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Topic 3: Chemistry of Life

3.1: Chemical Elements & Water

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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

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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

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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

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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?