Disaccharides and Polysaccharides

• Carbohydrates contain carbon, hydrogen, and oxygen
• The general formula for carbohydrates is Cx(H2O)y, where x and y are variable numbers

Structure of Carbohydrates

• Carbohydrates include monosaccharides, disaccharides, and polysaccharides

Monosaccharides

• Monosaccharides cannot be further hydrolyzed into simpler carbohydrates
• The general formula for monosaccharides is (CH2O)n
• Examples of monosaccharides include glucose, fructose, and galactose (hexose sugars)

Disaccharides

• Disaccharides form through condensation reaction between two monosaccharides, with the removal of one water molecule
• Common examples of disaccharides:
• Maltose consists of glucose + glucose
• Lactose consists of glucose + galactose
• Sucrose consists of glucose + fructose
• The bond between two monosaccharides is called a glycosidic bond

Polysaccharides

• Polysaccharides are polymers of monosaccharides
• Common examples of polysaccharides:
• Starch which is a storage polysaccharide in plants
• Glycogen which is a storage polysaccharide in humans
• Cellulose which is a structural polysaccharide in plants
• Starch is a polymer of glucose with amylose and amylopectin components
• Amylose has a straight chain structure with a helical shape and consists of thousands of glucose units joined by glycosidic bonds
• Amylopectin is compact, branched structure with glucose units held together by glycosidic bonds and has twice as many glucose units as amylose

Functions of Carbohydrates

• Carbohydrates are a source of energy
• Sucrose transports sugar in phloem of plants, and its ability to flow at high concentrations because it is soluble; also it is chemically unreactive
• Polysaccharides like starch and glycogen are good storage molecules that are easily hydrolyzed into monosaccharides
• Cellulose is a structural polysaccharide, found in all plant cell walls, has good tensile strength, and is permeable to water and solutes
• Monosaccharides are required for nucleic acid synthesis (deoxyribose is a constituent of DNA, ribose is a constituent of RNA), disaccharides and polysaccharides

Tests for Carbohydrates

• There are different methods to detect reducing sugars, non-reducing sugars, and starch

Test for Reducing Sugars

• All monosaccharides and some disaccharides (e.g. lactose and maltose) are reducing sugars
• Sucrose (common table sugar) is a non-reducing sugar
• Benedict's test can detect the presence of reducing sugars
• Reducing sugars reduce copper from a valency of 2 to 1
• Benedict's test involves an alkaline solution of copper(II) sulfate (CuSO4), which is reduced to insoluble copper(I) oxide (Cu2O), forming a brick-red precipitate
• The test is semi-quantitative, greater reducing sugar levels result in more brick-red
• The colour changes with increasing concentrations of reducing sugar goes as follows:
• No reducing sugar: Blue solution
• Large amounts of reducing sugar: Green -> Yellow -> Brown -> Red (suspension)
• If there is no precipitate, there are no precipitate at all; If there is a lots of precipitate, there are large amounts of precipitates

Test Non-Reducing Sugar (Sucrose)

• If the reducing sugar test is negative (remains blue), the test for non-reducing sugars can proceed
• Method:
• Add a few drops of hydrochloric acid to 2 cm³ of the test sample in a test tube
• Heat the mixture for 2 minutes in a hot water bath to hydrolyze sucrose into glucose and fructose
• Neutralize the acid by adding sodium hydrogen carbonate until effervescence stops
• Carry out the Benedict's test again
• If non-reducing sugars were present, the Benedict's test will turn positive

Test for Starch

• Method:
• Add a few drops of iodine-potassium iodide solution to the test sample in a test tube or on a white tile
• Observe any colour change
• Observations:
• The brown (or yellow) solution turns dark blue if starch is present
• Iodine takes up position in the centre of the amylose helix to form a dark blue starch-iodide complex
• The solution remains brown (or yellow) if starch is absent

Explore the structure, formation, and function of carbohydrates, including disaccharides and polysaccharides. Learn about glycosidic bonds, storage polysaccharides like starch and glycogen, and structural polysaccharides like cellulose. Understand the properties of monosaccharides and their role in forming complex carbohydrates.