Summary of AQA A Level Biology Water topic in Unit 1 (Biological Molecules) topic
A* and A quality
As with all my notes, the underlining is the wording from the AQA specification so is mark scheme specific
Concise and perfect revision poster on all of the water topic
Water is major component in cells. It has several properties that are important in Biology
Approximately 80% of the cytoplasm is water
Properties of an individual water molecule:
- POLAR this means that electric charge is not equally spread across the molecule
- Oxygen atom has a slight negative charge and hydrogen atom has a slight positive charge
- When multiple water molecules are placed in close proximity these opposite charges attract each
other
- These intermolecular forces are called hydrogen bonds, and they cause water molecules to bind
loosely together
- POLAR means it makes an excellent solvent
Properties of water that make it important to Biology:
1) Water is a metabolite in many metabolic reactions, including condensation and hydrolysis reactions
Many metabolic reactions happen inside aqueous cytoplasm of cells
Water is a reactant and product in many of these
2) Water is an important solvent in which metabolic reactions occur
Polar molecules and ions will only dissolve in liquid polar substances; non-polar substances will only
dissolve in non-polar substances
Vast majority of biologically important molecules are polar, therefore dissolve in water – said to be a
UNIVERSAL solvent
Exceptions: lipids are hydrophobic molecules that are non-polar so DO NOT dissolve in water
3) Water has a relatively high specific heat capacity, buffering changes in temperature
This means that water can gain or lose a lot of heat energy, without changing temperature
This is a result of hydrogen bonding between water molecules being relatively hard to disrupt
Changes in internal temperature are therefore buffered, so temperature does not rise too quickly or
too slowly when placed in a hot/cold environment
4) Water has a relatively large latent heat of vaporisation, providing a cooling effect with little loss of
water through evaporation
This means that a lot of heat energy is required to cause water to change state from liquid to gas
Heat energy is used to break the hydrogen bonds between water molecules as the molecules move
apart on becoming gases
This is the basis on which sweating cools you down – a lot of body heat energy is used to evaporate
water on the skin, causing a cooling effect with little loss of water
5) Water has strong cohension between water molecules: this support columns of water in the tube-like
transport cells of plants and produces surface tension where water meets air
Water displays cohesion – neighbouring molecules “stick together” due to hydrogen bonding
Produces surface tension where water meets air and helps explain water transport in the xylem by
cohesion tension theory
^^ GOOD ESSAY LINK
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