This lesson is on the cell membrane and selective permeability.
In this lesson, we’ll learn what is a cell membrane, what it means to be selectively permeable, and why the cell membrane is selectively permeable.
What Is the Cell Membrane?
Think about the building you’re sitting in right now. What separates you from the outside world? The walls, right? They keep you safe from the elements, but they also let certain things in or out. In a classroom, the doors let students into the right classes, but can keep students out that belong somewhere else.
Let’s look at how this translates to cells. Cells are the basic units of life that make up all living things. Cells, just like the rooms they are named for, have an outer barrier to keep the good stuff in and keep the bad stuff out. This barrier is called the cell membrane.
The cell membrane has tiny gates that act like doors and windows in a room. This property of only letting certain things in and out is called selective permeability. Let’s look closer at what this means.
What Does Selectively Permeable Mean?
The cell membrane is selectively permeable, meaning it only lets certain things in and out of the cell. If you think about our building analogy, this can be a very important thing. You wouldn’t want just anyone running in and out of the White House, for example.
It’s important to have guards who determine who’s allowed in and who’s not. A cell is a living thing and needs just the right balance of nutrients and water, called homeostasis. The selective permeability of the membrane allows the cell to stay in homeostasis.
But how does the cell membrane do this?
Why Is It Selectively Permeable?
The components that make up the cell membrane allow it to be selectively permeable. The cell membrane is made of proteins and a special type of fat called phospholipids . The phospholipids form a bilayer, or double layer. A phospholipid has two parts: the head, which likes water (hydrophilic) and the tail, which doesn’t like water (hydrophobic). The phospholipid tails come together, protecting themselves from the water, leaving the heads on the outside.Because the phospholipids are tightly packed together, really big things like sugar can’t just drift through the cell membrane. Tiny things that are also hydrophobic, like carbon dioxide, can squeeze through, but very few things have hydrophobic properties.
So the phospholipid bilayer keeps things out, but what lets them in, acting as the doors to the cell? The answer is proteins. Proteins dot the surface of the bilayer, floating like rafts. Some of these proteins have channels, or doors between the cell and the environment.
The channels let larger things that are hydrophilic and normally couldn’t pass through the membrane into the cell.Some channels are open all the time and let things flow naturally in and out, or diffuse, according to homeostasis. Other proteins, like carrier proteins, need a signal to open. The signal might come from other cells in the area or from something in the environment.
Let’s look at an example of selective permeability in action to make things clear. Glucose is a type of sugar your body uses to make energy. Glucose is found in most carbohydrates we eat, like bread, pasta, and sweets. When we eat, our stomach digests the food and the nutrients enter our blood where they circulate to the liver. When our blood has plenty of glucose, our body produces a hormone called insulin. Insulin tells our body it has plenty of food and we should take the glucose from our blood and store it in our cells. Insulin is made in the pancreas and moves around in the blood.
It attaches to special proteins on the cell membrane of muscle and liver cells. These proteins tell the cell to open up transport proteins in the cell membrane that let glucose into the cell. Glucose is too big to enter the cell without this transport protein, so unless all the signals are clear, glucose won’t get in. The cell is selectively permeable for glucose, letting it in only when we have enough of glucose for storage.
In summary, the cell membrane is a thin, flexible barrier outside all cells.
The cell membrane is selectively permeable, meaning it only lets certain things in and out. The structure of the phospholipid bilayer prevents random things from drifting through the membrane, and proteins act like doors, letting the right stuff in and out. Glucose is an example of a substance that the membrane selectively lets in and out. A chemical called insulin tells cells to bring glucose into the cell using a transport protein only when there is enough glucose around for storage.