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?
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: What is the sucrose concentration of a
potato?
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.
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:
(New Value - Old Value)
Percent Change 
 100
Old Value
• 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/2000pxTurgor_pressure_on_plant_cells_diagram.svg.png
Procedure 3 Overview
1. Obtain slices of potato core. Don’t take the ones with the skin on
them.
2. 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.
3. Take the mass of three potato core slices – individually.
• Why individually? Two reasons:
1.
2.
We need to take the average of the three pieces.
We will add in a statistic called standard deviation that requires individual
numbers.
4. Put all three in one sucrose solution. Repeat for the other
solutions.
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!
Lab Manual Questions to Address
Answer these within the Data and Analysis Section
• 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 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 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).
• Not in your lab manual: Calculate standard deviation
and standard error for your potato slices.
Last Thing to Remember
• 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.
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: What is the sucrose concentration of a
potato?