Photosynthesis, Part 3 - Chromatography lab

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Objective

Students will understand how chromatography separates two or more pigments found in plants.

Big Idea

Even though that leaf appears green, it isn’t the only pigment in that spinach leaf.

Warm-Up

5 minutes

Warm-up: What do you know about the term, chromatography based on its word parts?

Ask this question to set the stage for the lesson and assess how well students are able to decode a term that they may not know.  Remind students to use their word stems to aid them in decoding the term’s meaning.  If necessary, use the word in a sentence so that students will be able to use the outside/in strategy to help them decode the term’s meaning. Even if students are not able to put the parts together for a complete meaning, look for students to be able to correctly identify that chromato  means “color” and graph means “to write”.  Some might be able to determine that chromatography means writing in colors.  

Introduce New Material

15 minutes

Distribute copies of a scientific text, The chemistry of autumn colors.

Assign groups of students to read specific parts of the text, making sure that no one group is assigned to read the entire article.  Instruct students to take 5 minutes to perform a close read of the section that they are assigned; making sure that they write down significant ideas and curiosities from the section they read.

At the end of the timed period, inform students that they will now perform a “jigsaw” activity in which they will find groups who read different sections of the article to learn other significant ideas from the text.  The jigsaw is a good way to allow students to pool their reading abilities to learn the “gist” of the entire text without having to read the entire text.  Before allowing students to move into the jigsaw, make sure that students know what the movement and behavior should be for the jigsaw activity.  Count down 3-2-1, start and allow students to circulate the room and share with other groups for 3-4 minutes.

Instruct students to return to their seats and ask for a few volunteers to share key points from each section of the assigned reading.  Make sure that students note that there are other pigments found in plants besides chlorophyll and that they indicate why leaves change color.

Guided Practice

10 minutes

Ask students, “If plants contain pigments other than chlorophyll, how can we prove it?  Allow them “brainstorm” and share ways in a whole class discussion without correcting any of their thoughts. Inform students that they will investigate whether plants contain pigment other than chlorophyll.

Show students a brief clip, Separation of Photosynthetic Pigments by Paper Chromatography.

After viewing, ask students to explain what they saw  Look for students to identify that the pigments were extracted through use of a solution that separated the colors.

Inform students that they will work in groups of two to participate in a lab to observe the pigments found in spinach leaves.  Begin by displaying and distributing Chromatography lab instructions to students.  As a way to provide differentiated instructions to the various learner types, provide a verbal summary of the lab materials and procedures, and also project the instructions using a LCD projector.   

Ask a student to read the background aloud to the class.  Summarize the materials needed for the lab and demonstrate how to set up the lab.

Modeling is an effective way to not only tell, but show students how to complete a task. Before releasing students to work in their small groups, reinforce the safety rules. 

                                       

Independent Practice

25 minutes

Allow students to collect their lab materials 1-2 groups at a time to minimize disruption.

Instruct students to read the chromatography lab before releasing them to begin the lab. Walk around as students work to observe their lab procedure and safety practices.  Observe students’ behaviors to ensure that everyone is wearing goggles and that no one is purposely inhaling the solvent.   Listen to students’ comments as they work. Use the observations to formatively assess students’ knowledge, but skills and techniques. Address student misconceptions as they heard and correct safety violations.  Redirect students who appear to be off-task.  I also like to ask questions to reinforce learning as I walk around.

Note: One bag of spinach should be sufficient for an entire day of classes.  I cut the leaves in half to make sure that there is enough for the day’s lesson.

The student work samples indicate that students were able to observe that plants contain more than green pigment.  Their responses also reflect the need to persist with writing assignments that will build their skills with crafting constructed responses to questions. I assess students' writing, not only to determine if what they have written about is accurate but I monitor their writing skills over time to assess whether improvement is occurring or not.

Close

5 minutes

Display a closing question and ask, “Which of the pigments was the most soluble in the solvent?  How do you know?”

Engage students in a discussion.  Look for students to be able to identify that beta carotene was the most soluble because it moved the farthest up the paper.  Students should be able to explain that the pigments are not equally soluble in the solvent because they were not carried to the same location on the paper.