Which layer surrounds the cytoplasm of a cell and regulates what enters and exits?

The cell membrane is the layer that surrounds the cytoplasm of a cell and plays a crucial role in regulating what enters and exits the cell. This semi-permeable membrane is made primarily of phospholipids and proteins, which allows it to selectively allow substances to pass through while keeping others out. This selectivity is vital for maintaining the cell's internal environment, enabling it to carry out necessary functions and respond to changes in the external environment.

The cell membrane also contains various proteins and cholesterol, which assist in communication with other cells and provide structural support. This functional aspect distinguishes it from the other structures mentioned, such as the cell wall, which is found in plant cells and provides extra protection but does not have the same selective permeability function. The nucleus is an organelle that houses the cell’s genetic material and is not involved in regulating substance movement in and out of the cell, and the cytoplasm is the jelly-like substance inside the cell where various cellular processes occur but is not a regulatory layer.