life11e_ch02

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Review

2.4 The acid–base properties of water and other compounds in cells allow reversible changes that affect biological functions.

Scientists are developing methods to sample a person’s breath and test it for higher-than-normal levels of compounds that contain ketones. The buildup of these compounds in the body can indicate type I diabetes. This work could lead to development of a portable device that will allow earlier diagnosis of diabetes, especially in children.

Insulin is a hormone that stimulates cells in the body to take up glucose from the blood. Cells use glucose as a source of energy for normal cell function. In type I diabetes, insulin is not produced, and glucose does not enter cells from the blood. Lacking an incoming supply of glucose, cells begin breaking down fats as a source of energy. When fats break down, several ketone-containing compounds are produced and they can be detected by a breath analysis device. A ketone has the general structure RCOR′, where R and R′ are the same or different groups of atoms.

One of the ketone-containing products of fat breakdown is acetoacetic acid, which has the structure below. Acetoacetic acid is very harmful when it builds up in high concentration in the body.

Questions

Question 1

In water, acetoacetic acid can ionize to form protons and acetoacetate ions. Write the chemical structures for this reaction.

Question 2

Now write out the reversible chemical reaction for the ionization of carbonic acid in blood, which can act as a buffer. Would the pH of blood change if a small amount of acetoacetic acid produced normally in tissues were added to the blood? Explain.

HCO₃^– + H⁺ ⇌ H₂CO₃

The pH would not change. As H⁺ was added due to ionization of acetoacetic acid, it would push the equilibrium of the above reaction to the right, as shown below.

HCO₃^– + H⁺ → H₂CO₃

added H⁺ reacts with HCO₃^– to shift equilibrium to the right

That is, bicarbonate ions would attach to the added H⁺, so the overall free H⁺ would not change.

Question 3

What would happen to the pH of the blood if more and more acetoacetic acid built up, as in a case of untreated diabetes? Explain, using what you found in Question 2 above. What would happen to the body’s ability to maintain a constant blood pH? How does this explain why people with this condition become severely ill?

Eventually the bicarbonate buffer system would be overwhelmed. It would no longer be able to take up the additional H⁺ ions building up in the blood and shuttle them into carbonic acid. At that point, H⁺ ions would accumulate, which would cause the blood pH to drop. The body would not be able to maintain a constant pH and its cells would be exposed to lower than normal pH conditions in the blood. In this case, the person would become severely ill because the cells would be exposed to an abnormal environment that then disrupts normal cell functions.

Question 4

Acetone is one of the compounds produced from fat breakdown and expelled in the breath. It is targeted for detection by a breath-analysis device. Its chemical structure is shown below. Analyze the structure to determine whether it would be expected to affect the carbonic acid or bicarbonate buffer system in the blood.

Acetone does not contain an acidic hydrogen, so it would not be expected to affect any acid or base buffering systems in the blood.

Question 5

Suppose scientists succeed in developing a device that detects ketone buildup in the breath. If a person tested positive for higher-than-normal ketones with the breath test, what are some blood tests that could be done to follow up? Suggest three or four different blood tests, other than acetoacetic acid or ketone levels, that could be used to detect the existence of insulin insufficiency. Describe the outcome of each test in the case of a positive result.

Four test possibilities are: insulin level (lower than normal), blood glucose level (elevated), blood pH (lower than normal), and bicarbonate level (lower than normal).