The Plasma Membrane and the Cell Wall
Welcome to the second lesson of this course, in which we will continue exploring the minuscule units that make us who we are. Today, we shall tackle what can be thought of as the “skin” of our cells.
The Plasma Membrane
Cells are champions in borderline and “customs” control, since they must ensure that only the stuff they need can be allowed in, and block entry of molecules that they do not require or might be dangerous for them.
But this is not an ordinary layer of organic molecules; it has the task of limiting the cell’s “innards” from its surrounding medium. Why is this a nearly insurmountable task? The cell lives in a water-based environment, and it also is one. That is, everything inside and outside the cell exists in an aqueous solution.
Now, this poses a problem: How is it possible for two water-based solutions of molecules to coexist without one being diluted into the other? Life has solved this in a way that is elegantly simple, as genius always is: by the fact that water and oil do not mix.
Indeed, all cells have a plasma membrane that is made of both polar (water-soluble) and non-polar (fat-soluble) elements. Particularly, this membrane features proteins both on the outside and on the inside of them, and they create a sandwich with a phospholipid (a kind of fat) stuffing. This allows for the strict separation of cell contents and cell surroundings.
Some cells, like many bacterial, plant, and fungi ones, have another protective layer outside their plasma membranes, which is formed by special secreted molecules. Such a layer determines the shape of cells, as well as conferring rigidity and protection from osmotic pressure damage.
Cells have a very thin line—literally—separating what is part of them and what constitutes the surrounding milieu.
While some small, non-charged molecules like oxygen, carbon dioxide, and water are able to non-selectively traverse the plasma membrane in a rather free fashion, other atoms and molecules must traverse it via specialized passage tunnels. This transport is passive, i.e. doesn’t require an energy source, since the movement follows the direction from where it is more concentrated to where there is less of it. But if the cell requires moving something in the opposite direction, this is against a concentration gradient, and it has to spend energy to achieve this countercurrent transport. This latter kind works like a pump and is called active transport.
Did You Know?
Even though cells were discovered in the 17th century, a more or less precise description of the plasma membrane structure was published only in the 20th century. We now know more than ever about it, including the fact that the main lipid involved in its structure is cholesterol. Cholesterol is a kind of lipid that, when too elevated, can raise our cardiovascular disease risk, but nevertheless, it is an absolutely key element for every cell that makes us up.
The delicate layer that separates what constitutes the cell from its surroundings is called the plasma membrane. This important part of the cell acts both as a limit and as a very active customs officer, allowing only what the cell requires to enter, while maintaining its cytoplasm composition. Some cells feature an additional layer called a cell wall that confers additional rigidity and helps them maintain a specific shape.
Tomorrow, we will dive into the cell nucleus and begin exploring what makes us altogether similar to other members of our species but unique as individuals at the same time: our genetic code.
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