The Phospholipids Between Us (Part 2)

Transport through Cell Membranes

Lesson Plan developed by Kai Orton, PhD and Apurva Naik, PhD (Northwestern University) and based on the PhET Interactive Simulation: Membrane Channels

(http://phet.colorado.edu/en/simulation/membrane-channels)

Keywords: cell membrane, membrane transport, transport channel, permeability, osmosis, homeostasis, equilibrium, concentration gradient, and diffusion

Level All high school levels of biology

Purpose This activity introduces students to the use of interactive simulations to model how different types of transport occur through a cell membrane. Specifically, this lesson uses a modeling simulation from the PhET Biology library called Membrane Channels, as a means of investigating differences between osmosis, passive, facilitated diffusion mechanisms. The students will use a PhET model to observe when and how molecules move across a cell membrane and the effects of different types of transport channels on concentration gradients.

Overview The activity focuses on student understanding of the biology concept of passive diffusion. Each activity is divided into three main CT activities: 1) recording observations and gathering data and 2) analyzing data and 3) testing different simulation scenarios to understand a problem.

Student Outcomes Learner Objectives: 1. Students will understand how the cell membrane regulates what comes in and out of the cell. 2. Students will understand the process of passive diffusion and facilitated transport and how/when

molecules and particles will move through a membrane and when they will not. 3. Students will be able to predict rates of diffusion and ion transport based on the number of protein

channels present. 4. Students will understand the differences between passive and selective diffusion and facilitated

transport through a cell membrane. Standards addressed

- CT-STEM Skills (from CT-STEM Skills Taxonomy) 1a. Collecting Data 1b. Create Data 1c. Analyzing Data 1d. Visualizing Data 2a. Using computational models to understand a concept 2b. Using computational models to find and test solutions 3d. Assessing different approaches/solutions to a problem 4b. Understanding the relationships within a system

     

-Next Generation Science Standards HS-LS1 From molecules to organisms: Structures and Processes LS1.A: Structure and Function

-Common Core Math Standards Mathematics

K-MD.A.2: Directly compare two objects with a measurable attribute in common to see which object has more of or less of the attribute and describe the difference.

Time 2-3 class periods Materials and Tools Computer Systems Requirements: Windows Macintosh Microsoft Windows XP/Vista/7 Sun Java 1.5.0_15 or later

OS 10.5 or later Sun Java 1.5.0_19 or later

1. Students will use a PhET Interactive Simulation: Membrane Channels (http://phet.colorado.edu/en/simulation/membrane-channels)

2. In the simulation, students will drag the channels onto the membrane and then investigate what happens to molecules/particles.

3. The concentrations graphs are meant to give students qualitative, relative information to help them understand diffusion.

Preparation Begin the lesson by reviewing the differences between the cell wall and cell membrane, which can easily be confused by students. This is also an important topic because these two structures are very important to diffusion and osmosis. Ask questions such as, “What is the difference between the cell wall and cell membrane?” Explain how the cell wall is the outside lining of the cell that helps support cell structure and holds the cell together. For plant cells the cell membrane lies within the cell wall, while it is the outer structure for animal cells. The cell membrane has the quality of being semi-permeable so it can allow various substances to go in and out of the cell. A great example of a very simple cell everyone is familiar with is an egg, one of the largest cells. The shell is the cell wall and the thin lining we sometimes peel off the shell is the cell membrane. The white of the egg (clear runny part) is the cytoplasm and the yolk or yellow part is the nucleus. Plants have both a cell wall and a cell membrane. Animal cells usually do not have a cell wall. Bird eggs are an exception. After they have a firm understanding of this topic you can move on to the concept of diffusion and equilibrium. Next, discuss the topic of osmosis and how these two topics relate to one another. Setting up the simulation: Open the “Membrane Channels” interactive simulation on the computer: 1. Type http://phet.colorado.edu/en/simulations into the Internet browser. 2. Click “Run Now” (bright green icon) Background 1. Students should understand concepts of the cell and organelle function 2. Student should have a clear understanding of the differences between a cell wall and cell membrane 3. Students should be familiar with concepts like concentration, equilibrium and diffusion.

     

4. Students should have already studied concepts in osmosis like tonicity and osmotic pressure.

Teaching Notes 1. A computer lab with at least one computer per two students is recommended, but groups may be

formed for classrooms with less computer resources 2. Keep in mind that a strong student may dominate the group work. 3. Students should work in small groups (even when they have their own computer) to compare their

observations 4. In a real cell, leakage channels are always open, whereas gated channels only open in response to

some stimulus. Some gated channels respond to the presence of a certain molecule (ligand gated), some respond to a change in membrane potential brought about by changes in ion concentrations (voltage gated), some respond to changes to tension in the cell membrane (mechanically gated), and some respond to light (light gated).

5. In real cells, channels do not actively move things through them; they only allow things to diffuse through them. This is true in the simulation too, though in some cases it may appear that an ion is being pulled across a channel. Teachers should make sure that students understand that the motion through the channels is passive – a result of diffusion and that membrane channels do not ‘pump’ anything across the membrane.

6. In real cells, channels do not actively move things through them; they only allow things to diffuse through them. This is true in the simulation too, though in some cases it may appear that an ion is being pulled across a channel. Teachers should make sure that students understand that the motion through the channels is passive – a result of diffusion and that membrane channels do not ‘pump’ anything across the membrane.

7. In the virtual lab activities, students are asked to input a certain number of green circles or blue diamonds. A few molecules more or less won’t affect the equilibrium conditions.

8. In the virtual lab activities, students are asked to state when they observe equilibrium. A few molecules more or less on either side of the membrane will not matter for equilibrium conditions, but these fluctuations may appear significant to the students when they graph. You can ask the students to take an average of the values after equilibrium is reached to make a smoother curve.

9. Equilibrium point is reached when there are approximately equal numbers of blue diamonds on each side of the membrane or when both the blue bars in the concentration plot appear to be of the same size.

10. A timer is required for most of the activities. If the students don’t have hand stop watches, they may make use of cellphones with timers or also use a window with timer. The PhET simulation window doesn’t have to take up the whole screen.

11. Within the simulation environment the placement of the channels (into the membrane) may affect the rate at which equilibrium is reached, although there isn’t be a big difference in the students’ general observations.

Assessment How will the teacher evaluate if the objectives have been met? Include homework assignment handouts, in-course discussion questions, and/or exam questions. Handouts begin on the following page.

     

Student Guide: Name: ___________________________________

The Phospholipids Between Us: Transport through Cell Membranes

Oil in water by Tetine

     

Student Guide Name: ____________________________________________

Transport through Cell Membranes

Using a PHET simulation of homeostasis and different types of cell membrane diffusion

The net movement of molecules from an area of high concentration to an area of low concentration is called diffusion. In a liquid, a solute (dissolved molecules) will diffuse in a solvent (dissolving agent, most often water in biological systems) and eventually become uniformly distributed throughout the solution.

In the context of a cell, diffusion of a substance, such as molecules and ions, happens across the cell membrane and continues until equilibrium is reached. At the point of equilibrium, the concentration of the substance is same on either side of the membrane. As this is a spontaneous process requiring no input of energy, it is also referred to as passive diffusion. The cell takes in nutrients like glucose, ions and water and eliminates wastes through the cell membrane. Cell membranes are composed of phospholipids and a variety of proteins. Many of these proteins form selective channels that help move material across the cell membrane, for example glucose ions and ATP. Cell membranes are selectively permeable, meaning that some substances can pass across them and others cannot. The cell membrane’s ability to permit the exchange of gases or water molecules is called permeability. Fig. 1 illustrates the types of passive transport mechanisms.

Fig.  1  Types  of  passive  transport  mechanisms  

Diffusion  through  lipid  bilayer  

Selective  Diffusion   Facilitated  Diffusion  

     

Diffusion of water through a selectively permeable membrane is called osmosis. Unlike many other substances, water molecules do not require protein channels to move across the membrane but diffuse through the lipid bilayer. The direction of net movement of water depends on the concentration of solute inside and outside the cell, that is, whether the cell’s environment is isotonic, hypotonic, or hypertonic. Figs. 2a, 2b and 2c show the effect of different environment on human red blood cells. The first image in Fig. 2a shows the red blood cells depleted of water. This happens when concentration of solute is greater outside the cell and the concentration of water outside is correspondingly lower. As a result, water inside the cell flows outwards to attain equilibrium, causing the cell to shrink. The second image shows appearance of red blood cells in isotonic environment where the concentration of solute is equal on either side of the membrane. In this case, the movement of water into the cell balances the movement of water out of the cell and the cell retains its donut like shape. Cells in the third image in Fig. 2a appear to be swollen because of the low concentration of solutes outside the cell compared to inside the cell. In an attempt to balance the concentrations of solutes across the cell membrane, water will rush into the cell, causing it to swell and possibly burst. Fig 2b shows how the red blood cells actually look like when observed under a microscope.

Fig.  2a  Effect  of  different  solutions  on  red  blood  cells  

Fig.  2b  Microscope  images  of  red  blood  cells  in  different  solutions  

     

Red blood cell membranes are characterized by internal channels that allow glucose molecules to

pass through them. Because these channels are selective in their transport, only glucose can pass through and can move in either direction. The membrane protein channel is said to facilitate, or help the diffusion of glucose across the membrane. This process, shown in Fig. 1 is called facilitated diffusion. Although facilitated diffusion is fast and specific, a net movement of molecules across a cell membrane will occur only if there is a higher concentration of the particular molecules on one side than the other side.

Depending upon the material, diffusion across the membrane happens through either ‘leakage channels’, or ‘gated channels’. Leakage channels are always open, while gated channels open in response to some stimulus (ion concentration or light).

In the red blood cells, a specific protein that spans the membrane has an internal channel that allows glucose to pass right through it. Only glucose can pass through this channel, and it can move in either direction. Fig. 3 below shows the presence of these protein channels when observed in florescent light under a microscope.

This activity will introduce you to an interactive simulation of different ways molecules, such as ions, travel across cell membranes. In this virtual lab, you will be using the ‘Membrane Channels’ model to simulate and visualize the dynamics of simple and facilitated diffusion across a semi-permeable membrane. Setting up the model A screenshot of the simulation window is shown in Fig. 4. In the simulation, the top half of the blue space is considered the inside of the cell. The bottom half of the blue screen is considered the outside of the cell. The ‘Membrane’ separates the inside of the cell from the outside. The blue diamonds are water molecules and the green circles are solute molecules (sugar, salt, proteins etc.). You can add these to the cell environment by pressing the red buttons on the left. The slider at the bottom of the screen controls the speed of the simulation. ‘Check’ the 'Show Concentration' box to see the concentration of solute and water on either side of the membrane. The concentration of each substance is represented by a color-coded bar. The bar will increase or decrease in length as the simulation progresses, indicating the corresponding change in concentration. This visual representation is also known as a bar graph.

Fig. 3 Presence of protein channels on red blood cells (This image is courtesy of Heiko Loche)

 

     

1. Add approximately 20 green circles, which represent molecules, to the inside of the cell. Observe the

behavior of the molecules. 2. Pause the simulation. 3. Drag and drop 1-2 “leakage” green channels onto the membrane. Press the start button. Observe. Question 1: What do you observe when green leakage channels are added to the membrane? 4. Drag and drop 1-2 “leakage” blue channels onto the membrane. 5. Add 20 blue diamonds to outside of cell membrane. 6. Click on the box next to 'Show Concentration' tab to observe concentration of molecules on each side

with help of the bar graph. 7. Let the simulation continue for 2 minutes. Question 2: At the end of 2 minutes, how did the bar graph look like for the blue diamonds and why?

Inside  of  the  cell  

Outside  of  the  cell  

Simulation  speed  slider  

Molecule  selection  buttons  

Fig.  4  Screen  shot  of  simulation  window  

     

Activity 2: Selective Diffusion The difference in concentration of molecules on either side of the membrane is called a concentration gradient. The concentration gradient is responsible for driving transport of molecules across the semi-permeable cell membrane. Exploration 1: Understanding how concentration gradient affects rate of selective diffusion In this exploration, you will observe how the concentration of a particular molecule, for example chloride ions, in the cell affects the rate of diffusion. The effect of presence of different molecules in the cell on the rate of diffusion of another molecule will also be examined. Setup a simulation where chloride ions diffuse in and out of the cell. In this exercise, chloride ions will be represented by green circles. Press the “reset all” button to remove all of the molecules and channels. 1. Add 20 chloride ions inside and 5 chloride ions to the outside of the cell and set the animation speed

(by moving the slider at the bottom) at the default speed. 2. Check the box 'SHOW CONCENTRATION'.

Question 1: Observe at the bar graph. Which side of the cell has a higher concentration gradient?

Question 2: How much is the concentration gradient? 3. Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green leakage channels into

the membrane. 4. Observe the bar graph on the right side of the simulation screen. 5. Begin a timer and click the PLAY icon at the same time. Keep observing the bar graph as the

simulation progresses. 6. Stop the timer and note the time when you think concentration (the number) of the chloride ions is

equal on both sides; that is, when equilibrium is reached.

Question 3: How much time is required to achieve equilibrium across the membrane? Earlier in the exploration, you made a hypothesis. You estimated the amount of time it will take to reach equilibrium by observing the bar graph. Now, you will use another method. 1. Press the “reset all” button to remove all of the molecules and channels. 2. Add 60 chloride ions (green circles) inside and 15 chloride ions to the outside of the cell and set the

animation speed (by moving the slider at the bottom) at 170. 3. Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green leakage channels into

the membrane. 4. Begin a timer for 20 seconds and click the PLAY icon at the same time. After every 20 seconds,

pause the simulation and count the number of green circles on each side of the cell membrane. Do this for about 5 minutes. Complete Table 1 given below.

     

5. Press the “reset all” button to remove all of the molecules and channels. 6. Follow the steps mentioned earlier but add 40 chloride ions inside the cell. 7. Begin a timer for 20 seconds and click the PLAY icon at the same time. After every 20 seconds,

pause the simulation and count the number of green circles on each side of the cell membrane. Do this for about 5 minutes. Complete Table 2 given below.

Time Chloride ions (Green circles) inside the cell

Chloride ions (Green circles) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

Question 4: From the values you filled in Table 1, can you estimate how much time is required to reach equilibrium? How does this compare with the time value you estimated for Question 3?

Time (seconds) Chloride ions (Green circles) inside the cell

Chloride ions (Green circles) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

Table  1   Table  2  

     

You will now test if the presence of different molecules in the cell affects the rate of diffusion of a particular molecule. Setup a simulation where a cell contains sodium ions in addition to chloride ions. In the model, blue diamonds will represent sodium ions. 1. Press the “reset all” button to remove all of the molecules and channels. 2. Follow the steps mentioned earlier but add 20 chloride ions (green circles) and 20 sodium ions (blue

diamonds) inside the cell. 3. Begin a timer for 20 seconds and click the PLAY icon at the same time. After every 20 seconds,

pause the simulation and count the number of green circles on each side of the cell membrane. Do this for about 5 minutes. Complete Table 3 given below:

Table 3

Time (seconds) Chloride ions (Green circles) inside the cell

Chloride ions (Green circles) outside the cell

Sodium ions (Blue diamonds) inside the cell

Sodium ions (Blue diamonds) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

     

Activity 2 (Graphs)

Exploration 1: Plot a curve on the graph paper provided to you based on the values entered in Table 1, Table 2 and Table 3 for ‘Time’ on the X-axis and the ‘Number of chloride ions outside the cell’ on the Y-axis.

     

Activity 2 (Questions)

Exploration 1:

Question 5: How does the increase of concentration affect the rate of diffusion? Explain with the help of the curves you plotted.

Question 6: How does the presence of another type of molecule affect the rate of diffusion? Explain with the help of the curves you plotted.

Question 7: In the third and last simulation, if the concentration of sodium ions is increased, how will it affect the rate of diffusion of chloride ions?

     

Molecules like water and lipids diffuse freely through the cell membrane. However, most molecules cannot diffuse similarly because of their size or electric charge and require protein channels that facilitate their transport across the cell membrane. As shown in Fig. 3, certain types of protein channels present on the surface of red blood cells facilitate the exchange of glucose molecules. Similarly, other types of protein channels facilitate the transport of ATP molecules in and out of the cell. Like glucose, ATP molecules are a source of energy for the cell. Fig. 5A and 5B show how red blood cells look like when observed under a scanning electron microscope.

Fig. 5A shows how red blood cells appear when they are immersed in a solution in which the concentration of ATP is equal on either side of the membrane. In this case, there is adequate ATP for the cell to carry out its functions and the cell is healthy and retains its donut like shape. Fig. 5B shows the red blood cells depleted of ATP. The cell, deprived of energy fails to carry out its functions and loses its shape and it shrinks. Eventually it dies.

Exploration 1: Understanding how concentration gradient affects rate of facilitated diffusion In this exploration, you will observe how concentration of glucose molecules affects its rate of diffusion. Setup a simulation where glucose molecules are diffusing into the cell. In this exercise, glucose molecules will be represented by green circles. 1. Press the “reset all” button to remove all of the molecules and channels. 2. Add 5 glucose molecules inside and 20 glucose molecules outside of the cell and set the animation

speed (by moving the slider at the bottom) at 170. 3. Check the box 'SHOW CONCENTRATION'. 4. Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green gated channels into

the membrane. 5. Begin a timer for 20 seconds and click the PLAY icon at the same time. After every 20 seconds,

pause the simulation and count the number of green circles on each side of the cell membrane. Do this for about 5 minutes. Complete Table 1 on the next page.

Fig. 5A RBCs with adequate supply of ATP  

Fig. 5B RBCs when deprived of ATP  

     

Question 1: Look at the bar graph. Which side of the cell has a higher concentration gradient?

Question 2: How much is the concentration gradient? 6. Press the “reset all” button to remove all of the molecules and channels. 7. Follow the steps mentioned earlier but add 40 glucose molecules inside the cell. 8. Begin a timer for 20 seconds and click the PLAY icon at the same time. After every 20 seconds,

pause the simulation and count the number of green circles on each side of the cell membrane. Do this for about 3 minutes.

Time Glucose molecules (Green circles) inside the cell

Glucose molecules (Green circles) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

Time Glucose molecules (Green circles) inside the cell

Glucose molecules (Green circles) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

Table  1   Table  2  

     

Exploration 2: Understanding how number of protein channels affect rate of diffusion In addition to concentration gradient, rate of facilitated diffusion also depends on the number of specific carrier protein molecules in the membrane. Considering the previously stated example of diffusion of glucose: No matter how much the cell “needs” the glucose- no matter how great the concentration gradient- the rate at which the glucose can diffuse into the cell has a limit because of the limited number of glucose carrier protein molecules in the lipid bi-layer. To understand how number of protein channels available for diffusion affect rate of diffusion, again setup the previous simulation where glucose molecules are diffusing into the cell with the help of the carrier proteins present in the cell membrane. 1. Add 5 glucose molecules (green circles) inside the cell. Add 20 glucose molecules outside of the cell. 2. Set the animation speed (by moving the slider at the bottom) at 170. 3. Check the box 'SHOW CONCENTRATION'. 4. Click the PAUSE icon (next to the slider) to pause the simulation. Place 2 green gated channels into

the membrane. 5. Begin a timer for 20 seconds and click the PLAY icon at the same time. After every 20 seconds,

pause the simulation and count the number of green circles on each side of the cell membrane. Do this for about 3 minutes. Complete Table 3 below.

Time Glucose molecules

(Green circles) inside the cell

Glucose molecules

(Green circles) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

Table  3  

     

Complete Table 4 for the previous scenario with glucose molecules having 4 channels for diffusion. Complete Table 5 for the previous scenario with glucose molecules having 8 channels for diffusion.

From tables 3, 4 and 5 find out how much time was needed in each case to reach equilibrium and fill in the blanks in the table below. If you have a time range, take the value at the middle of that range as the required time.

Time Glucose molecules (Green circles) inside the cell

Glucose molecules (Green circles) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

Time Glucose molecules (Green circles) inside the cell

Glucose molecules (Green circles) outside the cell

20 sec

40 sec

1 min

1 min 20 sec

1 min 40 sec

2 min

2 min 20 sec

2 min 40 sec

3 min

3 min 20 sec

3 min 40 sec

4 min

4 min 20 sec

4 min 40 sec

Number of leakage channels for each molecule 2 4 8

Time required to reach equilibrium

Table  6  

Table  5  Table  4  

     

Activity 3 (Graphs) Exploration 1: Plot your experimental data as a line graph on the graph paper provided to you based on the values entered in Table 1 and Table 2 for ‘Time’ on the X-axis and the ‘Number of glucose molecules outside the cell’ on the Y-axis.

     

Exploration 2: Plot a bar graph based on the data values in Table 6. Plot number of gated channels for glucose molecule on the X-axis and the time required for equilibrium on the Y-axis.

   

     

Activity 3 (Questions)

Exploration 1:

Question 3: How does the increase of concentration affect the rate of diffusion? Give an explanation based on the data that you have plotted.

Question 4: How would the presence of another molecule, such as ATP, affect diffusion of glucose molecules if:

A: ATP molecules and glucose require different channels for transport.

B: ATP molecules and glucose use the same channels for transport.

Exploration 2:

Question 1: How does the number of channels available affect the rate of diffusion for one type of molecule? Explain with the help of the bar graph that you have plotted.

 Question 2: Will the rate of facilitated diffusion of glucose molecules increase or decrease if:

A: There were 12 channels.

B: There were only 1 channel.