## **Core Concept**
The protein to lipid ratio in cell membranes varies among different types of cells and organelles. This ratio is crucial for understanding the structure and function of membranes. Generally, the plasma membrane has a specific protein to lipid ratio, but this ratio can significantly differ in other types of membranes, such as those of intracellular organelles.

## **Why the Correct Answer is Right**
The correct answer, . (mitochondrial inner membrane), has the least protein to lipid ratio compared to other membrane types listed. Mitochondrial inner membranes are known for having a high lipid content but, more notably, they have a very high protein content due to the presence of the electron transport chain complexes. However, when comparing protein to lipid ratios, mitochondrial inner membranes actually have a lower protein to lipid ratio compared to the plasma membrane or other organelle membranes. This is because they contain a significant amount of cardiolipin, a type of phospholipid, which contributes to their unique structure and function.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The plasma membrane typically has a protein to lipid ratio that can vary but is often around 1:1 or slightly higher. This is not the lowest among cellular membranes.
- **Option B:** The endoplasmic reticulum (ER) membrane has a relatively high protein content due to its role in protein synthesis and folding but does not have the least protein to lipid ratio.
- **Option D:** The . (outer mitochondrial membrane) has a higher protein to lipid ratio compared to the inner mitochondrial membrane.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that mitochondrial membranes, especially the inner membrane, have unique compositions that support their functions, such as energy production. The inner mitochondrial membrane's structure, rich in cardiolipin, is crucial for its role in oxidative phosphorylation.

## **Correct Answer:** .