Cells are the basic biological unit. Humans have an estimated 75-100 trillion cells. Our skin, our lungs, our liver, our eyes are made of cells. The cell membrane is the outer lining of the cell which keeps the inner workings compact and separated from each other and the outside world.
I spend several posts talking about the cell membrane, because it leads to some very important conclusions as to whether the cell, and subsequently life itself, could have come about by chance. Several subjects need to be considered concerning the cell membrane, including the environment inside the cell that is needed for its biochemical processes to take place. For example, the level of acidity, usually called pH, is critical for these chemical reactions. If the cell is too acid inside, the balance of particles, such as the hydrogen atom, proton and electron, will not be maintained at the proper levels.
In the picture at top, the layer in light blue is called a phospholipid bilayer. Lipids are fats, and two fat molecules (long strings) are attached to a molecule called a phosphate 
(chemical name for specific arrangement of phosphorus atom with oxygen atoms). The bilayer means that 2 rows of phospholipids line up with the fats inside and the phosphate part outside. The Wikipedia link for "phospholipid" HERE shows a close-up of the atoms with further links. I have an image at left which I accessed in April 2009. The part on the left has the fats, which continue to the left with carbon and hydrogen atoms (represented by R), and the right part is the phosphate. The fat is called apolar because it repels water (which is polar, described in Wikipedia at "chemical polarity" HERE). And the phosphate is polar because it attracts water (and other molecules which are charged).
(chemical name for specific arrangement of phosphorus atom with oxygen atoms). The bilayer means that 2 rows of phospholipids line up with the fats inside and the phosphate part outside. The Wikipedia link for "phospholipid" HERE shows a close-up of the atoms with further links. I have an image at left which I accessed in April 2009. The part on the left has the fats, which continue to the left with carbon and hydrogen atoms (represented by R), and the right part is the phosphate. The fat is called apolar because it repels water (which is polar, described in Wikipedia at "chemical polarity" HERE). And the phosphate is polar because it attracts water (and other molecules which are charged).
These molecules have to be made by the cell so there will be enough of them when the time comes for the cell to divide (reproduce) and grow. I will not get into details of this mechanism at least for now because there are enough details to consider with the membrane itself.
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