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8.12Problems

 

 

ATP is known as “the currency of biological energy.” Explain the reasons for this in connection with the overall processes of intracellular metabolism.

 

 

In eukaryotic cells, H+ released by glycolysis and the citric acid cycle produces energy via oxidative phosphorylation in the electron transport chain. Briefly explain the mechanism of ATP synthesis in mitochondria using the terms below (and drawings if desired). Terms: inner membrane, intermembrane space, matrix, F-ATP synthase

The oxidation of sugar molecules in cells follows the equation below:
C6H12O6 (glucose) + 6O2 → 6CO2 + 6H2O + energy
Indicate whether the following statements are correct, and provide reason for your answers.

A)

Energy is produced by the oxidation of carbon atoms.

B)

The water required by cells is largely supplied by this reaction.

C)

In cells, this reaction is a several-step process.

D)

Reaction with oxygen occurs in many of the steps of sugar molecule oxidation.

E)

In some organisms, the reverse reaction occurs.

F)

Some cells produce CO2 while growing in an O2-free environment.

 

 

Explain how material transport in the mitochondrial inner membrane occurs, and how this mechanism contributes to mitochondrial functions.

 

 

In chloroplasts, the pH of the external solution is increased by photosynthetic electron transport, whereas in mitochondria it is reduced by electron transport. However, in both cases, ATP synthase is arranged facing the stroma or matrix. Explain the topological differences regarding this H+ movement.

The energy stored in the mitochondrial inner membrane (H+ motive force, pmf) is expressed by the equation below:
ΔG = 2.3 RT [pH (inside) - pH (outside)] + ZFΔψ
where R is the gas constant, T is the absolute temperature, Z is the electric charge (e.g., H+ = 1),
F is the Faraday constant and ψ is the membrane potential.

1)

Calculate the value of ΔG when the membrane potential is 0.168 V (inside - negative), the pH difference between the inside and outside of the membrane is 0.75 and the temperature is 37 ˚C.

2)

The free energy change required to synthesize ATP from ADP "in vivo" is slightly larger than that required in standard conditions. Assuming that the value of this change is 45 kJmol-1, give the minimum number of H+ ions that need to be transported to synthesize one ATP molecule.

 

 

ATP is known as “the currency of biological energy.” Explain the reasons for this in connection with the overall processes of intracellular metabolism.

In a cell, the energy obtained from the oxidative decomposition of nutrients is stored as ATP molecules, and the energy required for the synthesis of high molecular compounds, muscular contraction, active transport, luminescence and electrical generation (among others) is obtained from ATP hydrolysis. Thus, electrical, mechanical and chemical cellular activities are mediated by ATP, which is known as the currency of biological energy. See in 7.2.

 

 

In eukaryotic cells, H+ released by glycolysis and the citric acid cycle produces energy via oxidative phosphorylation in the electron transport chain. Briefly explain the mechanism of ATP synthesis in mitochondria using the terms below (and drawings if desired). Terms: inner membrane, intermembrane space, matrix, F-ATP synthase

The H+ ions generated by glycolysis and the citric acid cycle are stored as electron energy in the form of NADH and FADH2. The high-energy electrons derived from NADH and FADH2 move from one protein complex to another in the electron transport chain of the mitochondrial inner membrane, finally reaching oxygen and reducing it to water. The energy released during this process is used to actively transport H+ from the matrix to the intermembrane space, thus creating an electrochemical gradient across the mitochondrial inner membrane. This electrochemical H+ gradient facilitates the influx of H+ ions (needed when F-ATP synthase phosphorylates ADP to ATP) into the matrix (see Fig. 8-2).

The oxidation of sugar molecules in cells follows the equation below:
C6H12O6 (glucose) + 6O2 → 6CO2 + 6H2O + energy
Indicate whether the following statements are correct, and provide reason for your answers.

A)

Energy is produced by the oxidation of carbon atoms.

Correct. Carbon atoms in glucose can be oxidized further, and are in a relatively reduced state.

B)

The water required by cells is largely supplied by this reaction.

Incorrect. The amount of water generated by this reaction is extremely small compared with the total water content of cells.

C)

In cells, this reaction is a several-step process.

Correct. See Fig. 7-3 and others. The merit of this reaction occurring in multiple steps is that it becomes slower, allowing efficient use of the energy generated in the reaction.

D)

Reaction with oxygen occurs in many of the steps of sugar molecule oxidation.

Incorrect. Reaction with oxygen gas (molecular oxygen) occurs only in the last step of the electron transport chain.

E)

In some organisms, the reverse reaction occurs.

Correct. This is known as photosynthesis.

F)

Some cells produce CO2 while growing in an O2-free environment.

Correct. Some anaerobes (bacteria that do not use oxygen) and yeasts exhibit this behavior.

 

 

Explain how material transport in the mitochondrial inner membrane occurs, and how this mechanism contributes to mitochondrial functions.

A mitochondrion has outer and inner membranes. The outer membrane has many large channels, and most small molecules in the cytoplasm can pass through them. The inner membrane, on the other hand, blocks most substances other than those actively transported by membrane transport proteins. These characteristics mean that the substrates necessary in the citric acid cycle and the electron transfer chain are actively transported to the matrix surrounded by the inner membrane, and the H+ concentration gradient is maintained.

 

 

In chloroplasts, the pH of the external solution is increased by photosynthetic electron transport, whereas in mitochondria it is reduced by electron transport. However, in both cases, ATP synthase is arranged facing the stroma or matrix. Explain the topological differences regarding this H+ movement.

Originating from endosymbiosis, both chloroplasts and mitochondria are surrounded by a double layer of outer and inner membranes. The inner mitochondrial membrane is compartmentalized into many corrugations (or cristae), and the space surrounded by the inner membrane is called the matrix. With chloroplasts, on the other hand, the material inside the inner membrane is called the stroma, and contains membrane structures called thylakoids. The matrix and the stroma are therefore homologous compartments, and H+ is transported by electron transfer from the matrix through the inner membrane to the intermembrane space in mitochondria, and from the stroma through the thylakoid membrane to the inside of the thylakoid lumen in chloroplasts. Furthermore, the ATP synthase of mitochondria and chloroplasts, coupled with H+ transport that follows the H+ concentration gradient, synthesizes ATP in the respective homologous compartments. See Figs. 8-2 and 8-5 and 8.10.

The energy stored in the mitochondrial inner membrane (H+ motive force, pmf) is expressed by the equation below:
ΔG = 2.3 RT [pH (inside) - pH (outside)] + ZFΔψ
where R is the gas constant, T is the absolute temperature, Z is the electric charge (e.g., H+ = 1),
F is the Faraday constant and ψ is the membrane potential.

1)

Calculate the value of ΔG when the membrane potential is 0.168 V (inside - negative), the pH difference between the inside and outside of the membrane is 0.75 and the temperature is 37 ?C.

21 kJmol-1
ΔG = 2.3 × R × T × [pH (Inner) -pH (Outer)] + ZFΔΨ
= 2.3 × 8.315 × 10-3 × 310 × 0.75 + l × 96.5 × 0.168

2)

The free energy change required to synthesize ATP from ADP "in vivo" is slightly larger than that required in standard conditions. Assuming that the value of this change is 45 kJmol-1, give the minimum number of H+ ions that need to be transported to synthesize one ATP molecule.

Based on the Nernst equation, the free energy change of one H+ ion passing through the membrane is:
ΔG = -nFΔE = -1 × 96.5 × 0.168 = -16.212
45 > 2 × 16.212
The number of H+ ions is therefore three or more.

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