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4.1Biological Molecules

Biological molecules — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

5 min read 6 parts Lesson 1 of 3 1 quiz

📚 Cambridge IGCSE Biology 0610 Hub / Topical Revision
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Lesson summary

Biological molecules — IGCSE BIOLOGY notes: State the elements in carbohydrates, fats and proteins, the small units they are built from, and the role of water as a solvent.Three main groups — most molecules in living organisms are carbohydrates, proteins and lipids (fats and oils…

  • Distinguishing element
  • Functions of the carbohydrates
  • Cell chemistry
  • Amino acid
  • Two glucose molecules joined form maltose (a disaccharide)
  • Many glucose molecules joined form a polysaccharide: starch, glycogen or cellulose

Part 1 of 4Elements in biological molecules

  • Three main groups
    • most molecules in living organisms are carbohydrates, proteins and lipids (fats and oils).
    • All contain carbon, so they are called organic molecules.
MoleculeElements
CarbohydrateCarbon (C), hydrogen (H), oxygen (O)
Fats and oils (lipids)Carbon (C), hydrogen (H), oxygen (O)
ProteinCarbon (C), hydrogen (H), oxygen (O) and nitrogen (N); some also contain sulfur (S) or selenium (Se)
  • Distinguishing element
    • nitrogen is in proteins but not in carbohydrates or fats.

Part 2 of 4Large molecules are built from small units

Large moleculeSmaller units
Starch, glycogen, celluloseGlucose
Fats and oilsFatty acids and glycerol
ProteinsAmino acids
Large molecules are built from small units
Carbohydrates
Carbohydrates are long chains of simple sugars (monosaccharides).
  • Glucose is a simple sugar and the main respiratory substrate.
  • Two glucose molecules joined form maltose (a disaccharide).
  • Many glucose molecules joined form a polysaccharide: starch, glycogen or cellulose.
Functions of the carbohydrates
.
  • Starch — the energy store in plants.
  • Glycogen — the energy store in animals, kept in the liver and muscles.
  • Cellulose — forms plant cell walls.
Fats and oils
  • Fats and oils most are triglycerides, each made of one glycerol joined to three fatty acid chains.
  • Fats are solid at room temperature; oils are liquid.
  • They store energy, insulate and form membranes.
Proteins
Proteins are long chains of amino acids.
  • There are about 20 different amino acids; each has the same basic structure with a different R group.
  • The sequence of amino acids determines the protein's shape and function.
  • Even a small change in sequence gives a different protein, e.g. enzymes and antibodies.

Part 3 of 4Water as a solvent

Solvent
  • Solvent water dissolves many substances, so they can be moved and reacted.
  • Digestiondissolved food molecules move from the alimentary canal to cells.
  • Transportdissolved substances move in blood plasma and in xylem and phloem.
  • Excretionurea and excess salts dissolve and leave in urine.
Cell chemistry
  • Cell chemistry water is part of the cytoplasm, where metabolic reactions take place.

Part 4 of 4More from the specification

1Nucleotide bases in biological samples
  • Nucleotide bases in biological samples extracting an organic tissue sample permits qualitative identification of pyrimidine and purine nitrogenous bases.
2Double helix
  • Two DNA strands wound around each other.
3Iodine
  • Element needed to make thyroxine.
4Nucleotide
  • The unit of DNA made of a phosphate, a deoxyribose sugar and a base.

Key terms

TermMeaning
CarbohydrateA molecule of carbon, hydrogen and oxygen, including sugars, starch, glycogen and cellulose.
GlucoseA simple sugar; the unit that starch, glycogen and cellulose are built from.
Fats and oilsLipids made of fatty acids and glycerol, used for energy storage, insulation and membranes.
ProteinA long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
Amino acidThe unit from which proteins are made; the sequence decides the protein's shape and function.
SolventA liquid, such as water, in which other substances dissolve.

Avoid these mistakes

1
Starch, glycogen and cellulose are made of different units — all three are polymers of glucose.
2
Proteins and fats contain the same elements — proteins also contain nitrogen.
3
Fats are made of amino acids — fats are fatty acids plus glycerol; proteins are amino acids.
4
Glycogen is the energy store in plants — glycogen is in animals, starch is in plants.

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