This leaf is made up of cells. These cells are shielded around a cell wall and a cell membrane. The cell membrane of these cells and all other plant cells has the base of a phospholipid bilayer. A phosopholipid is a lipid with a hydrophillic phosphate head and two hydrophobic phosphate tails. The bilayer means that there are two rows of these phospholipids facing opposite directions. The thing that is so cool about this phospholipid bilayer is that it is a semi-permeable membrane. Semi-permeability means that only certain things can go through. These certain things that can go through are hydrophobic molecules. These hydrophobic molecules are able to pass through hydropobic tails of the phospholipids. Because of this unique ability this phospholipid bilayer can prevent anything that it doesn’t want coming in from coming in. You might be wondering what hydrophobic means. Hydrophobic means that the polarity of the molecule is constant around all parts. These molecules that are hydrophobic are bonded by non-polar covalent bonds. Water has a polar covalent bond and the polarity is not the same from each atom to atom. Because of these differences any hydrophobic molecule will repel a water moleucle. Hence the name hydrophobic because hydro means water and phobic means afraid of.