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.

• Technically it’s “concentrated;” strong means something else.

– 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?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: How does osmosis affect plant cells?

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

• Find the block of agar up on the main desk and cut yourself three pieces:• 0.5 cm on a side.

• 1 cm on a side.

• 2 cm on a side.

• The agar is made using, well, agar.– Importantly, this agar is also spiked with NaOH and

phenolphthalein.

• Cut your agar cubes using an old gift card or small ruler.

Procedure 1 – Diffusion

• Back at your lab station, calculate the surface area and volume of each cube and record.

– Be accurate – if you measured poorly, measure again.

• Predict: If they were all placed into a solution, that solution would first 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

1. Observe phenolphthalein’s effect in HCl and NaOH using two test tubes.

2. Cut up agar cubes, predict which one will experience complete diffusion first. Determine what test you will use to confirm your prediction.

3. Test your prediction. Record results in your notebook.

4. Use a ruler to determine the approximate rate of diffusion. Remember, diffusion happens from all sides at once, so you may need to adjust some dimensions a little.

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.

– We don’t know their concentrations, however, 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 to just cover the slices; don’t go nuts.

• 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 each 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.

100Value Old

Value) Old - Value (New ChangePercent •

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 Overview1. Obtain slices of potato core. Don’t take the ones with the skin on

them.2. Into each of six beakers, pour a thin layer 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.

3. Take the mass of three potato core slices – individually.• Why individually? Two reasons:

1. We need to take the average of the three pieces.2. We will add in a statistic called standard deviation that requires individual

numbers.

4. Put all three in one sucrose solution. Repeat steps 3-4 for the other solutions, each with three potato slices.

5. Figure out how you’ll get the concentration of each solution and of the potato tomorrow.• Hint: You need a graph, remember?• Hint: The 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!

Last Thing to RememberAnswer these within the Discussion/Reflection/Conclusion Section

• Don’t forget the bigger meaning here:

– Why are we doing this lab?

– Why am I not just giving you notes?

– What in biology does this lab illustrate for us?

• No offense, agar fans, but agar is not the real learning goal here.

Let’s Talk About the Lab Report

• First things first: follow the rubric.

– It’s available online if you need it.

• That’ll cover little things like numbering pages, whether or not you should be writing in pencil, page numbers, et cetera…

• Now then, the features of this lab report.

– Model future informal lab reports on this one.

Let’s Talk About the Lab ReportNeed not be in order…

• Introduction (4 points)– Include an “Objective” section.

• What are we researching in each procedure? Why?

– Add some background knowledge.• What do you already know about all this stuff?

– Provide a hypothesis.• For each procedure!

– For the potatoes, explain how you will figure out the solutions’and the potato’s molarities.

– Answer preliminary questions (none assigned here).

Let’s Talk About the Lab ReportNeed not be in order…

• Procedure (3 points)

– Include materials and describe, loosely, theprocedure of the two experiments we’re doing.

• Draw the setup if it’s worthwhile.

Let’s Talk About the Lab ReportNeed not be in order…

• Data and Analysis (7 points)– Procedure 1: Calculate the surface area, volume, and surface

area to volume ratio for your cubes. (see next slide)– Procedure 1: Calculate the diffusion rate. (see next slide)– Procedure 3: Calculate averages and percent changes for the

potato slices between Day 1 and 3.– Procedure 3: Calculate the standard deviation and standard

error for all 18 potato slices BEFORE and for all 18 slices AFTER their time in the beakers.

– Procedure 3: Draw a graph relating molarity to percent change.

– Procedure 3: Identify which color solution had which molarity.

– Procedure 3: Identify the potato’s molarity.

Lab Manual Questions to AddressAnswer these within the Data and Analysis Section

• Not in your lab manual: Actually calculate the rate of diffusion.

– Note: “5 mm per 4.5 minutes” is not an acceptable rate of diffusion.

• Same reason why, if a police officer asks you how fast you were going, saying, “120 miles per 0.5 hours” isn’t acceptable.

• 5 mm per 4.5 minutes = 5 mm/4.5 min = 1.11 mm/min

• Not in your lab manual: Actually calculate the SA to V ratio (surface area to volume).

Let’s Talk About the Lab ReportNeed not be in order…

• Discussion/Reflection/Conclusion (7 points)

– Answer the lab manual questions identified on the next slide.

– Write more than just the answers to those questions:

• What’s the bigger meaning here?– See big and small ideas on the last slide…remember?

• Why do this lab?

• What could have been improved?

• What was unexpected?

• What sources of error?– Do not utter the phrase “human error.” Ever.

• Procedure 1:

– Which solution used was 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 AddressAnswer these within the Discussion/Reflection/Conclusion Section

Lab Manual Questions to AddressAnswer these within the Discussion/Reflection/Conclusion 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.

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: How does osmosis affect plant cells?