Q.Write the chemical formula of glucose sugar and draw its chemical structure. OR How do the prosthetic groups of enzymes differ from coenzymes?
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From Simple Sugars to Giant Chains
Imagine you have a box of identical Lego bricks. Each brick is a monosaccharide — a single sugar unit like glucose. By itself, a glucose molecule is small, sweet, and dissolves easily in water. But living things need to store energy for later, or build strong structural materials. You cannot do that with loose bricks; you need to snap them together into long chains.
That act of snapping — the chemical linkage that joins one sugar to the next — is the glycosidic bond.
The Intuition: A Dehydration "Handshake"
Every monosaccharide has hydroxyl (−OH) groups sticking off its carbon atoms. When two sugars come together, one sugar loses an −OH group and the other loses a hydrogen atom (−H) from its own −OH group. These two pieces combine to form a water molecule (H2O) that leaves. In their place, an oxygen bridge — a single oxygen atom — now connects the two sugar rings.
This is a dehydration reaction (also called condensation). You remove water to form the bond. To break the bond later, you add water back — that is hydrolysis.
So a glycosidic bond is simply a C–O–C link between two monosaccharide units, formed by the loss of water.
The Precise Statement
A glycosidic bond is a covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate. In the context of polysaccharides, it is specifically the bond between the anomeric carbon (the carbon that was part of the carbonyl group in the open-chain form) of one monosaccharide and a hydroxyl oxygen of another monosaccharide.
The bond is named by the numbers of the carbons it connects, plus a descriptor for the orientation of the anomeric carbon:
- α (alpha) — the −OH on the anomeric carbon points down (in the standard Haworth projection for D-sugars).
- β (beta) — the −OH on the anomeric carbon points up.
For example, a bond between carbon-1 of one glucose and carbon-4 of the next glucose, with the anomeric carbon in the alpha orientation, is written as an α–1,4–glycosidic bond.
Monosaccharide1–O–Monosaccharide2
The oxygen is the bridge. The bond is named: α/β – (carbon1 number) , (carbon2 number).
Polysaccharides: The Chains They Build
A polysaccharide is a long polymer of monosaccharides linked by glycosidic bonds. The properties of the polysaccharide — whether it is digestible, whether it stores energy or provides structure, whether it branches or stays straight — depend entirely on which glycosidic bonds are used.
Starch (Energy Storage in Plants)
Starch is a mixture of two polymers, both made entirely of glucose:
- Amylose: Long, unbranched chains with α–1,4 bonds. The chain coils into a helix, which packs tightly.
- Amylopectin: Branched chains. The main chain uses α–1,4 bonds, but every 24–30 residues a branch is attached via an α–1,6 bond.
The α–1,4 bond makes the chain flexible and helical. The α–1,6 bond creates a branch point — like a side street off a main road.
Humans have enzymes that break α–1,4 and α–1,6 bonds, so we can digest starch.
Glycogen (Energy Storage in Animals)
Glycogen is like amylopectin but much more branched. It uses the same α–1,4 and α–1,6 bonds, but branches occur every 8–12 residues. This high branching allows rapid release of glucose when energy is needed — many chain ends mean many sites for enzymes to work simultaneously.
Cellulose (Structural Support in Plants)
Cellulose is also a polymer of glucose, but the bond is β–1,4. This small change has enormous consequences. The β linkage forces each glucose to flip 180° relative to its neighbour, producing a long, flat, ribbon-like chain. These chains lie parallel to each other and form hydrogen bonds between adjacent chains, creating strong, crystalline fibres.
Humans cannot digest cellulose. We lack the enzyme (cellulase) that breaks β–1,4 bonds. Cows and termites can, because they host microbes that produce cellulase.
Inulin (Energy Storage in Some Plants) …
Glucose is a six-carbon monosaccharide sugar with the molecular formula C6H12O6, which can exist as an open (straight) chain with an aldehyde group and five hydroxyl groups, or in a cyclic (ring) pyranose form. …
Glucose (C6H12O6) is an aldohexose whose open-chain structure has an aldehyde group at C1 and hydroxyl groups at C2-C5, and a -CH2OH at C6; it usually cyclizes into a six-membered pyranose ring in solution.
Glucose is the most common monosaccharide and has the molecular formula C6H12O6 — six carbons, twelve hydrogens, and six oxygens.
Open-chain structure (numbering carbons 1 to 6):
C1: -CHO (aldehyde group, making it an aldose)
C2: -CHOH
C3: -CHOH
C4: -CHOH
C5: -CHOH
C6: -CH2OH
So the straight-chain structure is written as: CHO-CHOH-CHOH-CHOH-CHOH-CH2OH.
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- CBSE 2025Set ANNUAL1 markMCQQ.Inulin is a polymer of :(a) Glucose(b) Fructose(c) Galactose(d) None of these
›Reveal solutionSolution
Inulin is a polysaccharide made entirely of fructose units — a "fructan" — used by some plants as a storage carbohydrate in place of starch.
Polysaccharides are polymers of monosaccharides joined by glycosidic bonds. While the commonest storage polysaccharide, starch, is a polymer of glucose, some plants (notably members of the Compositae/Asteraceae family, e.g. dahlia, Jerusalem artichoke) store their reserve carbohydrate as inulin, a fructan built from chains of fructose molecules (with a terminal glucose).
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- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following saccharides makes exoskeleton of members of arthropoda?(a) Chitin(b) Cellulose(c) Glycogen(d) Starch
›Reveal solutionSolution
Chitin, a structural polysaccharide, forms the exoskeleton of arthropods.
Among biomolecules, polysaccharides serve both storage and structural roles. Cellulose forms the cell wall of plants; starch and glycogen are storage polysaccharides in plants and animals respectively. Chitin is a nitrogenous polysaccharide made of repeating units of N-acetylglucosamine. It is secreted by the epidermis of arthropods (e.g …
- CBSE 2025Set ANN1 markQ.Inulin is a polymer of ______ .
›Reveal solutionSolution
Inulin is a polymer of fructose.
Inulin is a polysaccharide (storage carbohydrate found in the roots/tubers of some plants such as dahlia). On hydrolysis it yields molecules of the monosaccharide fructose. Hence inulin is a polymer of fructose (a fructan). It is often used clinically to estimat …
- CBSE 2024Set ANNUAL1 markMCQQ.Glycosidic bond occurs in(a) fats(b) proteins(c) carbohydrates(d) amino acids
›Reveal solutionSolution
The glycosidic bond joins two sugar units via a condensation (dehydration) reaction and is the linkage that builds di- and polysaccharides.
A glycosidic bond forms when two monosaccharide molecules are joined together through a condensation reaction, in which a molecule of water is removed and an oxygen bridge (–O–) links the anomeric carbon of one sugar to a hydroxyl group of the other. This bond is the basic linkage in carbohydrates, connecting monosaccharides into disaccharides (e.g., the glycosidic bond between glucose and fructose in sucrose) and further into larg …
- CBSE 2024Set ANNUAL1 markMCQQ.Which structure of the starch molecule forms blue colour with I2?(a)(a) Helical(b)(b) Branched(c)(c) Linear(d)(d) Spherical
›Reveal solutionSolution
[!TLDR]
(a) Helical
Why
The helical coiled structure of amylose (a component of starch) traps iodine molecules inside its helix, producing …
- CBSE 2023Set ANNUAL1 markQ.What are the carbohydrates found in milk called?
›Reveal solutionSolution
Milk's principal carbohydrate is lactose, a disaccharide of glucose and galactose joined by a glycosidic bond.
Carbohydrates are classified as monosaccharides, disaccharides, or polysaccharides based on the number of sugar units. Lactose is a disaccharide — on hydrolysis, it breaks down into one molecule of glucose and one molecule of galactose. It is the characteristic sugar of mammalian milk (hence "milk sugar") …
- CBSE 2021Set ANNUAL1 markMCQQ.Glycogen is a polymer of:(a) Glucose(b) Fructose(c) Galactose(d) Amino acid
›Reveal solutionSolution
Glycogen is a branched storage polysaccharide made entirely of repeating glucose units.
Glycogen is the principal storage polysaccharide in animals and fungi (analogous to starch in plants). It is a highly branched polymer built from thousands of glucose monomers joined by α-1,4-glycosidic bonds along the chain, with α-1,6-glycosidic bonds at branch points. It is …
- CBSE 2020Set ANNUAL1 markMCQQ.The simplest form of carbohydrate is :(a) Amino acid(b) Fatty acid(c) Nucleic acid(d) Glucose
›Reveal solutionSolution
Glucose is a monosaccharide, the simplest form of carbohydrate, and serves as the basic building block for larger carbohydrates like starch, cellulose, and glycogen.
Carbohydrates are classified by complexity into monosaccharides (simplest, single sugar unit, e.g. glucose, fructose), oligosaccharides (a few linked units, e.g. sucrose), and polysaccharides (many linked units, e.g. starch, cellulose). Glucose, being a single 6-carbon sugar unit that cannot be broken down further by hydrolysis into smaller carbohydrate units, is the simplest carbohy …
- CBSE 2020Set ANNUAL1 markMCQQ.Inulin is a polymer of :(a) Glucose(b) Fructose(c) Galactose(d) None of these
›Reveal solutionSolution
Inulin is a polysaccharide made up of repeating fructose units, not glucose.
Polysaccharides are long chains of monosaccharides joined by glycosidic bonds. Most familiar storage polysaccharides — starch (in plants) and glycogen (in animals) — are polymers of glucose. Inulin, however, is an exception: it is a fructan, a polymer built from fructose molecules, and serves as a storage carbohydrate in the roots/tubers of many plants such as dahlia, chicory, and Jerusalem artichoke. Because humans lack the enzyme to hyd …
- CBSE 2020Set ANNUAL1 markQ.The individual monosaccharides are linked by which bond in a polysaccharide?
›Reveal solutionSolution
Monosaccharides in a polysaccharide are joined together by glycosidic bonds.
A polysaccharide (e.g., starch, cellulose, glycogen) is a long chain, often branched, made up of hundreds to thousands of monosaccharide units. Each unit is linked to the next through a glycosidic bond, which forms when the hydroxyl (-OH) group of one sugar's anomeric carbon reacts with a hydroxyl group of another sugar molecule, releasing a water molecule (a condensation/dehydration synthesis reaction). This is analogous to how amino acids are linked by peptide bonds in proteins. The specific type of glycosidic linkage (e.g., alpha-1,4 in starch …
- CBSE 2020Set ANNUAL1 markQ.Fill in the blank: Cellulose is a __________ biomolecule. (choose: Polysaccharide / disaccharide)
›Reveal solutionSolution
Cellulose is a polysaccharide, not a disaccharide.
Polysaccharides are polymers made of many (poly-) monosaccharide units linked by glycosidic bonds; they are typically insoluble, non-sweet, and function in structure (cellulose) or storage (starch, glycogen). Cellulose consists of unbranched chains of beta-D-glucose units linked by beta-1,4-glycosidic bonds, which pack into strong microfibrils forming the plant cell wall. A disacc …
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