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Essay on Carbohydrates
Essay Contents:
- Essay on the Introduction to Carbohydrates
- Essay on the Functional Importance of Carbohydrates
- Essay on the Synthesis of Carbohydrates
- Essay on the Forms of Carbohydrates
- Essay on the Absorption of Carbohydrates
- Essay on the Role of Vitamins on Carbohydrate
- Essay on the Metabolism of Carbohydrates
Essay # 1. Introduction to Carbohydrates:
A carbohydrate is generally defined as a neutral compound made up of carbon, hydrogen and oxygen, the last two elements remaining in the same proportion as in water.
The general formula is Cn(H2O)n. But there may be exceptions. For instance, rhamnose (C6H12O5) is a carbohydrate in which H and O remain in a different proportion. Also there are certain other compounds, such as formaldehyde (HCHO), acetic acid (CH3COOH), lactic acid (CH3CHOHCOOH), etc., which have got the same empirical formula but are not carbohydrates. Thus chemically, carbohydrates can be defined as the aldehyde and ketone derivative of higher polyhydric alcohol (having more than one ‘OH’ group).
Essay # 2. Functional Importance of Carbohydrates:
i. It is the readily available fuel of the body.
ii. It also constitutes the structural material of the organism.
iii. It also acts as important storage of food material of the organism.
iv. Carbohydrate also plays a key role in the metabolism of amino acids and fatty acids.
Essay # 3. Synthesis of Carbohydrates:
1. From Fats:
It is certain that the glycerol, portion of fat, which makes up about 10% of the fat molecule, is converted into glucose in the body but the conversion of fatty acid portion of fat molecules to glucose is a matter of dispute specially in animal body as contrast to plants.
Some of the evidences are given below:
(a) During hibernation the marmot shows a very low respiratory quotient-about 0.3-0.4. The excess oxygen intake is explained on the assumption that oxygen-poor substance (fat) is being converted into an oxygen—rich substance (carbohydrate). But these findings and conclusions are not beyond question.
(b) The same type of conversion is believed to take place in diabetic subjects where a low respiratory quotient is found, and
(c) When fatty acids containing odd number of carbon atoms, such as, propionic, valeric and heptoic acids, etc., are administered to starving rats they are converted into glycogen in the liver. But these fatty acids are not found in the natural fats.
Natural fats, which contain fatty acids with even number of carbon atoms only, do not produce this effect. From this it can be concluded that the synthesis of carbohydrates from fats is not beyond possibility but it takes place indirectly.
2. From Proteins:
There is sample evidence to show that formation of glucose and glycogen may take place from proteins. The administration of certain amino acids in a depancreatised dog raises the urinary glucose. These amino acids are called antiketogenic amino acids. On the whole it is generally accepted that about 60% of the food proteins can form sugar. As to the chemical process of synthesis of carbohydrates from proteins, nature of the mechanism is different with different amino acids.
Some examples are given below:
Essay # 4. Forms of Carbohydrates:
The different forms of carbohydrates which are generally included in diet are as follows:
i. Polysaccharides – Starch, dextrin, glycogen and cellulose.
ii. Oligosaccharides (Disaccharides) – Lactose, maltose, sucrose.
iii. Monosaccharides – Glucose and fructose.
Of these types, cellulose containing β-1, 4 linkages cannot be appreciably digested in the human alimentary canal. Monosaccharides need no further digestion, because all carbohydrates are absorbed in the form of monosaccharides. Hence, digestion of carbohydrates includes the digestion of polysaccharides and oligosaccharides.
Digestion of polysaccharides and oligosaccharides starts in the saliva and is completed in the succus entericus. Digestion of oligosaccharides (disaccharides) chiefly takes place in the succus entericus, but may occur to a slight extent by other digestive juices.
The brief details of the digestion of starch and disaccharides are as follows:
I. Digestion in the Saliva:
Saliva contains – (a) chiefly salivary amylase or ptyalin, and (b) traces of maltase (its presence in saliva is doubtful). Salivary amylase (α-type) whose origin in the saliva, acts on starch (which is mostly amylopectin type) and contains straight chains held by 1, 4′-α glucosidic linkages and branch chains whose branch points are 1, 6′- α glucosidic linkages. Maltase acts on maltose.
Ptyalin:
1. Conditions and Peculiarities of Ptyalin Action:
a. Ptyalin acts on boiled starch only. It cannot penetrate the intact cellulose covering of the un-boiled starch particle.
b. Optimum reaction is slightly acid (pH 6.5), but it can also act in neutral or slightly alkaline medium.
c. Strong acid (such as HCl of gastric juice) destroys ptyalin.
d. Optimum temperature is about 45°C., at 60°C., it is destroyed.
e. Effects of salts (such as chlorides) are necessary for ptyalin action.
f. Ptyalin digests starch up to the maltose stage only.
2. Site of Ptyalin Action:
Although digestion of starch starts in the mouth, yet ptyalin action chiefly takes place in the stomach (before the HCl concentration becomes high). On an average it continues for 30-40 minutes, upper favourable conditions, starch is converted into maltose, isomaltose and maltotriose.
3. Stages of Ptyalin Digestion:
Ptyalin which hydrolyses only α-1, 4′ linkages but not the α-1,6′ linkages and splits the more central linkages, α- and β amylases supplement each other’s action upon amylopectin as β-amylase splits maltose from the end groups and a-amylase splits central linkages to form more end groups.
By its action isomaltose (containing 3 glucose molecules in which there is one α-1, 6′ linkages), maltose (glucose-glucose), maltotriose (glucose-glucose-glucose) and a mixture of dextrins (containing the α-1, 6′ branches and averaging six glucose residues per molecule) are produced.
The stages are briefly as follows (Fig. 9.57):
II. Digestion in the Gastric Juice:
Gastric juice does not possess any carbohydrate splitting enzyme, but gastric HCl can carry on some hydrolysis of sucrose.
III. Digestion in the Pancreatic Juice:
Pancreatic juice contains two enzymes acting on carbohydrates:
i. Pancreatic Amylase:
Pancreatic amylase acting on starch and dextrin. Hopkins has divided the amylases into two groups – α-amylase or endoamylase is of animal origin and β-amylase or exoamylase is of plant origin, α-amylase acts on the polysaccharide on the interior of the chain and β-amylase from the non-reducing end.
ii. Maltose:
Maltose (in traces) acting on maltose.
Pancreatic Amylase:
1. Conditions of action of pancreatic amylase are as follows:
i. It can act on both boiled and un-boiled starch.
ii. Its action is much more rapid than ptyalin. Most of the starch is converted into maltose within a few minutes.
iii. Optimum reaction ranges from pH 6.7 – 7.0, i.e., slightly acid or neutral. It can also act in slightly alkaline medium.
iv. Optimum temperature is about 45°C.
v. Salts and CI ions are essential for its action.
vi. Amylase is not present in the pancreatic juice of infants up to the age of about 6 months. Hence, during this period the baby should not be given any starchy food.
2. Stages of Action of Pancreatic Amylase:
Stages of action of pancreatic amylase are same as that of ptyalin. All starch and dextrins when exhaustively acted both by salivary and pancreatic amylase successively are converted into maltose, dextrin and isomaltose. The latter two are not hydrolysed to maltose and ultimately glucose due to the absence of oligo-α-1, 6′ glucosidase and appreciable amount of maltase in either saliva or pancreatic juice. After this, digestion by intestinal juice begins.
IV. Digestion in the Succus Entericus:
Succus entricus contains oligo-α-1, 6′ glucosidase which splits α-1, 6′ linkages of dextrin formed by the action of a-amylase thus providing scope of further activity of α-amylase and of isomaltose converting it to glucose. It also contains maltase which hydrolyses maltase to glucose.
So the starch is completely hydrolysed to glucose by joint action of α-1, 4′ amylase present in saliva and pancreatic juice and α-1, 6′ glucosidase and maltase in the succus entericus. The other disaccharides taken in food are hydrolysed by lactase and sucrase (invertase) present in this juice.
The enzymes, and their substrates upon which they act, and the respective end products are given below:
i. Sucrase (Invertase):
Acts on sucrose producing one, molecule of fructose and one molecule of glucose.
ii. Lactase:
Acts on lactose giving one molecule of glucose and one molecule of galactose.
iii. Oligo-1, 6′ Glucosidase:
Converts isomaltose into glucose and splits α-1, 6′ linkages of dextrin.
iv. Maltase:
Acts on maltose giving two glucose molecules.
(It has been suggested that there are more than one kind of sucrase, lactase or maltase).
v. Amylase:
Traces of this enzyme may be present in the succus entericus. The presence of this enzyme at a low concentration has been established. It is supposed to act on that little quantity of starch and dextrin which might have escaped pancreatic digestion.
Evidences indicate that many or all intestinal enzymes are intracellular. Their presence in the juice is due to cell desquamation.
Thus, the digestible carbohydrates are all converted into monosaccharides in which form they are absorbed. The process of starch digestion is slow and prolonged. So monosaccharides are slowly produced as observed by restricting disaccharides in the diet. Thus their absorption becomes slow and a high rise of blood sugar is prevented. Consequently, this can be considered as a process for maintaining blood sugar level within a constant range.
Essay # 5.
Absorption of Carbohydrates:
End products of carbohydrate digestion are all monosaccharides, such as glucose, laevulose, galactose, xylose, mannose, arabinose, etc. It is in this form that carbohydrates are absorbed. It is believed that little quantities of disaccharides may also be absorbed, but the still higher forms are not absorbed at all.
Mechanism of Absorption:
The monosaccharides being highly soluble in water, the physical forces like osmosis, diffusion, filtration, etc., certainly play a considerable part in their absorption. But if these were the only forces concerned in the process, the rate of absorption should have been directly proportional to their concentration in the gut.
But this linear relation is found to hold only in the cases of xylose, arabinose, and mannose. With other sugars this principle does not apply; for instance, the amount of glucose and galactose absorbed per hour is entirely independent of their concentrations in the gut. Laevulose occupies an intermediate position.
Moreover, the rate of absorption varies with the nature of sugars. If the rate of glucose absorption is taken as the standard (i.e., 100%), then galactose is found to be absorbed much more rapidly (225%); whereas the rate of absorption of laevulose is about half that of glucose, (i.e., 44%); of mannose (33%) and xylose (30%) is about one-third that of glucose.
Other sugars are still more slowly absorbed. Hydroxyl group at position 2 of the sugar molecule seems to be essential for their active transport. It is found that when the temperature of the mucous membrane is brought down between 0°C., to 20°C., the difference in the rates of absorption between glucose, galactose, laevulose and other sugars disappears. Under such conditions, they are all absorbed slowly, but at the same rate.
These observations indicate that the preferential treatment done towards glucose, galactose and laevulose is due to some chemical activity going on inside the cells. Phloridzin (phlorrhizin) or iodo-acetic acid retards the high rate of glucose and galactose absorption. Since, these substances paralyse the activity of phosphatase it is probable that this enzyme is intimately concerned with the process.
Verzer has suggested that during absorption, phosphorylation of sugars takes place and the corresponding hexose phosphates are formed. The formation of these compounds keeps down the concentration of free glucose, etc., in the cells and thereby ensures their rapid absorption.
In this way the high rate of absorption of these sugars can be explained. It is almost certain that on the other side of the epithelial cells a reversible reaction takes place (dephosphorylation) in which the hexose phosphate is broken down, free glucose enters the blood stream and the phosphoric acid is retained in the cells for further phosphorylation. It is interesting to note that the same mechanism also takes place in the reabsorption of glucose by the renal tubules.
Thus from the above it can be concluded that in addition to the physical forces the following factors are necessary for rapid absorption of sugars:
i. Complete digestion into monosaccharides.
ii. Presence of phosphoric acid and phosphokinase enzyme in the epithelial cells.
iii. Adrenal cortex and insulin which probably control the process of phosphorylation.
iv. Anterior pituitary whose exact role is not known, but which may act indirectly through adrenal cortex.
v. Sodium salts also exert some effects. Because in adrenalectomised animals the rate of absorption of glucose can be restored to normal by giving sodium salts.
vi. Vitamins Thiamine, pantothenic acid and pyridoxine may help in the process of absorption.
vii. Blood glucose level has got no effect on absorption of glucose.
Pathway of Absorption:
Carbohydrates are almost completely absorbed through the portal system, because portal blood always shows a higher concentration of sugar during absorption. An inconspicuous amount may pass through the lymphatics.
Site of Absorption:
Glucose is maximally absorbed in jejunum.
Essay # 6. Role of Vitamins on Carbohydrate:
1. Thiamine (Vitamin B1):
Thiamine acts as a coenzyme of the carboxylase which helps in oxidative decarboxylation of pyruvic acid. It has a potential role in the oxidation of sugar in tissues including brain. In its absence, pyruvic and lactic acids fail to be metabolised with a result of accumulation of these substances in blood and tissues. This metabolic disorders produced by this vitamin deficiency, results beriberi. In the tissues thiamine exists as thiamine pyrophosphate ester and helps in decarboxylation of α-ketonic acid as a coenzyme.
2. Riboflavin:
Since riboflavin is related with tissue oxidation, so it takes part in carbohydrate metabolism. In the tissues this vitamin exists as FMN and FAD. These two coenzymes in combination with apoenzyme play a great role in a number of enzyme systems. Deficiency of this vitamin results disorder of intermediary metabolism leading to condition known as cheilosis.
3. Nicotinic Acid (Niacin):
It remains as a prosthetic group of at least two enzyme systems- NAD and NADP, and takes part in tissue oxidation. Niacin helps in the formation of fats from carbohydrates. The deficiency of this vitamin results disorder of intermediary metabolism leading to condition known as pallegra.
4. Pantothenic Acid (Vitamin B3):
Since pantothenic acid is a component of coenzyme A, so it takes part in carbohydrate metabolism. The condition of alopecia and certain gastro-intestinal disorders are produced by the deficiency of this vitamin.
5. Cyanocobalamin (Vitamin B12):
This vitamin acts as a cofactor (cobamide) for the enzyme methyl malonyl CoA isomerise which is concerned for the conversion of methyl malonyl CoA to succinyl CoA or succinyl CoA to methyl malonyl CoA. Thus this vitamin is essential in the biochemical conversion of carbohydrate to fat or fat to carbohydrate. After administration of B12 hyperglycaemia may be corrected.
6. Ascorbic Acid (Vitamin C):
Ascorbic acid takes part in the tissue oxidation probably by acting as hydrogen-carrier. Deficiency of this vitamin results disorders of metabolism leading to condition known as scurvy (scorbusis).
Essay # 7. Metabolism of Carbohydrate:
Although the products of carbohydrate digestion, absorbed into the blood, are mainly hexose monosaccharides, i.e., glucose, fructose, galactose as well as partly pentose sugars but animal cells utilize mostly glucose so monosaccharides are converted into glucose by the cells mainly for their oxidation and partly for their storage as glycogen (through UDPG cycle).
As fructose and galactose are utilized to a lesser extent in the body so they are reconverted from glucose as evident from the following:
i. Large quantities of galactose are found in the brain tissue.
ii. The brain probably uses carbohydrates as galactose only.
iii. During the normal process of glucose utilization, fructose diphosphate is formed as an intermediary step.
iv. When liver function is deficient, fructose and galactose fail to be converted into glucose and are turned out into the blood stream as such. Due to their low renal threshold they are excreted in the urine. The fructose and galactose tolerance tests of liver function depend upon this principle.
Since carbohydrate is utilized by the cells of all animals including man mainly as glucose so carbohydrate metabolism is meant essentially as metabolism of glucose and other substances, convertible to glucose or vice versa.
Pentose sugars, viz., xylose, arabinose and ribose present in the diet may be absorbed but their fate is not known whereas α-ribulose and α-2-deoxyribose after absorption are utilized in the body for the synthesis of nucleoproteins.
Normally glucose metabolism not only supplies major amount of energy for the body but also- (1) other forms of monosaccharides (viz., fructose, galactose, α-ribulose and α-2-deoxyribose, etc.), disaccharides (lactose) and polysaccharides (glycogen); (2) reserve depot fats, (3) tissue glycolipids, mucopolysaccharides, and (4) amino acids are formed from glucose and reversely protein and lipids are metabolized through glucose pathway.
Thus it appears likely that glucose metabolism holds a central key position in carbohydrate metabolism which is closely associated with the metabolism of protein and fat.
The metabolism of carbohydrate, i.e., mainly of glucose, in the mammalian organism is discussed under the following points (fate and functions):
i. Storage (by the Process of Glycogenesis):
When glucose is not immediately required by the tissues, it remains stored. The chief form of storage is glycogen. Total about 500-700 gm of glucose may remain stored in this form. The main storehouses are liver and muscles in which glycogen is almost equally distributed. Some glucose may remain stored as such temporarily in the skin and subcutaneous tissue.
ii. Sources of Energy (By the Process of Glycolysis and Oxidation of Pyruvic Acid through TCA Cycle):
Carbohydrates are oxidized in the tissues and supply energy. One gram yields 4 C of energy. The major part of daily energy requirement under normal conditions is derived from carbohydrate oxidation. It is the most readily available source of energy and its combustion involves the least oxygen requirement.
iii. Maintenance of Blood Sugar (Homeostasis):
Glucose remains in blood and its level is always kept within a narrow range. Normal blood sugar varies from 80-120 mgm per 100 ml whenever it tends to fall, glucose is mobilized from glycogen and blood sugar is maintained.
iv. Synthesis of Hexose Phosphate:
This is an intermediary step formed during oxidation of glucose, absorption of glucose from intestine and reabsorption from kidney tubules. It is a very important form in which hexose exists in the body. It is present in large quantities in the muscles, intestines, liver, etc.
v. Synthesis of Lactose:
In the lactating mother lactose is synthesized from blood glucose. The mammary glands convert glucose into galactose and then unite the latter with another molecule of glucose to form lactose.
vi. Synthesis of Glycoproteins.
vii. Synthesis of Complex Fats Containing Sugar:
For instance, cerebrosides are synthesized by the nerve cells from galactose and fats. Galactose is locally synthesized from blood glucose.
viii. Synthesis of Fat (Lipogenesis):
It is an established fact that the body can convert carbohydrate into fats.
ix. Synthesis of Proteins:
Simple amino acids may be formed by uniting ammonia with pyruvic acid, etc., which may easily be derived from carbohydrates.
Excretion of Glucose:
Glucose is not excreted from the body in normal health and Fehling’s test is negative. It is a high threshold substance (170 mgm per 100 ml of blood). In metabolic disorders (e.g., diabetes mellitus), glucose appears in the urine and Fehling’s Test is positive. When blood sugar level gives higher value than renal threshold value, the glucose appears in the urine, the condition is known as glycosuria.
In lactating mother, lactose may be found in the mother’s urine. Pentose is found in a condition known as pentosuria. Glucose may be found in urine in higher quantities in some pathological conditions.