Water Structure & Function
Biology: Cambridge International A Level, topic 2.4.1
Function of Water
Water makes up 60-70% of the human body. Life also evolved in a watery environment. Water's chemical structure gives it properties that are vital for life.
Uses of water
- Water is one of the most useful molecules for life.
- Water acts as a reactant in cells, for example in photosynthesis and hydrolysis reactions.
- Water provides structural support in cells, for example by keeping them turgid.
- Water helps keep organisms cool, maintaining body temperature.
Hydrogen bonding in water
- Water molecules are polar, with a slightly negative oxygen atom and slightly positive hydrogen atoms.
- A polar molecule has an unequal spread of charge.
- The charges allow water molecules to form hydrogen bonds with each other.
- Hydrogen bonding gives water properties that are vital for life.
Cohesion and adhesion
- Hydrogen bonds cause cohesion in water.
- Cohesion is a pull between water molecules.
- Cohesion pulls water up through xylem vessels in plants.
- Water also shows adhesion, a pull between water and other polar surfaces.
- Adhesion helps water stick to the walls of xylem vessels.
Thermal properties
- Water has a high specific heat capacity.
- A high specific heat capacity means water needs lots of energy to warm up.
- High specific heat capacity keeps aquatic habitats at a stable temperature.
- Water has a high latent heat of vaporisation.
- Evaporation of sweat cools the body down.
Water as a solvent
- Water is a good solvent.
- Water's polarity attracts charged and polar substances.
- Hydrophilic substances are polar or ionic and dissolve easily in water.
- Hydrophobic substances are non-polar and do not dissolve in water.
- Water carries dissolved substances around the body.
Density of water
- Ice is less dense than liquid water.
- Hydrogen bonds hold the molecules further apart in ice than in liquid water.
- Lower density means ice floats on top of liquid water.
- Floating ice insulates the water below, protecting aquatic organisms from freezing.
The Structure of Water
The structure of a water molecule helps us to understand hydrogen bonding and the function of water.
Contents of a water molecule
- Water molecules (H2O) are made from:
- One oxygen atom.
- Two hydrogen atoms.
Polarity
- Water is a polar molecule.
- The oxygen atoms in water are slightly negatively charged.
- The hydrogen atoms in water are slightly positively charged.
Hydrogen bonding
- The polarity of water molecules means that a hydrogen atom on one water molecule is attracted to the oxygen atom on another water molecule.
- This attraction is called hydrogen bonding.
Useful Properties of Water
Water is a major component of cells and is essential to life as we know it (60–70% of the human body is made up of water). The properties of water that make it such a useful substance are:
High latent heat of vaporisation
- The latent heat of vaporisation is the amount of energy needed to change one gram of a liquid substance to a gas.
- As liquid water heats up, hydrogen bonding makes it difficult to separate the water molecules from each other. This means that a lot of energy is needed for water to evaporate.
- When water evaporates, energy is used up - this cools the environment where the evaporation is taking place.
- This is why sweating helps with body temperature regulation.
High heat capacity
- Specific heat capacity is the amount of heat one kilogram of a substance must absorb or lose to change its temperature by 1oC.
- Water has a high heat capacity so takes a long time to heat and cool.
- The specific heat capacity of water is much larger than sand. This is why land cools faster than the sea.
- Water is used by warm blooded animals to more evenly disperse heat in their bodies.
Good solvent
- Water is a good solvent because ions and polar molecules can easily dissolve in it.
- Water is a polar molecule. This means that the positive end of the water molecule attracts negative ions and the negative end will attract positive ions.
Good metabolite
- Water is used or formed in many metabolic reactions, such as condensation and hydrolysis reactions.
- ATP + H2O → ADP + Pi + energy
- A bond is broken and a water molecule is used up and so this is a hydrolysis reaction.
- ADP + Pi + energy → ATP + H2O
- A new bond is formed and a water molecule is released and so this is a condensation reaction.
Cohesive properties
- The strong attraction between water molecules due to hydrogen bonds is called cohesion.
- Cohesion produces surface tension where water meets air.
- This is why water forms droplets when placed on a dry surface rather than being flattened out by gravity.
- Plants use this natural phenomenon to help transport water from their roots to their leaves.
Test yourself on Water Structure & Function
Decide which statement about water's properties is correct in each pair.
What % of the human body is made up of water?
What are the components of a water molecule?
What is the attraction between water molecules?
Which of the following statements about water are true?