The membrane of all cells is made of a phospholipid bilayer. This is a double layer of phospholipid molecules, with their hydrophilic (water-loving) heads facing outwards towards the watery environment inside and outside the cell, and their hydrophobic (water-fearing) tails facing inwards.

Here's a breakdown of the key components:

* Phospholipids: These are the main building blocks of the cell membrane. They have a phosphate head group that is hydrophilic and two fatty acid tails that are hydrophobic.

* Proteins: Proteins are embedded within the phospholipid bilayer. They have various functions, including:

* Transport proteins: Help move molecules across the membrane.

* Receptor proteins: Bind to signaling molecules and transmit information into the cell.

* Enzymes: Catalyze chemical reactions within the cell.

* Cholesterol: Cholesterol is a lipid that helps to maintain the fluidity and stability of the membrane.

The phospholipid bilayer forms a barrier that separates the inside of the cell from the outside environment. This barrier is selectively permeable, meaning that it allows some molecules to pass through but not others. The proteins embedded in the membrane play a crucial role in regulating this selective permeability.