As previously discussed, a cell needs some sort of a membrane to keep all its important stuff inside and to function properly. That role is filled by the plasma membrane, also known as the cell membrane. But what is the cell membrane made of? What do all those names in the drawing above mean? We are going to discuss that today.
The basic building block of the cell membrane, both in plant and animal cells, is the lipid bilayer. A lipid bilayer is essentially two layers of fatty molecules put together, forming a membrane. Yes! Your cell membranes are mostly made out of fat! But not just any fat. The lipids of the cell membrane are equipped with a hydrophilic head. Hydro meaning water, and philic meaning to like. So the hydrophilic head ‘likes’ water.
But what does that really mean? Why does it matter? For that, we need to dive into a little bit of biochemistry. You see, lipids, are large organic molecules which do not dissolve in water. Fats are in fact one of the most well known type of lipids. Because of their molecular structure and the atoms they are made out of, water molecules cannot dissolve them. That is the reason why you can never truly mix water and olive oil together, for example.
Now, as a cell, you will be dealing with water a lot. It wouldn’t make sense if your membrane was strictly hydrophobic (water-fearing, not dissolving in water). Therefore the membrane has a hydrophilic head. The heads face outside the cell, where water is present, and inside the cell, because the cytoplasm is also made out of water; while the hydrophobic lipid chains are facing away from water and facing each other! Now using such lipid bilayer, you can have the best of both worlds: you can work with both water soluble and water insoluble molecules. A fun fact to consider (since we are discussing hydrophobic and hydrophilic molecules): ever wondered how you can clean a hydrophobic liquid when water obviously can’t dissolve and take it away? You clean it with another hydrophobic solution! Of course, it’s a bit more complicated cause not any solution would do, but as a general rule of thumb, you can say: hydrophilic dissolve in hydrophilic and hydrophobic dissolves in hydrophobic.
The lipid bilayer alone can only let certain molecules out of the cell and let very small molecules in such as water molecules, steroid hormones (like testosterone), oxygen and carbon dioxide. Obviously, the cell needs more than just small molecules to survive. How can it pick and choose? This is where the membrane proteins come in! Some are open channels which let larger molecules or hydrophilic molecules which cannot pass through the bilayer to go through. This is called Facilitated diffusion. Molecules like proteins from the food you ate, sugars, amino acids etc pass through these channels.
Another form of facilitated diffusion is one in which a protein selectively allows certain molecules in. This often happens when more of a molecule is outside the cell than inside the cell. These proteins then use energy to bring molecules from outside to inside. How do they select? Most of these proteins have shapes which accommodates the molecule they want to select, thus letting these molecules fit in nicely and pushing them inside the cell!
Thus this concludes the essentials of our cellular membrane. To summarize: our cell membranes are made out of lipid bilayers that have hydrophobic and hydrophilic parts. Most molecules can pass through but for those that can’t, special proteins exist which facilitate the transition. We will cover more cell organelles in the upcoming posts. Cheers!
Kent, M. (2016). Advanced biology.
Alberts, B. (2008). Molecular biology of the cell. New York: Garland Science Taylor & Francis.