1
Topic 3: Chemistry of Life
3.1: Chemical Elements & Water
2
The Chemistry of Life
Which elements are most commonly found in the body?
Element Percentage composition of the human body
Oxygen 65%
Carbon 18%
Nitrogen 10%
Hydrogen 3%
Other elements exist in trace amounts in the body
Trace elementsElement Example role in
plantsExample role in animals
Example role in prokaryotes
Sulfur In some amino acids
In some amino acids
In some amino acids
Calcium Co-factor in some enzymes
Co-factor in some enzymes & component of bones
Co-factor in some enzymes
Phosphorus Phosphate groups in ATP
Phosphate groups in ATP
Phosphate groups in ATP
Iron In cytochromes In cytochromes and in haemoglobin
In cytochromes
Sodium In membrane function
In membrane function and sending nerve impulses
In membrane function
Pearson, Pearson Baccalaureate, Higher Level Biology, 2007, p. 47
5
The Key to Life: Water
o It acts as a solvent, dissolving nutrient molecules and transporting them in blood plasma (which is 90% water).
o Many chemical reactions take place in water and/or require water to break the bonds in macromolecules.
o Water helps maintain body temperature – sweating to decrease temperature, and shivering to increase body temperature.
o Water is a major component of body lubricants – sweat, tears, saliva, and synovial fluid in joints.
o 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 -
+ +
O
H H
Water is a polar covalent molecule
Why is water so special?
-61-8634H2SHydrogen sulphide
+19-9220HFHydrogen fluoride
+100018H2OWater
-33-7817NH3Ammonia
-161-18416CH4Methane
Boiling point / °C
Melting point/ °C
Molecular mass
FormulaMolecule
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
9
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 organismssolvent propertiesremains a liquid over a large range of
temperatures Water is also an important medium for
biochemical reactions
Cohesion
“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
Public Domain image
Eucalyptus
grandis
NSW Australia
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 © Shirley Burchill 2007
Water skater
Gerris gibbifer
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
19
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:
1. 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?
2. 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?
3. Some people suffer from a condition that makes it impossible to perspire. How might this limit their physical activity?
Top Related