Water Lecture-BIOL 483 PDF

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Second Lecture notes -Prof Erb -on water properties...

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STUDY GUIDE PART 1 WATER • Water has special properties that make it ideal for living systems: hydrogen bonding, solvation properties, dielectric constant, and heat capacity.  Water interacts with different molecules through hydrogen bonding- it allows it to interact different molecules through hydrogen bond interactions. Water can act as a solvent for many different molecules. High dielectric constant (the amount of dipoles are in the solvent).  Water has a high dielectric constant and high heat capacity which prevents all water to evaporate in the body at our standard living temperature.  Oxygen has 6 valence electrons, when binds with two H= 2 lone pairs, 2 H-bonds.  Bond angle between H-O-H is 104.5. Electrons are in sp3 orbital. The two lone pairs on the oxygen distorts the shape of the tetrahedral and repel the hydrogens.

 Oxygen is partially negative charge and the hydrogen has a partially positive charge. Dipole moment from oxygen toward hydrogen.  Water is a good solvent for charged and polar substances. Poor solvent for nonpolar substances.  More electronegativity = more polar

HYDROGEN BONDS: not actual bond, more of force, strong dipole dipole interaction  In order to form a hydrogen bond, you need a H donor and acceptor. Donor is going to be any atom bind to a H and is going to be electronegative (commonly oxygen or nitrogen). Oxygen has partial negative charge. H-bond acceptor is going to be an electronegative with lone pair of electrons (commonly oxygen, nitrogen). Donor and acceptor has to be in proximity to each other.  Carbon is not a donor or acceptor.  H- bond in a straight structure is a stronger H-bond than a H-bond bent in 108 angle (weaker H-bond)

 Each water molecule can make up to 4 hydrogen bonds. Water has whole network of H-bonding which leads to extreme attraction, makes water to have a high BP and MP. Takes more energy to break the bonds.  Each oxygen: # lone pairs= # of H-acceptors. More H-acceptors, more soluble with water.  Positively charged ion attracts negative charge of the water.  H-bonds are seen in hydroxyl group, carbonyl group, polypeptide, and DNA  Entropy increase as ordered crystal lattice is dissolved.

AMPHIPATHIC COMPOUNDS (IONIC)- hydrophobic and hydrophilic  Fatty acids are major component in membranethey’re amphipathic. They have a hydrophobic tail (nonpolar) and a hydrophilic head (polar). COO-

(polar) has hydrogen bond acceptors. The hydrocarbon tail be surrounded by water, but when stuck with water, water makes H-bonds (no H-bond acceptors/donors) with the CH groups.  In order to avoid interaction, water becomes ordered-water adapting in order.  Water molecules interacting near fatty acids are in ordered shells which help to limit the minimize the amount of interaction between water molecules and nonpolar group. In order to reduce surface area, nonpolar groups will cluster and the SA of water is smaller.  Individual fatty acids in water: Fatty acids cluster together and the SA decreases- entropy (disorder) of water increase.  Entropy (disorder) is higher when it is thermodynamically favorable. Hydrophobic solutes have low solubility.  Hydrophobic effect: hydrophobic interact together to avoid interaction with water. Don’t make favorable interactions with the water. Clustered to minimize interaction with surrounding.

 Van der Walls interactions: at all distances there are interactions between all atoms-weak

Ionization of Water

+ H O ->H + 2

 Protons hop from water molecule to another water molecule which allows acid and base reactions to occur very quickly.  Keq is the equilibrium constant, products over reactants.  Concentration of water in water is 55.5 M.  Ionic product of water: Kw=Keq * [H2O]=[H+] [OH-]=1*10-14 M2.  pH is the negative log of hydrogen ion concentration.

pH=-log [H+]

pH+pOH=14  In neutral solution, [H+]=[OH-] and pH 7. pH can be negative.

BUFFERS

 Weak acids and weak bases partially dissociates in water- they are good buffers. Buffers allow reaction to occur in a constant pH environment. o Ex: acetic acid- has a proton that dissociates and will form a hydronium ion and left with acetate. Weak acid gives up a proton.  When the numerator value is less than denominator (small fraction), means more reactant which means weak acid/base.

pKa= -log Ka Ka= acid dissociation constant  pKa: How strongly the acid is going to dissociate. Lower pKa, easier proton will dissociate.  Buffers are mixtures of weak acids and conjugate base. They resist change in pH, neutralizes mixture  When the pH= pKa, buffer is at its greatest capacity. The buffering capacity is lost when the pH differs from pKa by more than 1 pH unit- beyond that 1 unit they are not able to buffer.  Buffering systems work best at their pK. Bicarbonate buffers blood and phosphate buffers cells.

 A weak acid will be titrated with a strong base. As you move along x-axis, more base is being added. Change in pH is shown along the y axis.  Hyperventilation results in low CO2 in blood, pH increases because the [H+] lowers (alkosis). Hypoventilation causes acidosis, more acid in blood and pH will decrease.  To manage pH in cells, phosphate is the primary intracellular buffer. Has three buffering zones. Equilibrium at pKa=7.21....