JESS'S A LEVELS: Biology: Chapter 2: Biological Molecules: Water

Biology: Chapter 2: Biological Molecules: Water

In covalent bonds, atoms share electrons. Each shared electron is one covalent bond.
However electrons are not shared equally. Some particles have a higher electron affinity (electron attraction) than others.
In water, oxygen has a higher electron affinity, so the electron spends more time with the oxygen atom, making it slightly negative, which is called delta minus (δ-)
Hydrogen has a weaker electron affinity, so the electron spends less time with them, making them slightly positive - delta plus (δ+)
This unequal distribution of charge is called a dipole.

The negatively charged oxygen of one H20 is attracted to the positively charged hydrogen of another.
This is called a hydrogen bond.
Weaker than a covalent bond, but still has a significant effect.
Hydrogen bonds are not inclusive to water; dipoles can occur in many different molecules, particularly ones with -NH-, -CO-, or -OH- groups.
Important for the structure of proteins and carbohydrates.
Molecules with dipoles are called polar molecules; they are attracted to water hydrophilic) because water is also polar.
Molecules without dipoles are called non-polar molecules; they are not attracted to water (hydrophobic) .

Excellent solvent ( substance that dissolves a solution) for ions and polar molecules (unevenly charged molecules) because of water is also a polar molecule and is therefore attracted to ions and polar molecules.
The H2O molecules collect around the molecule and separates them, which we can also called dissolving. This allows the separate ions or chemicals to move freely and react with other chemicals.
Most chemical processes in living organisms take place this way.

Water can take part in some chemical reactions
Photosynthesis: Sunlight is used to separate hydrogen from water. The hydrogen is then used as a fuel to produce energy needed for the plant - eg. making glucose (C6H12O6), which is rich in energy,
The plant produces oxygen as a waste product, which is then used by aerobic organisms for respiration.
Water is also used for hydrolysis reactions, where water is needed for large molecules to be broken down eg. digestion.

To raise the temperature of water or change it to a gas, molecules need to gain energy to move more rapidly. Hydrogen bonds prevent water molecules from moving too much, so they need to be broken first which requires more energy.
Allows water to store more energy for the given temperature.

Specific heat capacity: Amount of heat required to raise it's temperature by a given amount; in water, it is the amount of heat energy needed to raise the temperature of 1kg of water by 1C.
Water has a high heat capacity because of it's hydrogen bonds.
Water has a more constant temperature that is less likely to be affected by air temperature.
Temperatures of the water inside living organisms is more constant so chemical reactions can happen at constant rates and are less likely to be affected by changes in outside temperature.
Large bodies of water ( eg. lakes and oceans) less likely to be affected by air temperature, providing more stable environments for aquatic organisms.

Latent heat of vapourisation: Measure of heat needed to change a liquid to gas.
Water has a high latent heat of vapourisation because the hydrogen bonds need to be broken
Water absorbs a lot of heat from it's surroundings before evaporating.
Living organisms use this as a cooling mechanism, by producing water droplets we call sweat.
A large amount of heat can be lost for a small amount of water, which reduces risk or dehydration or overheating.
Plants also use this to cool leaves during transpiration.
Works the other way - Water is less likely to freeze, so aquatic animals' bodies are less likely to freeze

Ice is less dense than water.
Below 4C, density of water starts to decrease,
Ice floats on water, insulating the water underneath it.
Reduces chances of large bodies of water (eg. lakes) to freeze completely, and makes it more likely for aquatic life surviving in cold conditions.
Changes in water density move nutrients in the ocean.

Cohesion: Tendency of molecules to 'stick' to each other.
Water has high cohesion because of hydrogen bonding.
Helps water move in long, unbroken columns through and up vascular tubing in plants.
This also results in high surface tension at the surface of water
Small organisms, such as pond skaters, can 'skate' over water without falling in, providing a good habitat for them.