The cell membrane consists of layer of phospholipids with the facing outward.

The cell membrane or the plasma membrane is the biological membrane surrounding the cytoplasm of both the prokaryotic and the eukaryotic cells. It separates the interior of the cell from the external environment, helping in protecting and organizing the cell.

It is a thin, semi-permeable membrane which is lipopretienaceous in nature. It can be best explained with the help of fluid mosaic model proposed by S.J. Singer and G.L. Nicolson in the year 1972. According to this, the cell membrane consists of a bilayer of phospholipids with the hydrophobic head facing outward and proteins.

The phospholipid in the lipid bilayer is made up of a water loving or hydrophilic polar head facing outward and a hydrophobic nonpolar, fatty acid tail. The polar head is contains a phosphate group and the fatty acid tails are made up of carbon and hydrogen. Since it contains both the polar and non polar regions, the phospholipids are called amphipathic.

Thus, the cell membrane consists of a bilayer of phospholipids with the hydrophobic head facing outward and proteins.

the answer is:

carbon dioxide