Biological Molecules 2 :

Protein structure: Primary structure This is the order of amino acids in a polypeptide chain. Polypeptides hae up to 20 different types of amino acids. They can be joined in any number , order or structure. The primary structure is determined by the base sequence on one strand of the DNA molecule. Different groups will have different properties e.g some will be charged, uncharged. The way a protein will fold will depend on the R group. Polypeptides will form hydrogen bonds. (They will form a spiral)
1. Secondary structure: This is the shape that the polypeptide chain form as a result f hydrogen bonding between =0 on CO groups and the -H on -NH groups in the peptide groups along the chain. This causes a long peptide chain to be twisted in a 3D shape . The spiral shape is an alpha helix structure. Another, structure less common arrangement is the beta pleated sheet. The proteins are fibrous (rope like structures) e.g alpha helix and keratin has a high proportion of alpha helix and the protein fibrin in silk has a high proportion of bete pleated sheet.
  1. Tertiary structure: The secondary structure is taken and made more complex. They are less fibrous and more globular shaped. The alpha helix of the secondary protein structure can be folded and twisted to give a more compact, 3D structure. The two hydrophobic stay together. The hydrogen bond is folded round. Globular protein are metabolically active. Bond types: hydrogen bonds, ionic bonds, disulphide bonds and hydrophobic interactions.These bonds are important in giving globular proteins e.g enzymes their shape. (CYS contains sulphur -sulphide link is covalent and very strong.)All enzymes are made up of proteins immoglobulin which take bacteria and destroy them, steroid hormones and blood proteins (albumins)
    1. Quaternary structure: Different primary structure proteins folded into globular shape and joined together by disulphide links e.g haemoglobin. Some polypeptide chains are not functional unless they are in combination. In some cases, they may combine with another polypeptide chain such as an insulin molecule, which has two chains. They may also be associated with non- protein groups and form large complex molecules such as haemoglobin.
      1. Globular proteins: They are compact and folded into spherical molecules. They are soluble in water so they have different functions including enzymes, antibodies, plasma proteins and hormones. Haemoglobin is a globular protein consisting of 4 polypeptide chains and the centre is iron which is a group containing haem.
      2. Fibrous Proteins: They are long thin molecules and their shape makes them insoluble in water. So they have structural functions as in the bone. The polypeptides are in parallel chains or sheets with many cross linkages forming long fibres e.g keratin the protein hair. Fibrous proteins are strong and tough. Collagen is a fibrous protein providing strength and toughness in tendons. A single fibre, sometimes called tropocollagen, consists of 3 identical polypeptide chains twisted like a rope. The 3 chains are linked by a hydrogen bond, making the molecule stable.
    2. Testing for the presence of sugars:All monosaccharides are reducing sugars. The benedicts solution detects reducing sugars in a solution The glucose and benedict's solution are heated at 70 degrees Celsius. The solution will turn from blue, green yellow. orange and red. The solution will turn form a blue to a red precipitate. The sugars donate an electron to reduce copper sulphate to red copper oxide.
      1. Testing for sucrose:As sucrose is a dissacharide so it is a non reducing sugar and gives a negative result. There will be no colour change with benedict's solution.Sucrose will be detected if the glyosidic bond is broken to its constituent monosaccharide of glucose and fructose. It is put into a water bath with Hydochloric acid to break down the glycoside bond between glucose and fructose. Both these are reducing agents and will react with benedits solution after sodium carbonate which is an alkali is added to chane the colour from blue to red
        Testing for the presence of starch:Iodine solution which is iodine dissolved in aqueous solultion. Potassium reacts with starch which is brown turns to black/blue. This can be used in plants t detect for the presence of starch
        Test for protein:(the biuret test).The test is for the bonds that holds the amino acids together called a peptide bond. It is called the biurets test because the solution is blue and doesn't need to b heated to tuen to violet. A few drops of biuret reagent are added (NaOH) and (CuSO4) are added. The (NaOH) and copper sulphate are added to make blue copper hydroxide which interacts woth the peptide bonds present in the protein to make a biuret which is purple.
        Test for lipids:Add a drop of olive oil in a test tube and use ethanol to dissolve the lipid and shake the test tube to get a colourless liquid. Add water to get a cloudy white colour. The dissolved lipids come out of the solution because they are insoluble in water. They form emulsion, making the sample cloudy white it is called the ethanol emulsion test.

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Biological Molecules 2 :

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	Created by Wania Mariam about 6 years ago