biological molecules



  • small organic molecules 
  • building blocks 
  • hexose sugars C6H12O6 e.g glucose
  • ring structures in water
  • glucose, a form and B form
  • main functions- 
  • source of energy in respiration via breaking of carbon-hydrogen bonds to make ATP from ADP
  • building blocks for larger molecules 
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  • two monosaccharide units linked with a glycosidic bond and the elimination of water this is a condensation reaction.
  • a glucose + a glucose = maltose
  • glucose+ fructose = sucrose
  • glucose + galactose = lactose
  • used for storage and transport
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Testing For Sugars

  • benedicts reagent
  • heat  
  • reducing sugar present= blue>green>yellow>orange>red
  • non reducing sugars need to be broken down to monosaccharides by heating with hydrochloric acid in order for test to work
  • qualitative / semi quantitative test
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  • large complex molecules called polymers
  • large numbers of monosaccharides linked with glycosidic bonds.
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  • glucose dissolves 
  • converted into starch as it is insoluble
  • compact 
  • found in plant cells as grains
  • a glucose molecules polymers
  • amylose is linear and coils
  • amylopectin is branched and fits inside
  • known as glycogen in animals
  • starch and glycogen is readily hydrolysed
  • test: orange iodine turns black
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  • structural polysaccharide
  • cell walls
  • long parallel chains of B glucose cross linked by hydrogen bonds
  • 60-70 molecules form bundles called microfibrils
  • microfibrils bundle into fibres
  • cell wall has several layers in different directions
  • very strong but freely permeable


  • insects
  • glucosamine
  • forms mucopolysaccharide
  • glucose with N acetyl group 
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  • non polar
  • insoluble in water
  • triglycerides= 1 glycerol 3 fatty acids
  • condensation reaction removes 3 moecules of water
  • ester bonds
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Properties of Lipids

  • differences in fats and oils come from variations in the fatty acids
  • no carbon-carbon double bonds the fatty acid is saturated
  • double bonds = unsaturated
  • saturated fats contribute to heart disease
  • animal lipids= saturated
  • plant lipids= unsaturated (oils)
  • insoluble in water but dissolve in organic solvents like alcohols
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Roles of Lipids

  • plasma membranes
  • energy reserves
  • insulator against heat loss
  • protection around delicate organs
  • triglycerides can produce a lot of metabolic water 
  • waterproofing 


  • one of the fatty acid chains is replaced by phosphate 
  • hydrophilic head, hydrophobic tail
  • polar
  • plasma membranes
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  • contain nitrogen
  • large compounds made of amino acids
  • 20 different types of amino acids
  • shape of a protein is determined by specific sequence of amino acids in a chain
  • amino acids have amino group -NH2 and carboxyl group -COOH but differing R groups
  • amino group of one amino acid reacts with carboxyl group of another eliminating water 
  • peptide bond is formed making a dipeptide
  • proteins are very large molecules of many amino acids joined to make polypeptides
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Structure of Proteins


  • the sequence of amino acids in a chain
  • large number of combinations of the 20 amino acids
  • differ in variety, numbers and order of amino acids


  • the shape the polypeptide chain forms as a result of hydrogen bonding
  • causes a 3D shape
  • spiral a helix 
  • B pleated sheet


  • the a helix of the secondary protein structure being folded and twisted to give a more complex, compact 3D shape
  • maintained by disulphide, ionic, and hydrogen bonds
  • globular proteins
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Structure II and Globular & Fibrous Proteins


  • a combination of two or more polypeptide chains in tertiary form
  • large complex molecules e.g. haemoglobin

Fibrous Proteins

  • structural functions
  • polypeptides in parallel chains/sheets with  numerous cross linkages to form fibres
  • insoluble in water, strong and tough
  • 1 fibre = 3 polypeptide chains twisted like a rope linked by cross bridges
  • keratin in hair, collagen in tendons

Globular Proteins

  • enzymes, antibodies, plasma proteins, hormones
  • compact and folded as spherical molecules 
  • soluble in water

testing- biuret reagent, blue > purple

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  • aquatic habitat
  • key elements in solution
  • medium for metabolic reactions
  • important in plants animals and cells
  • 'stick' by forming hydrogen bonds
  • hydrogen bonds affect its properties
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Solvent, Density and Transparency

  • excellent solvent, water is slightly polar and attracts charged particles
  • chemical reactions can take plac in solution
  • can be transported
  • oxygen is 2d-
  • hydrogen is d+


  • max density at 4oC
  • when solid as ice it is less dense than water- floats 
  • insulates water underneath
  • reduces liklihood of whole bodies of water freezing completely
  • organisms can survive beneath


  • light can pass through, aquatic plants can photo synthesise
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Thermal Properties

  • high specific heat
  • large amount of energy needed to raise the temperature of water
  • hydrogen bonds restrict movement
  • prevents large fluctuations in water temperature- keeps aquatic habitats stable
  • enzymes can work
  • high latent heat 
  • great deal of heat energy needed to turn it from liquid to gas
  • important in temperature control e.g. sweating
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Cohesion & Surface Tension

  • water is polar with no overall charge
  • oxygen is slightly negative hydrogen slightly positive
  • 2 molecules close together their opposing charges attract
  • forms a hydrogen bond
  • individually hydrogen bonds are weak but there are many of them forming a lattice
  • this sticking together is cohesion
  • columns of water can be drawn up xylem in tall trees
  • at ordinary temps, water has high surface tension
  • cohesion between molecules produces surface tension 
  • body of an insect can be supported on surface of pond
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Inorganic Ions

  • micronutrient are needed in trace amounts
  • macronutrients are needed in small amounts
  • magnesium- chlorophyll in plants
  • iron- haemoglobin in red blood cells
  • Phosphate- making nucleotides and phospholipids
  • calcium- bones
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