2.1.2 BIOLOGICAL MOLECULES

• Describe the structure of a water molecule.

  Two hydrogen atoms and one oxygen atom covalently bonded. Due to unequal sharing of electrons, the hydrogen atoms are δ+ve and the oxygen atom is δ-ve. This causes hydrogen bonds to form btwn the H and O atoms of adjacent water molecules.
• How does water dissolve polar substances?


  Because water is polar +ve ions are attracted to the δ-ve O and -ve ions are attracted to the δ+ve H in each molecule. Important because metabolism requires ions in solution and, as non-polar molecules don't dissolve, stability of the membrane is maintained.
• Why are the thermal properties of water important?

  Hydrogen bonds absorb a lot of energy so water has high specific heat capacity. Large amounts of energy needed to heat water, or to change its state. Important in cooling via evaporation of sweat, and acts as temperature buffer.
• Why are the freezing properties of water important?

  Water has maximum density at 4OC. As the temperature of a pond decreases below this, water becomes less dense, due to the formation of hydrogen bonds in an open structure, and rises to freeze as ice at the surface, insulating water and aquatic organisms below.
• Why are the surface tension properties of water important?

  Water has a high surface tension as H-bonds attract molecules together. At the surface of a solution, water molecules are pulled down by H-bonds forming just below the surface. This causes the liquid's surface to contract. Cohesive properties important for transport in xylem of plants (molecules stick together) and, because it is not easily compressed, water provides turgor in plants, support in an earthworm's hydrostatic skeleton and buoyancy for large aquatic mammals. Surface tension provides a habitat for small organisms e.g. pond skater.
• Explain water's importance in metabolism

  Water is a reagent in hydrolysis and a source of H+ ions in photosynthesis. It is produced in condensation.
• List other important properties and functions of water.

  Transparent so light can penetrate aquatic environments, this is also important in light transmission in the eye, present in protective fluids like mucus and lachrymal fluid (tears), acts as lubricant in joints, osmosis is important in stomatal opening and kidney function, it is a habitat and aids reproduction in aquatic organisms, germination in seeds and dispersal of seeds and spores.
• How are iron ions important?

  Component of haemoglobin which transports oxygen in RBCs
• How are hydrogen ions important?

  Determine and regulate pH. Increased H+ = more acid.
• How are calcium ions important?

  Co-factor for enzymes involved in blood clotting, involved in insulin release from the pancreas and in transmission of nerve impulses at a synapse where they promote release of neurotransmitter.
• How are sodium ions important?

  Involved in cotransport of glucose and amino acids, important in generation of the resting and action potential in nerves as well as regulating water potential
• How are potassium ions important?

  Activate enzymes for photosynthesis in plants, important in generation of the resting and action potential in nerves as well as regulating water potential.
• How are ammonium ions important?

  Absorbed from soil by plants as a source of nitrogen to be assimilated into protein and nucleic acids.
• How are phosphate ions important?

  Phosphate group in nucleic acids, ATP, phospholipids and calcium phosphate in bones. Bonds btwn phosphate groups in ATP store energy - involved in respiration and photosynthesis.
• How are nitrate ions important?

  Absorbed from soil by plants as a source of nitrogen to be assimilated into protein and nucleic acids
• How are hydrogen carbonate ions important?

  Act as a buffer for pH of blood.
• How are chloride ions important?

  Co-factor for amylase enzyme in digestion, involved in nerve impulses and in the chloride shift in gas exchange.
• How are hydroxide ions important?

  Affect pH. Increased OH- = more alkali.
• What is a monomer?


  Small, repeating subunit, built into larger molecules.
• What is a polymer?

  Large complex molecule, made from a large number of repeating monomers.
• What is a condensation reaction?

  Joining molecules with a chemical bond, producing a larger molecule, eliminating water
• What is a hydrolysis reaction?


  Breaking a chemical bond in a molecule with the addition of water, to produce smaller molecules
• Why do branched molecules hydrolyse faster?

  They have more ends on which enzymes can act
• What is a carbohydrate?


  Organic molecule containing C H O in the ratio 1:2:1.
• What is a monosaccharide?


  A single sugar
• What is a triose sugar?

  A sugar with 3 C atoms. E.g. aldo/keto triose.
• What is a pentose sugar?

  A sugar with 5 C atoms. E.g. ribose, deoxyribose.
• What are the structures of these pentose sugars? .

  
• What is a hexose sugar?

  A sugar with 6 C atoms. E.g. glucose, fructose, galactose.
• What are the structures of the two isomers of glucose?

  
• What is a disaccharide?


  Double sugar. E.g. maltose (α glycosidic bonds), sucrose (αβ glycosidic bonds), lactose (β glycosidic bonds).
• What are the properties of mono and disaccharides?

  Soluble in water, sweet, form crystals
• What are the functions of mono and disaccharides?

  Yield energy, lots stored in chemical bonds. Small and soluble so easily transported.
• What bond joins monosaccharides?

  Glycosidic.
• What is a polysaccharide?


  Multi sugar. E.g. starch (αglucose as amylose and amylopectin), glycogen (αglucose) and cellulose (βglucose).
• What are the properties of polysaccharides?

  Insoluble in water, not sweet, not crystallisable.
• What is important about the structure and function of starch?

  Made of 30% amylose helix (1-4 glycosidic bonds) and 70% branched amylopectin (1-4 and 1-6 glycosidic bonds). Side branches mean enzymes can easily hydrolyse glycosidic bonds to release glucose quickly. OH groups and H-bonds project inside the helix. Highly coiled and compact so lots of energy is stored in a small space. Insoluble so exerts no osmotic effect and cannot diffuse out of storage. Exists as grains.
• Why is amylose more soluble than amylopectin?

  Amylose has more OHs so more readily dissolves.
• What is important about the structure and function of glycogen?

  1-4 and many 1-6 glycosidic bonds. Many side branches so enzymes easily hydrolyse glycosidic bonds to quickly release glucose for respiration in animals. Highly coiled and compact so lots of energy stored in small space. Insoluble so exerts no osmotic effect and cannot diffuse out of storage. Prevents too high glucose levels in cells as glucose converted to glycogen. More soluble than starch so exists as tiny granules in liver and muscle cells.
• What is important about the structure and function of cellulose?


  1-4 glycosidic bonds Alternate βglucose monomers are flipped 1800, causing OH groups to stick out from the chain in opposite directions and H-bonds to form between neighbouring chains. This produces a lattice of parallel, unbranched, straight chains with many cross-links. These form microfibrils. This provides high tensile strength and is difficult to digest protecting plants from predator