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Most of the chemistry of life occurs in water, which has unique properties that make it an ideal medium for supporting life. These properties include hydrogen bonding between molecules, high specific heat, and cohesion between molecules. Aqueous solutions can be acidic or basic, depending on the concentration of hydrogen ions. The cells and tissues of organisms are buffered, however, because changes in pH can change the properties of biological molecules.
learning outcomes
You should be able to:
Explain why water’s solvent properties are important in understanding events taking place inside cells.
Perform quantitative analyses of biochemical compounds using the mole concept.
Predict and explain changes in pH caused by changes in concentration of weak acids or bases in living tissue.
What is a solution, and why do we call water “the medium of life”?
A solution contains a solvent (e.g., water) and a substance dissolved in the solvent (e.g., table salt, NaCl). Water is the medium of life because most molecules in living organisms either dissolve in water or interact with it.
A substance in a cell has a concentration of 0.00000001 molar. What does this mean in terms of the number of molecules in a cell that has a volume of 0.000001 liters?
A 1 molar (1 M) solution has a concentration of 1 mole per liter, which means it has 6.02 × 1023 molecules per liter. A 10–8 M solution has 6.02 × 1015 molecules per liter. If there is 10–6 liters, the number of molecules is 6.02 × 109.
In human tissues, CO2 is formed as a by-
This reaction increases the H+ concentration in the blood and thereby decreases the pH. To increase the pH, and thereby decrease the H+, the reverse reaction could be used as a buffer:
H+ + HCO3– →CO2 + H2O