Science · Grade 7 · The Cellular Basis of Life · Term 2

Cell Membrane and Transport

Understanding how the cell membrane regulates the movement of substances into and out of the cell.

• Ontario Curriculum Expectations
• MS-LS1-2

About This Topic

The cell membrane serves as a selectively permeable barrier that regulates what enters and leaves the cell, maintaining internal balance. Grade 7 students examine passive transport methods like diffusion, where molecules move from high to low concentration without energy, and osmosis, the diffusion of water across the membrane. They contrast these with active transport, which uses ATP to move substances against their concentration gradient, such as sodium-potassium pumps. Predicting cell responses in hypertonic solutions, where water leaves causing shrinkage, reinforces these ideas.

This topic anchors the unit on the cellular basis of life, linking structure to function and introducing homeostasis. Students practice modeling concentration gradients and energy roles, skills that support inquiry-based science. Connections to real-world examples, like plant wilting in salt water, make concepts relevant.

Active learning benefits this topic greatly. Hands-on models using dialysis tubing or eggs in solutions let students observe transport directly, turning abstract processes into visible events. Collaborative predictions and discussions build accurate mental models and address misconceptions through evidence.

Key Questions

1. Explain how a cell decides what enters and leaves its boundaries.
2. Differentiate between passive and active transport mechanisms.
3. Predict the outcome for a cell placed in a hypertonic solution.

Learning Objectives

• Explain the function of the cell membrane as a selectively permeable barrier.
• Compare and contrast passive transport (diffusion, osmosis) with active transport, identifying the role of energy.
• Predict the effect of placing a cell in a hypertonic solution on its water movement and overall shape.
• Model the movement of substances across a cell membrane using different concentration gradients.

Before You Start

Structure of a Cell

Why: Students need to identify the cell membrane as a key component of the cell before understanding its function.

Concentration and Solutions

Why: Understanding the concept of concentration is fundamental to grasping how substances move from areas of high to low concentration.

Key Vocabulary

Selectively Permeable Membrane	A barrier that allows certain molecules or ions to pass through it by means of active or passive transport.
Diffusion	The net movement of anything generally from a region of higher concentration to a region of lower concentration. The process does not require energy.
Osmosis	The movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration.
Active Transport	The movement of molecules across a cell membrane against their concentration gradient, requiring energy in the form of ATP.
Hypertonic Solution	A solution that has a higher solute concentration, and thus a lower water concentration, than the cell it is placed in.

Watch Out for These Misconceptions

Common Misconception

The cell membrane lets everything pass through equally.

What to Teach Instead

The membrane is selectively permeable, allowing small nonpolar molecules like oxygen to diffuse easily but blocking larger ones. Active learning with dialysis tubing shows size-based passage, helping students test and revise ideas through observation.

Common Misconception

All transport across the membrane requires cell energy.

What to Teach Instead

Passive transport relies on concentration gradients alone, while active transport needs ATP. Egg osmosis labs demonstrate passive water movement without energy, and group predictions clarify distinctions during discussions.

Common Misconception

Cells shrink in hypertonic solutions because they lose solutes.

What to Teach Instead

Shrinking results from water leaving via osmosis due to higher external solute concentration. Modeling with solutions lets students measure and see water movement directly, correcting focus on water over solutes.

Active Learning Ideas

Demonstration: Egg Osmosis Lab

Place a peeled hard-boiled egg in vinegar overnight to dissolve the shell, then transfer to corn syrup (hypertonic) or distilled water (hypotonic). Students measure mass changes daily over three days and graph results. Discuss why the egg shrinks or swells.

45 min·Small Groups

Stations Rotation: Transport Models

Set up stations for diffusion (food coloring in water), osmosis (gummy bears in solutions), selective permeability (plastic bags with starch), and active transport (simulated pumps with pipettes). Groups rotate, draw observations, and explain mechanisms.

50 min·Small Groups

Pairs: Dialysis Tubing Experiment

Fill dialysis tubing with starch and glucose solution, place in iodine water. Observe color changes inside and test for glucose outside. Pairs predict and record which molecules pass through based on size.

30 min·Pairs

Whole Class: Concentration Gradient Game

Students act as molecules moving across a membrane line. Teacher calls 'diffusion' for free movement or 'active transport' requiring energy pushes. Debrief on directionality and energy use.

20 min·Whole Class

Real-World Connections

• Medical professionals, like nurses and doctors, use their understanding of osmosis and diffusion when administering intravenous fluids or managing dehydration. They must consider how the concentration of solutions affects water movement into and out of body cells.
• Food scientists use principles of osmosis and diffusion in food preservation techniques, such as salting fish or curing meats. The high salt concentration draws water out of microbial cells, preventing spoilage.

Assessment Ideas

Quick Check

Present students with three scenarios: 1) a cell in a hypotonic solution, 2) a cell in an isotonic solution, and 3) a cell in a hypertonic solution. Ask students to draw a simple diagram for each scenario showing the direction of water movement and the resulting cell shape. They should label the type of solution.

Discussion Prompt

Pose the question: 'Imagine you are designing a new type of medication delivery system that needs to get a drug across a cell membrane. Would you design it to use passive or active transport, and why? What challenges might you face?' Facilitate a class discussion where students justify their choices based on the properties of transport mechanisms.

Exit Ticket

On an index card, have students define diffusion and osmosis in their own words. Then, ask them to provide one example of each process that occurs within a plant or animal cell.

Frequently Asked Questions

How to explain passive vs active transport to grade 7 students?

Use everyday analogies: passive transport is like people spreading out in a crowded room without effort, active is pushing uphill with energy. Follow with labs like gummy bears in water for osmosis and pump simulations. Students compare results in pairs to solidify differences, building confidence in predictions.

What happens to a cell in a hypertonic solution?

In a hypertonic solution, water moves out of the cell by osmosis, causing it to shrink or crenate. This maintains equilibrium with the higher external solute concentration. Activities like eggs in syrup let students quantify mass loss, connecting observation to the concept of water following solute gradients.

How can active learning help students understand cell membrane transport?

Active learning makes invisible processes tangible through experiments like dialysis tubing for selective permeability and egg osmosis for solution effects. Students predict outcomes, collect data in groups, and revise models based on evidence. This approach boosts retention, addresses misconceptions, and develops scientific reasoning skills essential for the unit.

What materials are needed for cell transport activities?

Common items include eggs, corn syrup, vinegar, dialysis tubing, starch, iodine, food coloring, and gummy bears. These allow modeling diffusion, osmosis, and permeability at low cost. Prep stations ahead for rotations, ensuring safety with food-grade materials and clear protocols for measurements.