Biomolecules • Topic 1 of 3

Carbohydrates

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. The general formula Cx(H2O)y once suggested they were 'hydrates of carbon', but deoxyribose (C5H10O4) and acetic acid (C2H4O2, not a sugar) show the name is only historical. Chemically most are simply called saccharides (Latin saccharum, sugar).

Classification by hydrolysis. Monosaccharides cannot be hydrolysed to simpler sugars (glucose, fructose, ribose). Oligosaccharides give 2–10 monosaccharide units on hydrolysis — disaccharides such as sucrose, maltose and lactose are the common type. Polysaccharides yield a large number of monosaccharide units (starch, cellulose, glycogen). Sugars that taste sweet and are crystalline are loosely called sugars; polysaccharides are non-sugars.

Aldoses and ketoses. A monosaccharide carrying an aldehyde (–CHO) group is an aldose (glucose, an aldohexose); one with a keto (C=O) group is a ketose (fructose, a ketohexose). The number of carbons gives the prefix — triose (3C), tetrose, pentose (ribose), hexose (glucose, fructose).

Reducing vs non-reducing sugars. Any sugar that reduces Tollens' reagent (silver mirror) or Fehling's/Benedict's solution (red Cu2O) is a reducing sugar; this needs a free aldehyde or keto group (a free anomeric –OH). All monosaccharides and the disaccharides maltose and lactose are reducing. Sucrose is non-reducing because both anomeric carbons are locked in the glycosidic bond.

Structure of glucose. Open-chain D-glucose is an aldohexose, CHO–(CHOH)4–CH2OH, drawn as a Fischer projection. It exists mainly as a six-membered pyranose ring formed when the C5–OH adds to the C1 aldehyde, creating a new chiral centre at C1 (the anomeric carbon). This gives α-D-glucose (C1–OH down) and β-D-glucose (C1–OH up); in water they interconvert through the open chain — mutarotation. The cyclic structure is shown as a Haworth projection.

Fructose is a ketohexose; its C2 keto group reacts with C5–OH to give a five-membered furanose ring. Although fructose has no free –CHO, it is still a reducing sugar because in basic Tollens'/Fehling's medium it tautomerises (via an enediol) to glucose and mannose.

D/L configuration. The D or L label refers to the configuration of the chiral carbon farthest from the –CHO group (C5 in glucose), compared with D- or L-glyceraldehyde — –OH on the right means D. It does not tell the direction of optical rotation. Most natural sugars are D.

Disaccharides. Sucrose = α-D-glucose + β-D-fructose joined C1→C2 (both anomeric carbons used) — non-reducing; its hydrolysis gives an equimolar 'invert sugar' (the rotation inverts from +66° to −20°). Maltose = two α-D-glucose units (C1→C4); reducing. Lactose (milk sugar) = β-D-galactose + glucose (C1→C4); reducing.

Polysaccharides. Starch (plant store) = amylose (linear α-1,4 glucose, helical, gives blue with iodine) + amylopectin (branched α-1,4 with α-1,6 branches). Glycogen (animal starch, stored in liver/muscle) is like amylopectin but more highly branched. Cellulose (plant cell wall) = linear β-1,4 glucose; the β-links make straight chains held by H-bonds, giving rigid fibres humans cannot digest. Importance: glucose is the body's chief fuel; starch/glycogen store energy; cellulose gives structure and dietary fibre; sugars also build nucleic acids and glycoproteins.

Haworth projection of alpha-D-glucopyranoseOC5C4C3C2C1OH(anomeric)OHOHOHCH2OHα-D-Glucopyranose
Solved Examples
1
Worked Example
Classify the following and state whether each is reducing or non-reducing: glucose, sucrose, maltose, cellulose.
Solution
  1. Glucose — a monosaccharide (aldohexose); has a free –CHO, so reducing.
  2. Sucrose — a disaccharide; both anomeric carbons are tied in the glycosidic bond, so non-reducing.
  3. Maltose — a disaccharide of two glucose units leaving one free anomeric –OH, so reducing.
  4. Cellulose — a polysaccharide (non-sugar), not classed as a reducing sugar.

Answer: Reducing: glucose, maltose. Non-reducing: sucrose. Cellulose is a polysaccharide.

2
Worked Example
Why is glucose called an aldohexose, and what does the prefix D in D-glucose indicate?
Solution
  1. Glucose has six carbon atoms (hex-) and an aldehyde group (–CHO), hence aldo + hexose.
  2. The D/L prefix is fixed by the configuration of the chiral carbon farthest from the –CHO, i.e. C5.
  3. In D-glucose the –OH on C5 lies on the right in the Fischer projection (matching D-glyceraldehyde).

Answer: Six carbons + aldehyde group make it an aldohexose; D refers to the C5 –OH being on the right, not to the sign of rotation.

3
Worked Example
What is mutarotation? Illustrate with the two anomers of glucose.
Solution
  1. When α-D-glucose (specific rotation +111°) is dissolved in water, its rotation slowly falls.
  2. Pure β-D-glucose (+19°) dissolved in water has its rotation slowly rise.
  3. Both reach the same equilibrium value (+52.5°) because each anomer opens to the open chain and recloses, interconverting α and β.

Answer: Mutarotation is the spontaneous change in optical rotation as α- and β-glucose equilibrate through the open-chain form to a +52.5° mixture.

4
Worked Example
Fructose has no free aldehyde group, yet it reduces Tollens' reagent. Explain.
Solution
  1. Tollens'/Fehling's reagents are basic.
  2. In base, the keto group of fructose tautomerises through an enediol intermediate.
  3. The enediol rearranges to give aldose forms (glucose and mannose), which have a free –CHO.
  4. This –CHO then reduces Ag(I) (silver mirror) / Cu(II).

Answer: In basic medium fructose isomerises via an enediol to glucose/mannose bearing a free –CHO, so it acts as a reducing sugar.

5
Worked Example
On hydrolysis sucrose changes the sign of optical rotation from +66.5° to −20°. Account for this 'inversion'.
Solution
  1. Sucrose (+66.5°) hydrolyses to an equimolar mixture of D-glucose and D-fructose.
  2. D-glucose is dextrorotatory (+52.5°); D-fructose is strongly laevorotatory (−92°).
  3. The fructose contribution dominates, so the mixture is net laevorotatory (about −20°).
  4. The sign of rotation has reversed, hence the product is called invert sugar.

Answer: The strongly laevorotatory fructose outweighs the dextrorotatory glucose, inverting the net rotation to −20°.

6
Worked Example
Contrast starch and cellulose in terms of glycosidic linkage and digestibility in humans.
Solution
  1. Starch is built from α-D-glucose joined by α-1,4 (and α-1,6 branch) linkages.
  2. Cellulose is built from β-D-glucose joined by β-1,4 linkages, giving straight, H-bonded fibres.
  3. Human digestive enzymes (amylase) hydrolyse α-links but not β-links.

Answer: Starch has α-1,4/α-1,6 links and is digestible; cellulose has β-1,4 links and is not digestible by humans (it acts as fibre).

Key Points

  • Carbohydrates are optically active polyhydroxy aldehydes/ketones or compounds that hydrolyse to them; classed as mono-, oligo- and polysaccharides.
  • Aldoses (glucose) carry –CHO; ketoses (fructose) carry C=O. Reducing sugars (all monosaccharides, maltose, lactose) reduce Tollens'/Fehling's; sucrose is non-reducing.
  • Glucose is an aldohexose existing mainly as the pyranose ring; the anomeric C1 gives α/β forms that interconvert by mutarotation. Fructose forms a furanose ring.
  • D/L is set by the configuration of the carbon farthest from the carbonyl (C5 in glucose) and does not give the sign of rotation.
  • Starch = amylose + amylopectin (α-links, digestible store); glycogen = animal store; cellulose = β-1,4 glucose (structural, indigestible fibre).
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Which of the following is a non-reducing sugar?
Explanation: In sucrose both anomeric carbons are involved in the glycosidic bond, leaving no free aldehyde/keto group, so it is non-reducing.
Q2.The anomeric carbon in α- and β-D-glucopyranose is:
Explanation: Ring closure makes C1 (the former aldehyde carbon) a new chiral centre, the anomeric carbon, giving α and β forms.
Q3.Glucose and fructose are best classified respectively as:
Explanation: Glucose has an aldehyde group (aldohexose); fructose has a keto group (ketohexose); both have six carbons.
Q4.Cellulose is not digested by humans because it contains:
Explanation: Cellulose has β-1,4 links, which human amylase cannot hydrolyse; only α-links (as in starch) are digestible.
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