2.2. Biological Molecules

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Water

  • Water is a polar molecule due to hydrogen and oxyegn have different electronegativities 
  • It is a metabolite as it is used in metabolic reactions such as condensation and hydrolysis reactions 
  • It is a solvent in which many metabolic reactions occur in 
  • Water has a high specific heat capacity which means lots of energy is required heat it up which allows for minimal temperature fluctuations in living organisms 
  • It has a relatively large latenet heat of vaporisation so that evaporation provides a cooling effect with little water loss 
  • It has strong cohesion between molecules which allows for effective water transport 
  • Water is incompressible which means it provides good support
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Monomers and Polymers

  • Monomers are small units which are the components of larger molecules e.g. monosaccharides in glucose, ammino acids, and nucleotides 
  • Polymers are many monomers joined together 
  • A condensation reaction is a reaction in which monomers are chemcally bonded together by the removal of a water molecule 
  • A hydrolysis reaction is the breaking of bonds between monomers by the addition of a water molecule
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Carbohydrates

  • Carbohydrates are molecules made of only: carbon, hydrogen, and oxygen 
  • They are long chains of sugar called saccharides 
  • There are 3 types of saccharides: monosaccharide, disaccharide, polysaccharide 
  • Monosaccharides can form disaccharides and polysaccharides by glycosidic bonds formed by condensation reactions 
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Monosaccharides

  • Glucose is a monosaccharide 
  • It contains 6 carbon atoms in each molecule 
  • It is the main substrate in respiration therefore it is of great importance 
  • C6H12O6 + 602 = 6CO2 + 6H2O 
  • It has two isomers: alpha glucose and beta glucose 
  • Alpha glucose, on the second carbon, has its hydrogen above the OH group 
  • Beta glucose, on the second carbon, has its Oh group above the hydrogen 
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Disaccharides and Polysaccharides

  • Maltose is a disaccharide formed by the condensation of two glucose molecules
  • Sucrose is a disaccharide formed by the condensation of glucose and fructose 
  • Lactose is a disaccharide formed by the condensation of glucose and galactose 
  • Glycogen and starch which are polysaccharides formed by the condensation of of alpha glucose 
  • Cellulose which is a polysaccharide isformed by the condensation of beta glucose 
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Glycogen

  • Glycogen is the main storage molecule in animals 
  • It's formed from many molecules of alpha glucose joined together by 1, 4, and 1, 6 glycosidic bonds 
  • It has a large number of side branches meaning that glucose and therefore energy can be released quickly 
  • It is a relatively large but compact molecule which maximises the amount o energy that it can store 
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Starch

  • Starch stores energy in plants 
  • It is a mixture of two polysaccharides called amylose and amylopectin 
  • Amylose is an unbranched chain of glucose molecules joined by 1,4 glycosidic bonds 
  • As a result of that amylose is coiled and therefore is very compact meaning it can store a lot of energy 
  • Amylopectin is branched and is made up of glucose molecules joined by 1, 4 and 1, 6 glycosidic bonds 
  • Due to the many side branches it is rapidly digested by enzymes releasing energy quickly 
  • Cellulose is a component of cell walls 
  • It's composed of long, unbranched, chains of beta glucose which are joined by glycosidic bonds 
  • Microfibrils are strong threads which are made of long cellulose chains joined together by hydrogen bonds and they provide structural support in plants 
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Lipids `

  • Lipids are only soluable in organic solvents such as alcohols 
  • Saturated lipids, found in animal fat, don't contain any carbon-carbon double bonds 
  • Unsaturated lipids, found in plants, contain carbon-carbon double bonds and melt at lower temperatures than saturated lipids 
  • The more unsaturated bonds, the weaker the intermolecular bonds resulting in lower melting point as less energy is needed to break the bonds 
  • As saturated fats don't contain any double bonds, they are solid at liquid temperature and unsaturated fats are liquid at room temperature 
  • Triglycerides are one molecule made up of one glycerol and three fatty acids 
  • They are joined by ester bonds in condensation reactions 
  • Triglycerides are used as energy reserves in plants and animals 
  • In phospholipids, one fatty acid is subsituted by a phosphate group 
  • Phosphate heads are hydrophillic and tails are hydrophobic 
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Inorganic Ions

  • Occur in solution (in the cytoplasm and bodily fluids of organisms)
  • Some essential ions include:
  • Hydrogen ions which determines the pH e.g. in blood sugar (higher the conc of H ions, the lower the pH)
  • Iron ions are a component of haemoglobin
  • Sodium ions are involved in co-transport of glucose and amino acids
  • Phosphate ions are a component of DNA and ATP
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Proteins

  • Amino acids are the monomers from which protein is made
  • They contain an amnio group -NH2-, a carboxyl group, and a variable R group which is a carbon-containg chain
  • Amino acids are joined by peptide bonds formed in condensation reactions
  • A dipeptide contains 2 amino acids and a polypeptide contains more amino acids
  • The primary structure of an amino acid is the order and number of amino acids
  • The secondary structure is the shape that the chain the amino acid takes- either an alpha helix or beta-pleated sheet and this is determined by hydrogen bonding
  • The tertiary structure is the 3D shape of the protein; it can be globular or fibrous
  • Globular proteins- such as enzymes - are compact whereas fibrous proteins - such as keratin - are long and therefore form fibres
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Collagen and Haemoglobin

  • Collagen - which is a fibrous protein - has great strength due to hydrogen and covalent bonds
  • Collagen molecules wrap around each other to form fibrils which form strong collagen fibres
  • Collagen forms the structure of: bones, cartilage, and connective tissue which is a main component of tendons which holds muscle to bone
  • Haemoglobin is a water soluable globular protein which consists of two alpha and two beta polypeptide chains each containing a haem group
  • It carries oxyegn in the blood as oxygen can binds to the haem (Fe2+) group and oxygen is then released when required
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Tests 1

  • Benedict's Test- finds non-reducing and reducing sugars
  • A reducing sugar is one which can donate electrons and includes all monosaccharides and some disaccharides
  • An alkaline solution of blue copper sulphate is added to the sugar and then heated
  • When there is a reducing sugar there is a red precipitate of copper oxide is formed
  • When there isn't a reducing sugar, the colour will remain blue
  • Biuret Test- finds proteins
  • A sample of the solution is added to test tube along with an equal amount NaOH is added
  • After, a few drops of dilute copper copper sulphate solution is added and gently mixed
  • In the presence of protein, the solution turns lilac as an indicator of peptide bonds
  • In the absence of proteins the solution will remain blue
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Tests 2

  • Emulsion Test- used to find lipids
  • 2ml of sample is added to 5ml of ethanol
  • The contents are mixed thoroughly by shaking to esure lipids are dissolved
  • Then 5ml of water is added and mixed too
  • The solution turns cloudy indicating the presence of lipids due to fomation of an emulsion where the light is refracted as it passes from droplets of oil to droplets of water
  • Iodine test- used to find starch
  • In the presence of starch, the colour of potassium iodide solution changes from yellow to black/blue
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