describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms

Polysaccharides and Lipids – Cambridge IGCSE/A‑Level Biology (Topic 2: Biological Molecules)

Learning Objectives

• Define key terminology (monomer, polymer, condensation, hydrolysis, reducing/non‑reducing sugar, etc.).
• Describe the molecular structures of the polysaccharides starch (amylose & amylopectin), glycogen and cellulose.
• Explain how structural features determine the biological functions of each polysaccharide.
• Identify the main classes of lipids (triglycerides, phospholipids, sterols, waxes) and relate their structures to membrane properties and energy storage.
• Carry out and evaluate practical tests for carbohydrates and lipids, including experimental design, data handling and sources of error.
• Write and balance the condensation and hydrolysis reactions that form and break glycosidic bonds.

1. AO1 – Core Definitions (Factual Knowledge)

• Monomer: the smallest unit that can join with others to form a polymer (e.g. D‑glucose).
• Polymer: a macromolecule composed of many monomers linked by covalent bonds (e.g. starch, glycogen, cellulose).
• Condensation (dehydration) reaction: formation of a covalent bond with loss of one H₂O molecule; in carbohydrates this creates a glycosidic bond.
• Hydrolysis: the reverse of condensation; H₂O adds across a bond, cleaving the polymer into smaller units.
• Reducing sugar: possesses a free hemi‑acetal (or hemi‑ketal) group that can reduce Cu²⁺ in Benedict’s solution (e.g. glucose, maltose).
• Non‑reducing sugar: both anomeric carbons are involved in glycosidic bonds, so no free hemi‑acetal is available (e.g. sucrose, the internal residues of a polysaccharide).
• Water (H₂O) – essential biological solvent:

  • High specific heat (cₚ) → stabilises temperature.
  • High heat of vaporisation → cooling by evaporation.
  • Excellent hydrogen‑bonding ability → dissolves polar molecules (sugars, amino acids).
  • Cohesion & surface tension → capillary rise in plants.

2. Ring Forms of D‑Glucose

D‑glucose cyclises in aqueous solution to give two pyranose stereoisomers:

• α‑D‑glucose: OH on C‑1 is trans to the CH₂OH group (down in Haworth projection).
• β‑D‑glucose: OH on C‑1 is cis to the CH₂OH group (up in Haworth projection).

These configurations dictate whether an α‑ or β‑glycosidic linkage is formed.

3. Formation & Hydrolysis of Glycosidic Bonds

Condensation (polymerisation)

\$\text{Glucose‑OH} + \text{Glucose‑OH} \xrightarrow{\text{enzyme}} \text{Glucose‑O‑Glucose} + \text{H}_2\text{O}\$

Linkage type (α‑1,4; α‑1,6; β‑1,4, …) depends on the orientation of the reacting –OH groups.

Hydrolysis

\$\text{Glucose‑O‑Glucose} + \text{H}_2\text{O} \xrightarrow{\text{hydrolase}} \text{Glucose‑OH} + \text{Glucose‑OH}\$

4. Practical Tests for Carbohydrates (AO2)

Designing a Carbohydrate‑Testing Experiment

1. Objective: Determine whether a food sample contains starch.
2. Materials: Test tubes, water bath, iodine solution, distilled water, sample (e.g., boiled potato), positive control (pure starch), negative control (water).
3. Procedure (outline):

   1. Prepare 3 test tubes: sample, positive control, negative control.
   2. Add 2 mL distilled water to each tube; stir to dissolve.
   3. Add 2–3 drops of iodine solution.
   4. Observe colour change and record.

4. Replication: Perform each test in triplicate to assess reproducibility.
5. Evaluation – possible sources of error:

   • Insufficient dissolution of sample → false negative.
   • Contamination between tubes → false positive.
   • Over‑heating or prolonged exposure to iodine → colour fading.
   • Subjective colour interpretation – mitigate by using a colour chart or spectrophotometer (absorbance at 620 nm).

6. Data handling: Convert colour intensity (or absorbance) into starch concentration using a calibration curve prepared from known starch standards.

5. Polysaccharides – Structure, Branching & Function

5.1 Starch (Plant Storage)

• Amylose

  • Linear chain of D‑glucose linked by α‑1,4‑glycosidic bonds.
  • ≈200–2 000 residues; adopts a left‑handed helix (≈6 residues per turn).
  • Relatively insoluble; forms a firm gel on heating with water (gelatinisation).

• Amylopectin

  • Branched polymer: α‑1,4 backbone with α‑1,6 branch points.
  • Branch every 24–30 glucose units (average).
  • Highly water‑soluble; gives starch granules a fluffy, porous appearance.

• Overall starch granule – mixture of ~20 % amylose and ~80 % amylopectin; the proportion influences texture (e.g., high‑amylose rice is firmer).

5.2 Glycogen (Animal Storage)

• Structurally analogous to amylopectin but far more highly branched.
• α‑1,4‑linked glucose backbone; α‑1,6 branches every 8–12 residues.
• Average molecular mass ≈10⁸ Da (≈10 000 glucose units); compact, extremely soluble.
• Stored mainly in liver (blood‑glucose regulation) and skeletal muscle (immediate ATP supply).

5.3 Cellulose (Plant Structural Polysaccharide)

• Linear polymer of β‑D‑glucose linked by β‑1,4‑glycosidic bonds.
• Each glucose is rotated 180° relative to its neighbours, allowing extensive inter‑chain H‑bonding.
• Forms rigid, insoluble microfibrils that give plant cell walls their tensile strength.
• Humans lack β‑glucosidase → cellulose passes unchanged through the gut (dietary fibre).

5.4 Summary Table – Structure ↔ Function

6. Enzymatic Degradation (AO2)

• α‑Amylase – endo‑enzyme; hydrolyses internal α‑1,4 bonds in amylose, amylopectin and glycogen → maltose, maltotriose, α‑limit dextrins.
• Glycogen phosphorylase – removes glucose units from the non‑reducing ends of glycogen, cleaving α‑1,4 bonds and releasing glucose‑1‑phosphate.
• Debranching enzyme (α‑1,6‑glucosidase) – hydrolyses α‑1,6 branch points, converting limit dextrins to free glucose.
• Cellulase (microbial) – hydrolyses β‑1,4 bonds in cellulose to glucose; absent in humans.

Reaction Schemes (simplified)

α‑Amylase:

\$\text{(Glc)}n \xrightarrow{\alpha\text{-amylase}} \text{(Glc)}{n-4} + \text{Maltotriose}\$

Glycogen phosphorylase (phosphorolysis):

\$\text{(Glc)}n + \text{P}i \xrightarrow{\text{GP}} \text{(Glc)}_{n-1} + \text{Glucose‑1‑P}\$

Debranching enzyme:

\$\text{α‑1,6‑branch point} \xrightarrow{\text{debranching}} \text{Glucose}\$

7. Biological Context

• Plants

  • Starch stored in amyloplasts (chloroplast‑derived organelles) as granules.
  • Amylose provides granule rigidity; amylopectin creates a large surface area for amylase during germination.

• Animals

  • Liver glycogen maintains blood glucose between meals (via glucose‑6‑phosphatase).
  • Muscle glycogen supplies ATP for contraction; cannot release free glucose into blood because muscle lacks glucose‑6‑phosphatase.

• Humans

  • Cellulose passes unchanged through the digestive tract, providing bulk and aiding intestinal health (dietary fibre).

8. Lipids – Structures, Properties & Functions (AO1 & AO2)

8.1 Triglycerides (Triacylglycerols)

• Glycerol backbone (three –OH groups) esterified with three fatty acids.
• Fatty acids may be:

  • Saturated – no C=C double bonds; straight chains → higher melting point.
  • Unsaturated – one or more C=C double bonds; kinks → lower melting point.

• Functions: dense energy storage (≈9 kcal g⁻¹), thermal insulation and protection of vital organs.

8.2 Phospholipids

• Glycerol backbone esterified with two fatty acids and a phosphate‑containing head‑group (e.g., choline, ethanolamine).
• Amphipathic: hydrophobic tails + hydrophilic head.
• Spontaneously form bilayers in aqueous environments – the basic structure of all biological membranes.
• Membrane fluidity: increased by:

  • Shorter fatty‑acid chains.
  • More double bonds (unsaturation).
  • Presence of cholesterol (in animal cells) which prevents tight packing at low temperature and restricts movement at high temperature.

• Permeability: phospholipid bilayers are permeable to small non‑polar molecules (O₂, CO₂) but impermeable to ions and polar solutes; transport proteins are required for the latter.

8.3 Other Lipid Classes (brief)

• Sterols (e.g., cholesterol) – rigid ring structure; modulates membrane fluidity and is precursor for steroid hormones.
• Waxes – long‑chain fatty acids esterified to long‑chain alcohols; highly hydrophobic, providing waterproofing (e.g., plant cuticle, animal fur).

8.4 Practical Tests for Lipids (AO2)

Designing a Lipid‑Detection Experiment

1. Objective: Determine whether a food sample contains triglycerides.
2. Materials: Test tubes, distilled water, ethanol, Sudan III solution, sample (e.g., butter), positive control (pure oil), negative control (sugar solution).
3. Procedure (outline):

   1. Add 1 mL of each sample to separate tubes.
   2. Add 2 mL water; shake – observe whether a layer forms.
   3. Add 2 mL ethanol; shake – lipid should dissolve.
   4. Add a few drops of Sudan III; look for red‑orange staining.

4. Controls & Replication: Include positive and negative controls; perform each test in triplicate.
5. Possible errors: Incomplete mixing, contamination between tubes, fading of Sudan III colour over time – mitigate by standardising timing and using fresh reagents.

9. Key Equations (LaTeX)

General condensation for a disaccharide:

\$\text{C}6\text{H}{12}\text{O}6 + \text{C}6\text{H}{12}\text{O}6 \xrightarrow{\text{condensation}} \text{C}{12}\text{H}{22}\text{O}{11} + \text{H}2\text{O}\$

Polymer of degree of polymerisation =n:

\$\bigl(\text{C}6\text{H}{12}\text{O}6\bigr)n \xrightarrow{\text{condensation}} \bigl(\text{C}6\text{H}{10}\text{O}5\bigr)n + n\,\text{H}_2\text{O}\$

Rate of glycogen synthesis (simplified kinetic expression):

\$\frac{d[\text{Glycogen}]}{dt}=V{\text{GS}}-V{\text{GP}}\$

where \$V{\text{GS}}\$ = activity of glycogen synthase, \$V{\text{GP}}\$ = activity of glycogen phosphorylase.

Michaelis–Menten equation for an amylase‑catalysed reaction (useful for data analysis):

\$v=\frac{V{\max}[S]}{Km+[S]}\$

10. Quick‑Check Revision Questions

1. What type of glycosidic linkage is present in cellulose? Why can humans not digest it?
2. Explain how the frequency of α‑1,6 branch points influences the rate of glucose release from glycogen compared with amylopectin.
3. Write the balanced equation for the hydrolysis of an α‑1,4 glycosidic bond by water.
4. In a Benedict’s test, a sample gives a brick‑red precipitate. What does this indicate about the sample?
5. Compare the energy density of triglycerides with that of starch, giving a numerical example.
6. Describe two structural features of phospholipids that determine membrane fluidity.
7. Outline an experiment to test for the presence of starch in a food sample, including controls and a source of error.

11. Suggested Diagrams (to be drawn in the exam or textbook)

• Haworth projections of α‑ and β‑D‑glucose.
• Helical representation of amylose and branched schematic of amylopectin.
• Highly branched glycogen molecule showing α‑1,4 and α‑1,6 linkages.
• Parallel β‑1,4‑linked cellulose chains with inter‑chain hydrogen bonds.
• Triglyceride structure highlighting ester linkages.
• Phospholipid molecule with polar head‑group and non‑polar tails; illustrate bilayer formation.
• Effect of unsaturation on fatty‑acid chain shape (kinked vs. straight).

Summary

All four major polysaccharides are polymers of D‑glucose, but the orientation of the glycosidic bond (α vs β) and the pattern of branching dictate their physical properties and biological roles. Amylose and amylopectin store glucose in plants; the high branching of glycogen allows rapid mobilisation in animals; β‑1,4‑linked cellulose provides structural support and dietary fibre. Lipids, though not polymers, share the common theme of amphipathicity: triglycerides store energy, while phospholipids form fluid, selectively permeable membranes whose properties are fine‑tuned by fatty‑acid chain length, saturation and cholesterol content. Mastery of the structures, reactions and experimental techniques described here equips students to meet all AO1 and AO2 requirements of the Cambridge IGCSE/A‑Level Biology syllabus.