The Importance of a Selectively Permeable Membrane


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My AP Biology Thoughts

Unit 2 Cell Structure and Function

Welcome to My AP Biology Thoughts podcast, my name is CJ and I am your host for episode #57 called The Importance of a Selectively Permeable Membrane. Today we will be discussing The Importance of Selective Permeable Membrane

Segment 1: Introduction to the Selective Permeable Membrane

  • Let’s start out on what a membrane is. When you look at a cell, or even think about a cell. There is typically a thin barrier surrounding the contents of the cell. Plant, animal, eukaryotic or even prokaryotic, they all have a membrane. This little barrier is made out of a phospholipid bilayer. The lipids bond their tails, which are hydrophobic. Their hydrophilic heads bond together as well creating a wall. Sometimes in the cracks of this wall are proteins. These proteins are either integral, or goes all the way through the wall, or peripheral, which only goes into the wall a little. These proteins are the key to the cell wall being semi permeable. Now what does that word even mean? Semi permeable means the cell membrane only lets in specific molecules, depending on the size and shape of the molecules. The cool thing is that some proteins are also defined as active transport. Which means they need energy to allow molecules to move through the membrane. They act sort of like a clothespin, on the side that's on the outside stays open. When the right molecule enters the open chamber, it closes which in turn opens the other side, and allows the molecules to freely float past. The other type of transport is passive, which is just a protein with a specific size hole, that certain molecules can float freely past. It is key that the cell membrane filters what enters and exits the cell. The diffusion of substances through the cell membrane allows for the cell to be properly maintained and protected.

Segment 2: More About the Selective Permeable Membrane

  • The cell is a delicate arrangement of organelles that work together in harmony. Each organelle has a defined function that is essential for the survival of that cell, and neighboring cells. If the integrity of this cell is compromised, then surrounding cells along with that one could perish. It may not be that much of a problem because cells on us die every day. However, in either smaller organisms or essential cells, such as the heart or brain, this could be particularly harmful. In order to know why a selectively permeable membrane is so essential to the survival of cells, we have to look closer as to what it does. Let's start with the excrement of substances. Depending on the metabolic activity of the cell, the byproduct of the reaction doesn’t just stay in the cell. It passes through the membrane and exits the cell. It is important that the cell loses the byproduct because it is useless to the cell. An abundance could cause the cell to rupture because of the lack of space. The other role the membrane has is keeping the cell protected from outside threats as well as maintaining homeostasis. The cell membrane serves as a line of defense because it forbids any useless molecules to enter and potentially harm the cell. This filter is essential for the life and efficiency of the cell. Allowing useful molecules for metabolic purposes, DNA replication, vitamins or even just to balance out the cell.

Segment 3: Connection to the Course

  • This connects with the rest of the unit by explaining specific functions of each organelle while even tying in macromolecules. The use of proteins for diffusion as well as the use of lipids for the protection of the cell, it all ties together. It explains why cells become hypotonic or hypertonic as a result of surrounding solute gradients. It explains how molecules such as glucose or oxygen either get in or out of the cell, helping it metabolize. And finally, it explains how complex cells really are. There are a lot of factors that come into play when dealing with cells. Just looking at the membrane, we can see how outside molecules get in and we see the first line of defense of every cell.

Thank you for listening to this episode of My AP Biology Thoughts. For more student-ran podcasts and digital content, make sure that you visit See you next time!

Music Credits:

  • “Ice Flow” Kevin MacLeod (
  • Licensed under Creative Commons: By Attribution 4.0 License

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