Biology - Biological Molecules 2

Biological Molecules Section 2

Starch, glycogen and cellulose

  • Starch in plant cells (alpha glucose)
    • made up of chains of a-glucose monosaccharides linked by glycosidic bonds
    • glycosidic bonds formed by condensation reactions
    • chain may be branched/unbranched
    • main roles of starch;
    • insoluble and doesn’t affect water potential
    • large and insoluble, it does not diffuse out of cells
    • compact, so a lot can be stored in a small space
    • hydrolysed to form a-glucose which is both easily transported and readily used in respiration
    • branched form has many ends each of which can be acted on by enzymes so glucose monomers can be released very rapidly

Glycogen in animal, bacteria cells (alpha glucose)

  • similar structure to starch but shorter chains and highly branched

  • stored as granules in animals, mainly in muscle and liver cells

  • its structure is suitable for storage because;

    • ==insoluble== and does not tend to draw water into the cells
    • ==insoluble==, so cannot diffuse out of cell
    • ==compact,== so a lot of it can be stored in small space
    • ==more highly branched than starch== so has more ends that can be acted on by enzymes so faster release of glucose monomers
    • ==faster release of glucose monomers== as animals have higher metabolic rate

  • Cellulose

    • made of b-glucose
    • cellulose a major component of plant cell walls and provide rigidity to the plant cell
    • structure of cellulose related to its function;
    • cellulose molecules are made up of b-glucose and so form long, unbranched molecules
    • the chains run parallel to each other and are crossed linked by hydrogen bonds which adds strength
    • these molecules are grouped to form microfibrils which in turn are grouped to form fibres which provides even more strength

 structure of a cellulose molecules

Lipids

  • Characteristics

    • contains C, H, O
    • proportion of ==oxygen== to ==carbon and hydrogen== is smaller than in carbohydrates
    • insoluble in water
    • soluble in organic solvents such as alcohol
    • main groups of lipids are triglycerides and phospholipids

  • Roles of lipids

    • contributes to the flexibility of the cell membranes and the transfer of lipid soluble substances across them
    • source of energy : when oxidised, they produce more than twice the energy the same mass of carbohydrate and release valuable water
    • waterproofing : lipids are insoluble in water so it is useful for waterproofing
    • insulation : fats are slow conductors of heat and when stored beneath the body surface helps to retain body heat. also acts as electrical insulators in the myelin sheath
    • protection : fat is stored around delicate organs such as kidney

  • Triglycerides

    • three fatty acids + glycerol = triglyceride
    • each fatty acid form ==ester bond (-COO-)== with glycerol in a condensation reaction

     

  • there are different variations of the triglycerides

    • no double bond ; saturated

    • one double bond ; mono-unsaturated

    • more than one double bond ; polyunsaturated

    • Structure of triglycerides related to their properties

  • high ratio of energy-storing carbon-hydrogen bonds to carbon atoms, so excellent source of energy

  • low mass to energy ratio, good storage molecules, more energy can be stored in a small volume

  • insoluble in water since they are large non-polar molecules. does not affect water potential of cells

  • high ratio of hydrogen to oxygen atoms, triglycerides release water when oxidised so it is important source of water

    • Phospholipids

  • similar to lipids but one of the fatty acids are replaced with a phosphate group

  • has hydrophilic head (phosphates) that are attracted to water

  • has hydrophobic tails (fatty acid molecules) that repels water but is fat soluble

     

  • Structure of phospholipids related to their properties

    • polar molecules ; in aqueous environment, phospholipid molecules form a bilayer within cell surface membranes, forming hydrophobic barrier
    • hydrophilic phosphate heads help to hold the surface of csm
    • phospholipid structure allows to form glycolipids by combining with carbohydrates within the membrane

  • Test for lipids - emulsion test

  1- take a completely dry and grease free test tube

  2- to 2cm^3 of the sample being tested, add 5cm^3 of ethanol

  3- shake the tube thoroughly to dissolve everything

  4- add 5cm^3 of water and shake gently

  5- a milky white emulsion indicates presence of lipids

Proteins

  • Structure of amino acid
    • amino acids : basic monomer units
    • they combine to form polymer; polypeptide
    • every amino acid has;
    • amino group (-NH2) a basic group
    • carboxyl group (-COOH) an acidic group
    • hydrogen atom (-H)
    • R(side) group , a variety of different chemical groups

       

  • The formation of a peptide bond

    • amino acids join together to form peptide bonds by condensation reaction and gives water
    • water made by combining an -OH from the carboxyl group of an amino acid and a -H from the amino group from another amino acid
    • peptide bond can be broken by hydrolysis forming its two constituent amino acids

  • Primary structure of proteins - polypeptides

    • many acid monomers joined together in polymerisation
    • forms polypeptide
    • sequence of the amino acids forms the primary structure of protein
    • sequence determined by DNA
    • primary structure determines shape so therefore its function
    • change in sequence of the amino acids lead to change/stops the function of the protein

  • Secondary structure of proteins

    • linked amino acids have both -NH and -C=O groups on either side of every peptide bond.
    • H in -NH (+ve), and the O in -C=O (-ve) form weak hydrogen bonds
    • this bond causes the polypeptide chain to twist into 3-D shape such as the a-helix coil

  • Tertiary structure of proteins

    • further twisting and coiling of the secondary structure
    • maintained by different bonds
    • disulfide bridges - very strong
    • ionic bonds - formed between any carboxyl group and amino groups that are not involved in forming peptide bonds. weaker than disulfide bonds
    • hydrogen bonds - numerous but easily broken
    • specific 3D shape gives its function and allow interaction with other molecules

     

  • Quaternary structure of proteins
    • a combination of a number of different polypeptide chains
    • there may also be non-protein (prosthetic) groups associated with the molecule
  • Test for proteins (blue - purple)
    • place a sample of the solution to be tested in a test tube and add an equal volume of sodium hydroxide solution at room temperature
    • add a few drops of very dilute (0.05%) copper II sulfate solution and mix gently
    • a purple coloration indicates the presence of peptide bonds and hence a protein. if no proteins are present, the solution remains blue