CARBOHYDRATES
LERANING OUTCOMES ● Classify carbohydrates into monosaccharide, disaccharide and polysaccharide with respect to their physical and chemical properties
LEARNING OUTCOME ● Classify monosaccharide according to the number of carbon atoms and the functional groups (i) triose e.g. glyceraldehydes, (ii) pentose e.g. ribose and deoxyribose, (iii) hexose e.g. glucose and fructose, ● illustrate the molecular structure of a monosaccharide and differentiate between the reducing and non-reducing ends
Carbohydrates (hydrates of carbon) are a large group of biochemicals which consist of carbon, hydrogen and oxygen in the proportion, 1 : 2 : 1. Carbohydrates are sugar-containing compounds. These are the most common organic compounds found on earth and in the largest amount as cellulose in plants. Their general formula is C x(H2O)y, such as glucose (C6H1206) and sucrose (C12H22011) Carbohydrates
CLASSIFICATION OF CARBOHYDRATES Carbohydrates are divided into three groups with respect to their physical and chemical properties: (a) Monosaccharides (one sugar unit) Monosaccharides are simple sugars, containing only one basic unit which can be used to form disaccharides and polysaccharides. They are the smallest carbohydrate molecules.
Monosaccharides have the following physical properties: i. sweet in taste. ii. dissolve in water. iii. can be crystallised. Physical properties of Monosaccharides MONOSACCHARIDES
Monosaccharides have the following chemical properties: They are either aldehydes which contain —CHO groups or ketones which contain C=O groups. Both have carbonyl groups (C=O). MONOSACCHARIDES
They are all reducing sugars. They can carry out reduction as carbonyl groups donate electrons. They react with Benedict's or Fehling solution to give a brick-red precipitate. They reduce alkaline copper(II) sulphate (CuSO4) into insoluble copper(I) oxide (Cu20). Monosaccharides
Monosaccharides reduce forming alcohol MONOSACCHARIDES
MONOSACCHARIDES Monosaccharides have a general formula of (CH 2O) They can be classified into trioses, pentoses and hexoses according to the number of carbon atoms and the type of functional groups. Functional groups can be either aldehyde group (-CHO) or keto group (-CO).
. TRIOSES (3C) — C3H603 Trioses are monosaccharides that contain three atoms of carbon e.g. glyceraldehyde and dihydroxyacetone. Their molecular structures are shown below:
Both trioses are intermediate products of metabolism during the breakdown of glucose (glycolysis) or synthesis (photosynthesis) of it. Glyceraldehyde exists as phosphoglyceraldehyde and dihydroxyacetone exists as posphodihydroxyacetone during glycolysis and carbon fixation during photosynthesis. They seldom exist long in the free state in cells. TRIOSE (C3) PROPERTIES
PENTOSES (5C) – C5H1005. (a) They are monosaccharides that contain five atoms of carbon. Examples include ribose, deoxyribose and ribulose.
(b) Pentose can either exist in an open chain form or a ring structure e.g. ribose and deoxyribose The structure of deoxyribose is the same as ribose except that the oxygen in carbon number 2 is missing.
PHYSIOLOGICAL ROLES OF PENTOSES Pentoses are used for the synthesis of nucleotides which is ribose for the formation of ribonucleotides in RNA and deoxyribose for the formation of deoxyribonucleotides in DNA. used for the synthesis of coenzymes such as NAD, NADP, FAD and coenzyme A. used for the synthesis of Polysaccharides. Ribulose biphosphate is used as a receptor for the fixation of CO 2 at the beginning of the light independent stage of photosynthesis.
(a) Hexoses such as glucose, galactose and fructose are monosaccharides that contain six atoms of carbon. Hexoses (6C) - C6F11206
Fructose and other ketoses have ketone (—CO) group as functional groups at one end. Both aldehyde and ketone groups have carbonyl (—CO) group as donor of electrons for reducing, at the reducing end. All of them are isomers, which have the same empirical formula of C6F11206. The glucose molecule can either exist in an open chain form or a ring structure as shown in figure 1.7. Hexoses (6C) - C6F11206
As shown in the open chain structure, glucose like all aldoses have aldehyde (—CHO) as a functional group at one end. As shown in the straight chain structure, the glucose is polyhydroxyaldehyde or aldohexose. D-glucose and L-glucose are optical isomers or stereoisomers. Hexose or pyranose ring structure of glucose can exist in two forms i.e. α-glucose and ß- glucose depending on whether the OH group at the carbon 1 atom is below or above the plane respectively. HEXOSES
Figure 1.7 Straight chain structure and ring structure of glucose HEXOSES
Hexoses act as a source of ATP in respiration especially glucose when completely broken down into water and carbon dioxide. balance water potential between the inside and outside of cells, so as to maintain the shape of cells e.g. red blood cells. used to synthesise disaccharides such as sucrose and lactose. used to synthesise polysaccharides such as starch, glycogen and cellulose. used to synthesise other substances such as fats and amino acids in the body when required. The physiological roles of hexoses
Table 1.1 Notice that each monosaccharide contains a carbonyl group (C=0) either at the first carbon atom (for aldose) or at the second carbon atom (for ketose). The rest of the carbon atoms have a hydroxyl group (OH) attached to them. The carbonyl group is part of the aldehyde or keto groups (highlighted) which have reducing properties. Therefore, all monosaccharides are reducing sugars which can reduce blue Benedict's solution to orange red