pHun with Phenolphthalein

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Students will be able to recognize the relationship between structure and function in living things.

Big Idea

Students dip agar containing phenolphthalein into ammonia revealing hot pink diffusion! Using these colorful cubes students can investigate how surface area to volume ratio affects diffusion in cells!

Connections to the NGSS and the Common Core

In this two day lesson, students cut different sized cubes of agar mixed with phenolphthalein and examine relationships between surface area to volume ratio and diffusion. After analyzing the data, students search for patterns in structure function relationships within a cell.  To close, students investigate the meanings of the NGSS Crosscutting Concepts and make connections between each concept and the lab.

This lesson is specifically designed to address the following NGSS and Common Core Standards:

MS-LS1-2  Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function. 

CCSS.ELA-LITERACY.RST.6-8.3  Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks.

CCSS.ELA-LITERACY.RST.6-8.1  Cite specific textual evidence to support analysis of science and technical texts.

CCSS.ELA-LITERACY.WHST.6-8.2  Write informative/explanatory texts, including the narration of historical events, scientific procedures/ experiments, or technical processes.

CCSS.ELA-LITERACY.WHST.6-8.1.B  Support claim(s) with logical reasoning and relevant, accurate data and evidence that demonstrate an understanding of the topic or text, using credible sources.

Scientific Practices:

The NGSS asks that students at any grade level should be able to ask questions of each other about the texts they read, the features of the phenomena they observe, and the conclusions they draw from their models or scientific investigations.  In this lab, the students are provided with opportunities to do this within their reading and their discussions (SP1).  Once collected, data must be presented in a form that can reveal any patterns and relationships and that allows results to be communicated to others.  In this lab, students look for patterns in data to identify the relationship between the surface area to volume ratio and the rate of diffusion (SP4).  In addition, in calculating surface area to volume ratios of different sized cubes, students use mathematics and conceptual thinking (SP5)

Crosscutting Concepts:

Specifically, this lesson focuses on the idea that the way an object is shaped or structured determines many of its properties and functions.  Students investigate how the size and shape of a cell or organelle can relate to their ability to do a specific job for the organism (Structure and Function).

However, this lesson relates to all of the Crosscutting Concepts.  Students develop their own definitions of each of the CCC's and then connect each concept to this lab.   It is a great opportunity for students to see that the concepts are truly crosscutting, they all can be applied to this one lab!

Connect to the Essential Question: What are you going to learn today?

5 minutes

Ask the students, "What are you going to learn today?".  Students should respond by referring to the Essential Question, "How do cells contribute to the function of living organisms?".  In my classroom, the EQ is posted both on the front board and on the students' Unit Plan.

Next, have the students turn to their Cells Unit Plan.  Over the course of this unit, students self assess on each of the five skills to reflect their level of mastery on a scale of 1 to 4 (4 being mastery).  Students change their self assessments as their level increases.  Explain to students that today's lesson will focus on Skill 3 (I can provide evidence that shows the relationship between cell structure and function).  

Have the students independently self assess on Skill 3.  In addition, allow them some time to reflect on their understanding of all of the skills so they can adjust and score those that they feel should be updated.  Remind students that learning is all about growth; it is a journey.  Taking the time to reflect on where they are with each skill will help them make a plan to take their own individual path to mastery.  It is ok to not be at a 4 yet - it's about having the awareness of where you stand so you can make a plan to reach your goal.


Pulling Information From Text: The Ladder of Discourse and Crosscutting Concepts

15 minutes

Students in my class have been provided with the Cell Transport reading and have read the first two pages of the document in a previous lesson.  Thus, my students already have background on diffusion, osmosis, and transport.  In this lesson, students read the second page of the document to gain an understanding of how the size, shape, and surface area to volume ratio of a cell are essential to cell function.

Students read the second page of the article and climb the Ladder of Discourse.  The Ladder of Discourse is a strategy I use in my class to help students think critically as they read.  For middle school students, informational reading can just become words on a page.  The Ladder of Discourse is a way to help students recognize what they should be thinking about as they read so that they can gain an understanding of the text.

The levels of the Ladder of Discourse are "Tweets" (text to self connections), "Huh?'s" (questions or concepts they do not understand), "Found It" (finding answers to questions through context clues or finding science answers), and "Discourse" (combining ideas to think beyond the text).  The resource Ladder of Discourse: Description of Rungs provides background about the "rungs" students use when reading.  

For middle school students, thinking critically during reading is a challenging task.  However, being able to think beyond the text is key to real understanding of scientific concepts.  This "thinking beyond the text" is what we call the rung "Discourse".  I have found that the NGSS has provided us with an invaluable tool in the Crosscutting Concepts.  These provide students with themes that can guide their critical thinking.  When they read, students have out their Ladder of Discourse: Description of Rungs document so that they have a description of the Crosscutting Concepts next to them with the CCCs fresh in their mind.  As they read, they try to make connections to the CCCs.  This could be in the form of a statement, idea for an experiment, a prediction, or a question.  In order to do this, students have to slow down when they read.  Every 2 - 3 sentences they stop and think about what connections they are making to the text and they document their thinking by "talking to the text".

I have found that with the implementation of this strategy, my students depth of understanding from reading has dramatically increased.  As students read, they "talk to the text", to document what they are thinking.

After reading and "talking to the text" (writing down their thoughts on their paper), have students talk in groups of 2 or 3 to discuss the discourse, or Crosscutting Concepts, that they were connecting to in the text.  Then, have the students write their discourse on sticky notes.  Posted on my wall are blank poster papers with the titles of each Crosscutting Concept.  Students write their discourse on the sticky note and place it on the appropriate poster it connects to.  

Now, I will say, this strategy takes practice!  It is not a magical strategy that makes reading easy.  It is a tool, however, that provides students with a structure and thought process for critical reading.  If you stick with it, I promise you that you will see an increase in the depth of thinking your students demonstrate while reading.  As another tool to help with this strategy, I have included a resource called Ladder of Discourse Sentence Starters that I used with my students when I first began implementing this to get them thinking about the Crosscutting Concepts.

Take a look at some of the ideas and questions that my students generated during this reading.  None of the comments on the sticky notes were stated in the text, it was all their own connections and conclusions beyond the text.  They are all ideas the students connected to because they were thinking about the Crosscutting Concepts as they read.  If you take the time to read each sticky note, I think you will see why I am so excited about this strategy.  Some of these sticky notes come from readers that have struggled.  I realize some of them aren't perfect, but the sheer idea that these are the thoughts going through their head when they are reading serves as evidence that the strategy is making a difference.

Systems and Models:

Stability and Change:

Energy and Matter:
Cause and Effect:

Structure and Function:
Scale, Proportion, and Quantity:

pHun With Phenolphthalein Lab

40 minutes

Explain to students that you have prepared agar that contains phenolphthalein in it.  Show them what the agar looks like and remind them that phenolphthalein is a base indicator that turns pink in the presence of a base. (My students have already completed a Chemistry Unit and have a lot of background in pH at this point in the school year.)  Let them know that in this lab they will be creating models of cells using the agar in order to determine how surface area to volume ratio affects diffusion.

**Teacher Tip** To make the agar, I brought 1000 mL of water to a boil and added 25 grams of nutrient agar and 10 grams of phenolphthalein powder.  Stir until it is all dissolved.  Be careful! When this mixture starts to bubble, it will bubble over quickly!  Pour the mixture into a baking tray.  Cover immediately and place in the refrigerator overnight.

Provide students with the procedures and lab documents and place them in groups of 3 - 5.  Emphasize to the students that reading the procedure for detail will be very important for obtaining valid data.  

**Teacher Tip**  I worked in conjunction with the math teachers in my school on this.  We saw a natural connection as the students in my class were working on calculating surface area and volume in their math class.  In addition, they had already completed a unit on ratios.  Thus, I did not do a separate mini lesson on measuring surface area and volume in this lesson.  If your students do not have that background, I would provide students with cubes (base ten blocks or children's building blocks work great) and have them practice each of these measurements.


1.  Put on a pair of goggles.

2.  Get a cup of the ammonia water.  As a group, discuss the pH level of ammonia.  Determine whether you believe it is an acid or a base.

**The ammonia water I use is very dilluted.  This will cut down on the smell and the safety risks when using ammonia.  In addition, I pre-pour the cups and place tin foil over them in order to cut down on the smell.

3. Carefully cut 3 different sized CUBES out of the agar:  one large, one medium, and one small.  It is very important that you cut your cubes to look like CUBES, not rectangular prisms!  Also, it should be obvious that each size cube is a different size from the others.  The small cube should be very small and the large cube should be significantly bigger!

4.  Carefully measure the approximate surface area and volume of each of the cubes and enter the values into your data table.  *As it is difficult to cut the agar, the cubes may not be perfect.  However, for the purposes of this lab, assume they are perfect cubes so that you can use the formulas below to calculate the SA:V ratio.  

a. Surface Area:  Length * Width * 6:  Measure one length of a cube in centimeters.  Multiple this number by itself (Remember that in a cube the length and width are the same.).  Multiply that product by 6.

b.  Volume:  Base * Length * Height:  Using the measurement you found when calculating the surface area, multiply the length by the length by the length.  (Remember in a cube, the base length and height have the same measurement.)

5.  In this experiment, you will be dipping these cubes into ammonia and measuring the percent diffusion that occurs in the cubes.  Write a hypothesis predicting the results on your lab sheet to predict which cube will have the highest rate of diffusion.  (If, then, because)

6.  Put the largest of the cubes on your paper clip platform and dip the paper clip into the ammonia.  Hold it there for 1 minute.  MAKE SURE NOT TO DROP YOUR CUBE!  LEAVE IT ON THE PLATFORM.

**To make the platform, unfold a paper clip to form a triangle at the end with a straight handle.  Take tape and wrap around the triangle to make the platform.

7. Cut the cube in half so that you can see the inside of the cube.  Estimate the % of the cube that turned pink and record in your data table.  Keep your cube to reference later!

8.  Repeat steps 6 and 7 for the medium and small cubes.  

**Notice that when looking at the cross section of the cubes, you can see that the ammonia did not reach the center of the cube.  The smaller the cube, the higher the percentage of the cube will turn pink.

A Look at Student Work: The Big Picture

Below is a look at some student work and criteria to look for in their writing.  

When looking at the data table, the place I find the students have the most difficulty is in getting the ratio to be  ____:1.  By getting the ratios in this format it is easy to make comparisons between the cubes; however, students will often need help with this process.

When looking at the hypothesis, this student does a nice job identifying all of the trial groups and in making a prediction about the dependent variable (% diffusion).  However, there is a common misconception that she demonstrates in the "because" part of her hypothesis.  Students can struggle with the idea of a ratio as a comparison of two variables.  This student only mentions volume as the cause of "less diffusion".  This student still needs to grasp the idea that it is the comparison between surface area and volume that is important.

This student did an excellent job recognizing that phenolphthalein is a base indicator and that ammonia is a base.  Keep in mind that my students have prior knowledge about acids and bases.  If your students have not covered these ideas yet, this question will take some teaching.

In question 6, it is critical that the students compare data to support their reasoning.  This student compares both the surface area to volume ratios as well as the % diffusion of the large and small cubes. I often have students that make the following mistakes:

- The student includes no data.

- The student uses general relationships as opposed to specific data.  For example, they say, "The cube with the largest SA:V ratio had the greatest diffusion while the cube with the smallest SA:V ration had the lowest diffusion."

- The student compares only the diffusion and not the surface area to volume ratio.  For example, the student may write, "The large cube diffused at 20% while the small cube diffused at 90%."  As the question is specifically asking for evidence of surface area to volume ratio, it must be included.

For questions 7 and 8 in the picture above, the key idea is that students recognize that in order for cells to function, important materials must transport in and out of the cell.  This makes the size, shape, and surface area to volume ratio important for cells to function.

Question 9:  The Big Picture 

The last question of the lab document asks the students to write a lengthy response to show their understanding of the big picture as relating to structure/function relationships and surface area to volume ratio.  Students answer the question, "Explain why size, shape, and surface area to volume ratio are essential for cells to function."  In the video below, I go over some tips for helping students plan for and organize their writing.  In the video, I reference an ABCDE format.  If you would like more information, I have included a "Quick Guide to the ABCDE Paragraph" in the resource section.  In addition, I have included a couple different student work examples of this last response if you wanted to access a hard copy in addition to the video.

Closure: Defining the Crosscutting Concepts and Making Connections

30 minutes

With the importance of the NGSS Crosscutting Concepts, I find that it is essential not only to take the time to establish a "7th grade definition" of each concept, but to take labs and connect to each concept.  To truly show students that these concepts are "crosscutting", I have found that it is very beneficial to ask students to take one specific lab and make connections to every Crosscutting Concept.  

This is no easy task!  In order to ensure success, I have incorporated a protocol for this process.

1.  Each group receives a description of one Crosscutting Concept (as unwrapped by the NSTA for grades 6-8). Each table receives only 1 of these concepts.

2.  The group reads one sentence aloud.  Then, the group breaks down the sentence word by word to discuss what the meaning of the sentence is.  This is repeated for all of the sentences on the description. (The Crosscutting Concepts Table Signs are written at a very high reading level.  It will take the students time to break them down and make meaning from them.)

3.  The group paraphrases the concept in a way that a typical 7th grader could understand.

4.  The group discusses how the lab connects to the concept using text and data from the lab to provide evidence of the connection.

5.  The group presents to the whole class their "7th grade definition" of the concept and their connection to the lab.

Before having students begin, I have found that it is important to model the process for them.  Thus, I use a "fishbowl activity" to provide that model.  I sit down with one group and work through the protocol while the rest of the class surrounds us, as if they are watching our interactions in a "fishbowl".   After the conversation of the fishbowl group, the observing students reflect on the strategies that allowed the group to be successful.  The video below is a segment of this activity.  I have added callouts to the video to identify the strategies that the class reflected about after the activity.


After reflecting on the fishbowl, the students return to their table groups and follow the protocol for their assigned concept.  Following the group discussions, the groups share out their "7th grade definitions" and connections to the lab to the whole class.

Check out what my 7th graders came up with!  You are going to notice that each group is at a different level.  This is not something that all students will master immediately.  The more consistent you are with the implementation and embedding of the Crosscutting Concepts, the better they will get.

Energy and Matter:

One of the great things about this activity is that each group of students comes up with a unique definition and connection.  It is so cool to compare the interpretations from class to class.  Check out these two videos if you want to see some different groups of students interpret "Energy and Matter":

Energy and Matter Student Share Out #2

Energy and Matter Student Share Out #3

Structure and Function:

Stability and Change:

Systems and System Models:


The more this protocol is used, the more effective your "7th grade definitions" and connections to the lab will become.  I reuse this strategy frequently and each time I give the table groups a different Crosscutting Concept.  That way, by the end of the year or unit, each group will have broken down each concept (Hopefully more than once!).  The more I utilize this, the more efficient the process is and the deeper the responses the students can provide.  Be patient with it!  The first time or two you try this, it may provide many challenges for the students.  However, you will see huge growth in learning and critical thinking in all that you do as a result!