Three general factors determine cell shape: the state of the cytoskeleton, the amount of water that is pumped into a cell, and the state of the cell wall. Each of these three factors is highly dynamic, meaning they are constantly in flux or can be suddenly changed. This dynamism is how cells can vary in shape.
It makes sense that cell membranes are made of phospholipids because water cant mix in with lipids, so the phospholipids will be able to effectively block out any unwanted water or chemicals suspended in water outside of the cell.
