Animal cells are separated from each other by a structure called the cell membrane. This membrane is a thin, flexible barrier that surrounds the entire cell and acts as a gatekeeper, controlling what goes in and out of the cell.

Here's a breakdown of the cell membrane and its role in separation:

* Structure: The cell membrane is primarily composed of a phospholipid bilayer. This means two layers of phospholipid molecules (containing a hydrophilic head and a hydrophobic tail) arrange themselves with their heads facing the watery environments inside and outside the cell, while their tails face each other in the middle. This structure creates a barrier that prevents most substances from passing through freely.

* Selective Permeability: The cell membrane is selectively permeable, meaning it allows certain substances to pass through while blocking others. This is achieved through various mechanisms:

* Passive Transport: Some substances can move across the membrane without the cell needing to expend energy. This includes diffusion (movement from high to low concentration), osmosis (movement of water across a membrane), and facilitated diffusion (movement with the help of membrane proteins).

* Active Transport: Other substances require the cell to use energy to move them across the membrane, typically against their concentration gradient. This is done through protein pumps that require ATP (cellular energy).

* Cell Adhesion: While the cell membrane keeps individual cells separate, it also plays a role in cell-to-cell communication and adhesion. Special proteins embedded in the membrane can bind to similar proteins on neighboring cells, holding them together in tissues and organs.

In summary: The cell membrane is the primary barrier that separates individual animal cells. Its selective permeability ensures that cells maintain their unique internal environment, while allowing for the necessary exchange of materials with their surroundings.