unit membrane  
 

 Molecules must be able to move into or out of a cell across its membrane. In this section you will 
learn how this is accomplished. Focus on the following principles as you learn the various ways 
that molecules are transported across the membrane. 

    1. Type of molecules ( water, small organic molecules, small inorganic molecules) moving across 
        the membrane. 

    2.  Passive process (does not require biological energy) or active process (does require 
         biological energy - ATP) moving the molecules. 

    3.  Accomplished by special carrier protein molecules in the membrane. 

Let's begin this study by writing some definitions in your notes. 

    Kinetic energy- energy of motion - all matter contains kinetic energy - increase 
                        temperature, allows matter to absorb energy and move faster - decrease 
                        temperature allows matter to lose energy and moves slower - 0 degree Celsius 
                        (-273 degree C) no kinetic energy - ice is water molecules (solid state) with less 
                        kinetic energy, when heated ice water molecules move faster and turns into liquid, 
                        when heated liquid water molecules move faster and turns into gas 

     Diffusion- molecules move from an area of high concentration to an area of low 
                      concentration - concentration is the number of molecules per volume (10% = 10 
                      molecules per 100) and amount is the number of molecules (10) 
                      All molecules possessing kinetic energy can diffuse. Gas molecules can diffuse 
                      through other gas molecules, liquids and solids. Molecules in the liquid state can 
                      diffuse through other liquid molecules, gas molecules and solid molecules. 
 
    Equilibrium- that point where there is no net movement of molecules - net movement is 
                         where there is more molecules moving in one direction compared to the opposite 
                         direction - when molecules move in both directions at the same rate there is no 
                         net movement. 
 

     Gradient- concentration differences between two regions of space - sugar placed in a cup 
                      of water initially has a high concentration of sugar around the undissolved sugar at 
                      bottom of cup compared to the region at the surface of the water 

Diffusion - Passive Transport 

Diffusion -Read the pages in the text indicated by B-19 and study behavioral objective 18.  Study the figure showing diffusion of a dye in water. Note - No membrane is involved. Dye molecules are moving from a region of high concentration (at dye particle) to a region of low concentration (water space away from particle). Keep in mind that water molecules are also diffusing; however, they are diffusing from the water space to the area around the dye particle. This will continue until equilibrium is reached (dye is completely dissolved in the water).  At equilibrium all of the liquid space in the beaker will have the same concentration of dye molecules; however, the dye and water molecules  will continue to move (kinetic energy). 
 

 
 In the figure above the pink dots on the left are diffusing to the right until the concentration of pink and yellow dots are the same. The pink dots are diffusing to the right and the yellow dots are diffusing to the left.  The dots are still moving at equilibrium but at the same rate. 

A      A     A      A 
___________ 
|      p   p         | A 
|      p p p        |            A 
|     p p    p      |A 
|    p      p p     | 
|      p        p    |       A 
|___________| 
 

In your notes write a description explaining the  diffusion of perfume molecules  from a bottle placed in the front of the lecture room with air molecules of the room after the lid is removed. See diagram above. (The "p" represent perfume molecules in bottle and "A" represent air molecules outside of the bottle). When the bottle is opened the perfume molecules will move out of the bottle due to kinetic energy and the air molecules will move into the bottle due to kinetic energy. Some perfume molecules which have left the bottle will move back in and some of the air molecules which moved into the bottle will move back out (random movement).  Since there will be more perfume molecules moving out of the bottle than into the bottle, the  net movement of perfume molecules will be out of the bottle. Using the same explanation  the net movement  of air molecules will be into the bottle. Both of the gaseous molecules are moving from a high concentration to a low concentration.  Will diffusion eventually come to a  stop? Yes, at equilibrium. When there is no net movement of either the perfume or air molecules. (Note- molecules will always be moving in and out.)  In this case when equilibrium in reached,  the number of perfume molecules leaving the bottle is equal to the number of perfume molecules entering the bottle.) 
 
 
In your description include concentration gradient, the initial direction of movement of the perfume molecules and air molecules, the movement of these molecule at equilibrium, the concentration of perfume molecules initially and at equilibrium.  Draw a figure showing this process. Click here to check answer.  click 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

     A       A 
A      p     A     p 
_                   _ 
|      p   p  p      | A 
|      p A p        |    A 
|    A p    A      |A 
|    p     p  p      |    p 
|    A    p   p     | 
|___________| 
 
 

1. Is this an active or passive process? 

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2. Initially is there a diffusion gradient? 

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3. Initially  which direction do the perfume molecules move?   - air molecules? 
 
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Draw a figure showing the arrangement of these gas molecules at equilibrium. 

Check your figure to the figure below. 
 

     p       A 
A      p     A     p 
_                   _ 
|      p   p  A     | A 
|      p A p        |    p 
|    A p    A      |A 
|    p     A p      |    p 
|    A    p   A    | 
|___________| 

 
 Note that the lid is open and that the concentration of air "A" molecules is equal to the concentration of perfume molecules "P". 

4. At equilibrium which direction does net movement of perfume molecules occur? 
          in, out of the bottle, not at all (explain) 

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5. At equilibrium where is the higher concentration of perfume molecules?  (in the bottle, in the 
    room, same) 

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6. At equilibrium where is the greater number of perfume molecules? (in the bottle, in the room, same) 

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7. Does diffusion stop at equilibrium? 
 
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Diffusion is a passive phenomena where molecules randomly move from one area of space to 
another area down a gradient. There is no membrane barrier involved during diffusion. The energy 
that drives diffusion is not biological energy but energy acquired from the environment as heat. 
 
 

Osmosis - Passive Transport of Water Across a Membrane 

What would happen if a selectively  permeable membrane separated the two systems undergoing diffusion. Selectively permeable refers to a membrane which allows only certain molecules pass through. 

Cells contain membranes that molecules must pass through. These membranes are selectively 
permeable because they allow some molecules pass through more rapidly than others due to the 
properties of the molecules and the membrane. In this section you will learn how various 
molecules pass across the membrane by osmosis, facilitated diffusion, and active transport. Read 
the pages indicated by B-19 and study behavioral objective 18   . 

Osmosis is the diffusion of water across a differentially membrane. Study the figures in the reading  demonstrating osmosis and answer questions concerning the following situations. 
 

Situation 1: 

A differentially permeable membrane bag containing 10% sucrose solution (10 g sucrose/90 g water) is placed in a beaker containing 100% water. The bag in impermeable to sucrose but allows water to pass through. 

8.  Will water move out of the bag through the membrane? 

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9.  Will water move into the bag through the membrane? 
 
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10.  In which direction will most of the water move (into the bag or out of the bag)?    This is 
       called net movement. 

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11.  Will sucrose move  out of the bag through the membrane? 

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12.  Will the solution in the bag ever reach equilibrium?  Explain. (equilibrium =  no net movement of 
        water in this example) 

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13. Is water still moving out of the bag at equilibrium? 

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14.  If membrane the does not burst will a pressure form inside the bag?  Define osmotic 
       pressure in your notes. 
 
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Examine the figures below representing  situation one  described above: initially and at equilibrium. 

Selective permeable membrane bag initially with 10 % 
sucrose place in a beaker containing water. 
 W = water molecules; s = sucrose molecules 

    W  W       W 
W      W     W    W 
____________ 
|    W s   s  W   | W 
|     W W  W    |    W 
|    W s    s       | W 
|    W   sW s    |  W 
|    s W  W   s  |      W 
|__________ |  W 
W      W     W    W 

   INITIAL 
------------------------------------------------ 

    W  W       W 
W      W     W    W 
____________ 
|    W s   W  W | W 
|     s W  s    W  |    W 
|    W s   s  W    | W 
|    W   sW s W |  W 
|    s W  W   W  |      W 
|___________ |  W 
W      W     W    W 

  EQUILIBRIUM 

Note: The number of sucrose molecules remained the same in the bag (8) but the number of water molecules increased; therefore, the concentration of sucrose in the bag  decreases to less than 10 %.  Because there is more water in the bag there must be an increase in pressure on the membrane. Observe  the  tight bag (turgor pressure, turgid, osmotic pressure). 

Draw these figures in your notes and click here to go back and review the questions concerning situation one. Click here to see questions concerning situation one.   click 
 

Situation 2: 

A differentially permeable membrane bag containing 100% water  is placed in a beaker 
containing 10% sucrose (10 g sucrose dissolved in 90g water) . The bag in impermeable to 
sucrose but not water. 

15.  Will water move out of the bag through the membrane? Concentration gradient 

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16.  Will water move into the bag through the membrane? 
 
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17.  In which direction will most of the water move (into the bag or out of the bag)? 
       This is called net movement. 

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18.  Will sucrose move out of the bag through the membrane? 

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19.  Will the solution in the bag ever reach equilibrium?  Explain equilibrium. No net movement 
       of water. 

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20.  Is water still moving out of the bag at equilibrium? 

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21.  If the membrane does not burst will a pressure form inside the bag? 
 
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Examine the figures below representing  situation two  described above: initially and at equilibrium. 

Selective permeable membrane bag, initially with water 
placed in a beaker containing 10% sucrose solution.. 
 W = water molecules; s = sucrose molecules 

    W  s       s 
W      s     W    W 
____________ 
|    W W  W    | s 
|     W W  W   |    W 
|    W W    W  | W  s 
|    W   W W   |  s   W 
|     W  W  W  |      W 
|__________ |  W 
W      s  W     W    s 

   INITIAL 
----------------------------------------------------------------------------------------------------------------------- 

    W  W    s   W 
W      W   s  W  s  W 
____________ 
|    W    W  W   | W s 
|      W      W   |    W 
|    W      W     | W s 
|    W         W  |  W 
|     W     W     |  s     W 
|____________|  W 
W   s   W s    W    W 

  EQUILIBRIUM 

Note: The number of sucrose molecules remained the same outside the bag (8). The number of water molecules outside the bag increases; therefore,  the concentration of sucrose in the bowl decreases to less  than 10 %.  Because there is less water in the bag there must be a decrease in the pressure on the membrane. (flaccid, negative osmotic pressure, plasmolysis) 

Draw these figures in your notes and click here to go back and review the questions concerning situation two.  Click here to see questions concerning situation two.  click 
 

Situation 3: 

A differentially permeable membrane bag containing 30% sucrose solution (30 g sucrose 
dissolved in 70 g water) is placed in a beaker containing 10% sucrose (10 g sucrose dissolved in 
90 g water). The bag in impermeable to sucrose but not water. 

22.  Will water move out of the bag through the membrane? Concentration gradient 

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23.  Will water move into the bag through the membrane? 
 
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24.  In which direction will most of the water move (into the bag or out of the bag)?    This is 
       called net movement. 

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25.  Will sucrose move out of the bag through the membrane? 

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26.  Will the solution in the bag ever reach equilibrium? 

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27.  Is water still moving out of the bag at equilibrium? 

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28.  If the membrane does not burst will a pressure form inside the bag? 
 
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Examine the figures below representing  situation three  described above: initially and at equilibrium. 

Selective permeable membrane bag, initially with 30% sucrose 
placed in a beaker containing 10% sucrose solution.. 
 W = water molecules; s = sucrose molecules 

    W  s       W 
W      W     W    W 
____________ 
|    W s  W   s  | s 
|     s W  s        |    W 
|    W W  s W   | W  s 
|    s     s W      |  W   W 
|     Ws  W  s   |      W 
|__________ |  W 
W      s  W     W    s 

   INITIAL 
---------------------------------------------------------------------------- 

    W  W    s   W 
W      W     W    W 
____________ 
|    WW sWW  | W s 
|   s   W  s   W |    W 
|s  W  W s W   |  s 
|  W s W  W W|  W 
| s    W s W s   |  s     W 
|___________|  W 
W     W s    W    W 

  EQUILIBRIUM 

Note: The number of sucrose molecules remains the same inside and outside the bag (8-5). However the concentration of sucrose will change. The water molecules move from the bowl into the bag (water moving from a high to low concentration. Since  water molecules were moving into the bag from the bowl the concentration of sucrose in the bag decrease and the concentration of sucrose in the bowl increase. This will continue until the concentration of sucrose in the bag and in the bowl are the same. At this point water will move into and out of the bag at the same rate; thus, equilibrium will be established. 

Draw these figures in your notes and click here to go back and review the questions concerning situation three. 
 
 Click here to go back to questions concerning  question three.  click 
 
 
 

 Think and Explain: During diffusion of gases  the concentration of particles will be the same at all points in the container and the room. During osmosis the concentration of particles may or may not be the same on both  sides of the membrane. 
 
 

Solutions: hypotonic, hypertonic, isotonic 

Read the section on solutions in the readings form B-19 and study behavioral objective 20. Solutions consists of two parts - solutes and solvents.  Solvent is a liquid capable of dissolving other substances in itself. The substance being dissolved is called a solute. In the above examples water is the solvent and sucrose is the solute. 

Solutions can be classified as hypertonic (hyperosmotic), hypotonic (hyposmotic)  or isotonic (isosmotic). Write in your notes the definition for these three terms. 

Note: You must compare at least two solutions when using these terms. 

Fill in the chart below to describe these three solution types. 
 
 

Solution Type	Amount of Solute (greater, lesser, same)	Amount of Solvent (greater, lesser, same)
Hypotonic
Hypertonic
Isotonic

 
 

29.  A sugar solution contains 15g glucose dissolved in 85 g water. Is this solution hypertonic, 
        isotonic, or hypotonic? 

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  30-35. Match the following terms to the correct statement listed below. 

         a) isotonic                          b) hypertonic                        c) hypotonic 
 
 

  30.  A solution with a higher concentration of solute molecules compared to a second solution. 

         a) isotonic                          b) hypertonic                        c) hypotonic 
 
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  31.   A solution with a lower concentration of solute molecules compared to a second solution. 
 
         a) isotonic                          b) hypertonic                        c) hypotonic 
 

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  32.    A solution with the same concentration of solute molecules as the second solution. 
 
         a) isotonic                          b) hypertonic                        c) hypotonic 
 
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 33.  In situation 3 above the solution in the bag is ________ compared to the solution in the 
        beaker. Check notes to see concentrations in the bag and beaker. 

         a) isotonic                          b) hypertonic                        c) hypotonic 
 
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  34.  The solution of the bag in situation 1 is _________ compared to the solution of the bag in 
       situation 2. Check your  notes to see concentrations is the bag and beaker. 

         a) isotonic                          b) hypertonic                        c) hypotonic 
 
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The U tube above shows a hypotonic  and hypertonic solution separated by a selectively permeable membrane.  Note the pressure that builds up on the hypertonic side. This is the osmotic pressure. 

  35. If a bag contains a hypertonic solution compared to the solution in the beaker, the bag will 
        __________ (swell up, shrink, remain the same). 
 
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Osmosis:  Transport of Water into Living Systems 
 
Now let us study osmosis in plant and animal cells.. 

The cell membrane is a selective permeable membrane. The solution of the cell's cytoplasm will 
react the same way the bags did in the above situations. For example study the figure in the 
readings (B-19) showing the effect of osmosis on red blood cells. 

   36. A red blood cell placed in a hypotonic solution will __________ (swell up, shrink, remain 
         the same). Since the cell membrane is not strong it will break releasing the cytoplasm 
        contents to the outside. This is called hemolysis. Lysis means to break. 

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   37. A red blood cell placed in an isotonic solution will __________ (swell up, shrink, remain the 
        same). 

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38.  A red blood cell placed in a hypertonic solution will __________ (swell up, shrink, remain 
       the same).  This is called crenation. If left in this condition very long the cell will die. 
 
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Explain what would occur if a plant cell was placed in these three conditions. 

39. A plant cell placed in a hypotonic solution will __________ (swell up, shrink, remain the same). 

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However the plant cell will not break due to the presence of a cell wall. The cell wall will become very tight due to the osmotic pressure. This pressure is called turgor pressure when plant cells are involved and the cell is described as being turgid. 
 

40. A plant cell   placed in a hypertonic  solution will __________ (swell up, shrink, remain the same). 

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 The cytoplasm will become separated from the cell wall (the cell wall does not shrink). This is called plasmolysis. The cell in this state is said to be flaccid. 

In your notes draw figures showing hemolysis, crenation, plasmolysis, turgid cell and write a 
paragraph explaining how these were formed by osmosis. 
 

41. Plant cells of a wilted plant are in what condition?  (plasmolyzed, turgid, crenated, hemolyzed) 

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                      o 

Draw these animal and plant cells in your notes with the proper terms. 
 

42. Is osmosis an active or passive process? Explain 
 
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Facilitated Diffusion: Transport of Ions and Small Molecules Across Membrane 

Facilitated Diffusion: Read the pages indicated by B-19 and study behavioral objectives 18  . In your notes write definition for facilitative diffusion. Facilitated  diffusion is exactly like osmosis except the molecules must use membrane proteins to be transferred across the cell membrane. This will allow for selectivity as to which molecules can pass through and allows for polar and larger molecules to pass through the membrane. 

Osmosis is the movement of water across the membrane. Other molecules such potassium ions, 
calcium ions, amino acids, and monosaccharides can only move across the membrane with the aid 
of channel proteins or carrier proteins. In your notes draw figures taken from the text and 
describe how  membrane  proteins help  transport these molecules. This type of transport will only 
move molecules down their concentration gradients similar to osmosis (from a high concentration to a low concentration). 
 
 
Identify the transport molecule "c" and the glucose (solute molecules). 

Membrane with carrier molecules "c" which will only transport specific molecules such as glucose "g".  The solution inside the cells consists of 10% ionic salt. The solution in the bowl contains 6% glucose (6) and 5% sucrose (5). 
 W = water molecules; s = sucrose molecules; I = inorganic salts; g = glucose molecules 

    W  s   g    W 
W      W  g   W    W 
___c____c___ 
|    W I  W   I   | s 
|     I W  I         |  g  W 
|    W W  I W   | W  s 
|    I     I W      c  W   W 
|     WI  W  I    |    g  W 
|_____c_____ c  W 
W g    s  W  g   W    s 

   INITIAL 
----------------------------------------------------------------------------------------------------------------------- 

    W  W    s   W 
W      W g    W    W 
___c_____c__ 
|    WW IWW   | W s 
|   I   W  I   g     |    W 
|I  g  W I W       |  s 
|  W I g  W W   c  W 
|  I    W I W I   |  s     g 
|_____c_____c  W 
W     W s    W    g 

  EQUILIBRIUM 

Note: The glucose molecules moved into the bag until the concentration of glucose in the bag was the same as glucose outside of the bag (both the solutions inside and outside contain 3% glucose (3)). The sucrose molecules and the inorganic ions did not  move across the membrane.  Why? 
 
 

Read the pages indicated by B-20 which give examples of these type of transport in humans. 
 

43. Carrier molecules located in the membrane will allow all small molecules to pass into the 
       cytoplasm? 
 
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Active Transport: Transport of Molecules Across Membrane Actively 
 

Active Transport: Read in the text the pages indicated by B-19   and study behavioral objective 19. Active transport is  similar to facilitative transport (uses carrier membrane protein) except biological energy ATP will be required for the carrier protein to  operate. Also the carrier molecules will allow selective transport in one direction which will allow molecules to move against a concentration gradient - low concentration to a  high concentration. Study the figure below. 
 
  

Membrane with energy driven carrier molecules "ce" which will only transport specific molecules such as glucose "g" into the cell.  The solution inside the cells consists of 10% ionic salt. The solution in the bowl contains 6% glucose (6) and 5% sucrose (5). 
 W = water molecules; s = sucrose molecules; I = inorganic salts; g = glucose molecules 

    W  s   g    W 
W      W  g   W    W 
___ce____ce___ 
|    W   I   W   I   | s 
|     I   W   I         |  g  W 
|    W  W   I  W   | W  s 
|     I      I  W      ce  W   W 
|     W I   W   I    |    g  W 
|_____ce______ ce  W 
W g    s  W  g   W    s 

   INITIAL 
----------------------------------------------------------------------------------------------------------------------- 

    W  W    s   W 
W      W s    W    W 
___ce_____ce__ 
|    W W I W W   | W s 
|   I  g W   I   g     |    W 
|I  g  W  I  W       |  s 
|  W I  g  W  W   ce  W 
|  I g  W g I  W I   |  s    W 
|_____ce_______ce  W 
W     W s    W 

  EQUILIBRIUM 

Note: All of the  glucose molecules moved into the bag. The sucrose molecules and the inorganic ions did not  move across the membrane.  Did the glucose molecules move against a concentration gradient? 
 
 
 

 In your notes write a paragraph comparing facilitative diffusion with active transport. Contrast active transport with facilatative transport  as to  use of biological energy and type of concentration gradient. 
 
 

Click here to learn how these concepts are used in the human respiratory system. 
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