describe the formation of a glycosidic bond by condensation, with reference to disaccharides, including sucrose, and polysaccharides

Cambridge A‑Level Biology 9700 – Biological Molecules

1. Glossary of Key Terms

• Monomer – Smallest unit that can join to form a polymer (e.g., a monosaccharide, amino‑acid, fatty‑acid).
• Polymer – Large macromolecule formed by repeated linkage of monomers (e.g., starch, protein, triglyceride).
• Macromolecule – A high‑molecular‑weight compound such as a carbohydrate, protein or lipid.
• Monosaccharide – Simplest carbohydrate unit (e.g., glucose, fructose).
• Disaccharide – Two monosaccharides linked by a glycosidic bond.
• Polysaccharide – Polymer of many monosaccharide units.
• Reducing sugar – Sugar that possesses a free anomeric carbon capable of acting as a reducing agent (e.g., glucose, maltose).
• Non‑reducing sugar – Sugar in which all anomeric carbons are involved in glycosidic bonds, preventing reduction (e.g., sucrose).
• Glycosidic bond – Covalent linkage formed between the –OH of one carbohydrate and the anomeric carbon of another; created by a condensation (dehydration) reaction.
• Hydrolysis – The reverse of condensation; water is added to break a covalent bond.
• Peptide bond – Covalent –CO–NH– linkage formed between the carboxyl group of one amino‑acid and the amino group of the next.
• Primary, secondary, tertiary, quaternary structure – Hierarchical levels of protein organisation.
• Hydrogen bond – Weak attraction between a hydrogen atom covalently bound to an electronegative atom (N, O, F) and another electronegative atom.
• Triglyceride (triacylglycerol) – Ester of glycerol with three fatty‑acid chains.
• Phospholipid – Glycerol esterified to two fatty acids and a phosphate‑containing head group; amphipathic.
• Steroid – Molecule built from four fused carbon rings (e.g., cholesterol).
• Water (H₂O) – Polar molecule that forms extensive hydrogen‑bond networks; high specific heat, high heat of vaporisation, excellent solvent.

2. Laboratory Tests for Biological Molecules (LO 2.1)

3. Carbohydrates – Structure, Formation of Glycosidic Bonds and Function (LO 2.2)

3.1. Formation of a Glycosidic Bond (Condensation)

The –OH of one monosaccharide attacks the anomeric carbon (C‑1 for aldoses, C‑2 for fructose) of another. A water molecule is eliminated, giving a covalent C–O–C linkage.

General condensation equation:

\$\text{Monosaccharide}1\;+\;\text{Monosaccharide}2 \;\xrightarrow{\text{condensation}}\; \text{Disaccharide} \;+\; H_2O\$

The relative orientation of the reacting –OH groups determines the bond type:

• α‑glycosidic bond – OH on the anomeric carbon is on the opposite side to the CH₂OH group (below the plane in the Haworth projection).
• β‑glycosidic bond – OH on the anomeric carbon is on the same side as the CH₂OH group (above the plane).

3.2. Hydrolysis of Glycosidic Bonds

Enzymes (e.g., sucrase, lactase, maltase) add a water molecule, cleaving the bond and regenerating the original monosaccharides.

\$\text{Disaccharide} \;+\; H2O \;\xrightarrow{\text{hydrolysis (enzyme)}}\; \text{Monosaccharide}1 \;+\; \text{Monosaccharide}_2\$

3.3. Reducing vs. Non‑reducing Sugars

• Reducing sugar: retains a free anomeric carbon that can open to an aldehyde/ketone form (e.g., glucose, maltose, lactose).
• Non‑reducing sugar: both anomeric carbons are locked in glycosidic bonds, preventing the open‑chain form (e.g., sucrose).

3.4. Disaccharides – Examples, Linkage Types and Reducing Status

3.5. Detailed Formation of Sucrose

Sucrose is produced when the α‑anomeric carbon (C1) of glucose reacts with the β‑anomeric carbon (C2) of fructose. Both anomeric centres are consumed, so the molecule lacks a free reducing end.

\$\text{Glucose}{\alpha\text{-C1}} \;+\; \text{Fructose}{\beta\text{-C2}} \;\xrightarrow{\text{condensation}}\; \text{Sucrose}{\alpha\text{-C1}\rightarrow\beta\text{-C2}} \;+\; H2O\$

3.6. Polymerisation to Form Polysaccharides

Repeated condensation of glucose units (or other monosaccharides) builds polysaccharides. For a polymer of n glucose residues:

\$n\;\text{Glucose} \;\xrightarrow{\text{condensation}}\; (\text{Glucose})n \;+\; (n-1)H2O\$

3.7. Major Polysaccharides and Their Glycosidic Linkages

• Starch (plant storage)

  • Amylose: linear α‑1,4‑linked glucose → helical, relatively compact.
  • Amylopectin: α‑1,4 backbone with α‑1,6 branch points (≈5 % of residues) → highly soluble, branched.

• Glycogen (animal storage)

  • Predominantly α‑1,4‑linked glucose with α‑1,6 branches every 8–12 residues → extremely branched, rapid mobilisation.

• Cellulose (structural)

  • β‑1,4‑linked glucose → straight chains that align side‑by‑side, forming extensive intermolecular hydrogen bonds → rigid, insoluble fibres.

3.8. Linking Structure to Function

4. Lipids – Structure, Bonding and Biological Significance (LO 2.2)

4.1. Main Classes of Lipids

• Triglycerides (triacylglycerols)

  • Glycerol esterified to three fatty‑acid chains.
  • Saturation: Saturated fatty acids contain no C=C bonds → straight chains → pack tightly → higher melting point. Unsaturated fatty acids contain one or more C=C bonds → kinks → lower melting point.
  • Primary long‑term energy store (≈9 kcal g⁻¹).

• Phospholipids

  • Glycerol esterified to two fatty acids and a phosphate‑containing head group (e.g., choline, serine).
  • Amphipathic: hydrophilic head + hydrophobic tails.
  • Self‑assemble into bilayers that form biological membranes.

• Steroids (e.g., cholesterol)

  • Four fused carbon rings; largely non‑polar.
  • Insert between phospholipid tails, modulating membrane fluidity.
  • Precursor for steroid hormones (e.g., cortisol, estrogen).

4.2. Ester Bond Formation (Condensation)

Each fatty‑acid chain is linked to glycerol via an ester bond (–CO–O–). The reaction is a dehydration synthesis:

\$\text{Fatty acid} \;+\; \text{Glycerol‑OH} \;\xrightarrow{\text{condensation}}\; \text{Ester (fatty‑acid‑glycerol)} \;+\; H_2O\$

Three such condensations give a triglyceride; two give a phospholipid (the third OH is esterified to a phosphate group).

4.3. Functional Relevance of Lipid Structure

• Energy storage – Triglycerides are densely packed, water‑insoluble, providing long‑term energy without adding bulk.
• Membrane structure – Phospholipid bilayers create selective barriers; cholesterol intercalates to prevent excessive rigidity (at low temperature) or excess fluidity (at high temperature).
• Insulation & protection – Subcutaneous fat reduces heat loss and cushions organs.
• Signalling – Steroid hormones derived from cholesterol act as powerful regulators of metabolism, development and reproduction.

5. Proteins – Structure, Formation and Function (LO 3.1)

5.1. Amino‑Acid Structure

General formula:

\$\text{NH}_2\!-\!\text{CH}(R)\!-\!\text{COOH}\$

• R group – side chain that determines the character of the amino‑acid (non‑polar, polar uncharged, acidic, basic).
• At physiological pH the amino group is protonated (–NH₃⁺) and the carboxyl group is deprotonated (–COO⁻), giving a zwitterion.

5.2. Peptide Bond Formation (Condensation)

The –COOH of one amino‑acid reacts with the –NH₂ of the next, releasing water:

\$\text{Amino‑acid}1\;-\;\text{COOH} \;+\; \text{NH}2\!-\;\text{Amino‑acid}2 \;\xrightarrow{\text{condensation}}\; \text{Amino‑acid}1\!-\!\text{CO–NH}\!-\;\text{Amino‑acid}2 \;+\; H2O\$

The resulting –CO–NH– linkage is planar and rigid, giving the peptide backbone its characteristic shape.

5.3. Levels of Protein Structure

1. Primary structure – Linear sequence of amino‑acids; determines all higher‑order structure.
2. Secondary structure – Regular folding of the backbone into α‑helices (stabilised by i→i+4 hydrogen bonds) or β‑pleated sheets (inter‑strand hydrogen bonds).
3. Tertiary structure – Three‑dimensional shape of a single polypeptide, stabilised by:

   • Hydrogen bonds
   • Hydrophobic interactions
   • Ionic (salt‑bridge) interactions
   • Disulfide bridges (–S–S–) between cysteine residues

4. Quaternary structure – Association of two or more polypeptide subunits into a functional protein complex.

5.4. Types of Protein Based on Shape and Function

• Globular proteins – Compact, roughly spherical; soluble in water; perform catalytic (enzymes), transport (haemoglobin), regulatory and defensive roles.
• Fibrous proteins – Long, filamentous; insoluble; provide structural support (collagen, keratin) or protective functions.

5.5. Required Examples

5.6. Inter‑Molecular Forces in Proteins

• Hydrophobic interactions drive folding of non‑polar side chains into the interior.
• Hydrogen bonds stabilise secondary structures and contribute to tertiary/quaternary packing.
• Ionic bonds (salt bridges) form between oppositely charged side chains.
• Disulfide bridges provide covalent cross‑linking, especially in extracellular proteins.

6. Water – Properties Relevant to Biology (LO 2.4)

• Polarity & hydrogen bonding – Each water molecule can form up to four hydrogen bonds, creating a cohesive network.
• High specific heat – Large amount of energy required to raise temperature (4.18 J g⁻¹ °C⁻¹); buffers temperature changes in organisms and environments.
• High heat of vaporisation – Energy absorbed during evaporation (≈2260 J g⁻¹) enables evaporative cooling (e.g., sweating).
• Universal solvent – Polar nature dissolves ionic compounds and most polar molecules, facilitating transport of nutrients, waste and biochemical reactions.
• Density anomaly – Ice is less dense than liquid water because hydrogen‑bonded lattice expands; ice floats, insulating aquatic life in winter.

7. Summary of Key Points

1. Carbohydrates are built from monosaccharide monomers; glycosidic bonds are formed by condensation (dehydration) and broken by hydrolysis.
2. The α/β orientation of glycosidic bonds dictates the physical properties of polysaccharides: α‑linkages give flexible, digestible storage polymers (starch, glycogen); β‑linkages produce rigid, insoluble structural polymers (cellulose).
3. Sucrose is a non‑reducing disaccharide formed by an α‑1,2 bond between glucose and fructose; it is hydrolysed by sucrase.
4. Triglycerides store energy densely; phospholipids form bilayers essential for membranes; cholesterol modulates membrane fluidity; steroids act as hormones.
5. Proteins are polymers of amino‑acids linked by peptide bonds; their function depends on four levels of structure and on intermolecular forces. Haemoglobin (globular) transports O₂; collagen (fibrous) provides tensile strength.
6. Water’s polarity, hydrogen‑bonding, high specific heat, high heat of vaporisation and solvent ability underpin virtually all biological processes.
7. Standard laboratory tests (Benedict’s/Fehling’s, iodine, emulsion, Biuret) enable rapid identification of carbohydrates, lipids and proteins in samples.