IMAT Biology · Biomolecules and Enzymes

Sugars, Glycosidic Bonds and Polysaccharides

Carbohydrates are built from simple sugars such as glucose; glycosidic bonds join them into disaccharides and into the polysaccharides starch, glycogen and cellulose.

All Biomolecules and Enzymes notes

Why this matters

The 2024 ministry paper asked two plain recall questions here: what kind of sugar glucose is, and which sugar is in RNA. The older papers asked which molecule is not a carbohydrate (2013, 2015) and which carbons of glucose form the bonds in starch (2012).

Concept 1 of 3: Monosaccharides: trioses, pentoses and hexoses

A monosaccharide is a single sugar unit, and the simplest way to name one is by its number of carbon atoms. Three carbons make a triose, five a pentose, six a hexose. Most have the formula

\mathrm{(CH_2O)}_n

, so once you know the carbon count you can write the formula.

Definition

Monosaccharides are the monomers of carbohydrates: sweet, soluble, formula usually $\mathrm{C}_n\mathrm{H}_{2n}\mathrm{O}_n$ .
Named by carbon number: triose (3 C), pentose (5 C), hexose (6 C).
Glucose is a hexose; ribose (in RNA and ATP) and deoxyribose (in DNA) are pentoses. Deoxyribose has one oxygen fewer than ribose.
Glucose, fructose and galactose share the formula $\mathrm{C_6H_{12}O_6}$ but differ in structure (they are isomers).
In its ring form glucose exists as α-glucose (the OH on carbon 1 points below the ring) and β-glucose (the OH on carbon 1 points above the ring). This small difference decides whether glucose builds starch or cellulose.

Sugar	Type	Formula	Where it is found or used
Glyceraldehyde	Triose	$\mathrm{C_3H_6O_3}$	Intermediate in respiration and photosynthesis
Ribose	Pentose	$\mathrm{C_5H_{10}O_5}$	RNA nucleotides, ATP, NAD
Deoxyribose	Pentose	$\mathrm{C_5H_{10}O_4}$	DNA nucleotides
Glucose	Hexose	$\mathrm{C_6H_{12}O_6}$	Main fuel for respiration; unit of starch, glycogen, cellulose
Fructose and galactose	Hexoses	$\mathrm{C_6H_{12}O_6}$	Fruit sugar; part of sucrose (fructose) and lactose (galactose)

Practice this conceptself-check · 4 quick reps

Glycerol and glucagon are not sugars

Glycerol is a three-carbon alcohol from lipids, and glucagon is a protein hormone that raises blood glucose. Neither is a carbohydrate, though both names sound like glucose. IMAT uses them as distractors in sugar questions.

Concept 2 of 3: The glycosidic bond and the three common disaccharides

Two monosaccharides join by condensation: an OH from each ring reacts, a water molecule leaves, and an oxygen bridge is left between the two rings. That bridge is the glycosidic bond. Which carbons it joins, and whether the sugar was α or β, is written in the bond's name.

Definition

A glycosidic bond joins two sugar units through an oxygen atom. It forms by condensation and is broken by hydrolysis.
α(1→4) glycosidic bond: from carbon 1 of an α-glucose to carbon 4 of the next unit.
Disaccharides have the formula $\mathrm{C_{12}H_{22}O_{11}}$ (two hexoses minus one water).
Reducing sugars (all monosaccharides, maltose and lactose) turn Benedict's reagent from blue to brick-red on heating. Sucrose is non-reducing: it gives a positive result only after it has been hydrolysed (by boiling with acid, then neutralising).

Disaccharide	Made from	Bond	Source
Maltose	α-glucose + α-glucose	α(1→4)	Digestion of starch by amylase; germinating seeds
Sucrose	α-glucose + β-fructose	α(1→2)	Transport sugar in plant phloem; table sugar
Lactose	β-galactose + glucose	β(1→4)	Milk sugar of mammals

Maltose and lactose are reducing sugars; sucrose is not.

Practice this conceptself-check · 3 quick reps

Sucrose is not a reducing sugar

Maltose and lactose turn Benedict's reagent brick-red, but sucrose does not, because the bond between glucose and fructose ties up both reactive groups. A negative Benedict's test therefore does not prove there is no sugar: sucrose may be present.

Concept 3 of 3: Polysaccharides: starch, glycogen and cellulose

Starch, glycogen and cellulose are all long chains of glucose, yet they do very different jobs. α-glucose chains coil into compact stores that enzymes can nibble from the ends. β-glucose units must flip alternately to bond, which makes straight chains that lie side by side and lock together with hydrogen bonds into tough fibres.

Definition

Starch (plant energy store) is a mixture of amylose (unbranched, α(1→4) bonds only, coils into a helix) and amylopectin (α(1→4) chains with α(1→6) branches).
Glycogen (animal and fungal energy store, in liver and muscle) is like amylopectin but more branched, so it has more ends where glucose can be released quickly.
Cellulose (plant cell walls) is made of β-glucose with β(1→4) bonds. Straight chains are cross-linked by hydrogen bonds into microfibrils of great strength. Humans have no enzyme to digest it.
A glucose unit in the middle of an unbranched chain uses its carbon 1 and carbon 4 for its two glycosidic bonds; a unit at a branch point also uses carbon 6.
Stores are insoluble and compact, so they do not draw water into the cell by osmosis. Iodine solution turns starch blue-black.

Polysaccharide	Monomer and bonds	Shape	Role
Amylose	α-glucose, α(1→4)	Unbranched helix	Compact plant store (part of starch)
Amylopectin	α-glucose, α(1→4) with α(1→6) branches	Branched	Plant store (part of starch)
Glycogen	α-glucose, α(1→4) with many α(1→6) branches	Highly branched	Animal store in liver and muscle
Cellulose	β-glucose, β(1→4)	Straight chains in microfibrils	Strength of plant cell walls
Chitin	Glucose with an N-containing group, β(1→4)	Straight chains	Insect exoskeleton, fungal walls

Practice this conceptself-check · 4 quick reps

Starch and cellulose differ in the glucose, not the sugar

Both are polymers of glucose. Starch uses α-glucose and cellulose uses β-glucose. An option saying cellulose is made of fructose, or of a different sugar from starch, is wrong.

Glycogen is the animal store, not starch

Animals and fungi store glucose as glycogen, mainly in liver and muscle. Plants store starch. Animals do not make starch.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Monosaccharides: trioses, pentoses and hexoses5 rows

Sugar	Type	Formula	Where it is found or used
Glyceraldehyde	Triose	$\mathrm{C_3H_6O_3}$	Intermediate in respiration and photosynthesis
Ribose	Pentose	$\mathrm{C_5H_{10}O_5}$	RNA nucleotides, ATP, NAD
Deoxyribose	Pentose	$\mathrm{C_5H_{10}O_4}$	DNA nucleotides
Glucose	Hexose	$\mathrm{C_6H_{12}O_6}$	Main fuel for respiration; unit of starch, glycogen, cellulose
Fructose and galactose	Hexoses	$\mathrm{C_6H_{12}O_6}$	Fruit sugar; part of sucrose (fructose) and lactose (galactose)

The glycosidic bond and the three common disaccharides3 rows

Disaccharide	Made from	Bond	Source
Maltose	α-glucose + α-glucose	α(1→4)	Digestion of starch by amylase; germinating seeds
Sucrose	α-glucose + β-fructose	α(1→2)	Transport sugar in plant phloem; table sugar
Lactose	β-galactose + glucose	β(1→4)	Milk sugar of mammals

Maltose and lactose are reducing sugars; sucrose is not.

Polysaccharides: starch, glycogen and cellulose5 rows

Polysaccharide	Monomer and bonds	Shape	Role
Amylose	α-glucose, α(1→4)	Unbranched helix	Compact plant store (part of starch)
Amylopectin	α-glucose, α(1→4) with α(1→6) branches	Branched	Plant store (part of starch)
Glycogen	α-glucose, α(1→4) with many α(1→6) branches	Highly branched	Animal store in liver and muscle
Cellulose	β-glucose, β(1→4)	Straight chains in microfibrils	Strength of plant cell walls
Chitin	Glucose with an N-containing group, β(1→4)	Straight chains	Insect exoskeleton, fungal walls

Watch out for (4)

Glycerol and glucagon are not sugars
→ Monosaccharides: trioses, pentoses and hexoses
Sucrose is not a reducing sugar
→ The glycosidic bond and the three common disaccharides
Starch and cellulose differ in the glucose, not the sugar
→ Polysaccharides: starch, glycogen and cellulose
Glycogen is the animal store, not starch
→ Polysaccharides: starch, glycogen and cellulose

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