Carbohydrates and Glycoconjugates Practice Questions

Correct answer: Polyhydroxy aldehydes or ketones

Carbohydrates are defined as polyhydroxy aldehydes, polyhydroxy ketones, or compounds that yield such units upon hydrolysis. They serve as the primary energy source (glucose), structural components (cellulose), and signaling molecules in biological systems.

Correct answer: Glucose

Glucose is a monosaccharide — a single sugar unit that cannot be further hydrolyzed into simpler sugars. Sucrose is a disaccharide (glucose + fructose), while starch and cellulose are polysaccharides composed of multiple glucose units.

Correct answer: Carbon count and functional group type

Monosaccharides are classified by the number of carbon atoms (triose, tetrose, pentose, hexose) and the type of carbonyl group (aldose for aldehyde, ketose for ketone). This classification reflects both their chain length and reactivity.

Correct answer: Fructose

Fructose is a ketose because its carbonyl group is a ketone located at carbon 2. Glucose, galactose, and ribose are aldoses — their carbonyl group is an aldehyde at carbon 1.

Correct answer: 1

In glucose, the C1 aldehyde carbon becomes the anomeric carbon when the open chain cyclizes into a ring. It is unique because it can adopt two configurations — alpha (axial OH) and beta (equatorial OH) — which differ in optical and biochemical properties.

Correct answer: Sucrose

Sucrose is a disaccharide formed by a glycosidic bond between glucose (C1) and fructose (C2), making it a non-reducing sugar. It is the most abundant transport sugar in plants and the common table sugar used in the diet.

Correct answer: Glycogen

Glycogen is the primary storage polysaccharide in animals, consisting of highly branched chains of α-glucose linked by α-1,4 and α-1,6 glycosidic bonds. It is stored mainly in liver (glycogen reservoir for blood glucose) and skeletal muscle cells (local fuel reserve).

Correct answer: Amylose and amylopectin

Starch consists of two glucose polymers: amylose, a largely linear chain linked by α-1,4 bonds, and amylopectin, a branched chain with additional α-1,6 branch points. The ratio of amylose to amylopectin varies by plant source.

Correct answer: The orientation of the glycosidic bonds

Both polymers are built entirely from glucose, a six-carbon sugar, and neither contains nitrogen. The difference is the anomeric configuration of the linkage: starch uses alpha-1,4 bonds (with alpha-1,6 at branch points) giving helical, digestible chains, while cellulose uses beta-1,4 bonds that force alternate glucose units to flip, producing straight ribbons that hydrogen-bond into fibres no human enzyme can cleave.

Correct answer: Cellulose

Cellulose forms long, rigid β-1,4-linked glucose chains that provide tensile strength to plant cell walls. It is the most abundant organic polymer on Earth. Glycogen and amylopectin are storage polysaccharides, while inulin is a storage fructan found in some plant roots.

Correct answer: Fungal cell walls and arthropod exoskeletons

Chitin is a structural polysaccharide made of N-acetylglucosamine units linked by β-1,4 bonds. It forms the cell walls of fungi and the hard exoskeletons of arthropods such as insects and crustaceans, providing rigidity and protection.

Correct answer: A free aldehyde or ketone group

Reducing sugars possess a free anomeric carbon that carries an aldehyde (aldoses) or can open to reveal a ketone (ketoses), enabling them to donate electrons in redox reactions. Common reducing sugars include glucose, fructose, maltose, and lactose. Sucrose is a notable non-reducing sugar because both anomeric carbons are locked in the glycosidic bond.

Correct answer: Sucrose

Sucrose is a non-reducing sugar because its glycosidic bond links the anomeric carbons of both glucose (C1) and fructose (C2), leaving no free reducing group. Glucose, fructose, and maltose all have free anomeric carbons and are therefore reducing sugars.

Correct answer: Protein and carbohydrate

Glycoproteins are proteins with one or more carbohydrate chains covalently attached, typically via N-linkage to asparagine or O-linkage to serine/threonine residues. They are abundant on cell surfaces and play key roles in cell recognition, signaling, and immune response.

Correct answer: Cellular membranes

Glycolipids are concentrated in cellular membranes, particularly the outer leaflet of the plasma membrane, where their carbohydrate chains face the extracellular space. They contribute to membrane stability, cell-cell recognition, and tissue formation.

Correct answer: Glycoproteins and glycolipids

Blood group antigens (ABO, Lewis, etc.) are specific carbohydrate chains displayed on glycoproteins and glycolipids embedded in the red blood cell membrane. The terminal sugar residues of these chains determine blood group specificity and are recognized by antibodies.

Correct answer: Two α-1,4-linked glucose units

Maltose is a disaccharide formed by an α-1,4 glycosidic bond between two glucose molecules. It is a reducing sugar produced during the hydrolysis of starch by amylases and is found in malted grains.

Correct answer: Glucose

Glucose is the primary and preferred energy substrate for most cells. It is catabolized via glycolysis to pyruvate, then fully oxidized through the citric acid cycle and oxidative phosphorylation, yielding up to 30-32 ATP per molecule.

Correct answer: Structural support and lubrication

Glycosaminoglycans (GAGs) are long, unbranched polysaccharide chains that attract water and form viscous gels, providing structural support and lubrication in connective tissues. Major examples include hyaluronic acid (joint fluid lubricant), chondroitin sulfate (cartilage), and heparin (anticoagulant).

Correct answer: They are long chains of monosaccharide units

Polysaccharides are high-molecular-weight carbohydrate polymers composed of many monosaccharide units joined by glycosidic bonds. They may be linear or branched and serve storage functions (glycogen, starch) or structural roles (cellulose, chitin). They are generally tasteless and may be soluble or insoluble depending on structure.

Correct answer: Glucose

Glucose is a monosaccharide and the simplest form of carbohydrate. It is a primary energy source for cells.

Correct answer: Sucrose

Sucrose is a non-reducing sugar because its anomeric carbons are involved in the glycosidic bond. It does not have a free aldehyde or ketone group.

Correct answer: Chitin

Chitin is a structural polysaccharide found in arthropod exoskeletons. It provides strength and protection.

Correct answer: Fructose

Fructose is a ketose sugar with a ketone functional group. Other common monosaccharides like glucose are aldoses.

Correct answer: Glucose

Glucose is the main energy source used in cellular respiration. It is metabolized to produce ATP.

Correct answer: Carbohydrates and proteins

Glycoproteins consist of proteins with attached carbohydrate chains. They play roles in cell recognition and signaling.

Correct answer: Glycoproteins and glycolipids

ABO blood group antigens are oligosaccharide chains attached to both glycoproteins and glycolipids on the red cell membrane; the immunodominant terminal sugars (for example N-acetylgalactosamine for the A antigen) determine specificity.

Correct answer: Ribose (a pentose sugar)

Ribose is a pentose sugar found in RNA. It forms part of the nucleotide structure.