Section 3.4 Review Sheet: Passive Transport (Diffusion and Osmosis)
Vocabulary for Section 3.4
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Osmosis
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Diffusion
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Passive transport
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Concentration gradient
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Isotonic – an isotonic solution has the same concentration of solutes as the cell, thus equal
amounts of water enter and exit the cell. The cell’s size does not change.
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Hypertonic – a hypertonic solution has more solutes than a cell, thus more water exits the cell in a
hypertonic solution. The cell shrivels.
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Hypotonic – a hypotonic solution has fewer solutes than a cell, thus more water enters a cell in
hypotonic solution. The cell expands.
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Solute – a substance dissolved in a solvent and is present at lower concentration than the solvent.
For example, salt water is made up of a solvent, water, and a solute, sodium (sodium is dissolved
in the water).
Review Questions
1.
What term describes the difference in concentration of a substance from one location to another?
What does
it mean for a molecule to diffuse down its concentration gradient?
Concentration gradient. The molecule, say sodium, moves from a region of high sodium
concentration to a region of low sodium concentration (thus the sodium has diffused down its
gradient).
2.
How does facilitated diffusion differ from diffusion? Explain why facilitated diffusion does not require energy from
a cell?
Molecules diffuse through protein channels. Energy is not needed because a molecule is still moving
from a high concentration region to a low concentration region.
3.
What term describes the diffusion of water through a semipermeable membrane down a concentration gradient?
Osmosis
4.
In which kind of solution would water move from the solution into the cell?
Hypotonic
5.
A cell is bathed in fluid. You notice that water is flowing out of the cell. In what kind of solution is this cell
immersed?
Hypertonic solution.
6. What would make a hypertonic solution isotonic?
Add more water. Adding more water would change the solution’s concentration gradient. The added
water would dilute the solution and change the concentration gradient. In an isotonic solution, the
cell has an equal amount of water enter and exit cell (its size stays constant). Remember, a solution
is isotonic to a cell if it has the same concentration of solutes as the cell.
7. When a person becomes dehydrated due to the loss of fluids and solutes, saline solution (water and
salts) is infused into the bloodstream by medical personnel. Why is saline solution used instead of
pure water?
Pure water would be hypotonic relative to the contents of blood cells and could cause the cells to
burst. The saline solution is isotonic relative to the cell contents.