yield 38ATP molecules (glycolysis 8, Pyruvate dehydrogenase complex 6, TCA cycle24).
Glycogenesis: -
Glycogenesis is the process of formation of glycogen from glucose. The major sites of glycogenesis are liver and muscles, but it can occur in every tissue in the body to some extent. Glycogenesis is a very important process as excess of glucose is converted to glycogen and is stored in this form for utilization at the time of requirement.
For synthesis of glycogen, a pre-existing glycogen chain (glycogen primer) is required. UDP-G transfers the glucose molecule to a pre-existing glycogen primer. C-1 of the glucose of UDP-G forms a glycosidic bond with the C-4 of a terminal residue of glycogen primer in the presence of enzyme glycogen synthase. UPD is liberated in this process. In this way an existing glycogen chain is repeatedly extended by one glucose unit at a time by the successive α 1-4 linkage. Glycogen synthase is a principal enzyme which regulates glycogen formation.
Glycogen synthase can add glycosyl residue only if polysaccharide chain already contains more than four residues. When the chain has become minimum of 11 glucose residues, another enzyme, branching enzyme transfers a part of α 1-4 chain (at least 6 glucose residues) to a neighboring chain to form α 1-6 linkage, thus forming a branching point in the molecule. The branches now grow further by further addition of α 1-4 glycosyl units and further branching. The glycogen formed is stored in liver and muscles.
Glycogenolysis: -
The breakdown of glycogen to glucose is called glycogenolysis. When the blood glucose level falls, glycogen is broken down to glucose to maintain the normal level of blood glucose.
The first step in the breakdown of glycogen is catalyzed by the enzyme glycogen phosphorylase which catalyzed the cleavage of terminal α1-4 linkage of glycogen to remove one glucose residue as glucose-1-phosphate.
The removal of glucose residues continues until about four to five glucose residues remain on either side of the α1-6 branch. Further degradation by glycogen phosphorylase can occur only after the debranching enzyme (a bifunctional enzyme), catalyzes two successive reactions: -
1). The transferase activity of the enzyme shifts a block of three glucose residues from the branch to a nearby non reducing end to which they reattached in α 1-4 linkage.
2). Then the 1-6 glucosidase activity releases a single glucose residue remaining at the branched point in α-1-6 linkage. After this glycogen phosphorylase activity can continue. Glucose1-phosphate formed on the phosphorylate cleavage of glycogen is converted into glucose 6-phosphate by phosphoglucomutase. The glucose 6-phosphate formed from glycogen in muscles enters into glycolysis and serve as an energy source to support the muscle contraction. In liver, enzyme glucose 6-phosphatase cleaves the phosphoryl group from glucose 6-phosphate to form free glucose which release into the blood when the blood glucose level drops.
Gluconeogenesis: -
It is the formation of glucose from non-carbohydrate sources, such as lactate, pyruvate, citric acid cycle intermediates and most of the amino acids. Gluconeogenesis mainly occurs in liver and to a lesser extent in kidney. It takes place only when carbohydrates are not available in sufficient amount from the diet. Though most of the reactions of gluconeogenesis are the reversal of glycolysis, there are certain reactions which are specific to gluconeogenesis. These are:
1). Conversion of pyruvate to phosphonyl pyruvate,
2). Conversion of fructose 1, 6, bisphosphate to fructose 6-phosphate,
3). Formation of glucose from glucose 6- phosphate
1. Conversion of pyruvate to phosphonyl pyruvate:
In this step pyruvate is first converted to oxaloacetate, which is then converted to phosphonyl pyruvate.As oxaloacetate is formed inside the mitochondria it must come out to the cytosol for conversion to phosphoenol pyruvate because the enzyme phosphoenol pyruvate kinase is present in cytosol. As oxaloacetate is not permeable to mitochondria it is converted to malate and cross the mitochondria. In the cytosol malate is again converted to oxaloacetate. Oxaloacetate can also combine to acetyl co-A in the mitochondria to form citrate which is permeable to mitochondria. In the cytosol, citrate is again converted to oxaloacetate. Once phosphoenol pyruvate is formed it goes into reverse glycolytic pathway to form fructose 1, 6, bisphosphate.
2. Conversion of fructose 1, 6, bisphosphate to fructose six phosphates:
3. Formation of glucose from glucose 6 phosphate
Glucose produced by gluconeogenesis in liver or kidney is released into blood stream to be carried to the tissues.
Pentose phosphate pathway
Pentose phosphate pathway (PPP) is also known as phosphogluconate pathway (PGP) and the hexose monophosphate shunt (HMP), operates in the cytosol o the cell.
The pathway serves following purpose:
✅ It provides cytosolic NADPH for use in biosynthesis of fatty acids and steroids,
✅ It synthesizes ribose phosphate, a precursor of the nucleotide biosynthesis.
✅The pathway ultimately yields glyceraldehyde 3-phosphate which can be then oxidized to produce ATP.
✅In dark reactions of photosynthesis, a variation of pentose phosphate pathway leads to the synthesis of glucose.
Pentose phosphate pathway comprises of some oxidative and some non-oxidative reactions. In oxidative phase, glucose 6-phosphate is converted to a pentose sugar, ribulose 5-phosphate with the elimination of NADPH and CO2. In non-oxidative phase, ribulose 5-phosphate molecules undergo a series of conversions and ultimately produce fructose 6-phosphate and glyceraldehyde 3-phosphate.
ELECTRON TRANSPORT SYSTEM OR RESPIRATORY CHAIN
During the process of respiration, both oxidation and reduction go on simultaneously. When a substance releases hydrogen, it is called hydrogen donor and said to be oxidized. The substance accepting hydrogen is called hydrogen acceptor and is said to be reduced.At various stages in glycolysis, in oxidation of pyruvic acid and Kreb’s citric acid cycle, hydrogen atoms are released.
✅ In glycolysis: 2 x 2H,
✅During decarboxylation of pyruvic acid and formation of acetyl coenzymes A: 2 x 2H,
✅During Kreb’s cycle: 2(4x2H.)
Each hydrogen atom consists of one proton and electron 2H→2H +2e. Protons are soluble in water but not the electrons. The protons are released into aqueous solution of cell, the electrons are accepted by the electron acceptors of electron transport system. ETS or electron transport system is a system of enzyme and coenzymes present in the inner mitochondrial membrane. These enzymes and coenzymes act as hydrogen and electron accepters. In ETS several hydrogen and electron acceptors are present alternately. Hydrogen and electrons are passed from one acceptor to another. Finally, hydrogen is accepted by molecular oxygen and water is formed.
There are three major classes of enzymes involved in electron transport in mitochondria,
(i) Pyridine linked dehydrogenase catalyzes reversible transfer of electron from intermediates of TCA cycle to NAD+ or NADP+ and produce NADH or NADPH respectively,
(ii) The flavin linked dehydrogenase catalyses transfer of electrons from either succinate or NADH to FMN or FAD and
(iii) The cytochromes acting in series and transferring electrons from flavoproteins to molecular oxygen Oxidative Phosphorylation in Electron Transport System Electrons released during transport of H+ along ETS are accepted by electron accepters of ETS and are passed on to Fo-F1 complex or ATPase complex where ADP is phosphorylated into energy rich ATP molecules. This process of ATP synthesis from ADP and inorganic phosphate (pi) during oxidation of acetyl coenzyme-A is called oxidative phosphorylation. During this oxidation energy is released in graded sequence. Some of this energy is utilized by F1 particles containing there coupling factors and ATPase enzyme for the synthesis of ATP molecules from ADP.
Thus, for the reduction of one molecule of oxygen, six protons are translocated from matrix to peri mitochondrial space. This creates an increase of proton concentration outside the inner membrane and sets up a proton gradient or electrical potential difference. The resulting potential difference forces the proton to return towards the mitochondrial matrix. They pass through the coupling factors of complex V or Fo-F1 complex or ATPase complex. This provides energy for synthesis of ATP for every pair of protons driven inwards one ATP molecule is synthesized.
SOURCES OF CARBOHYDRATE
Carbohydrates are the most abundant biological molecules. They are widely distributed molecules in plants and animals both tissues. Plants are considerably richer in carbohydrate in comparison of animal. Plants synthesize glucose during photosynthesis from carbon dioxide and water. It is stored in the form of starch. Cellulose, pectin and lignin are the common structural polysaccharides present in plants, which forms the plants framework, which is generally present in seeds, tubers and rhizomes of the plants. In animal cells, glycogen is present as a storage polysaccharide. Glucose is present in the human blood in a concentration of 1g/L. Chitin, hyaluronic acid and chondritin sulphates serve as structural polysaccharides in animals, present in the shells of lobsters, crabs, insects and in the cartilage, adult bones heart valves and cornea. Lactose occurs naturally in milk of mammals also detected in the flowers of some plants
BIOLOGICAL SIGNIFICANCE OF CARBOHYDRATES
- The most important information role of carbohydrates is the production of energy in the form of ATP both in plants and animals, the energy being derived as a result of their oxidation.
- They are indispensable for living organisms, serving as skeletal structures in plants (e.g. cellulose, hemicellulose, lignin and pectin) and animals (e.g. Chitin, hyaluronic acid chondroitin sulphates)
- They also occur as food reserves in the storage organs of plants (e.g. seeds, tubers and rhizomes) in the form of starch and inulin. Sucrose (sugar cane, beet), glucose (grapes) and fructose (fruits) are also stored. In animals, storage form of carbohydrate is glycogen. Glucose is the most important carbohydrate. The bulk dietary carbohydrates are absorbed in the form of glucose, which is present in blood as a most common carbohydrate in a concentration of 1g/L.
- Deoxyribose and ribose sugar form part of the structural framework of DNA and RNA.
- Carbohydrates also play role as mediators of cellular interactions.
- They play a key role in the synthesis of acids, lipids, fatty acids and steroids.
- They also serve to lubricate skeletal joints to provide adhesion between cells and to confer biological specificity on the surface of animal cells.
DEFICIENCY DISEASES OF CARBOHYDRATES
- Acidosis: In carbohydrate starvation, there is a shift from glycolysis (breakdown of glucose) to lipolysis (breakdown of lipids) and ketogenesis for energy needs. The resulting product, keto acids increase acidity in the blood and other body tissues. These changes in the pH of arterial blood outside 7.35 pH -7.45 pH result in irreversible cell damage.
- Ketosis: Carbohydrate deficiency causes the production of ketone bodies in liver formed by the breakdown of fatty acids and by the deamination of amino acids, leading to a state of ketosis. Ketosis results in chronic dehydration and reduced body secretions.
- Hypoglycemia: The non-availability of glucose due to severe lack of carbohydrate causes drop in the blood sugar levels. It occurs when blood glucose level drop under 70 mg/L with typical symptoms like giddiness, fatigue and distress.
- Constipation: Dietary fiber is an essential component of carbohydrate food, which is known to prevent recto colon cancer and help digestion. The absence of dietary fiber can cause constipation.
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