Cell Transport, part 2 - Starch and Iodine Lab

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Students will be able observe the diffusion of a substance across a semi-permeable membrane.

Big Idea

Diffusion explains why an onion in plastic wrap still emits a strong odor.


5 minutes

Warm-Up: When a child goes down a sliding board, what type of transport is that most like?  When a child goes up a sliding board, what type of transport is that most like?  Explain.

This warm-up question is a great way to assess how well students learned the content taught in the previous lesson. It allows students to recall an experience that they all likely have had and apply it to a scientific concept.  Take a minute and talk about your experiences with sliding boards.  Ask students how many of them have ever gone up a slide the wrong way.  Tell them to think of that experience before responding. 

Allow students to turn and talk for 1 minute with a seatmate.  Listen to students conversations as they share.

Look for students to identify that the movement down a sliding board is most like passive transport because the child does not have to use any energy to move with the direction of the slide from higher to lower because of gravity. Remind students to use the content specific vocabulary in their responses.  Look for students to use the term concentration gradient, in addition to passive and active transport.

Look for students to identify that the movement back up a slide is most like active transport because the child will have to expend energy to go against the slide’s usual direction (lower to higher).

Allow 1-3 students to share their thoughts with the class.  Listen for correct usage of vocabulary and affirm correct thinking.

Content Review

10 minutes

Display and distribute copies of a cell transport review table.  Give students 5-7 minutes complete the table for each of the six types of transport to the best of their ability without the use of their notes.  Instruct students to mark an X or checkmark in each square that correctly corresponds with the type of transport. Walk around and observe students as they work.  This serves as a formative assessment. After the 5 minutes have passed and before sharing the correct responses for the table, spend time sharing brief animations for each of the types of transport indicated on the table:

Ask students to consider it they want to change any their responses after viewing each of the animations and before you tell them the correct choices for the type of transport. Walk around to check to make sure students are making the correct notations on their review grids as shown on the student work.

This review activity allows students to revisit the information taught in Cell Transport, part 1 using animations, in order to facilitate a greater depth of understanding and concept mastery.

Guided Practice

10 minutes

Prep the lab and have all materials ready before students arrive.

Preform a pre-lab to help build students’ knowledge and provide them a base of understanding:

  1. Mix a small amount of starch and water in a beaker.
  2. Explain that iodine is an indicator for the presence of carbohydrates, turning from brown to purple.
  3. Before adding iodine to the starch solution, ask students to predict what will happen when the starch comes into contact with the iodine.  Students should be able to predict the iodine will change color from brown to purple.

Display and distribute copies of the diffusion lab.  Preview the lab and the lab materials.  Review safety rules and place students in groups of 2 to perform the lab.  The groupings of students can be ability-based, self-selected or randomly selected.  Make sure that the class dynamics are addressed with whatever grouping plan you select to ensure that class disruptions are minimized.

Independent Practice

20 minutes

Release the students to perform the lab. When students come up to get their materials, watch to see if they are able to use proper pipet technique.  Look for lab behavior that shows safe lab practice such as keeping the googles on at all times.

Walk around to observe students as they work. Make sure that students do not agitate the solution in the bag and that they allow the diffusion to occur on its own. Tell students to pay particular attention to the color of the water and the color of the solution inside the bag at the start of the lab.  They will need to observe what happens to the water outside the bag, as well as the solution inside the bag over time. 

While the diffusion is occurring, instruct students to work together on the lab analysis questions that don't require the completion of the diffusion process. This lab is a small group activity which allows students to share ideas and thoughts around the lab analysis and conclusion.  Listen to the conversations in the small groups to ensure that students are able to use the academic vocabulary when discussing why the water changes from clear to brown outside the bag. Also, listen for and correct any misconceptions that are identified. 

When reviewing the lab submissions, look for responses to the lab questions that show an understanding of diffusion and osmosis, the concepts of hypertonic, isotonic and hypotonic. Also, look for use of academic vocabulary in the responses. The student work shows that the student understands diffusion and osmosis.  The work also shows that the student is able to explain why the iodine moved outside the plastic bag.



5 minutes

I typically share a story about how I made a batch of cookie dough that had to be refrigerated for baking. When I baked the cookies, the cookies tasted like onions. Ughhh!!! I could not figure out why they tasted like onions until I realized that the cookies had been placed on a plastic bag of cut onions.

Ask students, “Why did that occur?  And, why does an onion emit an odor even when it is covered in plastic wrap? Students should be able to articulate that particles of the onion are small enough to cross the semi-permeable membrane of the plastic wrap, which results in the onion odor being emitted, even though it is covered with plastic wrap.

Listen to their responses to gauge how well they have mastered the concept of diffusion of particles.  Prompt students to use the specific vocabulary related to the concept when they share.