A Special Environment
Creating A Special Environment
Cells are special. They have lots of special chemical reactions going on inside of them that need a special environment in order to happen properly. They need all the special molecules that participate in these special reactions to be close together so that they can find each other and react in special ways.
The environment surrounding cells is not particularly special. It has lots of useless junk floating around in it, some of which is very toxic to cells. Outside of the cell, all of the special things that we need inside of cells are generally pretty few and far between: molecules that are important to life exist in the surrounding environment, but not in the amounts that we need or in close enough proximity to each other for life to happen. All of this “special” inside of cells and the “not special” in the environment is what makes cells living and the environment not living.
The environment surrounding cells is not particularly special. It has lots of useless junk floating around in it, some of which is very toxic to cells. Outside of the cell, all of the special things that we need inside of cells are generally pretty few and far between: molecules that are important to life exist in the surrounding environment, but not in the amounts that we need or in close enough proximity to each other for life to happen. All of this “special” inside of cells and the “not special” in the environment is what makes cells living and the environment not living.
So, how do we get the special molecules and ONLY the special molecules inside the cell? Well, to understand this, we first need to revisit a friend from chemistry.
Diffusion
Let’s remember back to when we learned about diffusion in chemistry. Molecules have a tendency to spread out so that they’re not concentrated. Basically, molecules don’t like being crowded. As long as they can spread out, they will go pretty much wherever they want.
The molecules that are important to life are no different: It’s not like they want to be inside of cells or even know that they’re inside of cells at all. They’re just molecules floating around trying to find their own space, not giving a care in the world about what kind of molecule they are or having any idea about how very special they are to life. They’re just molecules, and they’ll diffuse away like any other molecule, given the chance.
The Cell Membrane Prevents Unwanted Diffusion
So, how could life possibly exist? How do we get all of these molecules that don’t really like to be crowded to hang out together inside of a cell and not just diffuse away? That’s the role of the cell membrane.
The cell membrane is a very special barrier that surrounds the cell and decides what gets in and what stays out. It makes sure the things we need get into the cell in just the right concentration, while the things that we don’t need—or the things that would be really toxic to cells—stay out.
The cell membrane is a very special barrier that surrounds the cell and decides what gets in and what stays out. It makes sure the things we need get into the cell in just the right concentration, while the things that we don’t need—or the things that would be really toxic to cells—stay out.
To get a little more scientific about the role of the cell membrane, it is what we call a semipermeable membrane (“semi” meaning partially, and “permeable” meaning letting stuff in. A “membrane” is a very thin, flexible barrier). It’s not quite perfect at being semipermeable—sometimes bad stuff gets in or good stuff leaks out, despite the best efforts of the cell membrane—but it is really very good at it’s job. It’s kind of like the security at a super swanky event, the gates and fencing around the White House, or the annoying troll guarding the bridge in that one children’s story: stuff we want gets in, stuff we don’t want stays out (for the most part). We’ll talk more about how it does this later in the lesson.
This is one of the main roles of the cell membrane: to prevent unwanted diffusion. But, the cell membrane is more than just a wall between the outside world and the inside world. It is very much alive. Because it is alive, it can play an active role in deciding who goes where.
This is one of the main roles of the cell membrane: to prevent unwanted diffusion. But, the cell membrane is more than just a wall between the outside world and the inside world. It is very much alive. Because it is alive, it can play an active role in deciding who goes where.
Going Against the Concentration Gradient
Because cells are alive, they use and make energy. One of the ways that it can spend that energy is to force a molecule to go where it wouldn’t under the normal laws of diffusion. This is called active transport, and we’ll talk more about how it works later. For now, understand the important big picture: The membrane can force something inside the cell, even if it’s already concentrated in the cell. It can then prevent it from leaving. In this manner, the environment inside the cell can be different and special relative to the outside environment.
So, the cell membrane has a very special function. And because:
So, the cell membrane has a very special function. And because:
... the cell membrane must have a very special structure.
Cliffhanger! We’ll talk more about the special structure of the cell membrane in the next part of this lesson.
Summary
You should understand:
This video gives a great overview of the cell membrane. It goes into some detail, most of which we’ll talk about later. It might be helpful for you to have that little preview, but, for now, focus mostly on the big picture:
- The big picture of why the cell membrane is important to life: Keeps good stuff in, keeps bad stuff out, and even forces some good stuff in against its concentration gradient.
- That the internal cell environment is necessarily different from the external cell environment, and that this is essential for life. The molecules of biochemistry wouldn’t get in or stay in their biological “test tube” if not for the cell membrane.
- That the cell membrane is semipermeable.
This video gives a great overview of the cell membrane. It goes into some detail, most of which we’ll talk about later. It might be helpful for you to have that little preview, but, for now, focus mostly on the big picture:
Learning Activity
Contributors: Alan Li, Emma Moulton