Passive Transport

Essential Question

How do substances cross the membrane without using energy?

Cells move substances across their membranes all the time, and some of this movement costs the cell no energy at all. In this lesson, you’ll learn how diffusion, facilitated diffusion, and osmosis work, predict which way water will move, and use data from a potato experiment to find the concentration of solutes inside plant cells.

Diffusion

• Particles move randomly all the time
• Net movement is from high to low concentration
• No energy from the cell is needed
• At equilibrium, there is no net movement

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Particles are always moving randomly, but when there are more particles in one place, more of them move out than move in, so the particles spread out. The difference in concentration from one place to another is a concentration gradient, and diffusion moves particles down the gradient. Diffusion is a kind of passive transport, which means the cell does not use ATP. At equilibrium the particles are spread evenly and there is no net movement, though the particles keep moving.

Facilitated Diffusion

• Ions and polar molecules cannot cross the lipids
• Channel and carrier proteins help them cross
• Movement is still down the gradient
• No ATP is used

Many substances cannot cross the lipid part of a membrane. In facilitated diffusion, channel proteins and carrier proteins give them a path. The movement is still passive: particles move from high to low concentration, and the cell does not use ATP. Only the specific substances that fit a protein can use it, and the rate levels off when all the proteins are busy.

Osmosis

• Osmosis is the diffusion of water across a membrane
• Water moves toward the side with more solute
• Solute cannot cross, so water moves instead
• It continues until the concentrations are equal or pressure stops it

Osmosis is the diffusion of water across a selectively permeable membrane. When solutes cannot cross the membrane, water moves instead. The side with more solute has a lower concentration of free water, so water moves toward the side with more solute. The membrane in the figure lets water through but not sugar. Osmosis needs no ATP, so it is a form of passive transport.

Cells in Different Solutions

• Hypotonic: less solute than the cell, water moves in
• Isotonic: equal solute, no net movement
• Hypertonic: more solute than the cell, water moves out
• A cell wall keeps plant cells from bursting

To predict which way water moves, compare the solute concentration of the solution with that inside the cell. In a hypotonic solution (less solute than the cell) water moves into the cell. An animal cell swells and may burst, while a plant cell becomes firm, or turgid, because its cell wall resists. In an isotonic solution there is no net movement. In a hypertonic solution (more solute than the cell) water moves out, an animal cell shrinks, and a plant cell’s membrane pulls away from its wall.

Measuring Osmosis

• Soak potato cores in different sucrose solutions
• Find the percent change in mass of each core
• Gain means water entered; loss means water left
• No change estimates the isotonic concentration

In a common experiment, potato cores of equal mass are soaked in sucrose solutions of different concentrations. Water moves in or out by osmosis, and the percent change in mass shows which way and how much. Percent change = (final mass – initial mass) / initial mass x 100. A core that gains mass was in a hypotonic solution, and one that loses mass was in a hypertonic solution. The concentration at which the mass would not change estimates the solute concentration inside the potato cells.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Percent Change in Mass

A potato core with a starting mass of 2.40 g is soaked in distilled water for 24 hours and then weighs 2.76 g. A second core, also 2.40 g, is soaked in 0.6 M sucrose and then weighs 2.07 g. Find the percent change in mass of each core and say which way water moved.

Core	Start mass (g)	End mass (g)
Distilled water	2.40	2.76
0.6 M sucrose	2.40	2.07

Show the solution

1. Formula: percent change = (end mass – start mass) / start mass x 100.
2. Distilled water: (2.76 g – 2.40 g) / 2.40 g x 100 = 0.36 g / 2.40 g x 100 = +15.0%.
3. 0.6 M sucrose: (2.07 g – 2.40 g) / 2.40 g x 100 = -0.33 g / 2.40 g x 100 = -13.8%.
4. Direction: the core in water gained mass, so water moved in. The core in sucrose lost mass, so water moved out.
5. Answer: +15.0% in distilled water (water moved in) and -13.8% in 0.6 M sucrose (water moved out).

Example 2: Find the Isotonic Point

Potato cores of equal mass were soaked in sucrose solutions (sample data). Estimate the concentration at which the mass would not change. Write a claim, evidence, and reasoning.

Sucrose (M)	Start mass (g)	End mass (g)	Change (%)
0.0	2.00	2.34	+17.0
0.2	2.00	2.12	+6.0
0.4	2.00	1.90	-5.0
0.6	2.00	1.72	-14.0
0.8	2.00	1.58	-21.0

Show the solution

1. Find where the change is 0%: the change is +6.0% at 0.2 M and -5.0% at 0.4 M, so it crosses 0% between them.
2. Interpolate: 0.2 + 0.2 x (6.0 / (6.0 + 5.0)) = 0.2 + 0.11 = about 0.31 M.
3. Claim: the potato cells contain solute at about 0.3 M.
4. Evidence: cores gained mass (+6.0%) in 0.2 M and lost mass (-5.0%) in 0.4 M, so no change would occur near 0.3 M.
5. Reasoning: when the solution is isotonic there is no net movement of water. Below that concentration water enters the cells, and above it water leaves.
6. Answer: About 0.3 M.

Example 3: Predict the Water Movement

A red blood cell contains about 0.9% solutes. It is placed in a salt solution that is 3% solutes. Use the model to predict which way water moves and what happens to the cell.

Show the solution

1. Compare: the solution (3%) has more solute than the cell (0.9%), so it is hypertonic.
2. Direction: water moves toward the side with more solute, so water moves out of the cell.
3. Result: the cell shrinks.
4. Answer: Water moves out of the cell, and the cell shrinks.

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Practice

Predict how cells respond to different solutions, calculate percent change in mass of potato cores, read data to estimate an isotonic concentration, and explain why passive transport needs no ATP. Try it on your own first, then check your work with the answer key.

• Passive Transport Practice Worksheet (PDF)
• Passive Transport Practice Answer Key (PDF)

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Lesson Slides

• 6.6 Passive Transport Slides (PDF)

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Big Ideas

• Diffusion moves particles from high to low concentration without ATP
• Facilitated diffusion uses channel and carrier proteins and is still passive
• Osmosis is the diffusion of water; water moves toward more solute
• Hypotonic, isotonic, and hypertonic solutions predict how cells change
• Percent change = (final – initial) / initial x 100

Back to the essential question: passive transport moves substances down their concentration gradients without ATP. Diffusion works across the lipids, facilitated diffusion uses proteins, and osmosis moves water toward the side with more solute. Next, you’ll see how cells use ATP to move substances against a gradient.

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