Carbohydrate Chemistry and Tests

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Questions and Answers

What is the primary purpose of the Osazone test?

To test for the presence of enzymes

To identify non-reducing sugars

To measure the melting points of sugars

To differentiate between different reducing sugars (correct)

Which of the following sugars could potentially yield a false-positive result in the Osazone test when boiled for an extended time?

Sucrose (correct)

Fructose

Maltose

Glucose

Which component of the sugars primarily interacts with phenylhydrazine during the Osazone reaction?

The phenolic structure

The hydroxyl group

The phosphate group

The carbonyl group (correct)

What is another name for the Osazone test, reflecting the reagent used?

Phenylhydrazine test Signup and view all the answers

How does the Osazone test enable differentiation of reducing sugars?

By the time of appearance of the osazone complex Signup and view all the answers

What is the main chemical reaction occurring in the Molisch Test?

Dehydration of sugars Signup and view all the answers

Which of the following carbohydrates does NOT give a positive result in the Molisch Test?

Tetroses Signup and view all the answers

What color indicates a positive result for a ketose sugar in the Seliwanoff Test?

Deep cherry red Signup and view all the answers

Which substance is formed from pentoses during dehydration?

Furfural Signup and view all the answers

What happens when a sugar solution is mixed with alpha-naphthol in the Molisch Test?

A violet ring forms Signup and view all the answers

What reaction occurs with hydroxymethylfurfural under acidic conditions?

It forms furfural Signup and view all the answers

What type of bond forms between monosaccharides during glycosidic bonding?

Covalent bond Signup and view all the answers

Which of the following is a characteristic of an oligosaccharide?

Contains 3 to 10 sugar molecules Signup and view all the answers

Which of the following statements is true regarding the Seliwanoff Test?

Ketose sugars react with concentrated HCl and resorcinol to produce color. Signup and view all the answers

Which of the following statements about carbohydrates is NOT true?

Furfural is derived from ketoses only. Signup and view all the answers

In the process of glycosidic bonding, what byproduct is formed?

Water Signup and view all the answers

Which term describes a carbohydrate made up of many sugar units?

Polysaccharide Signup and view all the answers

What is the main feature of a ketone?

Carbonyl group attached to a carbon atom within the chain Signup and view all the answers

What is a polyhydroxy compound?

Contains multiple hydroxyl groups Signup and view all the answers

Which of the following correctly describes monosaccharides?

One sugar molecule Signup and view all the answers

Which carbohydrate classification can be hydrolyzed to yield single sugar units?

Disaccharides Signup and view all the answers

Which carbohydrate does not result in a color change when tested?

Maltose Signup and view all the answers

What observation indicates the presence of starch when tested?

Formation of blue-black color Signup and view all the answers

Which carbohydrate shows soluble hedgehog-shaped crystals?

Lactose Signup and view all the answers

What is a common feature of both maltose and lactose when tested?

Both are soluble in water Signup and view all the answers

What is the significance of the light blue color observed in the tests?

Indicates no reaction with iodine Signup and view all the answers

What shape of crystals is formed when glucose is tested in the osazone test?

Needle-shaped crystal Signup and view all the answers

Which sugar tested produces no crystal in the osazone test?

Sucrose Signup and view all the answers

Maltose crystals in the osazone test are characterized by which shape?

Hedgehog or ball-shaped Signup and view all the answers

What color precipitate indicates a positive reaction when using Nylander's test?

Black precipitate Signup and view all the answers

Which test can distinguish between aldohexoses and ketohexoses?

Seliwanoff's test Signup and view all the answers

Which reagent is used in the iodine test for detecting starch?

Lugol's iodine Signup and view all the answers

What result occurs when reducing sugars are heated with phenylhydrazine?

Formation of a yellow precipitate Signup and view all the answers

What type of sugar typically gives a brick red, brown, green, or yellow color in Benedict's test?

Reducing sugars Signup and view all the answers

What color does mucic acid yield in the presence of an acid?

Blue-black Signup and view all the answers

Which reagent is used to identify reducing sugars?

Benedict's test Signup and view all the answers

What is the effect of boiling aldohexoses with mucic acid?

Forms a product more slowly Signup and view all the answers

What is the result of treating starch with iodine?

Formation of a blue-black complex Signup and view all the answers

What characterizes the helical structure of glycogen and starch?

Iodine can fit into the helices Signup and view all the answers

Which of the following best describes the reaction of lactose with reagents?

Produces the same product but more slowly Signup and view all the answers

What is a property of glycogen compared to starch?

It forms tighter coils Signup and view all the answers

What kind of precipitate does bismuth produce when reacting with reducing sugars?

A brown-blue colored precipitate Signup and view all the answers

Signup and view all the answers

Study Notes

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

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Description

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.