Science · 7th Grade · The Architecture of Life · Weeks 10-18

Cellular Transport: Movement Across Membranes

Students investigate how substances move into and out of cells through processes like diffusion, osmosis, and active transport.

• Common Core State Standards
• MS-LS1-2

About This Topic

The cell membrane controls what enters and exits the cell through a combination of passive and active transport mechanisms. Diffusion is the movement of particles from an area of high concentration to low concentration without requiring energy. Osmosis is the specific case of water diffusion across a selectively permeable membrane. Active transport, by contrast, moves substances against the concentration gradient and requires the cell to spend energy in the form of ATP. MS-LS1-2 asks students to develop and use a model to describe the function of a cell as a whole and ways the parts work together.

US 7th graders benefit from learning these processes as a coherent system rather than three isolated definitions. All three mechanisms are working simultaneously in every living cell, and the balance among them determines whether a cell stays healthy, lyses (bursts), or shrinks. Students investigate osmosis through classic potato chip or dialysis tubing labs and explore diffusion using food coloring or perfume spreading through still water.

Transport processes involve particle behavior that is not directly visible, which is why active learning matters here. Physical simulations of concentration gradients, followed by hands-on osmosis experiments, give students direct evidence for processes happening at a scale far too small to observe.

Key Questions

1. Explain how the cell membrane regulates the passage of substances.
2. Compare and contrast passive and active transport mechanisms.
3. Predict the movement of water across a cell membrane in different solutions.

Learning Objectives

• Compare and contrast the mechanisms of diffusion, osmosis, and active transport in terms of energy requirement and direction of movement.
• Explain how the selective permeability of the cell membrane influences the movement of different substances.
• Predict the net movement of water across a cell membrane when placed in solutions of varying solute concentrations.
• Model the process of facilitated diffusion, illustrating the role of membrane proteins.
• Analyze experimental data to determine the type of transport occurring based on observed substance movement.

Before You Start

Structure and Function of the Cell Membrane

Why: Students need to know the basic components and role of the cell membrane before understanding how it regulates transport.

Introduction to Molecules and Concentration

Why: A foundational understanding of what molecules are and the concept of concentration is necessary to grasp movement from high to low concentration.

Key Vocabulary

Diffusion	The passive movement of particles from an area of higher concentration to an area of lower concentration across a membrane.
Osmosis	The specific diffusion of water across a selectively permeable membrane, moving from an area of higher water concentration to lower water concentration.
Active Transport	The movement of substances across a cell membrane against their concentration gradient, requiring cellular energy (ATP).
Concentration Gradient	The gradual difference in the concentration of a substance between two areas, from high to low.
Selectively Permeable Membrane	A membrane that allows certain molecules or ions to pass through it by means of active or passive transport.

Watch Out for These Misconceptions

Common Misconception

Osmosis is just water wanting to move to where there is more water.

What to Teach Instead

Osmosis follows concentration gradients: water moves from where its own concentration is highest (dilute solution) toward where its concentration is lowest (concentrated solution). Framing it as water wanting something adds inaccurate intent to a purely physical process. Graphing experimental osmosis data quantitatively helps students replace the anthropomorphic language with a precise model.

Common Misconception

Active transport is always better than passive transport because it is more active.

What to Teach Instead

Passive transport is often faster and more efficient for substances that need to move down their concentration gradient. Active transport is only necessary when a cell needs to move substances against the gradient. The distinction is about direction relative to the gradient, not efficiency. Scenario-based sorting activities help students identify when each type is needed.

Active Learning Ideas

Inquiry Circle: Osmosis and the Potato

Groups cut potato slices to uniform size, weigh them, and place them in solutions of different salt concentrations (plain water, 5%, 10%). After 30 minutes they weigh the slices again, graph the percent mass change against salt concentration, and explain the movement of water using the concept of osmosis and concentration gradients.

50 min·Small Groups

Simulation Game: The Human Diffusion Model

Mark a concentration line on the classroom floor and pack students densely on one side. On signal, students move randomly around the room. Students count how many are on each side every 30 seconds and graph the pattern over time, connecting the simulation to how food coloring spreads through still water without stirring.

25 min·Whole Class

Think-Pair-Share: Active vs. Passive Transport Decision Tree

Present students with four scenarios (a cell absorbing glucose against its concentration, oxygen moving into a blood cell, sodium being pumped out of a neuron, CO2 leaving a cell). Partners classify each as active or passive transport and justify their choice by identifying whether energy is required and whether movement is with or against the gradient.

20 min·Pairs

Gallery Walk: Healthy Cell, Unhealthy Cell

Stations show cells in hypotonic, hypertonic, and isotonic solutions with before-and-after diagrams. Student groups annotate what happened to the cell membrane and contents, predict whether the cell is still viable, and explain what the cell would need to do to restore its internal balance.

30 min·Small Groups

Real-World Connections

• Kidney dialysis machines simulate the selective permeability of cell membranes to filter waste products from the blood of patients with kidney failure, using diffusion and osmosis principles.
• Farmers use understanding of osmosis when irrigating crops; applying too much fertilizer can create a hypertonic soil solution, drawing water out of plant roots and causing wilting.
• Pharmaceutical companies develop drug delivery systems that utilize controlled diffusion to release medication slowly into the bloodstream over time.

Assessment Ideas

Quick Check

Present students with three scenarios: 1) a substance moving from high to low concentration without energy, 2) water moving across a membrane from high to low water concentration, 3) a substance moving from low to high concentration using energy. Ask students to label each scenario as diffusion, osmosis, or active transport.

Exit Ticket

On one side of an index card, have students draw a simple diagram illustrating osmosis. On the other side, ask them to write one sentence explaining why active transport is necessary for cell survival, even though it uses energy.

Discussion Prompt

Pose the question: 'Imagine a plant cell is placed in saltwater. What will happen to the cell, and which transport process is primarily responsible for this change? Explain your reasoning, referencing concentration gradients and water movement.'

Frequently Asked Questions

What is the difference between diffusion and osmosis?

Diffusion is the movement of any substance from high to low concentration. Osmosis is a specific type of diffusion where water molecules move across a selectively permeable membrane from an area of higher water concentration (more dilute) to an area of lower water concentration (more concentrated solution).

How does active learning help students understand cellular transport?

Cellular transport involves particle behavior too small to observe directly. Human diffusion simulations and osmosis labs give students evidence they can see and measure. When students predict what will happen to a potato slice in saltwater and then compare their prediction to their data, the abstract concept of water moving down a concentration gradient becomes concrete and testable.

What happens to a cell placed in saltwater?

In a saltwater (hypertonic) solution, water moves out of the cell by osmosis because the salt concentration outside is higher than inside. The cell loses water and shrivels. This is why eating very salty food makes you thirsty: cells in your body lose water to the saltier surrounding fluid.

Why does the cell membrane need to be selectively permeable?

The cell needs to let some substances in (like glucose and oxygen) and keep others out (like large proteins or certain ions). A membrane that let everything through could not maintain the internal chemical environment the cell's reactions need to function correctly.