Biological molecules



  • monosaccharides - single sugars. e.g. pentose (ribose and deoxyribose) and hexose (a-glucose, B-glucose and fructose)
  • disaccharides - double sugars. e.g maltose (2 a-glucose molecules) and sucrose (a-glucose and a fructose)
  • polysaccharides - amylose and amylopectin (plants) and glycogen (animals) are stores of a-glucose molecules

Bonds between a-glucose molecules:

  • 1,4-glycosidic bonds 
  • 1,6-glycosidic bonds - causes them to branch (amylopectin and amylose)

B-glucose molecules:

  • many bond to produce cellulose
  • straight chains
  • linked together by hydrogen bonds - grouped to produce microfibrils
  • each microfibril - 100s of cellulose molecules = immense tensile strength
  • form cell wall of plant cell and prevent cell from bursting in dilute solutions
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Triglycerides : lipids composed of 3 fatty acids joined to a glycerol by ester bonds

Fatty acids:

  • saturated
  • unsaturated - contains a double bond
  • release energy in aerobic respiration


  • glycerol + 2 fatty acids + phosphate
  • glycerol/phosphate 'head' = hydrophilic
  • fatty acid 'tails' = hydrophobic
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  • Amino acids joined by peptide bonds >>>> polypeptides
  • 20 different amino acids, so infinite variety of polypeptides possible

Amino acids consist of a carbon atom with 4 groups attached:

  • an amino group
  • a carboxylic acid group
  • a hydrogen atom
  • a residue group (R-group)

Proteins have different levels of structure:

  • Primary structure - sequence of amino acids
  • Secondary structure - a-helix (colied) OR B-pleated sheets
  • Tertiary structure - further folding of polypeptide, 3D shape. R-groups are held together by hydrogen bonds, hydrophobic interactions, ionic bonds and disulfide bonds.
  • Quaternary structure - two or more polypeptide chains
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Other Biological Molecules

Water molecules are dipolar, so neighbouring water molecules are linked by hydrogen bonds. The H-bonding of water molecules is known as cohesionThey are attracted to each other and surround ions and molecules with charged groups, which thus are dissolved by water.      Water is a good solvent caoable of dissolving a wide range of chemical substances

A solute dissolves in a solvent to produce a solution.

Role of ions and their atoms as components of biologically important molecules:

  • Nitrate (NO3-) >> Nitrogen in the amino group of amino acids and the organic base of nucleotides produced in plants
  • Sulfate (SO4 2-) >> Sulfur in the R group of the amino acid cysteine
  • Phosphate (PO4 3-) >> In a range of important molecules: adenosine triphosphate, ATP; nucleotides and so nucleic acids; phospholipids
  • Calcium (Ca2+) >> In calcium pectate, which contributes to the middle lamella of plant cell walls; in calcium phosphate in the bones of vertebrate animals
  • Magnesium (Mg2+) >> In the chlorophyll molecule
  • Iron (Fe3+) >> In the haemoglobin molecule
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