Summary of Carbohydrates: Monosaccharides, Disaccharides, Polysaccharides
Carbohydrates are molecules that are synthesized from carbon, oxygen, and hydrogen atoms. Some types of carbohydrates consist of a single unit consisting of a few atoms, while other carbohydrates consists of thousands of units linked together through chemical bonds. Glucose, maltose, and glycogen are three carbohydrates that are similar, but structurally different. Carbohydrates have the general molecular formula CH2O. Starch and cellulose are the two most common carbohydrates. Both are polymers (hence “polysaccharides”); that is, each is built from repeating units, monomers, much as a chain is built from its links. The monomers of both starch and cellulose are the same: units of the sugar glucose. Carbohydrates are one of the four major classes of organic compounds in living cells. They are produced during photosynthesis and are the main sources of energy for plants and animals. The term carbohydrate is used when referring to a saccharide or sugar and its derivatives. Carbohydrates can be simple sugars or monosaccharides, double sugars or disaccharides, or composed of many sugars or polysaccharides. Carbohydrates contain 3 elements:
Carbohydrates are found in one of three forms:
Functions of Carbohydrates:
Substrate for respiration.
Intermediate in respiration.
Energy stores (e.g. starch, glycogen).
Recognition of molecules outside a cell (e.g. attached to proteins or lipids
on cell surface membrane).
The simplest form of carbohydrates is the monosaccharide. ‘Mono’ means ‘one’ and ‘saccharide’ means ‘sugar’. Monosaccharides are either aldoses or ketoses. Aldoses such as glucose consists of a carbon backbone and a carbonyl group (C=O) located at the end of the chain. Ketoses such as fructose consists of a carbon backbone with a carbonyl group located at any other carbon in the chain. The remaining carbon atoms are bound to hydroxyl groups (-OH). General formula:
(CH2O)n where n is a number between 3 and 9. They are classified according to the number of carbon atoms. The monosaccharides you will have to know fall into these categories: •C = 3 = triose
•C = 4 = tetrose
•C = 5 = pentose
•C = 6 = hexose
•Trioses: (e.g. glyceraldehydes), intermediates in respiration and photosynthesis. •Pentoses: (e.g. ribose, ribulose), used in the synthesis of nucleic acids (RNA and DNA), co-enzymes (NAD, NADP, FAD) and ATP. •Hexoses: (e.g. glucose, fructose), used as a source of energy in respiration and as building blocks for larger molecules. All but one carbon atom have an alcohol (OH) group attached. The remaining carbon atom has an aldehyde or ketone group attached.
Due to the bond angles between the carbon atoms, it is possible for pentoses and hexoses to form stable ring structures. The carbon atoms are numbered 1 to 5 in pentoses and 1 to 6 in hexoses. Depending on the orientation of the OH group on carbon 1, the monosaccharide can have either α or β configurations.
Three common sugars share the same molecular formula: C6H12O6. Because of
their six carbon atoms, each is a hexose. They are: •Glucose, “blood sugar”, the immediate source of energy for cellular respiration •Galactose, a sugar in milk (and yogurt), and
•Fructose, a sugar found in honey.
Although all three share the same molecular formula (C6H12O6), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are known as structural isomers. Disaccharides
When two monosaccharides are joined together through a bond, a disaccharide is formed. Maltose is a sugar composed of two glucose molecules. Other common disaccharides include lactose and sucrose. The structure of carbohydrates featuring two or more monosaccharides is held together covalently with a glycosidic bond. Disaccharides and glycosidic bonds
These are formed when two monosaccharides are condensed together. One monosaccharide loses an H atom from carbon atom number 1 and the other loses an OH group from carbon 4 to form the bond. The reaction, which is called a condensation reaction, involves the loss of water (H2O) and the formation of an 1,4-glycosidic bond. Depending on the monosaccharides used, this can be an α-1,4-glycosidic bond or a β-1,4-glycosidic bond. Oligosaccharides can be formed from condensation reactions, these chains of monosaccharides are covalently linked together by glycosidic bonds, and they usually consist of 3-10 monomers, can be linear or branched and are relatively rare. The reverse of this reaction, the formation of two monosaccharides from one disaccharide, is called a hydrolysis reaction and requires one water molecule to supply the H and OH to the sugars formed. Examples of Disaccharides:
•Sucrose: glucose + fructose,
•Lactose: glucose + galactose,
•Maltose: glucose + glucose.
Sucrose is used in many plants for transporting food reserves, often from the leaves to other parts of the plant. Lactose is the sugar found in the milk
of mammals and maltose is the first product of starch digestion and is further broken down to glucose before absorption in the human gut. Biochemical tests
All monosaccharides and some disaccharides including maltose and lactose are reducing sugars. These can be tested for, by adding Benedict’s reagent to the sugar and heating in a water bath. If a reducing sugar is present, the solution turns green, then yellow and finally produces a brick red precipitate. Non-reducing sugars can also be tested for using Benedict’s reagent but first require addition of an acid and heating to hydrolyse (break apart) the sugar. The acid must then be neutralised using an alkali like sodium hydroxide before carrying out the test as described above.
Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain (condensation reaction) until very large molecules (polysaccharides) are formed. This is called condensation polymerisation, and the building blocks are called monomers. When dozens, hundreds, or even thousands of sugars are linked together, the molecule is considered a polysaccharide. Carbohydrate structures with a single repeating monosaccharide are considered homopolysaccharides, while carbohydrates that consist of more than one monosaccharide is considered a heteropolysaccharide. Polysaccharides can also be identified by whether or not the chain is linear or branched. Glycogen is an example of a homopolysaccharide that is branched. It consists of repeating glucose molecules. Glycosaminoglycan is an example of a heteropolysaccharide. It consists of repeating units of acetylglucosamine and glucuronic acid. Polysaccharides are insoluble and do not taste sweet, they also are used for storage or structural functions. The properties of a polysaccharide molecule depend on:
•Its length (though they are usually very long).
•The extent of any branching (addition of units to the side of the chain rather than one of its ends). •Any folding which results in a more compact molecule.
•Whether the chain is ‘straight’ or ‘coiled’.
However the properties of polysaccharides are:
Polysaccharides are complex carbohydrates made up of monosaccharides. Glycosidic bonds hold the monosaccharides together.
Chains of polymers are linked together by hydrogen bonds. These chains bond together to form microfribrils and in turn fibrils. Storage polysaccharides are hefty energy reserves in organisms, such as glycogen in animals and starch in plants. They are stored in our liver to be converted to energy when needed in the future. They are made up of glucose and when energy is needed hydrolysis breaks the polysaccharides into these glucose molecules for use in cellular respiration. The liver converts glucose into glycogen. This polysaccharide is ideal for energy storage because its branched and large size makes it insoluble and won’t pass through cell membranes. The 4 main polysaccharides:
•Main storage polysaccharide in plants.
•Made of 2 polymers – amylose and amylopectin.
•Amylose: a polymer of glucoses joined by α-1,4-glycosidic bonds. Forms a helix with 6 glucose molecules per turn and about 300 per helix. •Amylopectin: a polymer of glucoses joined by α-1,4-glycosidic bonds but with branches of α-1,6-glycosidic bonds. This causes the molecule to be branched rather than helical •Insoluble therefore good for storage.
•Helix is compact.
•The branches mean that the compound can easily hydrolysed to release the glucose monomers. Glycogen
•Main storage polysaccharide in animals and fungi
•Similar to amylopectin but with many more branches which are also shorter. •The number and length of the branches means that it is extremely compact and very fast hydrolysis. Cellulose
•Main structural constituent of plant cell walls
•Adjacent chains of long, unbranched polymers of glucose joined by β-1,4-glycosidic bonds hydrogen bond with each other to form microfibrils. •The microfibrils are strong and so are structurally important in plant
cell walls. Chitin
Occurs on some fungi and arthropods (as part of exoskeleton). Chitin is an analogous in structure to cellulose.
Consists of units of a glucose derivative (N-acetyl-D-glucosamine) joined to form a long, unbranched chain. Like cellulose, chitin contributes strength and protection to the organism.
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