Topic 3: Chemistry of Life 3.1: Chemical Elements & Water

The Chemistry of Life

Which elements are most commonly found in the body? Percentage composition of the human body Oxygen 65% Carbon 18% Nitrogen 10% Hydrogen 3% Other elements exist in trace amounts in the body

Trace elements Element Example role in plants Example role in animals Example role in prokaryotes Sulfur In some amino acids Calcium Co-factor in some enzymes Co-factor in some enzymes & component of bones Phosphorus Phosphate groups in ATP Iron In cytochromes In cytochromes and in haemoglobin Sodium In membrane function In membrane function and sending nerve impulses Pearson, Pearson Baccalaureate, Higher Level Biology, 2007, p. 47

The Key to Life: Water It acts as a solvent, dissolving nutrient molecules and transporting them in blood plasma (which is 90% water). Many chemical reactions take place in water and/or require water to break the bonds in macromolecules. Water helps maintain body temperature – sweating to decrease temperature, and shivering to increase body temperature. Water is a major component of body lubricants – sweat, tears, saliva, and synovial fluid in joints. Water is the basic component of all living tissues – cytoplasm, blood plasma, interstitial fluid, and lymph fluid. 5

The Key to Life: Water Formula H2O Structure Slightly () negative at the oxygen end and slightly positive at the hydrogen end Water is a polar covalent molecule  -  + O H

Why is water so special? Boiling point / °C Melting point Molecular mass Formula Molecule -161 -184 16 CH4 Methane -33 -78 17 NH3 Ammonia +100 18 H2O Water +19 -92 20 HF Hydrogen fluoride -61 -86 34 H2S Hydrogen sulphide Compared to molecules of similar size and properties water has a very high melting and boiling point - WHY?

The association between the polar water molecules Weak hydrogen bonds

So what? Hydrogen bonding gives water lots of unique properties: thermal properties & very high specific heat capacity universal solvent cohesion surface tension density All of these combine to allow us to function the way we do!

Thermal properties The molecules of water can absorb a lot of heat energy Water has a very high thermal capacity (4.184 J°C-1 g-1) The hydrogen bonding forms a lattice which does not easily fall apart as the temperature rises

Thermal properties and life Water is a very thermally stable medium Water helps living organisms resist changes in their environment To make water change from a liquid to a vapour requires a lot of energy (high boiling point) Evaporation of water on the surface of a body cools it down significantly

African elephants (Loxodonta africana) bathing © Shirley Burchill 2007

Solvent properties The polar properties of water make it a good solvent for: Polar molecules (e.g. sugars and alcohols) Positives are attracted to the O and negatives to the H Ionic compounds (e.g. salts, acids and bases) These dissociate into their component ions

Solvent properties and life Water is a very important transport medium for living organisms solvent properties remains a liquid over a large range of temperatures Water is also an important medium for biochemical reactions

Cohesion Results in high surface tension “Stickiness” between water molecules Stick together because of hydrogen bonds Cohesion of water allows it to be used as a transport medium (in the xylem of plants) by being sucked up to the tops of trees The tallest are at the physical limits of water trees (sequoia and eucalyptus are 100m tall) Results in high surface tension

Giant red wood Sequoiadendron giganteum California USA Eucalyptus grandis NSW Australia Public Domain image Public Domain image

Surface tension Water molecules hold together forming a skin at the surface This is strong enough for some organisms to be supported Water skater Gerris gibbifer Water Skater © Shirley Burchill 2007

Density Water is densest at 4°C whilst it is still a liquid So ice floats on the surface of water H-bonds result in open structure Organisms which live in water do not risk freezing solid so easily Water forms a good habitat for living organisms Iceberg, Antarctica © Shirley Burchill 2007

Density

Transparency Water is a transparent liquid, light passes though it Blue light, with the most energy, penetrates furthest, red light is the weakest and penetrates least Plants can photosynthesize under water Animals can use their visual systems Kelp forests (Macrocystis pyrifera) California © Mike Graham, Phycology Lab @ Moss Landing Marine Laboratories

Homework Choose any specific aquatic or terrestrial animal and make a list of all the ways that water is important to the animal Using the properties of water, explain each of the following situations: A well-watered plant is in freezing air temperature for an hour but shows no sign of damage. How did water help protect the plant? Areas of land close to large bodies of water often contain organisms that cannot life far from the body of water. These organisms are often limited to certain environmental temperatures. How is water related to this situation? Some people suffer from a condition that makes it impossible to perspire. How might this limit their physical activity?