Lab 4: Diffusion and Osmosis Agar + Potato = Learning

Lab Overview According to your lab manual, there are three parts to this lab. We are doing two of them – Procedure 1 and Procedure 3. Procedure 1 is an investigation of diffusion through a cube of agar. Procedure 3 is an investigation of osmosis in potato cores. You will be doing this as a lab table group (not partners). Also: Your lab manual talks a lot about water potential. We haven’t covered it yet. I know that. “Take a chill pill.”

Disclaimer How many of you haven’t had chemistry yet? If you haven’t, there’s one thing you need to know, which you kind of know already: M = molarity, and it’s a measure of the concentration of a solution. The higher the molarity, the “stronger” the concentration. Pronounce 1.0 M as “1.0 molar.”

Another Disclaimer This lab is highly inquiry-based. Translation? You guys need to figure stuff out. This is not a formal lab write-up. Translation? You’re writing this in your lab notebooks. Use the informal rubric. The questions in the lab manual should be used as a guide and can help you think of things to include in your conclusion/analysis/reflection. Key: Make sure you write down your procedure and OBSERVATIONS.

What are diffusion and osmosis? Really? Do I need to explain? Fine: Diffusion is the spreading out of particles till their concentration equalizes. Think of how food dye spreads out in a beaker of water until the water is uniformly that color. Osmosis is the diffusion of water and always involves some sort of membrane or barrier. Think about how plants stand up nice and tall and absorb water into their central vacuoles when watered well.

What are we investigating What are we investigating? Hint: This should comprise the bulk of your conclusion/reflection. This lab is a way to investigate four small ideas: Investigation 1: What is the rate of diffusion? Investigation 1: Which agar cube experiences complete diffusion first? Investigation 3: What is the molarity of a potato? Investigation 3: What are the molarities of the various “mystery solutions?” This lab is a way to investigate two big ideas: Investigation 1: Why are cells so small? Investigation 3: About how much sucrose does a potato have?

Procedure 1 – Diffusion The first thing to do is to investigate what phenolphthalein (an indicator) does in the presence of an acid or a base. You may remember this; just confirm it if you do. In the fume hood are three containers: 0.1 M NaOH 0.1 M HCl Phenolphthalein Place a few drops of HCl in a test tube, then add a few drops of phenolphthalein. Observe. Make sure your group can observe too. Place a few drops of NaOH in another test tube, then add a few drops of phenolphthalein. Observe again. Think about why I’m having you do this. It’s connected to the next part of Procedure 1.

Procedure 1 – Diffusion I’ll come by and give you a block of agar. The agar is made using, well, agar. Importantly, this agar is also spiked with NaOH and phenolphthalein. Using an old gift card (or slice of one), cut for your group 3 different-size cubes of agar. They should be smallish – think 2-3 cm to a side.

Procedure 1 – Diffusion Back at your lab station, calculate the surface area and volume of each cube and record. Predict: If they were all placed into a solution, that solution would diffuse entirely through which cube? As a group, decide how you’re going to test your prediction. I’m going to ask.

Procedure 1 Overview Observe phenolphthalein’s effect in HCl and NaOH using two test tubes. Cut up agar cubes, predict which one will experience complete diffusion first. Determine what test you will use to confirm your prediction. Test your prediction. Record results in your notebook. Use a ruler to determine the approximate rate of diffusion.

Procedure 2 Skipping it, remember?

Procedure 3 – Osmosis What happens if you put a freshwater fish into saltwater? What about the opposite? Throughout history, armies have sometimes “salted the earth.” Why? In this investigation we’ll be watching for osmosis into (or out of) a piece of potato. First, an introduction: Osmosis – Onion video (in lieu of microscope portion)

Procedure 3 – Osmosis The #Goal The colored solutions in the big jars around here all contain various concentrations of sucrose. They’re sugar water. That makes ‘em sticky. Remember that. However, we don’t know their concentrations, and more importantly, we also don’t know the concentration of a potato. All I can tell you is they’re in increments of 0.2 M between 0.0 M and 1.0 M.

Procedure 3 – Osmosis You’ll need to pour a small amount of sugar water into a beaker. Aim for just about ¾ of an inch? Record the mass of each of three slices of potato, then put them in the sugar water. Wait till tomorrow, then take the masses of them again. To determine % change, use this formula: Figure out how to determine the concentrations of each mystery solution and of the potato. Hint: You’ll need a graph.

Hint: Central Vacuole Support http://upload.wikimedia.org/wikipedia/commons/thumb/a/ab/Turgor_pressure_on_plant_cells_diagram.svg/2000px-Turgor_pressure_on_plant_cells_diagram.svg.png

Procedure 3 Overview Obtain slices of potato core. Don’t take the ones with the skin on them. Into each of six beakers, pour ¾” of one of each of the colored liquids (so each beaker has a different liquid). Keep a strip of paper towel under all the beakers with your names on it. You’ll have to visit other tables to get other solutions. Take the mass of three potato core slices – individually. Put all three in one sucrose solution. Repeat for the other solutions. Figure out how you’ll get the concentration of each solution and of the potato tomorrow. Hint: You need a graph, remember? Double hint: Your dependent variable nearly always goes on the Y-axis in biology.

For what am I looking? For this informal lab worth 25 points (secondary weight), I’m looking to see a thorough lab notebook. Data tables, calculations, work being shown. It need not be free of cross-outs. Questions from the lab book are answered (next slide). You need those big ideas from earlier. I’ll repeat those (last slide). Write this thing individually!

Lab Manual Questions to Address Answer these within the Data and Analysis Section Procedure 1: Which solution is an acid/base? What color is the dye in the base? In the acid? What is the surface area of each of your three cells? What is the volume of each of your cells? If you put each of the blocks into a solution, into which block would a solution diffuse throughout the entire block fastest?

Lab Manual Questions to Address Answer these within the Data and Analysis Section Procedure 3: Why are most cells small, and why do they have cell membranes with many convolutions? What organelles inside the cell have membranes with many convolutions? Why? Do you think osmosis occurs when a cell is in an isotonic solution? Explain your reasoning.

Lab Manual Questions to Address Answer these within the Data and Analysis Section Not in your lab manual: Actually calculate the SA to V ratio (surface area to volume).

What are we investigating What are we investigating? Hint: This should comprise the bulk of your conclusion/reflection. This lab is a way to investigate four small ideas: Investigation 1: What is the rate of diffusion? Investigation 1: Which agar cube experiences complete diffusion first? Investigation 3: What is the molarity of a potato? Investigation 3: What are the molarities of the various “mystery solutions?” This lab is a way to investigate two big ideas: Investigation 1: Why are cells so small? Investigation 3: About how much sucrose does a potato have?