Homeostasis and the Biological Environment

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1

A biochemist is studying two metabolic reactions at constant temperature and pressure. Reaction 1 is found to have a G value of . Reaction 2 has a G value of . Which statement is true about these reactions?

None of the other answers are true

Reaction 1 proceeds at a faster rate than reaction 2

Reaction 1 proceeds at a slower rate than reaction 2

Reaction 1 and reaction 2 proceed at equally fast rates

Reaction 1 and reaction 2 proceed at equally slow rates

Explanation

The G values given in the question relate to the reactions' thermodynamics. A negative G value means that a reaction is thermodynamically spontaneous. A spontaneous reaction can occur without further energy input. This does not tell us anything about the reaction's rate (kinetics). A spontaneous reaction may be slow or it may be fast.

2

What percentage of the human body is composed of interstitial water?

15%

5%

65%

60%

90%

Explanation

Interstitial water is the fluid on the outside of cells, and surrounds the cells of the human body. While estimates vary, the best answer is that 15% of the entire human body is composed of interstitial water. Overall, about 60% of the body (42L) is composed of water. Additionally, about two-thirds of the total body water is intracellular water. Finally, about 5% of the entire body is composed of plasma.

Remember that these percentages are estimates, and vary by individual. Sex and muscle mass are two factors that can significantly alter these estimates.

3

Which of the following statements about water is incorrect?

Because of hydrogen bonding, water has a low heat of vaporization.

Water has a tetrahedral structure when considering lone pairs and a bent molecular structure when not considering lone pairs.

Water has the potential to form a maximum of 4 hydrogen bonds.

Ice is a hexagonally packed network of water molecules and is less dense than water.

Explanation

Because of hydrogen bonding, water has a low high heat of vaporization. In other words, it takes more energy to free up water molecules from a hydrogen-bonded liquid state to a gaseous state.

4

Which of the following statements about water is incorrect?

Because of hydrogen bonding, water has a low heat of vaporization.

Water has a tetrahedral structure when considering lone pairs and a bent molecular structure when not considering lone pairs.

Water has the potential to form a maximum of 4 hydrogen bonds.

Ice is a hexagonally packed network of water molecules and is less dense than water.

Explanation

Because of hydrogen bonding, water has a low high heat of vaporization. In other words, it takes more energy to free up water molecules from a hydrogen-bonded liquid state to a gaseous state.

5

A biochemist is studying two metabolic reactions at constant temperature and pressure. Reaction 1 is found to have a G value of . Reaction 2 has a G value of . Which statement is true about these reactions?

None of the other answers are true

Reaction 1 proceeds at a faster rate than reaction 2

Reaction 1 proceeds at a slower rate than reaction 2

Reaction 1 and reaction 2 proceed at equally fast rates

Reaction 1 and reaction 2 proceed at equally slow rates

Explanation

The G values given in the question relate to the reactions' thermodynamics. A negative G value means that a reaction is thermodynamically spontaneous. A spontaneous reaction can occur without further energy input. This does not tell us anything about the reaction's rate (kinetics). A spontaneous reaction may be slow or it may be fast.

6

What percentage of the human body is composed of interstitial water?

15%

5%

65%

60%

90%

Explanation

Interstitial water is the fluid on the outside of cells, and surrounds the cells of the human body. While estimates vary, the best answer is that 15% of the entire human body is composed of interstitial water. Overall, about 60% of the body (42L) is composed of water. Additionally, about two-thirds of the total body water is intracellular water. Finally, about 5% of the entire body is composed of plasma.

Remember that these percentages are estimates, and vary by individual. Sex and muscle mass are two factors that can significantly alter these estimates.

7

A dialysis bag containing pure water is placed in a beaker containing a 1 molar glucose in water. The pore size of the dialysis bag is only large enough to allow water to pass through it. What will be the net flow of water?

Water will flow out of the dialysis bag into the glucose solution

Water will flow from the glucose solution into the dialysis bag

There will be no movement of water

Water will flow both directions at equal rates

Glucose will flow from the glucose solution into the dialysis bag

Explanation

Osmosis states that a solvent will move from a region of high concentration to a region of relatively lower concentration. In this case the solvent is water. The concentration of water inside the dialysis bag is higher than the concentration of water in the beaker because this water also contains 1 molar glucose. Therefore the water in the dialysis bag will flow out into the beaker to bring the concentration of water in both spaces toward equilibrium

8

What is the concentration in an solution with a pH of ?

Explanation

Here is the equation needed to find the correct answer to this question.

9

Calculate the pH of an ammonia buffer when the molar ratio of is . The pKa to be used is 9.75.

Explanation

This question requires you to use the Henderson-Hasselbach equation, one of the most important equations in biochemistry. The equation is:

where is the concentration of the conjugate base, and is the concentration of the acid. In this scenario, is the conjugate base, while is the acid. With the numbers given in this question, the equation should look like this:

10

Overall, water is a neutral molecule. However, it is polar due to electronegativity differences between oxygen and hydrogen. Which of the following could possibly be the partial positive and partial negative charges on a hydrogen and an oxygen in water, respectively?

Explanation

Oxygen is a more electronegative atom than hydrogen, and therefore attracts electrons within the molecule more strongly than hydrogen. Because electrons are negatively charged, oxygen must therefore contain a partial negative charge, while hydrogen must contain a partial positive charge.

Because water is neutral, the overall charge should add up to 0. However, there are two hydrogens in water, compared to one oxygen. Two positive charges of will cancel out a single negative charge of .

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