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Agua
Steven E. Massey, Ph.D.Associate Professor
Department of BiologyUniversity of Puerto Rico – Río Piedras
Office & Lab: Bioinformatics Lab
NCN343B787-764-0000 ext. 7798
Why does life require liquid ?
Organisms are about 70 % water
Many chemical reactions take place in liquids
Transport of chemicals is easier and controllable
The cell is flexible - important for movement, differentiation and cell division
Flexibility of proteins and other macromolecules is important in their function
Why is water ideal for life ?Because it is strange !!
Abundant
Surface tension – important for transport in plants and animals
'Correct' melting temperature – large liquid range, allows life in many habitats on earth. Only liquid (other than mercury and ammonia) to occur at planetary temperatures
High heat capacity – keeps environmental conditions steady
Expands as it freezes, meaning that ice floats
Largely inert but may participate in many biochemical reactions eg. hydrolysis, condensation, photosynthesis
'Universal solvent' for a wide range of chemicals both charged and polar, while it is immiscible with non-polar compounds
Polarity and hydrogen bondingWater is a bent molecule - because of this it has polaritygiven that the oxygen atom is electronegative
Hydrogen bonding by water is the result of two hydrogen atoms and two free electron pairs on the oxygen atom
Electron pairs in a water molecule form a tetrahedron
H-bonding between water molecules is responsible for the high melting and boiling points of water, the high heat capacity, surface tension, the ability to solvate polar molecules and the immiscibility of lipids
So, the asymmetry of the molecule produces properties conducive to life
Bonding with polar and chargedcompounds
Because of its polarity, water can form electrostatic interactions with polar and charged compounds
In NaCl the ions are hydrated by water molecules
Insolubility of non-polar compounds
Water does not bond with nonpolar compounds
Amphipathic compounds have a polar part and nonpolar part
The polar part bonds with water, but the nonpolar does not bond
Hydrophobic effect Important for the folding of proteins and the assembly of membranes
This is driven by an increase in entropy NOT by a change in bonding (enthalpy)
Folding of a protein or formation of a membrane minimizes the surface area in contact with water
This means that less water is ordered around the molecule
This means there is more disorder overall, and that the total entropy is higher
This means the reaction is more likely to take place according toGibbs free energy :
∆G = ∆H - T∆S
Hydrophobic effect explained
Van der Waals forces
Van der Waals forces arise from small fluctuations in the electron shell of atoms. These create small changes in charge that can lead to weak attraction
These are not the same as hydrophobic interactions, which are not bonds
Effect of solutes on thephysical properties of water
Melting temperature, boiling point, osmotic pressure and vapor pressure are all affected by solute concentrations
These are called colligative properties
They depend on the number of solute molecules present, not the type
The higher the concentration the higher the osmotic pressure and boiling point
and lower the vapor pressure and melting point
Difusión: Movimiento de partículas de un area de mayor concentración a un area de menor concentración.
Osmosis: Movimiento de agua a través de una membrana semipermeble causado por diferencias en presión osmótica
The difference between diffusion and osmosis
Ionization
H3O+
Hydronium ion
This means that water is a proton donor and so is a weak acid
pH
[H+] = 1 x 10-7 M in water
pKa
When [A-] = [HA] then
pKa = -log[H+]
If pKa < 7 then compound is acidic
If pka > 7 then compound is basic
BufferingSome compounds prevent changes in pH – these are termed buffers
At the pKa there is an equal mixture of acid and base
Henderson-Hasselbalch equation
Buffering in biological systems
Biological systems are usually buffered – this is a part of homeostasis
This can occur by acidic or basic functional groups on amino acids..........next lecture !
Robustness is an innate property of biological systems and alsocomplex systems as well eg. the internet is robust
There is currently a debate as to how this robustness evolved.....