ib biochemistry unit - properties of water

ib biochemistry unit - properties of water

1. The approximate proportions of the four main elements in living things are : • Carbon (19%)                         • Hydrogen (10%)                        • Oxygen (65%)                        • Nitrogen (3 %) Outside of the four main elements, living things may contain trace amounts of 20 or so other elements, including:  • Sulfur (0.25%)                         • Calcium (1.5%)                         • Phosphorus (1%)                        • Iron (0.7%)                         • Sodium (0.15 %)

2. Sulphur (S):  Found in certain amino acids ( cysteine and methionine ), allowing proteins to form disulphide bonds Calcium (Ca):  Found in bones and teeth, also involved in neurotransmitter release in synapses Phosphorus (P):  Component of nucleic acids and cell membranes Iron (Fe):  Found in haemoglobin (animals), allowing for oxygen transport Sodium (Na):  Involved in the generation of nerve impulses in neurons

3. Water (H 2 O) is made up of two hydrogen atoms covalently bound to an oxygen atom While this bonding involves the sharing of electrons, they are not shared equally  The oxygen atom, having more protons (+ ve ), attract the electrons (- ve ) more strongly (i.e. the oxygen has a higher electronegativity ) Thus the oxygen atom becomes slightly negative and the hydrogen atoms become slightly positive Water is polar

4. Hydrogen Bonding between Water Molecules Covalently bonded molecules that have a slight potential charge are said to be polar The slightly charged regions of the water molecule can attract other polar or charged compounds Water molecules can associate via hydrogen bonds (F/O/N bonding with H ) Hydrogen bonds are weaker than covalent bonds but powerful enough to give water special properties

5. Thermal Properties Water has a high specific heat capacity (the measure of energy required to raise the temperature of 1 g of substance by 1°C ) – difficult to change temperature of water – thermally stable habitats for aquatic organisms Water has a high latent heat of vaporisation (amount of energy absorbed per gram as it changes from a liquid to a gas / vapour ) – lots of energy absorbed when water goes from liquid to gas – water is a good coolant ( eg . Sweating)

6. Thermal properties continued High boiling point – water is liquid over a wide range of temperatures (0-100 deg C ), majority of earth’s habitats in this range Thermal properties occur as a result of the extensive hydrogen bonding between water molecules - this allows water to absorb considerable amounts of energy with little change in form (H-bonds need to be broken first)

7. Water vs methane Property Methane Water Formula CH 4 H 2 O Molecular mass 16 g/mol 18 g/mol Density 0.46 g/cm 3 1 g/cm 3 Specific heat capacity 2.2 J/(g deg C) 4.18 J/( g deg C) Latent heat of vaporization 760 J/g 2,257 J/g Melting point -182 deg C 0 deg C Boiling point -160 deg C 100 deg C

8. Cohesive Properties Water molecules are strongly cohesive (they tend to stick to one another) These properties occur as a result of the polarity of a water molecule and its ability to form hydrogen bonds with appropriate molecules Useful for transport of water in plants (xylem carry water to top of tallest trees >100 m)

9. Adhesion Water molecules will also tend to stick to other molecules that are charged or polar ( adhesion )  Effect of hydrogen bonds and dipolarity Water adheres to cell walls (cellulose) in leaves – keeps them moist so they can absorb CO 2 for photosynthesis

10. Solvent Properties Water can dissolve many organic and inorganic substances that contain electronegative atoms  (such as fluorine, oxygen and nitrogen) This occurs because the polar attraction of large quantities of water molecules can sufficiently weaken intramolecular forces (such as ionic bonds) and result in the dissociation of the atoms Cytoplasm is complex mixture of many dissolved substances needed for metabolism

11. Coolant Both plants and animals use the evaporation of water from the surfaces of their bodies to facilitate cooling (sweating and panting in animals, transpiration from leaves in plants) Water can be used to carry heat to cooler places in our bodies (countercurrent exchange of thermal energy ) Medium for Metabolic Reactions Water can dissolve many organic and inorganic substances to facilitate chemical reactions Water can also absorb thermal energy released as a by-product of many chemical reactions

12. Transport Medium The forces of attraction between water molecules help facilitate the transport of water up the xylem of plants Water is an effective transport medium for dissolved substances (in plants, minerals from the soil and sugars from the leaves can be transported in water through the xylem and phloem respectively; while in animals, water in the blood is used to transport oxygen, glucose and urea )

13. Hydrophobic and hydrophilic Hydrophobic substances (red) will be forced together so water can do more hydrogen bonding with itself – hydrophobic effect

14. Transport in blood plasma (liquid part of blood) How transported varies depending on solubility NaCl dissolves in water, Na+ and Cl - hydrophilic Amino acids – have + and – charges, some more soluble in water than others but all soluble enough in water to be carried dissolve in blood plasma Glucose – polar, soluble Oxygen – nonpolar , very little can dissolve, great majority must be carried by hemoglobin in red blood cells

15. Transport in blood plasma continued Fats and cholesterol are lipids ( nonpolar , insoluble in water) Must be carried by lipoproteins

16. Summary Water has hydrogen bonds High specific heat capacity High latent heat of vaporization Cohesive Adhesive Solvent Coolant Reactions happen there (medium for metabolism) Good for t ransport