Fick's Law and What Sets the Rate of Exchange · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Three dials that set the speed of crossing

Science · Organisms & Life Processes · ages 16-18
Name ______________________   Date ____________

  1. Which comparison identifies the driving gradient for oxygen diffusion between alveolar air and blood?

    • Oxygen partial pressure on each side
    • Total oxygen carried in each whole lung
    • Total air pressure inside and outside the chest
    • The number of blood cells beside each alveolus

  2. What does total net diffusion rate measure?

    • The speed of one molecule inside the barrier
    • The amount transferred overall each second
    • The time until every molecule stops moving

  3. At equal concentrations on both sides of a permeable barrier, molecules can still cross both ways even though net diffusion rate is zero.

    Circle one:   True   False

  4. Diffusion distance gets longer and all other factors stay fixed. What happens to total net diffusion rate?

    • The rate increases
    • The rate decreases
    • The rate remains unchanged

  5. A model doubles the number of identical alveolar exchange patches. Conditions within each patch stay fixed. How do total rate and rate per unit area change?

    • Both rates double because there are twice as many patches
    • Both rates stay fixed because each patch remains identical
    • Total rate doubles; rate per unit area stays fixed
    • Total rate stays fixed; rate per unit area doubles

  6. A membrane transfers 14 units per second. Exchange area doubles and all other factors stay fixed. What is the new total net rate, in units per second?

    Answer: ______________

  7. A gas crosses a membrane at 12 units per second. Membrane thickness doubles and all other factors stay fixed. What is the new rate, in units per second?

    Answer: ______________

  8. In the model, D and diffusion distance both double while area and concentration difference stay fixed. The total net rate stays unchanged.

    Circle one:   True   False

  9. A barrier transfers 16 units per second. Area triples, concentration difference halves and distance doubles. D stays fixed. What is the new rate, in units per second?

    Answer: ______________

  10. A gas crosses a barrier at 9 units per second. Diffusion distance triples and all other factors stay fixed. What is the new rate, in units per second?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Three dials that set the speed of crossing W1-mt_6JreZAFeb--s1

  1. Oxygen partial pressure on each side · Oxygen diffuses down its own partial-pressure gradient. Total oxygen content and total air pressure are different quantities.
  2. The amount transferred overall each second · Net rate measures overall transfer per second after allowing for movement both ways. It is not individual molecular speed.
  3. True · Equal concentrations give zero concentration difference and zero net transfer. Molecular movement need not stop.
  4. The rate decreases · Distance is in the denominator. Increasing it reduces rate when the other factors stay fixed.
  5. Total rate doubles; rate per unit area stays fixed · Twice as many identical patches give twice the total transfer and total area. Transfer per unit area remains unchanged.
  6. 28 · Rate is directly proportional to area. The new rate is 14 × 2 = 28 units per second.
  7. 6 · Thickness is the diffusion distance. Divide the original rate by 2: 12 / 2 = 6 units per second.
  8. True · D contributes a numerator factor of 2 and distance contributes a denominator factor of 2. Their combined factor is 1.
  9. 12 · The combined factor is 3 × 0.5 / 2 = 0.75. The new rate is 16 × 0.75 = 12 units per second.
  10. 3 · The distance factor is 3, so the new rate is 9 / 3 = 3 units per second.
Worksheet · LightMySky