A scientist added a chemical (cyanide) to an animal cell to stop aerobic respiration. Which of the following is most likely to have been affected by this treatment?

Options

A. Active transport of substances across the plasma membrane
B. Passive transport of substances across the plasma membrane
C. Diffusion of substances across the plasma membrane
D. The thickness of the plasma membrane

Answer

To solve the question regarding the effect of cyanide on aerobic respiration in animal cells, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Aerobic Respiration**:
- Aerobic respiration is a process that requires oxygen to produce ATP (adenosine triphosphate) from glucose. It primarily occurs in the mitochondria of cells and involves the electron transport chain.

2. **Role of Cyanide**:
- Cyanide is a potent inhibitor of aerobic respiration. It specifically inhibits cytochrome c oxidase (also known as cytochrome A3), which is a crucial component of the electron transport chain. This inhibition prevents the transfer of electrons to oxygen, effectively halting the entire process of aerobic respiration.

3. **Consequences of Stopping Aerobic Respiration**:
- When aerobic respiration is stopped, ATP production through oxidative phosphorylation is significantly reduced. Since ATP is the energy currency of the cell, its depletion can have widespread effects on cellular functions.

4. **Identifying the Most Affected Process**:
- One of the critical cellular processes that rely on ATP is the active transport of substances across the plasma membrane. Active transport requires energy (in the form of ATP) to move substances against their concentration gradient.

5. **Evaluating the Options**:
- **Option 1: Active Transport of Substances Across the Plasma Membrane** - This is likely to be affected due to the lack of ATP.
- **Option 2: Passive Transport of Substances** - This process does not require ATP and would not be significantly affected.
- **Option 3: Diffusion of Substances Across the Plasma Membrane** - Similar to passive transport, diffusion does not require ATP.
- **Option 4: Thickness of the Plasma Membrane** - This is a structural characteristic and is not directly affected by ATP levels.

6. **Conclusion**:
- The most likely process to be affected by the addition of cyanide, which stops aerobic respiration, is the active transport of substances across the plasma membrane.

### Final Answer:
**The active transport of substances across the plasma membrane is most likely to have been affected by the treatment with cyanide.**

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