Monosaccharides — Structure and Function

Carbohydrates are organic molecules composed of carbon, hydrogen and oxygen in the general formula (CH₂O)ₙ. They are classified by the number of sugar units they contain:

• Monosaccharides — single sugar units (n = 3 to 7)
• Disaccharides — two monosaccharides joined by a glycosidic bond
• Polysaccharides — many (hundreds to thousands) of monosaccharides joined together

This lesson covers OCR specification point 2.1.2 (b): the structure and function of monosaccharides, including glucose, fructose and ribose.

---

1. Classification of Carbohydrates

graph TD
  A[Carbohydrates] --> B[Monosaccharides<br/>Single sugar units]
  A --> C[Disaccharides<br/>2 monosaccharides]
  A --> D[Polysaccharides<br/>Many monosaccharides]
  B --> B1[Trioses — C3<br/>e.g. glyceraldehyde]
  B --> B2[Pentoses — C5<br/>e.g. ribose, deoxyribose]
  B --> B3[Hexoses — C6<br/>e.g. glucose, fructose, galactose]
  C --> C1[Maltose<br/>α-glucose + α-glucose]
  C --> C2[Sucrose<br/>α-glucose + fructose]
  C --> C3[Lactose<br/>β-galactose + α-glucose]
  D --> D1[Starch<br/>amylose + amylopectin]
  D --> D2[Glycogen]
  D --> D3[Cellulose]

Key Definition — Monosaccharide: The monomer unit from which larger carbohydrates are made. Soluble, sweet-tasting, reducing sugars with the general formula (CH₂O)ₙ.

---

2. Glucose — the Central Metabolic Sugar

Glucose (C₆H₁₂O₆) is a hexose monosaccharide and is the principal respiratory substrate in all living organisms. It has the following features:

• Molecular formula: C₆H₁₂O₆
• Soluble in water (many polar hydroxyl (–OH) groups form hydrogen bonds with water)
• A reducing sugar (its aldehyde group can reduce Cu²⁺ ions to Cu⁺ in the Benedict's test)
• Exists in solution predominantly in a ring form (cyclic), though it can interconvert with a linear chain form

2.1 Ring Structure of Glucose

When glucose dissolves in water, the C1 aldehyde group reacts with the C5 hydroxyl group to form a six-membered pyranose ring. During this cyclisation, C1 becomes an asymmetric (anomeric) carbon, producing two stereoisomers: α-glucose and β-glucose.

α-glucose (C1 –OH group points BELOW the ring, same side as C6 –CH₂OH is... actually OPPOSITE in this diagram)

          6 CH₂OH
             |
       5 C — O
      /       \
   4 C         C 1        <-- in α-glucose, OH on C1 is DOWN
     |         |
   OH         OH
      \       /
       3 C — C 2
          |   |
          OH  OH

β-glucose (C1 –OH group points UP, on the opposite side of the ring plane to α)

The only difference between α- and β-glucose is the orientation of the hydroxyl group on carbon 1 (the anomeric carbon):

Form	Position of C1 –OH
α-glucose	Below the plane of the ring (trans to C6 –CH₂OH)
β-glucose	Above the plane of the ring (cis to C6 –CH₂OH)

This tiny structural difference has enormous biological consequences:

• α-glucose polymerises to form starch and glycogen (energy storage)
• β-glucose polymerises to form cellulose (structural role in plant cell walls)

2.2 Numbering the Carbon Atoms

OCR candidates must know how carbons are numbered in a hexose ring. Start with the carbon to the right of the ring oxygen and number clockwise:

             6 CH₂OH
                |
          5 C — O
         /   |   \
      H       \   \
     4         \   1  <- anomeric carbon
      \       /
       \     /
        3 — 2

The glycosidic bonds between carbons are named by the carbons involved (e.g., 1,4-glycosidic bond between C1 of one glucose and C4 of another).

---

3. Other Important Monosaccharides

3.1 Fructose

Fructose is a hexose monosaccharide (C₆H₁₂O₆, same molecular formula as glucose, so they are structural isomers). Differences from glucose:

• Fructose forms a five-membered furanose ring (not six-membered like glucose).
• Contains a ketone group (C=O) at C2 rather than an aldehyde.
• The sweetest of all natural sugars — about 1.7 times sweeter than sucrose.
• Found in fruit, honey and as one of the two monomers in sucrose.
• Is a reducing sugar — gives a positive Benedict's test.

3.2 Galactose

Galactose is another C₆H₁₂O₆ hexose. It differs from glucose only in the orientation of the –OH on C4. It is a component of the disaccharide lactose (milk sugar) and of glycolipids in cell membranes. Galactose is a reducing sugar.

3.3 Ribose

Ribose is a pentose monosaccharide (C₅H₁₀O₅). Its ring contains four carbons and one oxygen (a furanose ring), with the fifth carbon (C5) projecting from the ring as a CH₂OH group.

Biological importance of ribose:

• Component of RNA (ribonucleic acid) — forms part of the sugar-phosphate backbone.
• Component of ATP (adenosine triphosphate) — the universal energy currency.
• Component of NAD, NADP and FAD — coenzymes in respiration and photosynthesis.

3.4 Deoxyribose

Deoxyribose (C₅H₁₀O₄) is ribose with the hydroxyl group on C2 replaced by a hydrogen atom (hence "deoxy"). It is the sugar in DNA (deoxyribonucleic acid). The absence of the 2′-OH makes DNA much more chemically stable than RNA, which is essential for a molecule that stores genetic information.

---