Problem 9 - AP Psychology Premium Prep 2021

Question

Two groups of cells were grown under identical conditions. Mitochondria from each group were isolated and half of them were placed in a low pH (approximately \(\mathrm{pH}\) 6.8) and the other half were placed in a neutral \(\mathrm{pH}\). Small molecules were allowed to diffuse across the outer membrane via facilitated diffusion. Both samples were exposed to oxygen bubbles through the growth media. What would you expect to see in terms of ATP production in the sample of cells placed in a low \(\mathrm{pH},\) with respect to the control population? (A) ATP production decreases. (B) ATP production increases. (C) ATP production stays the same. (D) ATP production ceases entirely.

Step-by-Step Solution

Answer

(A) ATP production decreases.

Step 1: Understand the role of mitochondria in ATP production

Mitochondria are organelles found in cells, often referred to as the "powerhouses" of the cell. They generate energy in the form of adenosine triphosphate (ATP) through the process of cellular respiration. The main components of this process are the electron transport chain, where protons are pumped into the intermembrane space, and the ATP synthase enzyme, which creates ATP when protons flow back into the matrix.

Step 2: Consider the effect of pH on the components of ATP production

To produce ATP, a proton gradient is established across the inner mitochondrial membrane. This gradient is created by the movement of protons (H+ ions) from the matrix to the intermembrane space through the action of the electron transport chain. ATP synthase then uses the energy from the flow of protons back into the matrix to drive the synthesis of ATP.

Step 3: Determine how low pH could affect ATP production

In a low pH environment, there is an excess of H+ ions (protons) in the surrounding media. This will affect the proton gradient across the inner mitochondrial membrane, as the concentration of protons in the intermembrane space will increase. As a result, the flow of protons back into the matrix through ATP synthase will be reduced since there is a smaller difference in proton concentration between the intermembrane space and the matrix. Less proton flow means less energy available to drive ATP synthesis.

Step 4: Choose the correct answer based on the effect of low pH on ATP production

Knowing that low pH reduces the proton gradient and consequently reduces the energy available for ATP synthesis, we can expect ATP production to decrease in the sample of cells placed in a low pH environment. Therefore, the correct answer is: (A) ATP production decreases.

Key Concepts

Mitochondria

Mitochondria are fascinating organelles located within the cells of most eukaryotic organisms. Often dubbed "powerhouses," mitochondria play a crucial role in producing energy. This energy is vital for various cellular functions. They achieve this by converting nutrients into energy, specifically in the form of adenosine triphosphate (ATP). Mitochondria have their own unique structures, consisting of an outer membrane, an inner membrane, and an intermembrane space which is essential for energy production.
When cells need energy, mitochondria step up to the plate by undergoing a process called cellular respiration. Cellular respiration involves a series of reactions where glucose and oxygen are used to produce ATP. This is where the mitochondria shine, especially the inner membrane, which houses proteins responsible for ATP generation. Additionally, mitochondria are intriguing because they have their own DNA, illustrating their ancient origin and symbiotic relationship with the cell.

ATP Production

Adenosine triphosphate (ATP) is the energy currency of the cell, crucial for powering many types of cellular work, such as muscle contraction and chemical synthesis. ATP production primarily takes place inside the mitochondria through a process known as oxidative phosphorylation. The electron transport chain, a series of protein complexes, plays a central role in ATP generation.

• Electrons are passed along the chain from one complex to another, releasing energy.
• This energy is then used to pump protons ( H^+ ions) across the inner mitochondrial membrane, creating a proton gradient.

The enzyme ATP synthase, embedded in the inner membrane, uses this proton flow to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate ( Pi ). This step is crucial as it harnesses the stored energy from the proton gradient, translating it into the high-energy phosphate bonds of ATP, thereby fueling cellular activities.

Proton Gradient

The proton gradient is a vital component of the ATP production machinery within mitochondria. During cellular respiration, this gradient creates a potential difference across the inner mitochondrial membrane, resembling a kind of biological battery. The electron transport chain, by transferring electrons, pumps protons ( H^+ ions) from the mitochondrial matrix into the intermembrane space. This forms a higher concentration of protons outside the inner membrane compared to the inside.
This disparity is crucial for ATP synthesis. As protons naturally move back to balance the concentration, they flow through ATP synthase. This flow is akin to water rushing through a dam, and it drives the mechanical rotation within the enzyme. The rotation facilitates the formation of ATP by combining ADP and a phosphate group. Without a robust proton gradient, ATP production would significantly decline, affecting the cell's energy supply.

pH Levels

pH levels, which measure the acidity or alkalinity of a substance, are central to various biochemical processes, including cellular respiration. Within the context of mitochondria, the pH level plays a crucial role in maintaining the proton gradient necessary for efficient ATP production.
The inner mitochondrial membrane separates two regions with different pH levels:

• The intermembrane space, which is more acidic.
• The mitochondrial matrix, which is relatively more alkaline.

When mitochondria are placed in a low pH environment, indicating high acidity, the external proton concentration increases. This can disrupt the normal proton gradient by reducing the difference in proton concentration across the inner membrane, a process known as proton motive force. As a result, less protons flow through ATP synthase, diminishing the amount of ATP generated. Therefore, maintaining an appropriate pH level is key to sustaining optimal energy production in cells.