2.1.2

• Hydrogen bonds


  Weak forces of attraction that can form between water molecules or between parts of a larger molecule
• Water molecule


  • It is polar, with an uneven distribution of charge
  • The oxygen end has a more negative charge (δ-)
  • The hydrogen end has a more positive charge (δ+)
  • The opposite delta charges attract water molecules together, producing hydrogen bonds
• Hydrogen bonds in water

  • Hold water molecules together loosely, allowing them to move past one another and remain a liquid between 0°C and 100°C
  • At 0°C, enough hydrogen bonds have formed to hold the water molecules in a stationary position, forming ice
  • Ice is less dense than water as it forms an open lattice structure, so it floats
• Properties of water important for living things

  • Thermal stability
  • Freezing
  • Evaporation
  • Liquid at most temperatures
  • Cohesion
  • Transparency
  • High density
  • Solvent
  • Reactant
  • Incompressibility
• Hydrogen bonds are not only found in water, they are also present in the secondary, tertiary and quaternary structures of proteins, in cellulose, and in DNA
• Monomer


  A smaller molecule that can be joined together to form a larger polymer molecule
• Polymer


  A large molecule made up of many similar smaller molecules (monomers) joined together
• Biologically important groups of polymers

  • Nucleic acids
  • Polysaccharides
  • Proteins
• Nucleotide


  The monomer of nucleic acids, made up of a phosphate group, a pentose sugar, and an organic base
• Organic bases in nucleic acids

  • Adenine
  • Cytosine
  • Guanine
  • Thymine
  • Uracil
• Condensation reaction


  Two molecules join to become one larger molecule via the formation of a covalent bond and the release of a water molecule
• Hydrolysis


  Molecules that were covalently bonded together are split apart using a molecule of water
• Chemical elements in biological molecules

  • Carbon
  • Oxygen
  • Hydrogen
• Types of carbohydrates


  • Monosaccharides
  • Disaccharides
  • Polysaccharides
• Monosaccharide


  A single sugar unit that can be used to build other carbohydrates
• Types of monosaccharides


  • Glucose
  • Pentose sugars (e.g. ribose, deoxyribose)
  • Hexose sugars (e.g. glucose)
• Disaccharide


  Two monosaccharides bonded together by a covalent glycosidic bond
• Polysaccharide


  Large insoluble molecules consisting of many monosaccharides joined together by condensation reactions
• Examples of polysaccharides


  • Starch (amylose and amylopectin)
  • Cellulose
  • Glycogen
• Amylose


  • Long unbranched chain of α-glucose subunits joined by 1,4 glycosidic bonds
  • Coils up, with hydroxyl groups hidden inside
  • Insoluble, compact, and used for glucose storage and energy in plant cells
• Amylopectin and glycogen


  • Long chains of α-glucose subunits with some 1,6 glycosidic bonds, making them branched
  • Insoluble, compact, and used for glucose storage and energy in plant and animal cells respectively
  • More branched structure of glycogen increases surface area for faster hydrolysis
• Cellulose


  • Long unbranched chain of β-glucose subunits joined by 1,4 glycosidic bonds
  • Hydroxyl groups exposed, allowing hydrogen bonding between adjacent cellulose molecules
  • Fibrous, strong, and insoluble, used in plant cell walls
• Lipids


  Not polymers like proteins and complex carbohydrates, a large group of compounds that includes triglycerides, phospholipids and steroids, insoluble in water, stored as droplets inside the cell
• Purposes of lipids in the body

  • Thermal insulation
  • Energy store
  • Protect organs from mechanical damage
  • Control exit and entry of molecules into cells
  • Component of steroid hormones
  • Buoyancy
  • Waterproof parts of the body
  • Source of water via respiration
  • Electrical insulation around neurones
  • Aid absorption, storage, and production of fat-soluble vitamins
• Triglyceride


  Macromolecule containing one glycerol molecule and three fatty acid chains, bonds can be broken by hydrolysis, rich in energy and used to store excess energy
• Triglycerides


  • Used to insulate animals in cold environments
  • Provide buoyancy for aquatic mammals
• Saturated fatty acid


  Each carbon has two hydrogen atoms attached, no double or triple bonds, found in animal fats, higher melting point, more solid at room temperature, can increase 'bad' cholesterol
• Unsaturated fatty acid


  Fewer hydrogen atoms, double or triple bonds between adjacent carbon atoms, found in plant fats and oils, lower melting point, more likely to be liquid at room temperature
• Phospholipid


  Similar to triglycerides but one fatty acid chain replaced by a phosphate group, hydrophobic 'tails' and hydrophilic 'heads', form bilayers that are the basis of cell membranes
• Amino acid


  Residual R group is the only part that differs between different amino acids, 20 different amino acids used in proteins
• Protein


  Polymers consisting of long, unbranched chains of amino acids held together by peptide bonds, four main types: enzymes, antibodies, transport proteins, and structural proteins
• Protein structure


  • Primary: sequence of amino acids
  • Secondary: folded and coiled into alpha helices and beta sheets
  • Tertiary: further folding and coiling due to interactions between R groups
  • Quaternary: proteins with more than one polypeptide chain
• Globular protein


  Highly folded to form a spherical shape, water soluble as hydrophobic groups are on the inside and hydrophilic on the outside, sensitive to temperature changes
• Haemoglobin


  Conjugated protein with four polypeptide chains and haem groups containing iron, transports oxygen as oxyhaemoglobin
• Enzyme


  Molecule with coiled alpha helices and folded beta sheets, contains an active site with a specific shape complementary to the substrate, requires a cofactor
• Hormone


  Insulin is a two polypeptide chain molecule held together by disulfide bridges, has a specific 3D shape complementary to a receptor
• Fibrous protein


  Regular sequence of amino acids repeated many times, less soluble in water, have quaternary structure, form strong fibres with structural functions
• Collagen


  Three polypeptide chains wound into a left-handed helix, contains covalent crosslinks, high proportion of glycine, insoluble, provides strength and flexibility
• Keratin


  Two polypeptide chains coiled together, strong, protects delicate parts of the body
• Elastin


  Produced by linking tropoelastin fibres, coiled like a spring, can stretch and recoil, used wherever stretching and recoil is required