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Instruction 1-1

Cell Membrane | Enzymes | Prokaryotic and Eukaryotic Cells | RNA's Role | The Role of the Endoplasmic Reticulum and the Golgi Apparatus | Energy Capture and Storage | Mitochondria | Macromolecules | What Determines the Eukaryotic Cell's Shape? | Summary

Cell Membrane
CA GR.9-12 Biology 1.a.

You are made up of cells. So are dogs and cats and trees and fish. Everything that lives is made up of cells. They are the basic building blocks of life. But all cells are not the same.  Human cells are different from cow cells or tree cells. 

Even inside us, the cells are different. Your body alone has about two hundred different kinds of cells. You actually have trillions of cells, made from those two hundred or so kinds of cells. There are brain cells and bone cells and stomach cells. Each kind of cell has its own specific purpose. There are acid secreting cells inside your stomach to help you break down the food you eat, and cells in your intestines that absorb the food that your stomach helped you to break down earlier. All cells from the stomach to the intestines, work together to form a living organism. Some organisms are made up of just one cell, and others, like you, are made up of trillions. Two very important things are needed to keep all cells alive.

Even single celled organisms must have at least these two things. Bacteria is the only exception. First, all cells need organelles (little internal laboratories with lots of chemicals inside). Bacteria actually lack true organelles. Secondly they need a way to transport the products made in those organelle laboratories.

Even though cells perform different jobs and make up millions of different organisms, most cells have three main parts.

The three main parts of a cell are:

  • The cell membrane (its outside or coating)
  • The nucleus (its center or brain)
  • The cytoplasm (everything in between), which is also called cytosol

In this instruction, we'll be talking about the outside of the cell - the cell membrane.

For plants, the cell membrane isn't the outside of the cell. Plants have something extra called a "cell wall" which goes all the way around the cell membrane. In fact, this cell wall is one of the big differences between plants and animals. Plants have them, animals don't.

In animals (like us), it's the membrane that keeps the cell together. But this cell membrane isn't solid. It has tiny openings that let things in and out. The word that scientists use for this is " semi-permeable," which means that the Cell membrane can be permeated, or entered, in various places. 

This is very important for life - for digesting food, getting rid of waste and other body functions.

The cell membrane is made up of two layers of molecules, so it's called "bilayer" ("bi" means two). The molecules in this bilayer are called phospholipids. Here is what they look like:

As you can see, each of these phospholipids has a head and a double tail. The head is hydrophilic. The tail is hydrophobic. Those are big words, so we'd better explain. 

"Hydro" means water. "Philic" means "loving." "Phobic" means "fearing." (When you have a phobia, that means you're afraid of something). So something that's hydrophilic is attracted to water. And something that's hydrophobic is afraid of it. 

When these phospholipids clump together to form the cell membrane they do so with their "hydrophilic" heads out and their "hydrophobic" tails in. 

Why? The human body is made up largely of water. That's what our cells float around in. It's called "the extra-cellular fluid." So the phospholipids clump together with the part that likes water pointing out and the part that doesn't like water pointing in. Here's what that looks like:

Of course all of this is very tiny. The cell membrane is approximately 5nm (nanometers) thick (a nanometer is one billionth of a meter). And, as we said, it isn't solid. 

There are many proteins inside each cell. Some of these proteins are found inside this membrane bilayer. These proteins stick out both sides of the membrane, and act as gatekeepers. The gatekeepers decide which traffic can move in and out of the cell. This is a rough idea of what a cell membrane looks like (this is an important diagram to remember):

 Protein is not simply what you get in red meat or soybeans! There are thousands of different kinds of proteins with specific jobs inside cells. Proteins create force inside muscles, carry oxygen in blood, help digest food and make up the sex hormones estrogen and testosterone. These are just a few jobs proteins do (more protein functions later).

There are four main kinds of protein involved with the cell membrane. Note that the first two appear but are not labeled on the diagram above:

  • Integral proteins - these proteins are part of the membrane and make contact with both the inside and the outside of the cell
  • Transmembrane channel proteins - these are similar to integral proteins but connect the cytoplasm to the extra-cellular fluid (They channel things in and out)
  • Surface membrane proteins - these are only on the outer layer of the membrane. Look for one on the diagram.
  • Inner membrane proteins - these are only on the inner layer of the membrane and make contact with the cytoplasm.

These proteins can do many things. They can give shape to a cell or attach cells together to form tissue. They can transport molecules into and out of the cells. This is important in understanding how medicines and other substances work in the body. For example, the molecules of ethanol (the chemical which gives alcohol its properties) are very small and permeate (enter) the cell wall very quickly. That's why people feel an effect when they drink alcohol. 

These proteins can also act as receptors (like radio receivers) for the chemical messages that cells send to each other. This is what initiates nerve impulses or hormone activity. Or they can take part in enzyme activity, which controls metabolism or fights disease.

The bilayer of the cell membrane is very fluid. Scientists call this the Fluid Mosaic Model, which is what
is shown in the chart above. (You might want to take another look at this chart before reading the next two paragraphs.) When a new receptor protein enters this fluid cell membrane, the molecules make room, much like floating ping-pong balls making room for a balloon.

It's called "mosaic" because of substances floating inside the phospholipid bilayer. Mosaic refers to a collection of different items. The items forming this mosaic include proteins, cholesterol and other molecules.

Note that there are molecules of cholesterol imbedded in the membrane. The cholesterol breaks up the bonds that can cause the phospholipid tails to stick together. Without cholesterol, these tails would bond to each other and cause “kinks” in the cell membrane.

Van der Waals bonds cause this “sticky” aggregation. If not for the cholesterol, the cell membrane could not remain fluid.

You'll also notice a dark blue “sting of beads” carbohydrate chain in the back right of the picture.

That chain, called a glycoprotein is attached to the integral protein. It gets its name because carbohydrates are really sugars. The prefix “glyco” means sugar and when you combine a sugar with a protein, you end up with a glycoprotein.

There is also something in the picture called a glycolipid. You guessed it, when you combine another carb, or sugar (glyco) with a lipid (phospholipid) you end up with a glycolipid. Notice the string of light blue beads trailing from the phospholipid heads.

If this seems like a lot to remember, you're right. It is! It is important that you understand the outer covering (cellular skin). When you understand this, it will allow you to learn how the cell is able to recognize the things it needs to let in, and block entry of the things it doesn’t need.

 It lets you know how certain medicines work and why sometimes just one malfunctioning protein can cause horrible results for a human being. When studying the cell membrane, you may see it referred to by different names. All of the following names mean the same thing:

Plasma Membrane

Fluid Mosaic Model

Cell Membrane

Truthfully, it can all sound a bit intimidating at first! All of these lessons are designed to present small puzzle pieces that will eventually help you to understand the whole picture.

To make sense of how organisms work, you have to understand what is happening at the smallest level of life, the cells.

For more information about the makeup and function of cells, click on any of the links listed in this instruction. Or go to:

Video Instruction
*Availability of You Tube video links may vary. eTAP has no control of these materials.


Experiments for Home and Classroom

This is fascinating, and ambitious, web site that invites students to take virtual tours of cells and their structures (especially the cell membrane) with helpful worksheets to guide them along their way. Tours are available in English, Spanish, French and Russian. Start with the Instruction page. Click: 
Then, to begin the tour, return to the home page at: 

Although at first this experiment appears to be simply a demonstration of how the senses of taste and smell work together, it evolves into a thorough examination of the structure and function of "taste cells" on the tongue. For this reason, it's a good idea to scroll down the entire page before beginning the experiment. Click: 

In this digital version of the classic microscope lab, available in Spanish or English, students are invited to classify onion-root cells into the appropriate phases of the cell cycle -- and then count the number of cells found in each phase. Go to Online Onion Root at: 

For two fascinating "Cell Cam" real-time explorations of cell division (involving cancer cells and bacteria), click: 

Reading List
from the California Department of Education
Rensberger, Boyce: Life Itself: Exploring the Realm of the Living Cell

for Students, Parents and Teachers

Now let's do Practice Exercise 1-1 (top).


Next Page:  Enzymes (top)