Electron transport systems involve:

Options

A. electron transfers
B. cell membranes
C. enzymes and cofactors
D. all of the above

Explanation

To answer the question "Electron transport systems involve:", we need to analyze the options provided and understand the role of the electron transport system (ETS) in cellular respiration.

### Step-by-Step Solution:

1. **Understanding Electron Transport Systems**:
- The electron transport system is a crucial part of cellular respiration, specifically during oxidative phosphorylation. It is primarily located in the inner mitochondrial membrane.

2. **Identifying Key Components**:
- The electron transport chain (ETC) consists of a series of protein complexes and other molecules that transfer electrons derived from nutrients. This process is essential for ATP production.

3. **Analyzing the Options**:
- **Electron Transfers**: This is a fundamental aspect of the ETS, as it involves the movement of electrons through various protein complexes.
- **Cell Membranes**: The electron transport chain is embedded in the inner mitochondrial membrane, making this option relevant.
- **Enzymes and Cofactors**: The proteins in the ETC act as enzymes, and cofactors are often necessary for these enzymes to function correctly.
- **All of the Above**: Since all the previous options are valid components of the electron transport system, this option would also be correct.

4. **Conclusion**:
- The correct answer is **All of the Above** because the electron transport system involves electron transfers, is located in cell membranes, and utilizes enzymes and cofactors.

### Final Answer:
All of the above.