Biological molecules

• • Benedict’s test for reducing sugars: Equal volume of sample being tested and Benedict’s solution are mixed and heated in a water bath up to 95C.
  • Positive: green → yellow → orange → brick red
  • Negative: blue
• • Biuret’s test used to detect the presence of proteins: Equal amounts of the sample and Biuret’s solution are added together, giving purple color over several minutes in the presence of proteins, and blue in its absence.
• • Emulsion test for lipids: The sample is added to 2cm3 of ethanol and mixed well until it dissolves (lipids are soluble in ethanol). This mixture is then placed into a test tube containing the same amount of water. A milky white emulsion will appear if lipids are present and remain clear if not.
• • Iodine test for the presence of starch: Iodine solution is orange-brown. Add drops of iodine in potassium iodide solution to the liquid that is to be tested. A blue-black color is quickly produced if starch is present.
• Glucose has the molecular formula C6H12O6. It is a hexose sugar. It is also the monomer from which Starch and Cellulose are made. 
• There are two different kinds of glucose monomers known as alpha- glucose and beta - glucose, and their difference lies in the position of an –OH group in their ring structures.‎
• Alpha - H-OH, beta- HOH

  A) H - OH

  
• • Monomer: a simple molecule that is used as a basic building block for the synthesis of a polymer; many monomers are joined together to make the polymer, usually by condensation reactions. e.g. of monomers: monosaccharides, amino acids, nucleotides.
• • Polymer: is a giant molecule made from monomers e.g. polysaccharides, proteins, nucleic acids
• • Monosaccharide: This molecule consists of a single sugar unit, the simplest form of carbohydrate and cannot be hydrolysed further. It has a general formula of (CH2O)n.
• • Disaccharide: a sugar molecule consisting of two monosaccharides joined together by a glycosidic bond.
• • Polysaccharide: a polymer whose subunits are monosaccharides joined together by glycosidic bonds.
• Glycosidic bonds: covalent bonds that occur between constituent monomers and are formed due to a condensation reaction involving removing a water molecule to form polysaccharides and disaccharides such as sucrose.
• • Starch is a macromolecule that is found in plant cells and is made up of two components: amylose and amylopectin. These components are polysaccharides made from a glucose molecules and contain 1,4 glycosidic bonds. Starch is highly compact and stores energy.
• Amylose - the soluble form of starch, consist of α 1,4 glycosidic bonds, helical and is more compact. Amylopectin - the insoluble for of starch, consist of α 1,4 and α 1,6 glycosidic bonds, branched.
• • Glycogen: a macromolecule that is used for the storage of energy in animal cells and is also made from α glucose molecules. The structure of glycogen is very similar to that of amylopectin; however, it is more branched and therefore contains more α 1,6 glycosidic bonds.
• Hydrogen bonds are also formed between parallel cellulose molecules. then 60,70 of them form microfibrils. Fibers increase tensile strength to withstand osmotic pressure, making the plant rigid and determining cell shape. 
• Triglyceride: forms by condensing 3 fatty acid chains and a glycerol molecule, forming an ester bond. Fatty acid chains are long hydrocarbon chains with a carboxylic head. Glycerol is an alcohol containing 3 OH groups.
• Unsaturated fatty acids: contain c=c bonds that are easier to break and melt easily
• Saturated fatty acids: contain c-c bonds that are solids at room temperature.
• • Role of triglyceride:
  • Better energy reserves than carbohydrates as more CH bonds
  • Acts as an insulator and provides buoyancy
  • A metabolic source of water gives CO2 and H20 to oxidation in respiration
• Proteins: Made of amino acids which only differ in the R- groups/ variable side chains and will always contain an amine group (basic), a carboxyl group (acidic) and a hydrogen atom attached to the central carbon atom.

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• A peptide bond is formed by condensation between 2 amino acids, forming a dipeptide. Many amino acids that join together by peptide bonds form a polypeptide.
• Primary structure: sequence of amino acids in a polypeptide/protein. 
• Secondary structure: the structure of a protein molecule resulting from α- helix and β- pleated sheet.
• Tertiary structure - : the compact structure of a protein molecule resulting from the three-dimensional coiling of the already-folded chain of amino acids.

  • Hydrogen bonds between wide varieties of R-groups (can be broken by PH and temperature changes)
  • Disulphide bridges between two cysteine molecules (can be broken by reducing agents)
  • Ionic bonds between R groups containing amine and carboxyl groups. (Can be broken by PH changes.)
  • Hydrophobic interactions between non-polar R groups.
• Quaternary structure: the three-dimensional arrangement of two or more polypeptides or of a polypeptide and a non-protein component, such as haem, in a protein molecule. The polypeptide chains are held together by bonds in the tertiary structure.
• Haemoglobin: a globular protein that has a quaternary structure with 4 polypeptide chains, 2 α-globin and 2 β-globin chains. Each chain has one prosthetic haem group containing an iron atom that reversibly binds to an oxygen molecule.