Membranes
Membranes, Diffusion
Classwork
1. How does a phospholipid membrane create an isolated internal environment?
2. Draw and label a phospholipid.
3. In what way do the screen on a window and a cell membrane serve a similar
function? What characteristic is used to describe this function?
4. What essential role does a cell membrane play in maintaining homeostasis?
5. If you stir a cube of sugar into a glass of water, which of these materials would be
a solute, and which would be a solvent?
6. If you have a solution consisting of 5 grams of NaCl in 200 mL of water, what is
the molarity of your solution?
7. What characteristic of passive transport makes it ‘passive?’
8. What role does a concentration gradient play in the process of passive transport?
9. Which image below is in a state of equilibrium? Explain your answer.
10. Dialysis tubing contains 0.5 M glucose solution. It is placed in a beaker of 1.0 M
glucose solution. Describe the direction of diffusion.
11. A cell with an O2 concentration of 8 mM and a CO2 concentration of 5 mM is
placed in a solution of 10 mM O2 and 1 mM CO2. Describe the direction of
diffusion of each gas.
Homework
12. Compare a phospholipid membrane to a chain link fence. How are these two
structures similar? What function do they both serve?
13. How do amphiphilic phospholipids prevent their hydrophobic ends from coming
into contact with water?
14. Why is it necessary for cellular health that a cell membrane be selectively
permeable?
15. Make a correction to the following statement to make it true:
A solute has the ability to dissolve a solvent.
16. If you have a solution consisting of 100 grams of C6H12O6 in 1000 mL of water,
what is the molarity of your solution?
17. Suppose you have a solution consisting of 20 grams of carbon dioxide in 750 mL
of water. What is the molarity of your solution?
18. What type of cellular transport would you use to describe a ball rolling down a
hill? Explain your answer.
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19. Suppose you spray air freshener into the corner of a room. Explain how the air
freshener will move throughout the room. Be sure to use the term concentration
gradient in your response. At what point will the air freshener molecules stop
moving through the room?
20. What impact does the state of equilibrium have on the rate of diffusion?
21. Suppose you have a cell that is freely permeable to H2O. There are more H2O
molecules outside of the cell than inside of the cell. In what direction will the net
movement of H2O molecules occur? Why?
22. A cell with an O2 concentration of 2 mM and a CO2 concentration of 10 mM is
placed in a solution of 9 mM O2. Describe the direction of diffusion of each gas.
Osmosis
Classwork
23. Since osmosis is passive transport, in which direction does water move related to
its concentration gradient? What direction does it move in relation to the solution
concentration gradient?
24. If a cell is placed in a hypertonic solution, in which direction will water flow in
relation to the cell?
25. Sometimes, doctors will recommend that patients experiencing a sore throat
should gargle saltwater to relieve their symptoms. Explain, in terms of solute
concentration, how gargling saltwater could help reduce swelling in the throat.
26. Suppose a cell is placed in an unknown solution. After examining the cell under
a microscope, you see that the cell membrane has expanded, like a tight water
balloon. What kind of solution, hypertonic, hypotonic or isotonic, is the unknown
substance? Why did you come to this conclusion?
27. If there is equal concentration of free water molecules inside of a cell compared
to its surrounding solution, what type of environment is the surrounding solution?
28. Suppose you have a houseplant that has begun to wilt. Would it be more
beneficial to water this plant with a solution that was hypertonic or hypotonic
when compared to the plant cells? Explain your answer.
29. A cell with a sucrose concentration of 0.65 M is placed in a 1.2 sucrose solution.
Describe the net flow of water.
Homework
30. Does osmosis require the input of energy? Why or why not?
31. What will happen to a cell that is placed in a hypertonic solution?
32. One way to preserve perishable food, such as meat, is to pack the food in a
heavy concentration of salt. Knowing that bacteria survive well in a moist
environment, explain why this method of food preservation can be effective.
33. If a cell lyses after being submerged in a solution, would you suggest this
solution is hypertonic, hypotonic or isotonic? Why?
34. If a cell membrane were impermeable to water molecules, how would this
change the process of osmosis in our cells?
35. What is the relationship between osmosis and diffusion?
36. A cell with an O2 concentration of 0.4 M and a glucose concentration of 0.1 M is
placed in a 0.5 M glucose solution. Describe the net flow of both solutes and
water.
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Plasma Membrane, Transport Through Proteins
Classwork
37. Explain why a cell that requires only passive transport of small molecules may
not require the presence of membrane proteins.
38. What similarities exist between peripheral vision and peripheral proteins? Why is
the same adjective used to describe both of these things?
39. Why might a polar molecule have a difficult time moving across a phospholipid
bilayer, even if it were a small molecule?
40. Explain why the term fluid mosaic is used to describe the structure of a
phospholipid bilayer.
41. Do you think an integral protein or a peripheral protein is more useful for the
transport of molecules across a cell membrane? Justify your answer.
42. Identify one similarity that exists between the processes of active transport and
facilitated diffusion.
43. Suppose you need to determine whether glucose is being transported using
active transport or facilitated diffusion. The only clue you have is that ATP
molecules are required for the movement to occur. Which type of transport do
you suggest is being used? Why?
44. Cl- ions often move across cell membranes through a membrane protein that
does not change shape to accommodate their transport. Is this protein more
likely a channel protein or carrier protein? Justify your response.
45. What type of cell transport is required to move a substance from an area of low
concentration to an area of high concentration?
46. Describe one function that peripheral proteins may provide for the cell.
47. In order for nerve cells to conduct electrical signals appropriately, certain ions
need to be transported against their concentration gradient. What type of cell
transport is necessary for this to occur? Is ATP required for this process to
occur?
Homework
48. Why are membrane proteins necessary for the movement of some larger
molecules across cell membranes?
49. What is the difference between a peripheral membrane protein and an integral
membrane protein?
50. Are the phospholipids and proteins that construct a cell membrane anchored in a
stationary position? What is the term used to describe the arrangement of a
phospholipid bilayer?
51. Why are integral proteins sometimes also called transmembrane proteins?
52. Even though ions are very small, they often require the assistance of a
membrane protein to enter or leave a cell. What characteristic of ions makes this
necessary?
53. Even though they both require the use of membrane proteins, if a molecule
needs to move against a concentration gradient, would active transport or
facilitated diffusion be a more likely method to accomplish this task? Explain
your answer.
54. What do facilitated diffusion and osmosis have in common?
55. Knowing that molecules will naturally move from an area of high concentration to
an area of low concentration, why do you think active transport may require the
input of energy?
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56. Suppose a cell needs to move a small, uncharged molecule against its
concentration gradient. What type of transport would be required to accomplish
this task?
57. In what way is active transport similar to pushing a ball up a hill?
Enzymes, Catalytic Cycle
Classwork
58. Explain the impact that an enzyme can have on a chemical reaction.
59. What is the relationship between enzymes and catalysts?
60. Which class of biological macromolecules would contain enzymes?
61. Suppose you are diagnosed with lactose intolerance. What enzyme would you
not find naturally occurring in your body? How would this affect the digestion of
lactose in your body?
62. What is an active site? Why is an active site important for the completion of
chemical reactions?
63. How is an ‘induced fit’ with an enzyme similar to having your shirt tailored?
Explain your answer.
64. How are the reactants altered over the course of a chemical reaction?
65. What is activation energy?
66. How do enzymes affect the amount of activation energy required for a reaction?
Homework
67. Suppose a friend of yours accidentally begins a food fight by flicking a grape off
of your lunch table. Identify the catalyst of the food fight.
68. Identify two ways in which an enzyme impacts a chemical reaction.
69. Explain the ‘lock and key’ relationship between enzymes and substrates.
70. How would activation energy be affected if a reaction occurred in the absence of
an enzyme?
Temperature, pH, Inhibition
Classwork
71. Explain the role that optimal temperature has on enzyme activity?
72. How is an enzyme affected when the environment exceeds its optimal
temperature?
73. If enzymes were a different kind of biological molecule, would temperature and
pH have the same affect on their activity? Explain your answer.
74. What role do cofactors have for enzyme activity?
75. How do competitive inhibitors interact with an enzyme?
76. Would the same solution for negating the impact of a competitive inhibitor work
for a non-competitive inhibitor? Why or why not?
Homework
77. How does increasing temperature affect enzyme activity?
78. Why does denaturing an enzyme impact its ability to influence a chemical
reaction?
79. Draw a graph of enzyme activity for an enzyme that has an optimal pH range of
6-8.
80. What is the relationship between cofactors and inhibitors?
81. Explain one way in which your body may negate the impact of a competitive
inhibitor. Explain why this would work.
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82. What affect does non-competitive inhibition have on the active site of an
enzyme?
Allosteric Regulation, Feedback Inhibition
Classwork
83. What is the difference between an allosteric site and an active site?
84. Explain how allosteric inhibitors function through feedback inhibition.
85. Even though allosteric regulation and denaturation both affect the shape of an
enzyme, they are very different processes. Explain the difference between these
two processes.
Homework
86. Explain why allosteric activators and inhibitors may be able to work on the same
enzyme.
87. Explain the process by which an allosteric inhibitor influences an enzyme.
88. Provide a hypothesis for why scientists use the term ‘feedback inhibition.’
89. Compare and contrast an allosteric activator and a coenzyme.
Free Response
1. Below is an illustration of a common lab set-up used to visualize and observe the
process of diffusion. The membrane of the bag is permeable to water and
glucose but is not permeable to starch. Using the illustration, and information
provided, respond to each of the following:
a. Will the starch move out of the bag? If so, why? If not, why not?
b. What reagent would we use to determine if starch moved out of the bag
and into the beaker solution? Explain the test you would use.
c. Will the glucose move into the bag? If so why? If not why not?
d. The beaker solution his composed of 360g of glucose (C6H12O6) in 500
ml of water. What is the molarity of the glucose solution?
Glucose
solution
Starch solution
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2. Below is an illustration of the catalytic cycle of an enzyme.
a. Complete the chart below by identifying the items labeled as a,b,c,d and e on
the illustration.
b. Describe the process seen in this illustration. Include the vocabulary specific
to this process in your response.
c
_______e
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3. Feedback is required in order to maintain homeostasis within cells and within
organisms. This feedback may be positive, inducing more production, or
inhibiting, inducing a stop to production.
a. In some processes, the products from one enzyme acts as the substrates
for a second enzyme, and so on. Explain how feedback inhibition can
occur to stop a series of enzyme catalyzed reactions.
b. Explain how feedback inhibition and allosteric inhibition are related.
4. Plasma, or cell, membranes have unique features that have made life possible.
Two of these characteristics are: the presence of transport proteins and a fluid
mosaic structure.
a. Peripheral proteins are located on one side only of the plasma
membrane. Why are integral proteins referred to as “transmembrane”
proteins?
b. Identify the two types of transport proteins
c. Explain how each type of transport protein functions in the process of
transport across the cell membrane. Describe whether the proteins move
molecules with or against the concentration gradient.
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Answer Key
1. The double layer of membranes creates a barrier that separates the cell from its
surroundings. The fact that the membrane is polar and non-polar also prevents
many molecules from freely entering and leaving the cell.
2. Draw and label a phospholipid.
3. Screens and cell membranes both allow certain things to pass through them, but
exclude other objects, depending on their size and other characteristics. We
describe these qualities as semi-permeable.
4. Cell membranes allow a cell to regulate its internal environment, regardless of its
surroundings. If the cell membrane did not exist, a cell would be directly subjected
to the changes of its environment and the ability to maintain stability would be
compromised.
5. The water is the solvent, sugar is the solute.
6. 0.43M NaCl
7. Passive transport involves moving substances down their concentration gradient,
and does not require the input of energy.
8. The concentration gradient, or distribution of molecules across a space,
determines which way, on average, a given molecule will move. In passive
transport, molecules move down their gradient, or from an area of high
concentration to an area of low concentration.
9. The image on the right is in a state of equilibrium.
10. Into the tubing
11. O2 in; CO2 out
12. Both the membrane and the fence are semi-permeable. They both function to
regulate the internal environment of an area.
13. The phospholipids are always oriented so their hydrophobic tails are facing each
other, and the hydrophilic heads are facing the water on the inside and outside of
the cell.
14. In order for a cell to obtain nutrients and other items necessary for proper function,
and to excrete wastes, the cell membrane must allow certain materials to pass
through. At the same time, the amount of these substances must be regulated,
and the cell membrane allows for this to occur. The cell membrane also excludes
foreign substances that could harm the cell, such as viruses and bacteria.
15. A solvent has the ability to dissolve a solute.
16. 0.6 M glucose
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17. 0.34 M CO2
18. A ball rolling down a hill could be similar to passive transport, as the ball does not
require energy to move down the “steepness gradient” just as molecules do not
require energy to move down their “concentration gradient.”
19. The particles of air freshener will move throughout the room from where they are
most concentrated, to where they are least concentrated, or down their
concentration gradient, until there is an equal distribution of air freshener particles
throughout the room.
20. Diffusion will continue until equilibrium is reached. Net diffusion will stop when
equilibrium is reached.
21. Into the cell
22. O2 in; CO2 out
23. Water moves down the concentration gradient, or from an area of high water
concentration to an area of low water concentration. This is also an area of low
solute concentration to high solute concentration.
24. The water will flow out of the cell. Since the surrounding solution is hypertonic to
the interior of the cell, there are more ‘free’ water molecules inside the cell, and
they will flow out, down their concentration gradient.
25. Saltwater is a hypertonic solution. Therefore, the concept is that the saltwater may
help to reduce the swelling in the throat, by drawing out excess fluids from the
affected cells, which is the source of the irritation.
26. The unknown solution is hypotonic. When placed in a hypotonic solution, the water
will flow into the cell, causing it to swell. This swelling is causing the observed
response in the cell.
27. It is an isotonic solution.
28. You would want to water the plant with a hypotonic solution. The plant cells will
more readily absorb water from a hypotonic solution. If surrounded by a hypertonic
solution, the plant cells will lose water.
29. Out of the cell
30. No. Osmosis does not require the input of energy. Osmosis is simply a form of
diffusion, which is passive transport, and does not require energy to occur, since
molecules are moving down their gradient.
31. The cell will shrivel as water leaves the cell.
32. The heavy salt concentration often will dry out the meat, and it also makes it
difficult for most bacteria to survive in the resulting hypertonic environment.
33. The solution is most likely hypotonic, as cells will swell when placed in a hypotonic
solution.
34. Water would not be able to flow freely in or out of the cell. Cells could not survive
like this, another method of transportation for water to move across a cell
membrane would likely have evolved.
35. Osmosis is the diffusion of water.
36. O2 and water will diffuse out of the cell; glucose will not diffuse
37. The molecules may be small enough to cross the cell membrane on their own, and
since they are moving down the gradient, there is no carrier protein required.
38. Peripheral vision is the edge of what you can see in your field of vision, peripheral
proteins are found on the edge of the cell membrane.
39. The polar molecules would not be able to cross the non-polar section of the cell
membrane. The polar molecules would need the help of a membrane protein.
40. Mosaic refers to the fact that the membrane is composed of a variety of different
components that creates a whole, and fluid refers to the fact that the components
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are not anchored in place, but have the ability to move and relocate throughout the
membrane.
41. More likely the integral protein would be useful for transport across a membrane.
Carrier proteins and channel proteins are both examples of integral proteins useful
in transport. While peripheral proteins can be useful in this process, they are more
likely to be used in cell-cell communication or enzymatic reactions.
42. Active transport and facilitated diffusion both use an integral protein.
43. Active transport. Active transport requires the input of energy to move substances
against their concentration gradient. This energy comes from ATP.
44. This is more likely a channel protein. Channel proteins do not change shape,
whereas carrier proteins do.
45. Active transport is required. In order to move substances against their gradient,
the input of energy is necessary. Active transport can accomplish this.
46. Signal transduction, cell to cell recognition or enzymatic activity are all acceptable
responses.
47. This requires active transport, which uses ATP.
48. The larger molecules, even if moving down their concentration gradient, cannot fit
in between the phospholipids and thus require a protein to pass through.
49. A peripheral membrane protein is found on the edge of the membrane, whereas an
integral protein is actually embedded within the membrane.
50. No. The phospholipids and proteins can shift throughout the membrane. This is
called the fluid mosaic model.
51. The integral proteins cross the cell membrane. They extend from end to the other.
52. Ions are charged, and thus cannot freely diffuse through the phospholipid bilayer.
53. Active transport. Molecules naturally want to move down their gradient, so in order
to move them in the opposite direction, energy is required.
54. Both are types of passive transport.
55. The energy is required to counteract the natural tendency of molecules to move
down their gradient. Often the energy is used to power the change of shape of the
carrier protein being utilized.
56. Active transport.
57. The ball naturally will move down the hill, just as particles will naturally move from
an area where they are highly concentrated, to an area where they are less
concentrated. In order to push the ball back up the hill, energy must be expended,
just as energy is expended to move particles against their gradient.
58. Enzymes increase the speed of a chemical reaction and lower the amount of
activation energy required to initiate a chemical reaction.
59. An enzyme is a biological catalyst
60. Proteins
61. Your body would not naturally produce lactase. As a result, your body would not
digest lactose as it properly should.
62. An active site is the physical location on the enzyme where the substrates will
interact to complete a chemical reaction. Without the active site, the substrates
would not have the physical proximity to complete the reaction.
63. An induced fit is where the enzyme slightly changes shape to create a better fit for
the substrates and enhance the chemical reaction, just like tailoring a shirt is
arranging the shape to make it fit slightly better
64. The reactants experience some form of molecular rearrangement and are
converted to products in the course of a chemical reaction
65. Activation energy is the energy that is required to initiate a chemical reaction.
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66. Enzymes reduce the amount of activation energy required to initiate a chemical
reaction.
67. Your friend flicking the grape would be the catalyst, because that action initiated
the food fight.
68. Enzymes speed up the rate of a chemical reaction and reduce the activation
energy required to initiate the reaction.
69. Just as only a specific key can open a specific lock, an enzyme only works on a
specific substrate.
70. The activation energy would not be reduced, and thus the amount of energy
required to initiate the reaction would be greater than if it occurred with an enzyme.
71. Optimal temperature is the temperature under which an enzyme will function most
effectively. Above or below this temperature will not achieve the same results.
72. The enzyme may become denatured, in which its shape is changed and it cannot
bind to its appropriate substrate.
73. They could, but it would be unlikely. Proteins are readily affected by change in
temperature and pH because of the complex bonding nature that occurs in the
structure of a protein.
74. Cofactors bind to an active site and help to increase the efficiency of enzyme
function by creating a more appropriate fit for the substrate.
75. Competitive inhibitors are the same shape as a substrate, so they will compete
with substrates for access to the active site of an enzyme.
76. No. You could potentially negate a competitive inhibitor by increasing the amount
of substrate available for the reaction. However, the amount of available substrate
will not affect a non-competitive inhibitor because it involves a change in shape at
the active site.
77. Increasing temperature will usually increase enzyme activity, until the temperature
exceeds optimal range, at which point enzyme activity will decline.
78. Denaturing an enzyme changes its shape, and will make it unable to bind to a
substrate at the active site, greatly reducing or stopping the course of the chemical
reaction.
79. Draw a graph of enzyme activity for an enzyme that has an optimal pH range of 68.
80. Cofactors and inhibitors work in opposite ways. Cofactors enhance chemical
reactions, inhibitors stop or slow them down.
81. Your body may be able to increase the amount of substrate available for a
chemical reaction to negate the effect of a competitive inhibitor. Since competitive
inhibitors compete with substrates for access to the active site, the more substrate
is available, the higher the percentage of a substrate binding to the enzyme.
82. Non-competitive inhibitors bind to an enzyme and change the shape of its active
site.
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83. An allosteric site is a location on an enzyme where an allosteric molecule can bind
and affect enzyme function, whereas the active site is the location where a
substrate(s) can bind to begin a chemical reaction.
84. In feedback inhibition, the product at the end of a series of chemical reactions is
also an allosteric inhibitor for one of the enzymes that is used early in the pathway.
This way when the product is available in ample supply, the pathway will be turned
off so as not to produce excess material and waste energy.
85. Allosteric regulation is reversible and denaturing of an enzyme is not. Allosteric
regulation can also work to the benefit of the enzyme, whereas denaturation is
always destructive for enzyme function.
86. Allosteric regulation is reversible, so an enzyme may experience both types of
molecules at different times.
87. An allosteric inhibitor binds to the allosteric site on an enzyme, changing the shape
of the active site and making it unable to bind to a substrate.
88. The final product in the series of reactions goes back to the beginning of the
pathway and stops the reaction process. Thus the product is being “fed back” to
the beginning and “inhibits” the pathway itself.
89. An allosteric activator binds to the allosteric site and induces a change of shape
that makes the active site more compatible with a substrate. A coenzyme has a
similar effect, but it binds to the active site to accomplish the task.
Free Response – Answer Key
1.
a. The starch will not move out of the bag. The bag is not permeable to the
starch molecules.
b. We would use Lugols (IKI) to test for the presence of starch outside of the
bag. I would take a sample of the beaker solutions and place it in a test tube.
Add a few drops of lugols reagent and swirl. If the solution stays a light
brown/amber color, then there is no starch present in the solution. If the
solution changes color to a blue/black, then that indicates the presence of
starch.
c. The glucose will move into the bag because the bag is permeable to the
glucose. Also, the concentration of glucose outside of the bag is higher than
the concentration of glucose inside the bag, therefore diffusion into the bag
will take place.
d. Find moles of glucose: 360g x 1 mole/180g = 2 moles of glucose
Change ml to L 500 ml = .5L
2 moles/.5L = 4M glucose solution
2. a. A – substrate; B – active site; C – enzyme-substrate complex; D – product; E enzyme
b. Enzymes are proteins that catalyze specific reactions. A substrate can attach to
the active site of an enzyme. When combined the activation energy for this reaction
is decreased. When the enzyme and substrate are attached, this is called the
enzyme substrate complex. The reaction occurs and a product or products is/are
produced. The product(s) are released and the enzyme remains the same, it is
unaltered and can perform this reaction over and over again.
3.
a. Many times there is a sequence of reactions catalyzed by enzymes.
i.
The products of one reaction are the substrates for the next reaction and so
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on, until the final endproduct is produced.
If there is enough of the product, and there needs to be a pause or a stop to
production, the end product then may act as an inhibitor.
iii.
The endproduct can do this by binding to the first enzyme allosterically to
halt production.
b. The end product of a series of enzyme catalyzed reactions can act as an allosteric
inhibitor. It can do this by binding to the subunits of an enzyme. When the end
product binds to the enzyme, the subunits shaoe is changed. The active site is no
longer able to bind to the substrate. This stops the sequence of production until
more product is needed.
4.
a. Transmembrane proteins span or cross the membrane from one side to the other.
Transmembrane refers to this crossing of the membran
b. The two types of transmembrane proteins are channel proteins and carrier proteins
c. Channel proteins provide openings that can allow a specific molecule or ion to cross
the cell membrane. They act as a corridor or tube. No energy is required. Solutes
still move from higher concentration to lower concentration. Carrier proteins change
shape slightly. A certain molecule binds to it and the carrier then moves the
molecule across the membrane. Carrier proteins can function in facilitated transport,
which does not require energy, or in active transport, where molecules are moved
against the concentration gradient and energy is required.
ii.
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