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

Plasma Membrane

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

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Tutorial

What's Covered

Welcome to today’s lesson on the plasma membrane. In this lesson today you are going to discuss the plasma membrane and its structure and function in a cell. Specifically you will 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. They're able to control what's going in and what's going out of the 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 is composing 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 that compose 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 and 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 page, one piece of paper, you would have to stack about 1,000 plasma membranes on top of each other and that should give you an idea of about how thin they actually are.

The plasma membrane, as mentioned before 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, receptors for signalling molecules, such as hormones, or they could be recognition proteins that help identify what type of cell it is.

The drawing above is an example of just one protein in the plasma membrane and is just a very simplified version of a plasma membrane. Actually a real one would have various types of proteins embedded in it but that protein could be a transporter protein that would transport a molecule from the outside of the cell to the inside of the cell.

The selective permeability of a plasma membrane allows some substances, such as oxygen, carbon dioxide, water, very small, nonpolar molecules, to cross through the plasma membrane on their own. However, some molecules that are larger or that are polar are not able to cross through this membrane on their own. They need to take advantage of these proteins that are embedded in the membrane, such as a transporter protein. So 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; which allows it to get across the plasma membrane either into or out of the membrane. Again there's various proteins that are embedded in the plasma membrane that all have very different roles.


Summary

In this lesson you saw an example of what a plasma membrane looks like and it’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

  • ​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

  • Plasma Membrane

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