Lab Exercise 2: Movements Through Membranes

Question 1

Define diffusion and identify physiological examples of diffusion

Answer 1

Diffusion is the passive, random movement of molecules with their concentration gradient. Respiratory gases (O2 and CO2) and ions move by diffusion. Glucose moves into cells by facilitated diffusion (requires insulin)

Question 2

How does the diffusion of gas like oxygen differ from the diffusion of an ion like sodium(Na)?

Answer 2

O2 is lipid soluble so it can pass through the phospholipid barrier without the help of a carrier or ion channel

Question 3

Define osmosis and identify physiological examples of osmosis

Answer 3

Osmosis is the passive movement of water molecules through a porous membrane with their concentration gradient (from hypotonic region to hypertonic region). All movement of water into and out of individual cells is by osmosis. Water movement out of a kidney tubules back into the blood stream is by osmosis.

Question 4

Define filtration and identify physiological examples of filtration

Answer 4

Filtration is the movement of molecules through a porous membrane using hydrostatic forces. Molecules leave capillaries by filtration

Question 5

Why is it possible to demonstrate diffusion, osmosis, and filtration in experiments that do not include living tissue?

Answer 5

Cellular energy in the form of ATP is not required

Question 6

How does the agar plate experiment demonstrate diffusion?

Answer 6

The KMnO4 molecules passively moved with their concentration gradient (from region of high concentration to region of low concentration)

Question 7

The results of the agar plate experiment suggest the two agar plates have different permeability. What would make membranes in your body have different permeability?

Answer 7

The type and distribution of ion channels present

Question 8

How does the dialysis experiment demonstrate the influence of time on the rate of osmosis?

Answer 8

The net change in volume gain and rate of volume change decreased as time passed. This is because the gradient gradually decreased.

Question 9

Could the dialysis bag experiment also demonstrate the influence of the size of the concentration gradient on the rate of osmosis? Explain

Answer 9

Yes. As water moved into the dialysis bag, the contents became more dilute, which decreased the gradient

Question 10

Because the salt molecules cannot pass through the dialysis tubing, the concentration of the solutions on each side of the dialysis tubing can never be the same but a state of equilibrium can still be achieved. Explain.

Answer 10

Because the water molecules are in random motion, some water molecules are also leaving the dialysis bag. At some point, the same volume of water will move into the dialysis bag as exits and the net change in volume will be zero.

Question 11

Summarize the results of the stimulation experiment

Answer 11

When blood cells are placed in a relatively hypotonic solution (low [NaCl]), they take on water from the
surroundings and swell. The enlarged (or perhaps ruptured cells) block less light, increasing
transmittance (plateau on upper left). Conversely, when blood cells are placed in a relatively
hypertonic solution (high [NaCl]), they lose volume to the surroundings and shrink. The shrunken cells
block more light, decreasing transmittance (plateau on lower right). Cells placed in a relatively isotonic
solution exchange water with their surroundings but do not swell or shrink (vertical section of tracing).

Question 12

In part C, was the solution in the beakers hypertonic, hypotonic, or isotonic to the solution in the dialysis
bags? Explain your answer.

Answer 12

Hypotonic. Water always moves with its concentration gradient from the relatively hypotonic region
toward the relatively hypertonic region.

Question 13

How does the benedicts/iodide experiment demonstrate filtration?

Answer 13

A solution was passed through a selective membrane by hydrostatic pressure. The water and small
sugar particles passed through the membrane while the larger charcoal pieces and starch molecules
were trapped by the membrane.

Question 14

List two reasons why it is necessary to test both the solution and the filtrate for sugar and starch.

Answer 14

1. To confirm that you know how to perform the tests and that all reagents are working properly
2. To have a positive control test for comparison of your “unknown” results

Question 15

Was chalk or charcoal present in the filtrate? Explain how you came to this conclusion.

Answer 15

No. Charcoal can be seen with the naked eye. No charcoal was present in filtrate; it was all trapped by
the filter paper.

Question 16

Was sugar present in the filtrate? how you came to this conclusion.

Answer 16

Yes. The Benedict’s test on the filtrate was positive for sugar because it turned green, yellow, orange,
or red.

Question 17

Was starch present in the filtrate? Explain how you came to this conclusion.

Answer 17

No. The IKI test on the filtrate was negative. It did not turn purple, dark green, or black.

Question 18

Explain why the above experiments and activities demonstrate good experimental design.

Answer 18

In parts B, C, and D, there was only one variable examined (elapsed time, initial concentration
gradient, [NaCl] into which blood cells are placed). In part E a control was performed. What all of the
above lacked was repetition, which is why we are averaging the results of all the lab sections together.

Question 19

A family went for vacation to the beach and spent the day swimming and playing in the ocean. One of the
children noticed that, unlike when she went to the swimming pool at home, her fingers and toes weren’t all
“wrinkly” when she got out of the water. Explain.

Answer 19

The ocean water contains more salt than does the pool at her house, so it is hypertonic to her skin.
Therefore, her skin loses water to the ocean. If the cells lose water, they shrink. [Think: if your
clothes shrink, they aren’t baggy and wrinkly, right? They are too tight.] At home the swimming pool
water is slightly hypotonic to her skin, so they cells take on water and increase in size slightly. [Think:
if you buy clothes a size too big, they are baggy and wrinkly, right?]

Question 20

Diseases like emphysema, tuberculosis, and pneumonia reduce the surface area available for gas exchange in
the lungs. Explain the effect this will have on diffusion.

Answer 20

Less surface area will slow the rate of diffusion. This is why individuals with these conditions may need
oxygen therapy. Oxygen therapy increases the concentration gradient to compensate for the loss of
surface area.

Question 21

The kidneys are where substances are filtered out of your blood, and some of these substances are removed
from your body in the urine. What will happen to filtration in the kidneys if your blood pressure increases?
Glucose molecules are relatively small and are very important as an energy source for the body. Penicillin is a
relatively large molecule and can be removed from the body only through urination. Which will be filtered?
Explain.

Answer 21

Increasing the driving force (hydrostatic pressure) will increase the rate of filtration. In your body
your blood pressure is the driving force behind filtration. Filtration is selective by size only, not need,
so glucose will filter while penicillin will not.