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Plasma Membrane
Next Generation: MS.LS1.2 MS.LS1.2

Plasma Membrane

Author: Sophia Tutorial
Description:

This lesson will describe the structure and function of the plasma membrane of a cell.

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Tutorial

What's Covered

In this lesson, we'll discuss the plasma membrane and its structure and function in a cell. Specifically, we'll look at:

  1. Plasma Membrane: Selectively Permeable
  2. Plasma Membrane: Lipid Bilayer
  3. Plasma Membrane: Structure

1. Plasma Membrane: Selectively Permeable

Plasma membranes are selectively permeable. What this means is that they can control what goes into and what comes out of a cell.

Terms to Know

Plasma Membrane

The outermost layer of a cell, made up of a phospholipid bilayer, which controls what enters and exits the cell

Selectively Permeable

A feature of the plasma membrane that allows it to regulate what crosses through it


2. Plasma Membrane: Lipid Bilayer

Another characteristic of the plasma membrane is that it is composed of a lipid bilayer; the prefix bi means two. This lipid bilayer is a double layer of lipids that makes up the plasma membrane; the specific lipid is a phospholipid.

Terms to Know

Phospholipid

A lipid made of a hydrophilic head and a hydrophobic head which make up the plasma membrane

Lipid Bilayer

A membrane created by two layers of phospholipids

Please refer to the drawing of a phospholipid below.

Phospholipids are the main lipid composing this lipid bilayer of plasma membranes. They are made of a hydrophilic head and two hydrophobic tails. The word hydrophilic means that they're attracted to water, while the word hydrophobic means that they're repelled by water. So the heads will face out towards the inside of the cell or towards the cytoplasm, and the tails will face inward, towards each other, away from where that water is.


3. Plasma Membrane: Structure

Take a look at the structure of a plasma membrane below.


Plasma membranes have a very fluid quality; they're not rigid at all, and they're actually very, very thin.

Did You Know

In order to equal the thickness of one piece of paper, you would have to stack about 1,000 plasma membranes on top of each other. That should give you an idea of about how thin they actually are.

The plasma membrane, as mentioned previously, is composed of phospholipids. But it also includes other lipids, like glycolipids and cholesterol.

In addition, the plasma membrane is embedded with several different proteins. These proteins might be enzymes, channels, transporter proteins (proteins that transport a molecule from the outside of the cell to the inside of the cell), receptors for signaling molecules (such as hormones), or they could be recognition proteins that help identify what type of cell it is.

The selective permeability of a plasma membrane allows some substances, such as oxygen, carbon dioxide, water, and very small non-polar molecules, to cross through the plasma membrane on their own. However, some molecules that are larger or polar are not able to cross through this membrane on their own. They need to take advantage of the proteins that are embedded in the membrane, such as transporter proteins. A transporter protein would be able to allow a molecule, such as sodium, which wouldn't be able to cross through the plasma membrane on its own, to get either into or out of the membrane. Again, there are various proteins that are embedded in the plasma membrane, and all of them have very different roles.


Summary

In this lesson, you saw an example of a plasma membrane's structure. You learned about the two major characteristics of the plasma membrane: that it is selectively permeable and that it has a lipid bilayer with two layers of phospholipids. Phospholipids have hydrophilic heads, facing the towards the cytoplasm of the cell, and hydrophobic tails, facing inward towards each other and away from water.


Keep up the learning and have a great day!

Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND

Terms to Know
Lipid Bilayer

A membrane created by two layers of phospholipids

Plasma Membrane

The outermost layer of a cell, made up of a phospholipid bilayer, which controls what enters and exits the cell

​Phospholipid

​A lipid made of a hydrophilic head and a hydrophobic head which make up the plasma membrane

​Selectively Permeable

​A feature of the plasma membrane that allows it to regulate what crosses through it