Sample essay markschemes
10. The biological importance of water OR The role of water in the lives of organisms
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Structure - dipolar nature, hydrogen bonds
Solvent - Hydrophobic/hydrophilic interactions leading to stability of membranes, proteins,
nucleic acids etc, diffusion of molecules, dilution of toxic compounds e.g. urea
Osmosis and turgidity and their effects on plant support
Transport medium - xylem, phloem, blood, lymph, secretion, excretion
High heat capacity - temperature regulation, constant external environment for aquatic
organisms
High heat of vaporisation - cooling effect e.g. sweating, panting, transpiration
Surface tension and cohesion - Translocation, mosquito larvae, pond skaters
Chemical reagent e.g. in P/S it is a source of Hydrogen, hydrolysis reactions
Incompressibility – hydrostatic skeletons, eyes, joints, seed germination, amniotic fluid,
shock absorption in brain
Density – floats when frozen and insulates
Transparent –light penetration for aquatic organisms
Medium for movement e.g. gametes, seed dispersal, oceanic migration, mucus in alimentary
canal
Protection e.g. lachrymal fluid, mucus
Factor for evolution e.g. terrestrial organisms have to adapt to conserve water
Water vapour can act as a greenhouse gas
11. The importance of proteins in living organisms
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Structure and chemical composition of amino acids, amphoteric nature
Essential and non-essential amino acids
Peptide bond, ionic bonds, disulphide bonds, hydrogen bonds
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Denaturation by heat, heavy metals, pH
Primary, secondary, tertiary and quaternary structures
Fibrous (Collagen in tendons and bone, myosin in muscle, silk in spiders webs, keratin in hair,
horn, nails and feathers) and globular (Enzymes, antibodies, hormones e.g. insulin, histones
for compacting DNA) proteins
Conjugated proteins e.g. phosphoproteins (Casein in milk), Glycoproteins
(Mucin),Nucleoproteins (viruses), chromoproteins (HB, Phytochrome, cytochrome),
Lipoprotein(Membranes and for lipid transport in the blood), Flavoprotein (FAD in ETS),
metal proteins(nitrate reductase in plants)
Structural collagen in connective tissue, keratin in skin etc., elastin in ligaments, sclerotin in
insect exoskeletons, mucoproteins in mucus, capsid proteins in viruses
Enzymes: RUBISCO, any named
Hormones: Insulin, glucagon
Transport: Hb, Mb, serum albumin for lipid transport
Protective: Antibodies, fibrinogen and thrombin for blood clotting
Contractile: myosin and actin
Storage: ovalbumin in egg white, casein in milk
Toxins: snake venom, diphtheria toxin