Diffusion Through A Membrane Lab Answer Key

Understanding the Diffusion Through A Membrane Lab

This is a standard biology lab that demonstrates how substances move across a semipermeable membrane. Dialysis tubing is soaked, filled with solutions like starch and glucose, then placed in water with iodine. Over time, small molecules pass through the tubing while larger ones stay trapped. The results are straightforward, but grading it consistently across twenty students is where things get messy. I have run this lab for years. The answer key questions tend to fall into a few predictable buckets, and students keep making the same mistakes on each one. Knowing what those are saves you time when you are actually grading.

Common Diffusion Through A Membrane Lab Answer Key Questions and Answers

Q1: What is a semipermeable membrane? It allows certain molecules to pass through while blocking others based on size and sometimes charge. Dialysis tubing has microscopic pores. Anything smaller than those pores diffuses through. Anything larger stays inside. Q2: Which molecules moved through the dialysis tubing in this experiment?

Glucose and iodine both moved across the membrane. Starch did not. You can verify this by testing the surrounding water after the experiment with Benedict's solution for glucose and Lugol's iodine for iodine. If you test the inside, the starch solution will turn blue-black when iodine enters the tubing. Q3: How do you know diffusion occurred? Color changes are the evidence. The water outside turns amber or brown if iodine leaves the tubing. The solution inside darkens if iodine enters. Glucose appearing in the outside water is confirmed by a positive Benedict's test, which changes from blue to green, yellow, orange, or brick red depending on concentration.

Q4: Why didn't starch diffuse through the membrane? Starch molecules are polysaccharides made of long glucose chains. They are simply too large to fit through the pores in the dialysis tubing. The pore size of standard lab tubing is roughly 1-2 nanometers. Glucose molecules are about 1 nanometer. Iodine ions are even smaller. Starch polymers are thousands of nanometers long. Q5: What is the difference between diffusion and osmosis in this lab?

Diffusion is the movement of solutes from high to low concentration. Osmosis is specifically the movement of water across a semipermeable membrane. In this particular setup, both processes happen simultaneously. Water moves into or out of the tubing depending on the solute concentrations inside versus outside. Most standard lab versions of this experiment focus on diffusion of the solutes, but osmosis is still occurring. Q6: If you measured the mass of the dialysis bag before and after, what would you expect? The bag would gain mass if the solution inside was hypertonic relative to the beaker water. Water would move into the bag by osmosis. It would lose mass if the inside was hypotonic. The exact change depends on the concentration gradient you set up. A 10% starch solution in a plain water beaker typically gains between 5-15% of its initial mass over two hours.

Q7: What would happen if you used a larger pore-size tubing? More substances would pass through, including smaller protein fragments or dextran chains that normally cannot cross. The selectivity of the membrane would decrease, and your results would be less clear-cut. You would see starch leaking out, which defeats the purpose of demonstrating selective permeability.

Practical Issues That Ruin This Lab

The biggest problem I encounter is tied tubing. If the knot is not tight or the tubing is nicked during handling, the starch leaks out and you get false positives in the beaker water. Every single student who gets glucose in their outside water without any actual diffusion happening usually has a compromised seal. I tell students to double-knot and inspect the tubing before starting. One student in my last class had the entire bag disintegrate because the ethanol in the soaking solution had been left too long and degraded the cellulose. That took twenty minutes of the lab period to clean up. Another issue is temperature. Diffusion rates change noticeably with temperature. A lab done at 20°C versus 30°C will show different rates of iodine migration. Most classrooms sit around 21-22°C, which is fine, but if your room is heated in winter and the beakers are near a radiator, your diffusion times will vary between groups. Standardize the water temperature if you want comparable results across sections. The iodine solution concentration matters more than most teachers realize. Standard Lugol's iodine is about 5% iodine and 10% potassium iodide. If you dilute it thinking it will be less harsh on the bench, you also slow down the visible diffusion enough that students think nothing is happening. Never dilute the test iodine. Keep it full strength for both the beaker solution and the testing solution.

Benchmarking Your Results

In a typical 45-60 minute lab period with 5% glucose and 10% starch inside the tubing and diluted Lugol's in the beaker, you should see a color change in the beaker water within 15-20 minutes. The inside of the tubing will darken noticeably within that same window. Complete diffusion equilibrium for the iodine takes about 45 minutes at room temperature. Glucose diffusion is slower to detect because you need Benedict's reagent and heat to visualize it, which adds another 5-7 minutes of active testing time per group. If you are using this as a graded assignment, here is what I look for in a solid lab report: correct identification of which molecules crossed and which did not, a clear explanation tying the result to molecular size and pore diameter, and a brief discussion of the concentration gradient as the driving force. Anything beyond that is usually padding. The Diffusion Through A Membrane Lab Answer Key I provide covers these core questions at a level appropriate for AP Biology or introductory college courses. If you need a version scaled for younger students, the answers should be shorter and avoid terminology like hypertonic and osmotic pressure. For advanced classes, I add questions about water potential calculations and osmotic coefficient adjustments, which most standard keys skip entirely.

Where to Get the Complete Diffusion Through A Membrane Lab Answer Key

I keep a current version stored on Google Drive. It includes the standard questions, the advanced variants, and a troubleshooting section for common experimental failures. The link is available through the shared folder on my department page. If you are a student looking for this, just search for the document title rather than asking your teacher to send it directly. Most instructors have it posted somewhere already. One thing worth noting: some online answer keys contain errors. I have seen keys that claim iodine cannot diffuse through dialysis tubing, which is wrong. Iodine absolutely does diffuse through. Keys that say starch breaks down into glucose during the lab are also incorrect unless heat and an enzyme were deliberately added. Always cross-reference your answers against your actual observations, not just someone else's typed document. The lab itself is reliable if the materials are fresh. Old dialysis tubing becomes brittle and cracks. Old Benedict's reagent loses sensitivity and gives weak positives even when glucose is present. Budget about ten minutes at the start of lab to check your supplies. It prevents half the problems you will otherwise spend the rest of the period debugging.