## **Core Concept**
The cell membrane, also known as the plasma membrane, is a biological membrane that separates the interior of a cell from its outside environment. It is composed mainly of **lipids and proteins**. The fluid mosaic model is the most widely accepted model describing the cell membrane's structure, where proteins are embedded within a phospholipid bilayer.

## **Why the Correct Answer is Right**
On a weight basis, the percentage of proteins in a cell membrane can vary depending on the cell type and its function. However, on average, cell membranes are known to contain approximately **50% proteins by weight**. These proteins perform a variety of functions, including acting as receptors, channels, and enzymes, and are crucial for cell signaling, transport, and maintaining the structural integrity of the cell.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a much lower percentage of proteins in the cell membrane. While it's true that some membranes may have less protein, 50% is a commonly cited average.
- **Option B:** This option suggests a higher percentage of proteins. While some specialized membranes may have a higher protein content, 50% is a generally accepted average for many cell types.
- **Option D:** This option also suggests a lower percentage of proteins compared to the commonly accepted average.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the cell membrane's fluidity and function are significantly influenced by its **protein and lipid composition**. Changes in this composition can affect cell signaling and transport, which are critical in various physiological and pathological processes.

## **Correct Answer:** . **50%**