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

stevenemassey@gmail.com

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.....