Acids and bases. Why are these important in biology?
It comes back to a number of biological processes. For example, enzymes work best at specific levels of acids or bases. Take your stomach, a very acidic environment. The enzymes that work in that environment could not work in your mouth. What would your food taste like if your mouth was also a very acidic environment?
Water is the main ingredient of many solutions. A solution is a mixture of two or more substances that has the same composition throughout. Some solutions are acids and some are bases. To understand acids and bases, you need to know more about pure water. In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. An ion is an electrically charged atom or molecule. The breakdown of water is represented by the chemical equation
2 H 2 O → H 3 O + + OH -
The products of this reaction are a hydronium ion (H 3 O + ) and a hydroxide ion (OH - ). The hydroxide ion, which has a negative charge, forms when a water molecule gives up a positively charged hydrogen ion (H + ). The hydronium ion, which has positive charge, forms when another water molecule accepts the hydrogen ion.
Acidity and pH
The concentration of hydronium ions in a solution is known as acidity . In pure water, the concentration of hydronium ions is very low; only about 1 in 10 million water molecules naturally breaks down to form a hydronium ion. As a result, pure water is essentially neutral. Acidity is measured on a scale called pH , as shown in Figure below . Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.
Acidity and the pH Scale Water has a pH of 7, so this is the point of neutrality on the pH scale. Acids have a pH less than 7, and bases have a pH greater than 7. Approximate pHs of examples are shown.
If a solution has a higher concentration of hydronium ions than pure water, it has a pH lower than 7. A solution with a pH lower than 7 is called an acid . As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is. Did you ever taste vinegar? Like other acids, it tastes sour. Stronger acids can be harmful to organisms. For example, stomach acid would eat through the stomach if it were not lined with a layer of mucus. Strong acids can also damage materials, even hard materials such as glass.
If a solution has a lower concentration of hydronium ions than pure water, it has a pH higher than 7. A solution with a pH higher than 7 is called a base . Bases, such as baking soda, have a bitter taste. Like strong acids, strong bases can harm organisms and damage materials. For example, lye can burn the skin, and bleach can remove the color from clothing.
Acids and Bases in Organisms
Acids and bases are important in living things because most enzymes can do their job only at a certain level of acidity. Cells secrete acids and bases to maintain the proper pH for enzymes to work. For example, every time you digest food, acids and bases are at work in your digestive system. Consider the enzyme pepsin, which helps break down proteins in the stomach. Pepsin needs an acidic environment to do its job, and the stomach secretes a strong acid that allows pepsin to work. However, when stomach contents enter the small intestine, the acid must be neutralized. This is because enzymes in the small intestine need a basic environment in order to work. An organ called the pancreas secretes a strong base into the small intestine, and this base neutralizes the acid.
- A solution is a mixture of two or more substances that has the same composition throughout. Some solutions are acids, some are bases.
- Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.
- Acids have a higher concentration of hydronium ions than pure water, and a pH lower than 7.
- Bases have a lower concentration of hydronium ions than pure water, and a pH higher than 7.
- Acids and bases are important in living organisms because most enzymes function best at a specific pH.
Use this resource to answer the questions that follow.
- pH at http://johnkyrk.com/pH.html .
- What is a strong acid?
- What is the pH scale?
- What is the pH range of most cellular processes?
Use the pH slider to find the pH of
- digestive juices.
- soapy water.
- sea water.
- pH scale at http://phet.colorado.edu/en/simulation/ph-scale .
1. What is the pH of a neutral solution?
2. Describe an example of an acid or a base that is involved in human digestion.
3. Assume that you test an unknown solution and find that it has a pH of 7.2. What type of solution is it? How do you know?