More Biological Molecules - Water

Question 1

What are water’s functions?

Answer 1

• Water is a METABOLITE in loads of important METABOLIC REACTIONS,
  including CONDENSATION and HYDROLYSIS reactions.
• Water is a SOLVENT, which means some substances DISSOLVE in it.
  Most metabolic reactions take place in solution (e.g. in the CYTOPLASM of eukaryotic and prokaryotic cells) so water’s pretty essential.
• Water acts as a TEMPERATURE BUFFER because it has a HIGH LATENT HEAT of VAPORISATION and a HIGH SPECIFIC HEAT CAPACITY.
• Water molecules are very COHESIVE (stick together), which helps WATER TRANSPORT in PLANTS as well as transport in other organisms.

Question 2

Describe the structure of water molecules.

Answer 2

One atom of oxygen covalently bonded with two atoms of hydrogen.

Because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge.

The unshared negative electrons on the oxygen atom give it a slight negative charge.

This makes water a POLAR molecule – it has a partial negative charge on one side and a partial positive charge on the other.

The slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules.

This attraction is called HYDROGEN BONDING and it gives water some of it’s useful properties…

Question 3

Describe and explain the useful properties of water.

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Answer 3

Water is an important METABOLITE.

Many metabolic reactions involve a condensation or hydrolysis reaction.

A hydrolysis reaction requires a water molecule to break a bond.
A condensation reaction releases a molecule of water as a new bond is formed.

For example, amino acids are joined together to make polypeptides (proteins) by condensation reactions.
Energy from ATP is released through a hydrolysis reaction.
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Water has a HIGH LATENT HEAT OF VAPORISATION.

It takes a lot of energy to break the hydrogen bonds between water molecules.

So water has a high latent heat of vaporisation –
– a lot of energy is used up when water evaporates (vaporises).

This is useful for living organisms because it means they can use water loss through evaporation to cool down (e.g. humans sweat to cool down) without losing too much water.
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Water can BUFFER (resist) CHANGES IN TEMPERATURE.

The hydrogen bonds between water molecules can absorb a lot of energy.

So water has a high specific heat capacity – it takes a lot of energy to heat it up.

This is useful for living organisms because it means that water doesn’t experience rapid temperature changes.
This makes water a good habitat because the temperature under water is likely to be more stable than on land.
The water inside organisms also remains at a fairly stable temperature –
helping them to maintain a constant internal body temperature.
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Water is a GOOD SOLVENT

A lot of important substances in metabolic reactions are ionic
(e.g. salt).
This means they’re made from one positively charged atom or molecule and one negatively charged atom or molecule
(e.g. salt is made from a positive sodium ion and a negative chloride ion).

Because water is polar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to a positive ion.

This means the ions will get totally surrounded by water molecules–
– i.e. they dissolve.

So water’s POLARITY makes it a useful solvent.
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There is a STRONG COHESION between water molecules.

Cohesion is the attraction between molecules of the same type
(e.g. two water molecules).
Water molecules are very cohesive (they tend to stick together) because they’re POLAR.

Strong cohesion helps water to flow, making it great for transporting substances.
For example, it’s how water travels in columns up the xylem
(tube-like transport cells) in plants.

Strong cohesion also means that water has a HIGH SURFACE TENSION when it comes into contact with air.
This is the reason why sweat forms droplets, which evaporate from the skin to cool an organism down.
It’s also the reason that pond skaters, and some other insects, can ‘walk’ on the surface of a pond.