Carbohydrate Chemistry and Tests

Experiment 5: Carbohydrates

• Biochemistry Laboratory 1st Semester A.Y. 2024 - 2025

Objectives

• To differentiate monosaccharides, disaccharides, and polysaccharides based on their properties and reactions to general and specific tests.
• To identify and classify an unknown carbohydrate sample by comparing its reactions with those of known test substances.

Carbohydrates

• Synthesized by plants via sunlight to convert CO₂ and H₂O to glucose and O₂.
• Polymers include starch and cellulose.
• Starch is a storage unit for solar energy.
• Most sugars have the formula Cₙ(H₂O)ₙ, a "hydrate of carbon."
• Sakkharon (Greek) - sugar

Biological Significance

• Staple in the human diet; oxidation is the central energy-yielding pathway in most non-photosynthetic cells (sugar and starch).
• Serves as structural and protective components in bacterial and plant cell walls as well as connective tissues and cell coats of animals.
• Lubricates skeletal joints and facilitates cell adhesion.
• Complex polymers attached to proteins or lipids act as signals, determining the intracellular location or metabolic fate of these glycoconjugates.

Glycoproteins and Glycolipids

• Carbohydrate chains enable them to act as receptor molecules.
• This allows binding with substances on the cell surface.
• Receptor types include signaling receptors (hormones and neurotransmitters), receptors involved in endocytosis and cell adhesion/stabilization.
• Cell adhesion allows cells to attach to each other to form tissues.
• Some act as cell markers or antigens for cell identification (e.g., to allow the immune system to determine if a cell belongs in the body or is a pathogen)

Biological Significance (cont.)

• Carbohydrates are used as fuel in the body. Their oxidation to ATP, carbon dioxide, and water is a major source of energy.
• Certain products of carbohydrate metabolism act as oxidation catalysts.
• Carbohydrates can be used as building blocks for synthesizing other types of compounds in the body.

Carbohydrates (cont.)

• Include both simple sugars (little ring-shaped molecules of carbon, hydrogen, and oxygen, either alone or in pairs) and complex carbohydrates (formed when these rings link together to make long chains).

Classification of Carbohydrates

• Simple carbohydrates (monosaccharides, oligosaccharides).
  • Monosaccharides (1 sugar unit): Glucose, Fructose, Galactose
  • Disaccharides (2 sugar units): Lactose, Sucrose, Maltose
  • Oligosaccharides (3-10 sugar units): Raffinose, Stachyose
• Complex carbohydrates (polysaccharides).
  • Polysaccharides (>10 sugar units): Glycogen, Starch, Dietary fiber

Examples of Simple Sugars

• Glucose: Most important; nourishes the brain, crosses the blood-brain barrier.
• Fructose: Commonly found in honey, fruits, and root vegetables.
• Galactose: "Milk sugar."
• Maltose: Found in molasses, used to ferment beer.
• Lactose: Found in milk.
• Sucrose: "Table sugar," found in sugar cane and beets.

Complex Carbohydrates

• Oligosaccharides: Galacto-oligosaccharide in soybeans.
• Polysaccharides: Most abundant carbohydrate type in food.
  • Starch
    • Amylose: Flexible long chain, helical or random coil shape.
    • Amylopectin: Branched glucose chains linked with alpha (1-6) glycosidic linkages. Each chain has 20-25 glucose units.
  • Glycogen: Similar to amylopectin, but highly branched with more branches than amylopectin.
  • Dietary Fiber/cellulose
    • Not used by humans for energy, but key for grazing animals..

Dietary Fibers

• Broken down by bacteria in the large intestine.
• Beta-glucan is good for heart health.
• Passes through the small intestine undigested.
• Slows absorption of simple sugars, maintaining blood glucose.
• Increases stool bulk, decreasing constipation.

Glycosidic Bonding

• Monosaccharides link together via glycosidic bonds.
• A glycosidic bond is formed when an OH group from one monosaccharide bonds with an H from another, releasing a water molecule.

Unlock Terms

• Hydroxyl group: Simple structures consisting of oxygen atom with two lone pairs bonded to a hydrogen atom (-OH).
• Polyhydroxy: Containing more than one hydroxyl group in the molecule.
• Aldehyde: Organic compound where the carbonyl group (C=O) is attached to a carbon atom at the end of the carbon chain.
• Ketone: Organic compound where the carbonyl group (C=O) is attached to a carbon atom within the carbon chain.

Classification of Carbohydrates (cont.)

• Monosaccharides are either aldoses (polyhydroxyaldehyde) or ketoses (polyhydroxyketone)
• Classified by the number of carbons in the chain and the configuration of the chiral carbon farthest from the carbonyl group.

Iodine Test

• Used to detect polysaccharides (primarily starch).
• Starch forms a dark-blue complex with iodine.
• Positive test: blue/purple color.
• Negative test: yellow/brown color.

Molisch Test

• Used as a general test for carbohydrates.
• A purple/violet colored ring forms at the point of contact between the sugar solution, alpha-naphthol, and concentrated H₂SO₄.
• The acid acts as a dehydrating agent, producing furfural derivatives.

Seliwanoff's Test

• Used to differentiate aldoses and ketoses.
• Rapid red color develops in the presence of a ketose sugar (e.g., fructose).
• Light-pink color develops in the presence of aldoses, typically slowly.

Mucic/Galactaric Acid Test

• Used to detect galactose and lactose.
• Forms an insoluble saccharic acid (mucic acid).
• Positive: Insoluble white crystalline precipitate.
• Negative: No precipitate.

Reducing Sugars

• Reducing sugars can reduce another compound, oxidizing themselves in the process.
• The carbonyl carbon in reducing sugars is oxidized to a carboxyl group; this is often observed by the aldehyde or ketone group.

Benedict's Test

• Detects reducing sugars.
• Positive test: A colored precipitate (brick-red, brown, green or yellow) forms.  
• Negative test: No color change or no precipitate.

Barfoed's Test

• Used to differentiate monosaccharides and disaccharides.
• Positive test: A red precipitate of Cu₂O forms rapidly for monosaccharides.
• Negative test, disaccharides: A red precipitate may form slowly, or not at all. 

Nylander's Test

• Used to detect reducing sugars. 
• Positive test: A dark-colored bismuth precipitate.

Osazone Formation Test

• Reducing sugars react with phenylhydrazine to form osazones (yellow precipitate).
• Different sugars form osazones with different crystalline structures (shapes and melting points) that can be used for identification.

Osazone Test

• Positive test: Characteristic crystalline structure under a microscope.
• Different sugars produce specific shapes.
  • Glucose: Needle-shaped
  • Fructose: Needle-shaped
  • Galactose: Thorny balls or spheres
  • Maltose: sunflower, or star shaped
  • Lactose: Cotton swab or puff-like

This quiz delves into the fundamental principles of carbohydrate chemistry, focusing on key tests like the Osazone test, Molisch test, and Seliwanoff test. Explore the chemical reactions, interactions, and results that help in differentiating various types of sugars. Test your knowledge on the outcomes of these essential reactions in biochemistry.