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B2.2 · Investigate movement of substances across a membrane

Learn to investigate movement of substances across a membrane through clear examples and targeted practice.

Ontario Grade 12 Biology

Biochemistry

How to model, measure, and interpret the movement of substances across a cell membrane

A cell takes in substances it needs and releases substances it no longer needs. These substances cross the cell membrane, a thin boundary around the cell. In SBI3U, you learned that cells are the basic units of life and that cell structures have different roles. Here, focus on a testable question: what affects the movement of a substance across a membrane? A model can help explain a result, but measurements are needed to decide whether the model fits the investigation.

What you will learn

Describe how a cell membrane affects the movement of substances into and out of a cell.
Distinguish diffusion, osmosis, facilitated diffusion, and active transport.
Plan and interpret a simple investigation of movement across a membrane.
Calculate and explain percentage change in mass or another measured quantity.

From cell boundary to investigation

The cell membrane separates the cell’s interior from its surroundings. It is selectively permeable: some substances can cross it more readily than others. This property helps a cell maintain conditions that support its activities. The membrane is not an open wall, and a substance’s movement depends on both the substance and the conditions on each side.

A useful starting point is a difference in concentration. Concentration describes how much of a substance is present in a given volume. If one side has a higher concentration of a substance than the other, the substance may spread from the higher-concentration side to the lower-concentration side. This overall spreading is diffusion. Individual particles move in many directions, but the net movement is from higher to lower concentration.

An investigation turns this idea into a question that can be measured. For example, does the concentration of a surrounding solution affect the mass of a piece of plant tissue? The independent variable is the factor deliberately changed; here, it is solution concentration. The dependent variable is the factor measured; here, it is tissue mass. Other conditions, such as tissue size, time in solution, and temperature, should be kept as similar as practical.

\text{net movement: higher concentration} \rightarrow \text{lower concentration}

A cell membrane is selectively permeable.
Diffusion is the net movement of particles from higher to lower concentration.
A fair investigation changes one factor and measures a relevant outcome.

Ways substances cross a membrane

The direction and route of movement depend on the substance and the membrane. Small particles such as oxygen and carbon dioxide can move directly through the membrane by diffusion. Water also moves across a selectively permeable membrane. The net movement of water across such a membrane is called osmosis. Water tends to move toward the side with a higher concentration of dissolved substances, when the membrane allows water through but restricts those dissolved substances.

Some substances cannot pass readily through the membrane’s fatty interior. They may cross through a membrane protein, a protein that provides a route through the membrane. When movement through a protein follows the concentration difference and does not require cellular energy, it is facilitated diffusion. It is still passive movement: passive means that the movement does not require energy supplied by the cell.

Active transport moves a substance across the membrane against its concentration difference, from lower concentration to higher concentration. It requires energy from the cell and uses membrane proteins. The key contrast is not simply whether a protein is involved: both facilitated diffusion and active transport can use proteins. The contrast is the direction relative to the concentration difference and whether energy is required.

Osmosis is the net movement of water across a selectively permeable membrane.
Facilitated diffusion uses a membrane protein but does not require cellular energy.
Active transport requires energy to move substances against a concentration difference.

Plan, measure, and interpret evidence

A membrane model can be a living cell, a piece of tissue, or a laboratory membrane that allows some substances through. These models are not identical. A laboratory membrane may help isolate the effect of size or concentration, but it does not reproduce every feature of a living cell. State what your model represents and what it cannot show.

For a tissue-mass investigation, blot each sample in the same way before weighing. This reduces the chance that surface liquid, rather than water movement into or out of the tissue, explains a mass difference. Record the starting and ending masses, use equal time intervals, and repeat measurements when possible. Repeated trials can show whether a result is consistent; they do not guarantee that every source of error has been removed.

Percentage change compares a change with the starting value. A positive result means the measured quantity increased; a negative result means it decreased. For mass, a gain is consistent with net water entering the tissue, while a loss is consistent with net water leaving it. This interpretation is strongest when other explanations, such as unequal blotting or damaged samples, have been considered.

Evidence supports a conclusion only within the conditions tested. If tissue gains mass in one solution, that does not show that all cells behave identically or that every substance crossed the membrane. Describe the observation, connect it to a reasonable model, and note uncertainty.

\text{percentage change} = \frac{\text{final value} - \text{initial value}}{\text{initial value}} \times 100\%

Keep relevant conditions consistent and record measurements with units.
Use percentage change to compare changes relative to starting values.
Separate an observed result from an explanation of that result.

Compare the movement models

Use the table to classify movement by direction, route, and energy requirement. A concentration difference is sometimes called a concentration gradient. In this lesson, that term means the difference in concentration between two locations.

A classification should match all the evidence given. For example, movement through a protein alone does not prove active transport, because facilitated diffusion also uses proteins. Look for whether movement is with or against the concentration difference and whether cellular energy is needed.

Check direction, route, and energy requirement together.
A concentration difference can drive net passive movement, but it does not by itself identify the route.

Movement across a membrane

Process	What moves and how	Direction relative to concentration	Cell energy required?
Diffusion	Particles spread directly across the membrane when able	Higher to lower	No
Osmosis	Water crosses a selectively permeable membrane	Net water movement toward the side with more dissolved substances, when those substances cannot cross	No
Facilitated diffusion	A substance crosses through a membrane protein	Higher to lower	No
Active transport	A substance crosses using a membrane protein	Lower to higher	Yes

Worked example

Calculate change in tissue mass

A tissue sample has an initial mass of 8.0 g and a final mass of 9.2 g after being placed in a solution. Calculate its percentage change in mass and give a cautious interpretation.

Find the mass change
Subtract the initial mass from the final mass. A positive difference means the sample’s measured mass increased.
$9.2\,\mathrm{g} - 8.0\,\mathrm{g} = 1.2\,\mathrm{g}$
Compare with the starting mass
Divide the change by the initial mass, then multiply by 100. The initial value is the reference for percentage change.
$\frac{1.2}{8.0} \times 100\% = 15\%$
Interpret the result
The sample’s mass increased by 15%. Net water entry by osmosis is one reasonable explanation if the membrane allowed water movement and the sample was blotted consistently. The mass result alone does not prove the cause.

Answer: The percentage change in mass is 15%. The increase is consistent with net water entering the tissue, but other measurement factors should be checked.

Check: The percentage is positive because the final mass is greater than the initial mass.

Worked example

Compare two solution conditions

Two equal pieces of tissue are tested for the same length of time. Sample A changes from 5.0 g to 5.5 g. Sample B changes from 5.0 g to 4.6 g. Calculate each percentage change and compare the results.

Calculate Sample A
The mass increased by 0.5 g. Divide that change by the 5.0 g starting mass to find the percentage change.
$\frac{5.5 - 5.0}{5.0} \times 100\% = 10\%$
Calculate Sample B
The mass decreased by 0.4 g, so the numerator is negative. Keeping the sign shows the direction of change.
$\frac{4.6 - 5.0}{5.0} \times 100\% = -8\%$
Compare the pattern
Sample A gained mass, while Sample B lost mass. If the samples and procedure were comparable, the pattern is consistent with net water entering A and leaving B. The results do not, by themselves, identify the exact solution concentrations.

Answer: Sample A changed by 10%; Sample B changed by -8%. Their opposite changes are consistent with water moving in opposite net directions under the two conditions.

Check: Both samples began at the same mass, so their percentage changes can be compared directly.

Worked example

Identify a movement route

A substance moves from a region of lower concentration to a region of higher concentration through a membrane protein. The cell supplies energy. Classify the movement and explain why.

Compare movement with the concentration difference
The substance moves from lower concentration to higher concentration. This is against the concentration difference, not with it.
$\text{lower concentration} \rightarrow \text{higher concentration}$
Use the energy evidence
The stated energy requirement supports active transport. A membrane protein is involved, but that fact alone would not distinguish active transport from facilitated diffusion.

Answer: This is active transport because the substance moves against its concentration difference and the cell supplies energy.

Check: Movement through a protein is not enough on its own to identify active transport.

Common mistakes and how to avoid them

Saying that particles stop moving when concentrations become equal.

Correction: Particles continue moving. At equal concentrations, there is no net movement in one direction.

Calling every protein-assisted movement active transport.

Correction: Facilitated diffusion also uses proteins. Active transport requires energy and moves against the concentration difference.

Treating a mass change as direct proof of osmosis.

Correction: Mass change is evidence consistent with water movement, but measurement conditions and other explanations must be considered.

Leaving the negative sign off a percentage decrease.

Correction: Use final minus initial. The negative sign communicates that the measured value decreased.

Lesson summary

A selectively permeable membrane allows some substances to cross more readily than others.
Diffusion and facilitated diffusion move substances from higher to lower concentration without cellular energy.
Osmosis is the net movement of water across a selectively permeable membrane.
Active transport uses cellular energy to move substances against a concentration difference.
A well-planned investigation controls relevant conditions, measures a clear outcome, and distinguishes evidence from interpretation.

Check your understanding

Question 1

A substance crosses a membrane protein from higher concentration to lower concentration without cellular energy. Which process best fits?

Active transport
Facilitated diffusion
Osmosis
Movement against a concentration difference

Show answer and explanation

Facilitated diffusion

Facilitated diffusion uses a membrane protein and moves down the concentration difference without cellular energy.

Question 2

A sample changes from 12.0 g to 10.8 g. What is its percentage change in mass?

10% increase
10% decrease
1.2% decrease
90% decrease

Show answer and explanation

10% decrease

The change is -1.2 g. Dividing by the initial 12.0 g and multiplying by 100 gives -10%, or a 10% decrease.

Question 3

Why should tissue samples be blotted in the same way before weighing?

To make all samples have the same initial mass
To remove a source of measurement variation from surface liquid
To make the membrane stop allowing water through
To ensure that every substance crosses at the same rate

Show answer and explanation

To remove a source of measurement variation from surface liquid

Consistent blotting reduces the chance that differences in surface liquid affect measured mass.

Key terms

Cell membrane: The boundary around a cell that controls the movement of substances into and out of it.
Selectively permeable: Allowing some substances to cross more readily than others.
Concentration: The amount of a substance in a given volume.
Diffusion: The net movement of particles from higher concentration to lower concentration.
Osmosis: The net movement of water across a selectively permeable membrane.
Facilitated diffusion: Passive movement across a membrane through a protein.
Active transport: Movement across a membrane that uses cellular energy to move a substance against its concentration difference.
Independent variable: The factor deliberately changed in an investigation.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation B2.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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