Monosaccharides — Structure and Function

Spec mapping — OCR H420 Module 2.1.2 — Biological molecules. This lesson develops the structure of monosaccharides, with particular emphasis on α- and β-glucose stereochemistry as the architectural starting point for the contrast between storage (starch, glycogen) and structural (cellulose) polysaccharides. Pentoses (ribose, deoxyribose) and ketose hexoses (fructose) are also covered (refer to the official OCR H420 specification document for exact wording).

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.

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:

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.

4. Why Glucose is Ideal as a Respiratory Substrate

Glucose is the primary energy source for cells because:

Soluble — hydroxyl groups form hydrogen bonds with water, so glucose can be transported in aqueous body fluids (blood plasma, xylem and phloem sap).
Stable — its ring structure is resistant to spontaneous breakdown, allowing it to circulate safely.
High energy yield — complete oxidation yields 2870 kJ mol⁻¹ (about 30–32 ATP molecules).
Easily metabolised — glycolysis, the first stage of respiration, acts directly on glucose.
Can be polymerised for storage as starch (plants) or glycogen (animals), making storage compact and osmotically inactive.

5. Structural Isomerism and Stereoisomerism

Monosaccharides illustrate two important concepts that OCR candidates must distinguish:

Structural isomers: same molecular formula, different arrangement of atoms (e.g., glucose and fructose).
Stereoisomers: same molecular formula and same bonding, but different spatial arrangement (e.g., α-glucose and β-glucose; glucose and galactose).

Exam Tip: Examiners often ask candidates to state the type of isomerism between two named sugars. Memorise: glucose/fructose are structural isomers; α/β-glucose are stereoisomers; glucose/galactose are stereoisomers.

6. Properties of Monosaccharides

Monosaccharides share several characteristic properties that follow directly from their structure:

Sweet taste — all monosaccharides taste sweet, with fructose being the sweetest.
White crystalline solids at room temperature — the high number of hydroxyl groups makes them crystallise readily.
Highly soluble in water — every hydroxyl group (–OH) can form hydrogen bonds with water molecules. For glucose, this means five hydroxyl groups plus the ring oxygen all contribute to hydration.
Reducing sugars — they can reduce Cu²⁺ to Cu⁺ in the Benedict's test.
Low molecular mass — small enough (typically 150–180 Da) to be transported through membranes via specific carrier proteins.
Optically active — most monosaccharides rotate plane-polarised light because they contain chiral (asymmetric) carbon atoms.

7. Transport of Glucose Across Membranes

Because glucose is a polar molecule, it cannot diffuse freely through the hydrophobic phospholipid bilayer. Cells have evolved two main mechanisms for glucose uptake:

7.1 Facilitated Diffusion (GLUT Transporters)

GLUT (glucose transporter) proteins are integral membrane proteins that provide a hydrophilic channel for glucose. There are several GLUT isoforms (GLUT1 to GLUT14), each expressed in different tissues:

GLUT1 — red blood cells, blood-brain barrier
GLUT2 — liver, pancreatic β-cells, kidney (low affinity, responds to blood glucose)
GLUT3 — neurones (high affinity, ensures constant brain glucose supply)
GLUT4 — skeletal muscle and adipose tissue (insulin-responsive)

7.2 Secondary Active Transport (Co-transport)

In the small intestine and kidney tubules, glucose is absorbed against its concentration gradient by co-transport with Na⁺ via the SGLT1 (sodium-glucose linked transporter) protein. The energy is provided indirectly by the Na⁺ gradient established by the Na⁺/K⁺ ATPase pump.

8. Monosaccharides in Other Key Biological Molecules

Beyond their role as respiratory substrates, monosaccharides are components of many essential biological molecules:

Molecule	Monosaccharide component	Role
ATP	Ribose	Energy currency
NAD / NADP	Ribose	Electron carriers in respiration/photosynthesis
FAD	Ribitol (derivative of ribose)	Electron carrier
DNA	Deoxyribose	Genetic material
RNA	Ribose	Protein synthesis, regulation
Glycoproteins	Various hexoses	Cell recognition, signalling
Glycolipids	Various hexoses	Membrane markers
Sucrose	Glucose + fructose	Phloem transport sugar
Lactose	Glucose + galactose	Milk sugar

Exam Tips

Always draw α-glucose and β-glucose with the –OH on C1 in the correct orientation. Many OCR mark schemes award a mark specifically for this.
When describing ring formation, mention that C1 (aldehyde) reacts with C5 (–OH) to form the pyranose ring.
Name specific coenzymes containing ribose: ATP, NAD, NADP, FAD, RNA.
State that ribose and deoxyribose differ only in the presence/absence of an –OH on C2.
When asked why glucose is a good energy source, cover solubility, stability, energy yield, ease of metabolism and storability.

Common Exam Mistakes

Confusing "isomer" with "polymer".
Drawing the –OH on the wrong carbon when showing α vs β glucose.
Writing "glucose is a reducing sugar because it has an OH group" — it is reducing because of the free aldehyde (or potential aldehyde in the cyclic form).
Saying "fructose has a different formula from glucose" — they have the same molecular formula (C₆H₁₂O₆).
Calling ribose a "hexose" — ribose is a pentose (C₅).
Describing glucose as "diffusing freely" through membranes — it requires GLUT transporters (facilitated diffusion) or co-transport.

9. Chirality, Pasteur and the Aldose/Ketose Distinction

Louis Pasteur (1848) demonstrated that biological molecules are chiral — they exist as one enantiomer rather than as a 50:50 racemic mixture. The chirality of monosaccharides is no accident: biological enzymes, themselves chiral proteins built from L-amino acids, can only act on D-sugars. Almost every sugar you will meet in biochemistry is the D-enantiomer: D-glucose, D-fructose, D-ribose.

Monosaccharides are also classified as aldoses (containing an aldehyde group, –CHO, at C1) or ketoses (containing a ketone group, C=O, at C2):

Class	Carbonyl position	Examples
Aldose	–CHO at C1	Glucose, galactose, ribose, deoxyribose, glyceraldehyde
Ketose	C=O at C2	Fructose, ribulose, dihydroxyacetone

Both classes are reducing sugars in dilute alkali because of enediol tautomerisation (a ketose can rearrange transiently to expose an aldehyde-like reducing group). This is why both glucose and fructose give a positive Benedict's test.

10. Synoptic Links

This lesson connects across the OCR H420 specification:

Lesson 2: Monosaccharides — α-Glucose, β-Glucose, Ribose and Fructose