Biology - properties of water and buffers

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  • Created by: Ivana
  • Created on: 27-11-13 20:03

Properties of water and buffers

-Solubilising properties of water help to keep polar molecules in solution and the nonpolar molecules are driven to adopt structures which avoid contact with water.

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Structure of water

-> Dipole: One pole is more electronegative than the other pole, allowing water molecules to interact. 
-> Hydrogen bonding: +ve ends are attracted to -ve ends. contributes to many properties of water
HYDROGEN BONDS (can form over distances) IS LONGER AND WEAKER THAN A COVALENT BOND (between hydrogen and oxygen).
-Fairly weak compared to covalent bonds in a compound that is solid at room temp
-Part of the reason water is a liquid at room temp
-Reason why water can evaporate
-> Surface tension: Due to hydrogen bonding and attractive forces between molecules. Surface molecules do not have other molecules on all sides of them so they cohere more strongly with those directly associated with them on the surface.Consequently forming a 'film' making it more difficult to move an object through the surface than when it is completely submersed 

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HYDROPHILLIC AND HYDROPHOBIC

->Hydrophillic (water loving/dissolve in water): POLAR COMPOUNDS
Glucose, Glycine, Aspartate, Lactate, Glycerol
-> Hydrophobic (water hating/ wont dissolve in water) NON-POLAR:
Typical wax.

-> Amphipathic (have both hydrophillic and hydrophobic parts:
Phenylalanine, Phosphatidylcholine

HYDROPHILIC - IONS-OH MOLECULES-DIPOLES-NONPOLAR MOLECULES -HYDROPHOBIC 
big spectrum between a molecule being hydrophillic or hydrophobic  

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Dielectric constant

Dielectric constant: a quantity measuring the ability of a substance to store electrical energy in an electric field. Meaning that substances whose molecules contain ionic bonds will tend to dissociate in water yelding solution contain charged species.
-> Water has a high dielectic constant:
Can easily overcome the attractive forces that hold oppositely charged particles together
Solubilises hydrophilic polar substances 

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Lipids/ oil droplets in water

OIL
-> immiscible in water (not forming homogenous mixture when mixed) 
->initially dispersed as small droplets which then reassociate and sit on top of the water
->thermodynamically unfavourable (gibbs free energy is negative) 
-> water forms highly strcutured (clatherate) complexes around the oil
highly ordered H20 molecles form cages around the hydrophobic akyl chains (alkane missing one H) 
dispersion of lipids in H20 - each lipid molecule forces surround H20 molecules to become highly ordered
clusters of lipid molecules - only lipid portions at the edge of the cluster force the ordering of water. Fewer H20 molecules are ordered and entropy is increased.  

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Solubilities of gas in water

> polar molecules dissolve far better even at low temperatures than non polar molecules do at high temperatures

>more gas can dissolve in water at low temperature

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Density of water

-The density of fresh water decreases as temperature increases
-Fresh water density peaks at 4C 

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Summary

MAIN PROPERTIES OF WATER:
> Hydrogen bonding:
-Longer than covalent bonding
-allows surface tension
>Solubilising properties :
-allows various polar molecules (solid, liquid and gas) to dissolve in water
>Lipid dispersion properties:
-lipids form clusters in water due to reoganisation of the water molecules

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Summary

MAIN PROPERTIES OF WATER:
> Hydrogen bonding:
-Longer than covalent bonding
-allows surface tension
>Solubilising properties :
-allows various polar molecules (solid, liquid and gas) to dissolve in water
>Lipid dispersion properties:
-lipids form clusters in water due to reoganisation of the water molecules

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