Which statement is true about the plasma membrane?

Options

A. Proteins make up the matrix of the membrane
B. Model can be likened to a sandwich where phospholipids are like the bread and proteins are like the filling
C. Fluid nature of the membrane is regulated by flip flopping of the phospholipids from one side of the membrane to the other
D. Movement of proteins and phospholipids can occur sideways within the plane of the membrane

Explanation

To determine which statement is true about the plasma membrane, let's analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding the Plasma Membrane**:
- The plasma membrane, also known as the cell membrane, is the outer boundary of the cell.
- It is present in both animal and plant cells.
- The plasma membrane is primarily composed of a phospholipid bilayer.

**Hint**: Remember that the plasma membrane separates the interior of the cell from the external environment and is crucial for maintaining homeostasis.

2. **Evaluating the Statements**:
- **Statement 1**: "Proteins make up the matrix of the cell membrane."
- This statement is incorrect. The plasma membrane does not have proteins as the main component of its matrix. Instead, it has an extracellular matrix that includes proteins and carbohydrates.

**Hint**: Consider the composition of the extracellular matrix and its role in the cell.

- **Statement 2**: "The model can be likened to being sandwiched where phospholipids are like the bread and the proteins are like the fillings."
- This statement is also incorrect. While it attempts to describe the structure, it oversimplifies the arrangement of proteins in the membrane. The membrane contains both integral and peripheral proteins, not just fillings.

**Hint**: Think about the arrangement of proteins in the phospholipid bilayer and their types.

- **Statement 3**: "The fluid nature of the membrane is regulated by the flip-flopping of the phospholipid from one side of the membrane to the other side."
- This statement is incorrect. The fluidity of the membrane is primarily due to the lateral movement of phospholipids within the same layer, not flip-flopping between layers.

**Hint**: Focus on how lipids move within the membrane and what affects their fluidity.

- **Statement 4**: "The movement of protein and phospholipid can occur sideways within the plane of the membrane."
- This statement is correct. The proteins and phospholipids can move laterally (sideways) within the plane of the membrane, contributing to its fluid nature.

**Hint**: Consider how proteins and lipids interact and move within the membrane structure.

3. **Conclusion**:
- After evaluating all the statements, the correct statement about the plasma membrane is: "The movement of protein and phospholipid can occur sideways within the plane of the membrane."

### Summary:
The true statement about the plasma membrane is that the movement of proteins and phospholipids can occur sideways within the plane of the membrane.