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Biological Molecules

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Edexcel IAL Biology  •  Unit: Biological Molecules

Biological Molecules

🧬 Big idea: Living things are built from a handful of carbon-based "monomer" building blocks (sugars, fatty acids, glycerol) that join together — releasing water — to make bigger, more useful molecules, and can be broken apart again — using water — whenever the cell needs energy or raw materials.

Summary — What This Chapter Covers

  • Water is a polar molecule that forms hydrogen bonds — this makes it a superb solvent, gives it cohesion/adhesion, and makes it the medium for all life's chemistry.
  • Saccharides (carbohydrates) come in three sizes: monosaccharides (1 sugar unit), disaccharides (2 units), and polysaccharides (many units) — all built from C, H and O.
  • Condensation reactions join monomers together and release a water molecule; hydrolysis reactions use water to split them apart again.
  • Glycosidic bonds link sugar monomers (e.g. 1,4 or 1,6 bonds); ester bonds link glycerol to fatty acids in lipids.
  • Starch (amylose + amylopectin) stores energy in plants; glycogen does the same job in animals and fungi — both are insoluble, compact, and branched for fast glucose release.
  • Benedict's test detects reducing sugars (colour change blue → brick-red); a modified version with acid + neutralisation detects non-reducing sugars like sucrose.
  • Iodine test detects starch (orange/brown → blue-black).
  • Both tests can be made semi-quantitative using serial dilutions + a colorimeter to build a calibration curve.
  • Triglycerides (fats/oils) are made of 1 glycerol + 3 fatty acids joined by ester bonds (esterification), releasing 3 water molecules.
  • Fatty acids can be saturated (no C=C double bonds, straight chains) or unsaturated (mono/poly, kinked chains, mostly plant oils).

1. The Importance of Water

Why water is polar

Water (H₂O) is one oxygen atom covalently bonded to two hydrogen atoms. Oxygen is far more "electron-greedy" (electronegative) than hydrogen, so the shared electrons in each covalent bond spend more time near the oxygen. That creates a slightly negative region on the oxygen (δ⁻) and slightly positive regions on each hydrogen (δ⁺).

Think of it like a tug-of-war where oxygen is much stronger than hydrogen — it doesn't rip the rope away completely (that would be an ionic bond), it just pulls the middle point closer to its own side. That uneven "middle point" is called a dipole, and a molecule with a dipole like this is called a polar molecule.

Key Term
A hydrogen bond is a weak electrostatic attraction between the δ⁺ hydrogen on one water molecule and the δ⁻ oxygen on a neighbouring water molecule. Individually weak — but water molecules form so many of them that, together, they're powerful.
Why this matters — properties that come from hydrogen bonding
  • Cohesion — water molecules stick to each other via hydrogen bonds. This is why water flows as a continuous stream and why it can be pulled up a plant stem in one unbroken column.
  • Adhesion — water molecules stick to other polar surfaces (like the walls of a xylem vessel). Cohesion + adhesion together explain capillary action.
  • Excellent solvent
Practice Question 1

Explain, in terms of electronegativity, why water is described as a polar molecule.

Exam Tips

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Also in the full note
  • 2. Saccharides (Carbohydrates)
  • 3. Core Practical 1: Testing for Sugars & Starch
  • 4. Condensation & Hydrolysis
  • 5. Triglycerides & Ester Bonds
  • What to Memorise
  • Concepts Checklist
  • The three levels of carbohydrate
  • Alpha (α) vs Beta (β) glucose
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