Acids and Bases
Acidic and basic conditions affect living organisms A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other: The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion ( ) The molecule with the extra proton is now a hydronium ion ( 3 O + ), though it is often represented as The molecule that lost the proton is now a hydroxide ion (O )
Fig. 3-UN3 + ydrogen bond + + O +
Fig. 3-UN2 Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed O O O O 2 2 O ydronium ion ( 3 O + ) ydroxide ion (O )
Though statistically rare, the dissociation of water molecules has a great effect on organisms Changes in concentrations of and O can drastically affect the chemistry of a cell
Effects of Changes in p Concentrations of and O are equal in pure water Adding certain solutes, called acids and bases, modifies the concentrations of and O Biologists use something called the p scale to describe whether a solution is acidic or basic (the opposite of acidic)
Acids and Bases An acid is any substance that increases the concentration of a solution A base is any substance that reduces the concentration of a solution
Fig. 3-UN5 Acidic [ ] > [O ] 0 Acids donate in aqueous solutions Neutral [ ] = [O ] 7 Basic [ ] < [O ] Bases donate O or accept in aqueous solutions 14
The p Scale In any aqueous solution at 25 C the product of and O is constant and can be written as [ ][O ] = 10 14 The p of a solution is defined by the negative logarithm of concentration, written as p = log [ ] For a neutral aqueous solution [ ] is 10 7 = ( 7) = 7
Acidic solutions have p values less than 7 Basic solutions have p values greater than 7 Most biological fluids have p values in the range of 6 to 8
Fig. 3-9 p Scale 0 O O Acidic solution O O O O O Neutral solution Neutral [ ] = [O ] 1 2 3 4 5 6 7 8 9 Battery acid Gastric juice, lemon juice Vinegar, beer, wine, cola Tomato juice Black coffee Rainwater Urine Saliva Pure water uman blood, tears Seawater O O O O O O O Basic solution 10 11 12 13 Milk of magnesia ousehold ammonia ousehold bleach Oven cleaner 14
Buffers The internal p of most living cells must remain close to p 7 Buffers are substances that minimize changes in concentrations of and O in a solution Most buffers consist of an acid-base pair that reversibly combines with
Example: Carbonic-Acid-Bicarbonate Buffer in Blood There are large concentrations of CO 3 - and CO 2 compared to the amount of added to the blood during normal activities. The blood p of 7.4 is maintained by the lungs and kidneys actively during strenuous activity.
Threats to Water Quality on Earth Acid precipitation refers to rain, snow, or fog with a p lower than 5.6 Acid precipitation is caused mainly by the mixing of different pollutants with water in the air and can fall at some distance from the source of pollutants Acid precipitation can damage life in lakes and streams Effects of acid precipitation on soil chemistry are contributing to the decline of some forests
uman activities such as burning fossil fuels threaten water quality CO 2 is released by fossil fuel combustion and contributes to: A warming of earth called the greenhouse effect Acidification of the oceans; this leads to a decrease in the ability of corals to form calcified reefs