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Water,pH,Buffer Cheat Sheet by



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
substance, proton (H+) donors
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.


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:


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