The Hydrophobic phospholipid "tails." Cell Membranes are composed of lines of hydrophilic (water loving) phospholipid heads that face outside the cell and inside the cell. On the inside of the membrane, however, are hydrophobic (water hating) phospholipid tails, which repel charged particles.
It is a Phospholipid Bilayer. Each phospholipid is composed of a hydrophilic head and 2 hydrophobic tails. The tails are lipids which are 'water fearing'. So the structure naturally forms with the Heads to the outsides and tails to the insides; forming a double layer structure (ie. bi-layered).
They have both hydrophilic and hydrophobic heads. In a lipid bilayer, the hydrophilic head of the phospholipid faces the outside of the membrane while the hydrophobic head faces the the hydrophobic head of another phospholipid.
The cell membrane acts as a barrier between a cell and its environment. This is composed of a phospholipid bilayer. The hydrophilic phosphate heads face outside and the hydrophobic lipid tails point towards each other in the centre of the membrane.
A plant cell does have a cell wall for support and protection. Animals have a cell membrane, which is composed of a phospholipid bilayer (made of polar, hydrophilic heads and nonpolar, hydrophobic tails) and do not have a cell wall. Animal cells never have cell walls.