Water

Hiya,

I've decided to bring back this blog for my A-levels because why not. Here's number 1; Biological Molecules; Water.

Water is a key biological molecule as it allows life on Earth to even exist. It has a variety of unusual properties that allow functions to occur, without dampening the topic, I'll dive straight in.

Water molecules are polar and form hydrogen bonds; 

Because of the significant difference in electronegativity between the hydrogen atoms and oxygen atoms in the molecules, permanent dipoles are created meaning there is a consistent charge imbalance.

This imbalance of charge causes the hydrogen atom of one molecule can form a hydrogen bond between the spare pair of electrons on another molecule. This is a strong bond between molecules in addition to the covalent bond within the water molecule.

Because water is polar, it is a polar solvent as a liquid meaning that polar substances can dissolve in it. This means useful biological molecules like mineral ions, sugars and molecules can dissolve in aqueous solutions. Why is this significant? It means water is a suitable medium for chemical reactions ; this is why cytoplasm is 80% water.

Hydrogen bonds lead to a variety of properties;

• Water is less dense as a solid- In most substances, solids are more dense than liquids because there are more particles packed into the same space; however, in water, because Hydrogen bonds are relatively long more hydrogen bonds are present at solid form therefore molecules are more spaced out by these hydrogen bonds and it becomes less dense. This is significant because it means Ice can float on water, preventing large areas of water from freezing over entirely. Also, Ice on top of water can allow the water to be insulated in extreme weather, maintaining a more stable water environment.
• Water has a high specific heat capacity- As water is a covalent molecule, it's not expected to have a high specific heat capacity (the amount of energy required to raise 1kg of water by 1 degree Celsius without changing state) but because it also has hydrogen bonds, the energy required to raise temperature (from breaking bonds) is significantly higher. This high specific heat capacity is useful because it means aquatic environments are relatively stable as lots of energy is required to change the temperature. This is valuable because it means metabolic processes can occur at their optimum temperatures for longer within organisms.
• Water has a high latent heat of vapourisation- Similarly to specific heat capacity, latent heat of vapourisation is the energy required to change liquid water into water vapour. This is high because of the additional hydrogen bonds therefore water remains in liquid state for most environments around the world. This allows aquatic environments to exist and also allows water to be an effective cooling agent as it keeps temperatures stable.
• Water molecules experience cohesion and surface tension- Water molecules can form droplets at hydrogen bonds pull molecules close together when on a surface. This is useful as it keeps water molecules together allowing processes like capillary action in the xylem to occur. Surface tension is where the water molecules in a liquid are more attracted to the molecules below than the air molecules above. This means that when light pressure is applied, the surface won't break. This is useful as it allows organisms like pond-skaters to walk on water, perhaps Jesus had this knowledge...