Agua Steven E. Massey, Ph.D. Associate Professor Department of Biology University of Puerto Rico Río Piedras Office & Lab: Bioinformatics Lab NCN343B 787-764-0000 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 bonding Water is a bent molecule - because of this it has polarity given 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 charged compounds 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)
Hydrophobic effect explained 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 to Gibbs free energy : G = H - T S
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 the physical 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
Buffering Some 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 also complex systems as well eg. the internet is robust There is currently a debate as to how this robustness evolved...