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

2. The Chemistry of Life

3. 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

4. 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

5. 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

6. 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

7. 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?

8. 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!

10. 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

11. 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

12. African elephants (Loxodonta africana) bathing © Shirley Burchill 2007

13. 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

14. 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

15. 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

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

17. 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

18. 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

20. 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 Moss Landing Marine Laboratories

21. 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?