2. molecular biology

• The amino acid sequence determines the three-dimensional conformation of a polypeptide by…

  • Being added / joined together always in the same sequence (at the ribosome)
  • Allowing the polypeptide to fold as it is being synthesised
  • Bonds form between amino acids / between R groups that hold the final shape/conformation
  • (In globular proteins) Hydrophilic amino acids end up at the edges / hydrophobic amino acids in the middle
  • (In fibrous proteins) The sequence of amino acids prevent folding
• The differences between the structures and properties of collagen and haemoglobin are…

  • Haemoglobin is globular; collagen is fibrous
  • Haemoglobin has 4 polypeptide chains; collagen has 3
  • Haemoglobin has a (iron-containing) prosthetic group / contains iron/Fe; collagen has none
  • Haemoglobin is soluble; collagen is insoluble
  • Haemoglobin binds reversibly with other molecules (oxygen); collagen does not
• The genome of an organism is fixed, whereas the organism's proteome can vary because…

  • All cells carry the same genome / set of chromosomes / DNA
  • Not all cells make/express the same proteins
  • The range of proteins made/expressed by a particular cell/tissue can vary during its lifetime / according to its developmental stage
  • The possible range of proteins an organism can make is far greater than the number of genes it possesses
• An explanation of the term, 'specificity' in the context of immunoglobulins and their mode of action is…

  • Sites on their two arms (of the Y-shaped molecule) bind to certain sites/antigens on pathogens
  • Only one type of immunoglobulin will bind to a particular antigen; [1 mark]
  • Because of complementary 3D-shape / structure of both immunoglobulin and antigen
  • Because of antigenic variability, many different immunoglobulins exist, each with a specific binding site
• Role of protein:

  • Enzymes / catalysis eg. known enzyme
  • Cell membrane proteins eg. carriers, channels, receptors
  • Hormones eg. insulin, oestrogen
  • Immunoproteins eg. antigens, antibodies, immunoglobulins
  • Transport proteins eg. haemoglobin
  • Structural proteins eg. keratin, collagen
  • Contractile proteins eg. myosin
  • Associated with DNA eg. histones (for compaction), gene expression factors
  • Blood clotting factors eg fibrinogen
• RuBisCo:

  • (Structure) Globular
  • (Structure) 16 polypeptide chains
  • (Function) Catalyses fixing of atmospheric CO2 during photosynthesis
• Spider silk:

  • (Structure) Fibrous
  • (Structure) High tensile strength / light
  • (Function) Strength for web-building (dragline silk)
  • (Function) Can also have some elasticity
• Immunoglobulins:

  • (Structure) Globular
  • (Structure) Y-shaped molecule with variable ends/tips
  • (Function) binds to / specific to antigens
• Rhodopsin:

  • (Structure) Globular
  • (Structure) contains retinal / light-sensitive part
  • (Function) to detect light stimulus
•  Low density lipoproteins (LDLs) raise the risk of a person suffering from coronary heart disease by:

  • (At times of high levels of LDLs in the blood,) LDL molecules are deposited on the inner walls of arteries
  • (These deposits) accumulate as fatty material / plaques
  • Plaques reduce the lumen size of the arteries / restrict blood flow
  • Coronary arteries can become blocked, causing coronary heart disease (CHD) / heart attack / stroke
  • High LDL/cholesterol levels in the blood also cause the hardening of arteries / atherosclerosis
• Similarities between glycogen and starch:

  • (Both are) polysaccharides
  • Compact
  • Folded
  • Storage molecules
  • Molecules can be variable in size
  • α- glucose monomers
• Differences between glycogen and starch:

  • Starch made in plants; glycogen in animals/fungi
  • Starch contains coils, glycogen more branched
  • (So) glycogen has more 'free ends' / can be hydrolysed faster
  • Starch long-term storage eg. in leaves/seeds/tubers as starch granules; glycogen is more a short-term storage