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Lesson 6 Membrane Transport

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Course: Science 10 [5 cr] - AB Ed copy 1
Book: Lesson 6 Membrane Transport
Printed by: Guest user
Date: Sunday, 7 September 2025, 6:45 PM

Table of contents

  Introduction

How does the cell membrane do its job of being a protective barrier?


A6.1 Cell membrane structure
The cell membrane is actually a very complex organelle. It has some very interesting properties, and its structure is something we have modelled many technologies after.

In this lesson, we will go through the structure of the cell membrane and how it allows some materials in but keeps others out. We will talk about the importance of the cell membrane in keeping materials balanced inside and outside of the cell. 

  Targets

By the end of this lesson, you will be able to

  • describe the role of the cell membrane in maintaining equilibrium while moving matter into and out of the cell
  • compare passive transport by diffusion and osmosis with active transport using the particle model of matter, concentration gradients, equilibrium, and protein carrier molecules
  • use models to explain and visualize diffusion and osmosis, endocytosis and exocytosis, and the role of the cell membrane

  Watch This


Cell Membranes Are Way More Complicated Than You Think © YouTube Ted-Ed  


Watch this video for information on the cell membrane. It goes into detail about the structure of the cell membrane and the function of the proteins found within it. You should focus on the fluid-mosaic model mentioned.
 

  Structure of the Cell Membrane

Before we talk about how materials are transported through the cell membrane, we need to understand the basic structure of it.


A6.2 Fluid-mosaic model
The model we use to describe the structure of the cell membrane is called the fluid-mosaic model. This model suggests the cell membrane is a collection of proteins floating within the membrane itself and the membrane is fluid in nature. This means

  • these proteins can move around in the membrane, depending on their job and the needs of the cell
  • the membrane itself can move, vibrate, and change shape when needed

The membrane itself is made of a phospholipid bilayer. This is a fancy way of saying the membrane contains two layers of particles called phospholipids. A phospholipid has a

  • round head made of the compound phosphate
  • tail made of lipids
A6.3 A phospholipid

A6.4 Structure of phospholipid bilayer
The phosphate head loves water while the lipid tail hates it. This means the two layers of the phospholipid form with the tails on the inside and the heads on the outside. The cell membrane is so strong because the lipid centre really does not want to get wet and so does everything in its power to keep the heads together as a membrane.

One of the compounds found in the membrane is cholesterol. Cholesterol is important to the structure of the membrane because it keeps the membrane fluid. This allows the other proteins in the membrane to move around and also allows particles that are needed to move into and out of the cell. The cell membrane is semi-permeable, meaning it lets some particles in and keeps others out.

  Read This

Please read pages 272 and 273 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on structure of the cell membrane and how it maintains equilibrium. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Why do you think the model used to explain the cell membrane structure is called the fluid-mosaic model?

Your answer should be a variation of the following: It is called fluid because the proteins in the membrane are floating around, as if they were fish swimming in water. It is called a mosaic because it has many different pieces held together by the phospholipid bilayer, just like a mosaic tile pattern.


A6.5 Mosaic tiles

  Maintaining Equilibrium

Besides protecting the cell, the cell membrane plays a large role in maintaining equilibrium.


© Biezl, via Wikimedia Commons 
A6.6 A cell moving toward equilibrium
Equilibrium is the idea that everything is balanced inside and outside the cell. This means the cell membrane works toward keeping all the materials, elements, particles, and compounds at the same concentration inside and outside of the cell.

The cell membrane also needs to maintain homeostasis. Homeostasis means staying the same or not changing. This means the cell membrane is trying to keep the concentration of everything inside the cell the same at all times. Some particles have a higher concentration inside of the cell due to what they are used for. For example, root cells in plants will have a larger nutrient concentration on the inside of the cell than outside the cell. Keeping this higher concentration inside the cell is the cell membrane maintaining homeostasis but not equilibrium.

The cell membrane is trying to keep both equilibrium and homeostasis. It wants to have the same concentration of most particles on either side of the membrane, but it also wants to stay the same. The particles that are needed more on the inside are brought inside despite equilibrium.

A6.7 Root vegetables have a high concentration of nutrients.

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What is the difference between equilibrium and homeostasis?
Equilibrium means keeping the concentrations of particles the same on either side of the cell membrane. Homeostasis means keeping the concentrations the same as they always are, even if that means they are not equal. For example, a cell membrane will keep the concentration of salt equal on both sides (equilibrium) but will keep a higher concentration of nutrients inside the cell (homeostasis).

  Passive Transport Through the Cell Membrane

How do materials, compounds, elements, and particles move through the cell membrane? 



There are two main classes of transportation through the cell membrane, and we will look at both. These are

  • passive transport—does not require energy to move particles
  • active transport—requires energy to move particles
There are a few different kinds of passive transport.
A6.8 Passive vs. active transport

Diffusion uses the concentration gradient to move small particles across the cell membrane. This means the particles move from where there is a higher concentration to where there is a lower concentration. For example, if there is more oxygen molecules surrounding the cell than on the inside, then the outside of the cell has a high concentration of oxygen. The oxygen will then move down the concentration gradient, moving from the outside of the cell to the inside of the cell.

© OpenStax, via Wikimedia Commons
A6.9 Molecules diffusing across the cell membrane
Oxygen is a very small particle, so it can slip between the constantly moving phospholipids in the membrane to get into the cell. This is possible because of the particle model of matter. Remember, the particle model of matter states all matter is made up of tiny particles and these particles are always moving. It also states particles have spaces between them, which the oxygen uses to slip past the membrane.

The oxygen will stop moving into the cell once the concentration inside the cell is equal to the concentration outside of the cell. In reality, this will never happen, as the cell uses oxygen almost as fast as the oxygen crosses the membrane.

  Did You Know?


A6.10 High concentration of students

Concentration does not just mean there is more of one kind of a particle than another. It is the amount of a particle in a given space. For example, if you and your friends are waiting for the school bus outside your school, there is a high concentration of students on the sidewalk. If you missed your bus and are now waiting for someone to come and pick you up, you are the only student left. That would be a low concentration of students on the sidewalk. The given space is the sidewalk, and the students are the particles!

smosis.jpg" data-lightbox="img1513639025528"> surprisesmosis.jpg" style="style:nomargin; margin: 0;" class="img-responsive" width="800" height="652">
A6.11 Osmosis
Osmosis is the movement of water across a concentration gradient. Often the particle dissolved in water is too large to fit through the membrane. Instead of the large dissolved particle moving across the cell membrane, the water moves across it to try to even out the concentration. This process works the same as diffusion, only it is the water moving across the membrane. One thing to note is that water is always moving across the membrane. If the water is mostly moving from outside the cell to the inside, then there is a higher concentration of the large dissolved particle on the inside of the cell. If water is mostly moving from the inside of the cell to the outside, then there is a higher concentration of the large dissolved particle on the outside. A good rule of thumb is that water moves to where the concentration of the large dissolved particle you are looking at is highest. It does this to try to dilute the concentration, or make the concentration smaller.

If water is moving the same amount into and out of the cell, then the concentration of the large dissolved particle is equal on both sides. This is called being in a state of equilibrium.

We have special names for each of the solutions described above:

Hypotonic: This is when there is more of the dissolved particle inside the cell, so water moves into the cell to try to even out the concentration. The water moves to the higher concentration of dissolved particles. “Hypo” means smaller or less, so there is a smaller concentration of the dissolved particle outside the cell. This will cause the cell to swell or burst.
A6.12 Hypotonicity
Hypertonic: This is when there is more of the particle outside the cell, so water moves out of the cell to try to even out the concentration. The water moves to the higher concentration of dissolved particles. “Hyper” means more. This will cause the cell to shrink or shrivel up.
A6.13 Hypertonicity

Isotonic: This is when the concentrations are equal, so water moves into and outside the cell at equal rates. “Iso” means the same. The shape of the cell will not change.
A6.14 Isotonicity



A6.15 What happens to a cell in different solutions


Large starch molecules cannot move through the cell membrane. If there was a higher concentration of starch on the outside of the cell, water would move from the inside of the cell to the outside. By doing this, it is lowering the concentration of the starch outside of the cell. This is a hypertonic solution since there is more starch outside the cell.

Now let’s say we are looking at a plant cell that creates starch. There would be a higher concentration of starch on the inside of the cell. Water would move into the cell to try to lower the concentration inside the cell. This is a hypotonic solution since there is a lower concentration of starch on the outside of the cell.

  Watch This

Osmosis © YouTube Amoeba Sisters


Watch this video for an overview of osmosis, the three different kinds of solutions, and real-world applications of this type of transport.


A6.16 Facilitated diffusion using carrier proteins
Facilitated diffusion is very similar to diffusion except instead of the particles passing through the membrane, they pass through special proteins in the membrane called channel proteins or carrier proteins.

Channel proteins go through the entire membrane and act as a passageway from one side of the membrane to the other. Due to their size and shape, they only allow certain molecules to pass through.

Carrier Proteins also go through the entire membrane, but they have a more complex structure. They only allow certain molecules to bind to them due to their shape. Once that molecule has bound to the carrier protein, the carrier protein changes shape and opens up on the other side of the cell. This allows the molecule to pass to the other side.

Facilitated diffusion works the same way as diffusion: The particles move along the concentration gradient. For example, glucose (a sugar used for energy) is too big to fit through the membrane without help. If there is a higher concentration of glucose on the outside of the cell, then the glucose will move from the outside of the cell to the inside of the cell using the carrier or channel protein that is designed for it. The glucose will continue to move into the cell until the concentrations inside and outside of the cell are the same.


Read This

Please read page 275 to 278, stopping at the “Active Transport” heading in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the different kinds of passive transport and how they work. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Draw a diagram explaining how each of these types of passive transport work.
Your diagrams should be a variation of these:

© OpenStax, via Wikimedia Commons
A6.9 Molecules diffusing across the cell membrane

smosis.jpg" data-lightbox="img1513639215330"> surprisesmosis.jpg" style="style:nomargin; margin: 0;" class="img-responsive" width="800" height="652">
 A6.11 Osmosis
A6.13 Facilitated diffusion using carrier proteins
A6.17 Facilitated diffusion using channel proteins


  1. Explain how the particle model of matter relates to diffusion, osmosis, and facilitated diffusion.
The particle model of matter states these four things:

i.    All matter is made of particles, but the particles in different substances may be different in size and composition.
ii.   The particles of matter are constantly moving or vibrating.
iii.  The particles of matter are attracted to one another or are bonded together.
iv.   Particles have spaces between them. These spaces may be occupied by particles by another substance.

Diffusion: Points i, ii, and iv are used here. Diffusion uses the movement of the particles to move them through the spaces between the particles in the membrane. This only works if the particle is small enough to fit in those spaces.

Osmosis: Points i, ii, and iv are used here. Osmosis uses the movement of the particles in water to move the water through the spaces between the particles in the membrane. Water particles are small enough to fit through the spaces in the membrane.

Facilitated Diffusion: Points i, ii, iii are used here. Facilitated diffusion uses the movement of particles and the attraction particles have for one another to move them through channel or carrier proteins The particles have to be of a particular size and shape to fit through the channel or carrier proteins..

  Virtual Lab


Osmosis, Diffusion, and Paramecium Homeostasis © Explore learning


Background Information:
These labs will allow you to observe how osmosis and diffusion work. You will complete a lab on osmosis, a lab on diffusion, and then a lab on a paramecium (a small single-celled organism) to see these two processes in action. They will also let you observe what happens in a hypertonic and a hypotonic solution.

Please note: if you scroll down while in the Gizmo you will see a list of questions. You DO NOT need to complete these questions. You are able to complete them for extra practice if you would like.

  1. Open the Osmosis Gizmo by clicking on the play button in this section. Print students can access the Gizmo in the Online Resources for Print Students section of their online course.
  2. The pink square represents a cell, the green dots represent water molecules, and the blue dots represent a large molecule such as starch or sugar.
  3. Check that the simulation has loaded with the default settings of five large molecules outside the cell and an initial cell volume of 30%. Take note of the initial concentrations inside and outside of the cell and the initial number of solute and solvent particles inside and outside of the cell. These are found on the right-hand side of the simulation. (You can take a screenshot of this information using the camera icon if you wish.)
  4. Click the play button located at the bottom of the screen to run the simulation. Watch what happens to the size of the cell as the simulation plays out. What is happening to the concentration inside and outside of the cell?
  5. After 20 seconds, take note of the final concentrations inside and outside of the cell, what happened to the size of the cell, and the number of solute and solvent particles inside and outside of the cell. (You can take a screenshot of this information using the camera icon if you wish.)
  6. Click on the reset button found next to the pause/play button.
  7. Change the solute outside to 10 molecules and the initial cell volume to 60%. Record or screenshot the initial information.
  8. Click on the play button to run the simulation. Watch what happens to the size of the cell as the simulation plays out. What is happening to the concentration inside and outside of the cell?
  9. After 67 seconds, click the pause button and record or screenshot the final information.
  10. Repeat steps 6 to 9, changing the initial cell volume to 20% and leaving the solute outside at 10 molecules. You only need to run this simulation for 30 seconds.
  11. Please return to the top of this activity and click on analysis to complete the analysis questions.


©Explore Learning
A6.18 Default Settings



©Explore Learning
A6.18 Camera icon
  1. What did you observe happening to the concentrations of each run of the simulation?

    Since the concentrations evened out at the end of each run, the cells were in isotonic solutions.


  2. Complete the following chart:
Run #
What happened to size of cell?
Why did the cell change that way?
Was the outside solution hypotonic or hypertonic to the inside of the cell?
1
2
3

Run #
What happened to size of cell?
Why did the cell change that way? Was the outside solution hypotonic or hypertonic to the inside of the cell?
1 It got bigger.
The concentration outside the cell was lower than inside, so the cell took in water to try to dilute the inside of the cell. Hypotonic
2
It got smaller.
The concentration inside the cell was lower than the outside, so water left the cell to try to dilute the concentration outside. Hypertonic
3
It got bigger.
The concentration outside the cell was lower than inside, so the cell took in water to try to dilute the inside of the cell.
Hypotonic
  1. At the end of each run of the simulation, were the cells in a hypertonic, hypotonic, or isotonic solution?

    Since the concentrations evened out at the end of each run, the cells were in isotonic solutions.




  1. Open the Diffusion Gizmo by clicking on the play button in this section. Print students can access the Gizmo in the Online Resources for Print Students section of their online course.
  2. Make sure the simulation opens with the default settings shown in image A6.19. The clear space in this simulation represents water, and the purple dots represent small particles.
  3. Click the play button in the bottom right half of the simulation. What happens to the number of particles in region B?
  4. Run the simulation for 60 seconds. What has happened to the number of particles in regions A and B?
  5. Click the reset button next to the pause/play button.
  6. Increase the number of y in B to 50 and decrease the wall height to 25%. You will now see purple and green particles.
  7. Click on the play button to run the simulation. What happens to the number of particles in regions A and B?
  8. Run the simulation for 120 seconds. What has happened to the number of particles in regions A and B? What has happened to the number of x and y particles in each region?
  9. Please return to the top of this activity and click on analysis to complete the analysis questions.




©Explore Learning
A6.19 Default settings in Diffusion lab
  1. In the first run of the simulation, what happens to the number of particles on each side of the wall? Why does this happen?

    The number of particles on each side of the wall even out as the simulation runs. This happens because the particles are trying to reach equilibrium. Once equilibrium is reached, there will be no net movement of particles.


  2. In the second run of the simulation, what happens to the number of particles on each side of the wall? What happens to the number of purple and green particles on each side of the wall? Why does this happen?

    The total number of particles on each side of the wall stays the same as both sides started at 50. The total number is in equilibrium, so it does not change. The number of purple and green particles changes to try to reach equilibrium. The purple particles move into region B, and the green particles move into region A. They will keep moving until equilibrium is reached. 


  3. Try changing the number of x and y particles in the simulation. If the particles are not even on both sides, what happens to the total number of particles as the simulation runs?

    Since the total number of particles are not in equilibrium, this number will change until they are in equilibrium.


  1. Open the Paramecium Homeostasis Gizmo by clicking on the play button in this section. Print students can access the Gizmo in the Online Resources for Print Students section of their online course.
  2. Check that the simulation has opened with a water solute concentration of 1% and paramecium controlled.
  3. Click on the play button located in the bottom left side of the simulation. What is happening to the contractile vacuole (the pink sphere)? Why do you think this is happening?
  4. Pause the simulation and change the water solute concentration to 2%. Click on the play button and observe what happens to the contractile vacuole.
  5. Pause the simulation again and change the water solute concentration to 0%. Click on the play button and observe what happens to the contractile vacuole.
  6. Pause the simulation and change the water solute concentration back to 1%. Change the simulation from “Paramecium controlled” to “User controlled.” You are now in charge of when the contractile vacuole does its job.
  7. Click the play button and observe what happens to the contractile vacuole and size of paramecium. Do not click on the contract button. What eventually happens to the paramecium?
  8. Reset the simulation, making sure the simulation is still user controlled. Click on the play button and try to click on the contract button at the right time to keep the paramecium healthy. If you would like a challenge, change the water solute concentration to 0%.
  9. Pause the simulation and change the water solute concentration to 2%.
  10. Click the play button and observe what happens to the size of the cell. (This is a small difference, so you really have to watch to see it!)
  11. Please return to the top of this activity and click on analysis to complete the analysis questions.

  1. What do you think the job of the contractile vacuole is based on your observations?

    The job of the contractile vacuole is to remove the excess water that is entering the Paramecium. This helps to keep the Paramecium from exploding. 


  2. Complete the following chart:
    Water Solute Concentration
    		Is water moving into or out of the cell?
    		How do you know?
    		Is the Paramecium in a hypertonic, hypotonic, or isotonic solution?
    1%
    2%
    0%

    Water Solute Concentration Is water moving into or out of the cell? How do you know? Is the Paramecium in a hypertonic, hypotonic, or isotonic solution?
    1% Moving into the cell
    • The contractile vacuole is working to push water out.
    • The paramecium gets bigger.
    		Hypotonic
    2% Moving out of the cell
    • The contractile vacuole doesn’t appear to be doing anything.
    • The paramecium is getting smaller.
    		Hypertonic
    0% Moving into the cell
    • The contractile vacuole is working very quickly to push water out.
    • The paramecium gets bigger until it explodes if the contractile vacuole does not do anything.
    		Hypotonic

  Active Transport Through the Cell Membrane

Active transport is broken down into the following types: 


A6.18 Active transport requires energy
Active transport requires energy because it moves against the concentration gradient. This type of transport is used for maintaining homeostasis rather than equilibrium. It is trying to keep the concentrations of certain particles the same rather than even out the concentrations.

This type of transport uses channel and carrier proteins, just like facilitated diffusion, but these proteins are more than just passageways in this case. They act more like a pump, pushing the particles across the membrane. This is very difficult; it would be similar to trying to ski uphill, so it requires energy from the cell.
Endocytosis and exocytosis are another type of active transport. “Endo” means in, so endocytosis is when particles are brought into the cell. “Exo” means out, so exocytosis is when particles are taken out of the cell. In both of these processes, vesicles are used to bring the particles in or take them out. These vesicles are needed because the particles that need to be transported are too big to pass through the membrane, even with the help of channel proteins.

In endocytosis, the cell surrounds the particle it wants to bring in. The cell membrane then fuses into a vesicle sac around the particle and “pinches off” from the main cell membrane. This creates a vesicle inside the cell with the particle inside. The vesicle will then transport the particle to the organelle that needs it.

A6.20 Exocytosis
Exocytosis is the opposite of endocytosis. In exocytosis, a vesicle travels to the cell membrane and fuses with it. The vesicle then becomes part of the main cell membrane, opening the vesicle to the outside of the cell and forcing the particles inside out.

Both endocytosis and exocytosis require energy to rearrange the cell membrane. The pinching off and fusing with the cell membrane is similar to breaking a stick or a branch. It is difficult to do, and energy is needed for it.
A6.19 Endocytosis
Make sure you understand how each of these five kinds of transport work. The five kinds of transport are

  • diffusion
  • osmosis
  • facilitated diffusion
  • active transport
  • endocytosis and exocytosis

  Read This

Please read pages 278 to 281 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the types of active transport and how they work. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Explain how the particle model of matter relates to active transport, endocytosis, and exocytosis.
The particle model of matter states these four things:

i.    All matter is made of particles, but the particles in different substances may be different in size and composition.
ii.   The particles of matter are constantly moving or vibrating.
iii.  The particles of matter are attracted to one another or are bonded together.
iv.   Particles have spaces between them. These spaces may be occupied by particles by another substance.

Active Transport: Points i, ii, and iii are used here. Active transport uses the fact that particles are always moving and that they are attracted to one another to move the particle to the channel or carrier protein. The particles have to be the right size and shape to attach to the channel or carrier protein to be pumped through to the other side of the cell.

Endocytosis and Exocytosis: Points i and ii are used here. These processes use the motion of the particles to move the membrane out and around the particles or away from the particles. They also use the motion of the particles to move the particle close enough to the membrane. The processes use the size and shape of the particles to trigger the membrane to surround the particles or to open to let the particles out.


  1. What is the difference between active transport and facilitated diffusion?
Active transport uses energy because it moves the particles against the concentration gradient. Facilitated diffusion does not use energy, because it moves particles along the concentration gradient.

  Importance of the Cell Membrane

As we have seen, the cell membrane is an important structure for a cell.


A6.21 Cell membrane structure
All cells have membranes and the membrane is involved in containing the inside of the cell and protecting the cell. Based on its structure, it allows certain particles in while keeping others out. The cell membrane does this through two main types of transport: passive and active transport. Passive transport does not require energy, and active transport does require energy. Diffusion, facilitated diffusion, and osmosis are all types of passive transport. Active transport, endocytosis, and exocytosis are types of active transport.

In the next lesson, we will apply this knowledge to determine what the ideal size of a cell is.

  Watch This

Cell Membranes and Cell Transport © YouTube Amoeba Sisters 


Watch this video for an overview of what we have learned in this lesson. The video goes through the structure of the cell membrane as well as the different kinds of transport across the membrane.


1.4 Assignment

Unit A Assignment Lessons 5-8

It is now time to complete the Lesson 6 portion of 1.4 Assignment. Click on the button below to go to the assignment page.

1.4 Assignment