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Water,pH,Buffer Cheat Sheet by

midterm!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Water

the solvent for virtually all of bioche­mistry, ~70% of the mass within each cell is water
Carbon has four electr­ons in the outer shell - these hybridise into 4 sp3 hybrid orbitals as tetrah­edron. If symmet­rical, the angle is 109.28 degrees.
WATER AS A SOLVENT (sub­sta­nces such as household sugar dissolve in water, means that their molecules sepa­rate from each other, each becoming surr­oun­ded by water molecu­les.)
When a substance dissolves in a liquid, the mixture is termed a solu­tion.
The dissolved substance is the solu­te, and the liquid that does the dissolving is the solv­ent.
Water is an excellent solvent for many substa­nces because of its polar bonds.
Covalent bond
Inside a molecule
Hydrogen bond
between molecu­les
Polar bonds (H2O)
uneven charge
Non-polar bonds (O2)
even charge

Hender­son­-Ha­sse­lbalch equation

 

pH (potential hydrogen)

The acidity of a solution is defined by the conc­ent­ration of H+ ions it possesses.
pH scale
pH = -log1­0[­H+]
pure water pH (7)
[H+] = 10-7 moles/­liter
acids
substance, proton (H+) donors
bases
substance, proton acceptors (such as OH-)
Water can act as both a weak acid and a weak base.
Acids in an aqueous enviro­nment
proton moves from one molecule to the other
pH is a measure of acidity (<7) or alka­linity (>7).
Higher amounts of protons in a solution
results in a lower pH (acidic)
Lower amount of protons
results in a higher pH (basic, or alkali)
Diff­erent Enzymes have different optimal pH according to their enviro­nment.
The strength of an acid is measured by its diss­oci­ation constant, Ka. The larger the Ka the more it diss­oci­ates and the stronger the acid.
The pH of a solution of a weak acid and its conjugate base is related to the conc­ent­ration of the acid and base and the pKa by the Hender­son­-Ha­sse­lbalch equation.
When ph < pKa, the weak acid predom­ina­tes. When pH > pKa, the conj­ugate base predom­ina­tes.
 

Buffers

A solution which pH resi­sts change upon addition of either small amounts of strong acid or strong base are added.
(consist of a weak acid and its conjugate base)
BUFFER CAPACITY - is related to the concen­tra­tions of the weak acid and its conjugate base,
The greater the concen­tration of the weak acid and its conjugate base, the greater the buffer capacity.
H2PO4- / HPO42- is the principal buffer in cells, H2CO3 / HCO3- is an important buffer in blood.
Buffers work because the concen­tration of the weak acid and base are kept in the narrow window of the titration curve.

Biological Buffer Systems

Mainte­nance of intrac­ellular pH is vital to all cells:
1. Enzyme­-ca­talyzed reactions have optimal pH,
2. Solubility of polar molecules depends on H-bond donors and acceptors,
3. Equili­brium between CO2 gas and dissolved HCO3- depends on pH.
Buffer systems in vivo are mainly based on:
1. Phosphate, concen­tration in millimolar range,
2. Bicarb­onate, important for blood plasma,
3. Histidine, efficient buffer at neutral pH.
Buffer systems in vitro are often based on sulfonic acids of cyclic amines:
HEPES, PIPES, CHES.
 

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