Materials & Prep
Prepare one dialysis-tubing bag containing starch solution and glucose solution in an iodine solution. Record the starting appearance of the bag contents and iodine solution. After at least 20 minutes, the bag contents should be blue-black while the iodine solution remains amber; test the outside solution with a glucose test strip if available. Prepare one set of transport cards labeled oxygen, carbon dioxide, water, glucose, sodium ion, and protein. Display a simple cell outline with an unlabeled membrane. Confirm local procedures for handling iodine and wear appropriate eye protection; students should not handle iodine unless permitted by school procedure.
Opening prediction
AskA cell is surrounded by a membrane. If the membrane were simply a wall with holes, what would determine whether a substance enters: the substance's size, its concentration, or the cell's energy use? Choose one and explain your prediction in two sentences.
Students commit individually, then compare predictions with a partner. Tell students to keep their original prediction because the evidence later will require them to revise or defend it. Listen for the misconception that the membrane is a rigid wall with permanent holes.
Phenomenon and misconception
DisplayShow the prepared dialysis bag before and after soaking in iodine. The starch solution inside changed from cloudy white to blue-black, while the iodine solution outside remained amber. If tested, the outside solution also gives a positive glucose test.
AskWhat crossed the membrane, and what did not? Students must separate observation from inference. An observation is that the inside changed color and glucose was detected outside. An inference is that iodine and glucose crossed the tubing while starch did not.
ConfrontA wall with holes would not explain why some substances cross and others do not. The cell membrane is a flexible, selective boundary. Its structure and transport proteins determine what can cross, and the cell may use energy to move some substances against a concentration difference.
Direct instruction
ModelDraw two layers of phospholipids. Each phospholipid has a water-attracting head facing the watery cytoplasm or extracellular fluid and water-repelling tails facing inward toward one another. Explain that the membrane is a phospholipid bilayer, not a solid wall. Embedded proteins form specific channels, carriers, or pumps.
AnalogyExplain the membrane's oily interior as a checkpoint with different routes. Small nonpolar molecules such as oxygen can pass through the lipid region, while charged particles such as sodium ions need a protein route. A protein route does not automatically mean energy is used.
Think aloudFor diffusion, particles move from higher concentration to lower concentration because of their random motion. If oxygen is higher outside a cell and lower inside, oxygen can move inward through the bilayer without cellular energy. Water can move through the bilayer and, in many cells, through aquaporin proteins. For facilitated diffusion, glucose can move through a carrier protein from higher concentration to lower concentration, also without cellular energy. For active transport, a pump uses ATP to move a substance from lower concentration to higher concentration. The common error is to call every protein-assisted movement active transport. The deciding question is whether movement is against the concentration gradient and requires energy.
Check for understandingFor each case, students show D for diffusion or A for active transport and explain why: oxygen moves from high concentration outside to low concentration inside; sodium ions are pumped from low concentration inside to high concentration outside. Require both the direction and the energy decision before confirming answers.
Guided membrane sort
Have students place each card at an entry route on the cell diagram and label the transport process: oxygen moving down its concentration gradient through the bilayer, water moving down its concentration gradient through the bilayer or an aquaporin, glucose moving down its gradient through a carrier protein, and sodium ions moving against their gradient through a pump using ATP. The protein card remains outside as a large molecule that cannot cross the bilayer directly.
StudentsFor each card, write three brief notes: concentration direction, membrane route, and energy use. They should use the sentence frame, “The substance moves from to through because .”
CirculateAsk, “What evidence tells you this is active rather than facilitated diffusion?” and “Is the protein itself the substance crossing, or is it providing a route?” Correct the wall-with-holes model by having students point to the lipid bilayer, a channel or carrier, and a pump as different structures.
ScaffoldProvide a completed example for oxygen: “high outside to low inside; through the phospholipid bilayer; no ATP.” Students then complete glucose and sodium ion with the same three-part organizer.
ExtensionChallenge students to explain why a cell might use active transport to maintain a sodium concentration lower inside the cell even when diffusion would move sodium inward.
Independent CER
WriteUsing the dialysis-bag observations and the transport examples, write a claim-evidence-reasoning response to this question: How does the cell membrane control what enters and leaves the cell?
Require the claim to name the phospholipid bilayer and membrane proteins, at least two pieces of evidence, and reasoning that distinguishes diffusion from active transport. Students must include one example of movement down a concentration gradient without ATP and one example of movement against a concentration gradient using ATP.
Look forStrong responses explain that membrane structure creates different routes and that selectivity depends on properties such as size, charge, polarity, concentration gradient, and the presence of a specific protein. Support students who need it with the frame, “The membrane is selective because . The evidence is . This shows because diffusion , whereas active transport .”
Closing check
Exit ticketDraw and label a phospholipid bilayer with one membrane protein. Then explain in three or four sentences how the membrane would handle these two substances: oxygen is more concentrated outside the cell, and sodium ions must be moved from lower concentration inside to higher concentration outside. Include the route, direction, and whether ATP is required.
ExpectedOxygen crosses the bilayer from high to low concentration without ATP. Sodium ions move through a membrane pump from low to high concentration and require ATP. Collect the tickets to check both the structural description and the diffusion-versus-active-transport distinction.