BIOLOGICAL MOLECULES: Water

Structure

Water is a polar molecuel (slightly charged). 

Water is bonded with both covalent bonds and hydrogen bonds.

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Roles

> Provides a habitat for living things in lakes, rivers, seas and other bodies of water.

> Forms a major component of tissue.

> Provides a reaction medium for chemical reactions.

> Provides an effective transport medium.

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Property: Density

> Ice is less dense than water because by 4 degrees the polar molecules align themselves so they turn into a solid form (ice).

> Because is is less dense than water, aquatic animals ahve a stable habitat during winter and ponds etc are insulated against extreme cold.

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Property: Solvent

> Water is a good solvent

> Because water is polar, the positive and negative parts of the molecule attract to the solute. 

> They cluster around the solute molecules keeping them apart.

> Because of this, molecules and ions can be transported around living things whilst dissolved in water.

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Property: Cohesion + Surface Tension

> Hydrogen bonding pulling water molecules together is cohesion. 

> Surface tension is as a result of  hydrogen bonds between molecules of the surface of water and molecules beneath the surface of water.

> The surface of the water contracts as the molecules are being pulled inwards.

Surface tension = the water thus has the ability to resist force put on it.

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Property: High Specific Heat Capacity

High specific heat capacity = the amount of heat energy required to raise the KE and temperature of molecules.

> 4.2kJ energy is required to raise the temperature of 1kg of water by 1 degree. 

> This is important for living things for enzyme-controlled reactions. 

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Property: Latent Heat of Vaporisation.

> Latent heat of vaporisation helps the molecules to break away from eachother when water evaporates. 

> Water is therefore able to help cool living things.

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Property: Reactants

> Water is a reactant in reactions. 

> For example: photosynthesis, hydrolysis reactions, digestion of starch, proteins and lipids.

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