Question 1

Which of the following best describes the process of diffusion as observed when a drop of colored solution is introduced into a still liquid?

Accepted Answer

Molecules move from regions of higher concentration to regions of lower concentration due to their thermal motion.

Question 2

Which equation gives the average distance $$S$$ from the starting point after $$N$$ collisions in a random walk, where $$L is $$the mean free path?

Accepted Answer

$$S = L \sqrt{N}$$

Question 3

A molecule in a liquid has a mean free path $$L = 10$$^{-7}$$ cm and a thermal velocity v$$ = $$10^4 cm/s. $$How long does it take, on average, for the molecule to diffuse a distance $$S = 10$$^{-5}$$ cm from its starting point?

Accepted Answer

$$t = \dfrac{S^2$}{L v} = \dfrac{(10$$^{-5}$$)^2$$}{$$10^{-7}$$ 	imes $$10^4$$} = $$10^{-3}$ s

Question 4

Which of the following statements about the diffusion coefficient $$D is $$correct?

Accepted Answer

$$D is $$inversely related to the square root of the molecular weight.

Question 5

The net flux $$J of $$molecules diffusing from region $$a ($$concentration $$C_1) to $$region $$b ($$concentration $$C_2) $$over a distance $$\Delta x is $$given by which equation?

Accepted Answer

$$J = D(C_1$ - C_2$)/\Delta x$$

Question 6

In the context of biological membranes, what does the permeability $$P of a $$membrane depend on?

Accepted Answer

Both the type of membrane and the diffusing molecule.

Question 7

Which of the following best explains why diffusion is sufficient for transport over small distances in cells but not over large distances in organisms?

Accepted Answer

Diffusion is a slow process over large distances, so organisms evolved circulatory systems for efficient transport.

Question 8

A cell membrane is permeable to water but not to solutes dissolved in water. What is the name of the process by which water enters the cell under these conditions?

Accepted Answer

Osmosis

Question 9

If the concentration of a solute is $$C_1 on $$one side of a membrane and $$C_2 on $$the other, and the permeability of the membrane is $$P$$, what is the net flux $$J$$ through the membrane?

Accepted Answer

$$J = P(C_1$ - C_2$)$$

Question 10

Which of the following real-world scenarios is an example of diffusion through a membrane being limited by the size of the diffusing molecule?

Accepted Answer

A large protein being unable to cross the glomerular membrane in the kidney.

Question 11

A scientist observes that a certain drug diffuses much more slowly through tissue than a small ion. Which of the following best explains this observation?

Accepted Answer

The drug has a larger molecular weight, resulting in a lower diffusion coefficient.

Question 12

Which of the following factors does NOT directly affect the mean free path $$L of a $$molecule in a liquid?

Accepted Answer

The temperature of the medium.

Question 13

If the mean free path $$L is $$doubled while all other factors remain constant, what happens to the diffusion coefficient $$D$$?

Accepted Answer

$$D is $$doubled.

Question 14

Which of the following best describes the relationship between molecular weight and diffusion coefficient as shown in the provided graph?

Accepted Answer

Diffusion coefficient decreases as molecular weight increases.

Question 15

A researcher wants to increase the rate of diffusion of a nutrient across a cell membrane. Which of the following changes would be most effective?

Accepted Answer

Increase the permeability of the membrane.

Question 16

Which of the following is a direct consequence of the inadequacy of diffusion for long-distance transport in multicellular organisms?

Accepted Answer

The evolution of the respiratory system.

Question 17

In a random walk model of diffusion, if a molecule takes $$N$$ steps of length $$L$$, what is the total distance traveled and the net displacement from the starting point?

Accepted Answer

Total distance: $$NL$$, Net displacement: $$L \sqrt{N}$$

Question 18

Which of the following best explains why water can enter a cell but many solutes cannot pass out through the same membrane?

Accepted Answer

The membrane is selectively permeable, allowing water but not certain solutes to pass.

Question 19

A patient is given a drug with a very high molecular weight. Based on diffusion principles, what can be predicted about the rate at which this drug will diffuse through tissue compared to a low molecular weight drug?

Accepted Answer

It will diffuse more slowly than the low molecular weight drug.

Diffusion: Mechanisms, Equations, and Biological Applications

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