Mono-, Di-, or Poly-? You Be The Judge
Lesson 8 of 11
Objective: SWBAT predict the identity and create a structural model for each type of carbohydrate based on the results of this laboratory investigation.
Students apply their prior knowledge regarding the structure of carbohydrates to complete the Pre-Lab Worksheet pertaining to the general structure of carbohydrates and the processes of hydrolysis and condensation. In order to complete this anticipatory activity, students read the text and respond in short answer to the accompanying comprehension questions. The students quickly review their responses with their neighbor with a pair-share and then in a whole-group discussion to ensure the accuracy of the responses.
Modeling To Make Meaning
As a follow-up, I demonstrate the processes of hydrolysis and condensation using the child's toy, pop-beads, pictured below. In order to allow two monosaccharides to form a single disaccharide, the process of condensation or dehydration synthesis must occur by removing a hydrogen (H) atom and a hydroxide (OH) molecule to open up the binding sites to chemically join the two structures. For the purposes of this activity, the water molecule that is removed will be represented by the blue pop-beads. The red and the yellow pop-beads represent the single monosaccharides that are now chemically bonded through the process of dehydration synthesis or condensation (both terms mean to make or remove water).
- The red and yellow beads represent the monosaccharides (single sugars).
- The blue bead represents a water molecule which was created during the condensation reaction. This chemical reaction removed a Hydrogen (H) and Hydroxide (OH) from the monosaccharides which opened up the binding sites so the two monosaccharides could chemically bond and the disaccharide to form.
- The red and yellow beads represent monosaccharides that have been chemically linked through the process of dehydration synthesis/condensation to form a polysaccharide chain. The water molecules (blue beads) are not pictured, but would have formed through this chemical reaction.
To demonstrate the process of hydrolysis, I break apart the disaccharide molecule and put the blue pop-beads back on the end of the monosaccharide models to represent the presence of hydrogen (H) atom and a hydroxide (OH) at the end of the model.
The students silently read the laboratory procedure from the Lab Procedure Worksheet as an introduction to the laboratory activity. I review the procedure in a whole-class discussion by demonstrating the procedural and investigative expectations for both stations.
Sample of Student Work - This student work sample demonstrates the student need to record their data for each test and their best effort to discern the identification of each unknown substance through the analysis of their data.
Sample of Student Diagrams - Students are also asked to create diagrams to represent the carbohydrate molecules that they identified in this lab activity.
- Station #1: The Benedict's Test - Students label 3 test tubes of known carbohydrate solutions, mono-, di-, poly-. Students also label 5 test tubes with the names of the common carbohydrate solutions and place two squirts of each of the eight solutions in the appropriate test tube. The students then place a squirt of the Benedict's solution into each of the eight test tubes. Students then place all of the labeled test tubes in a warm water bath on the hot plate assigned to their group. After 5 minutes, the students record their observations on their Student Answer Sheet. If the students observe a color change observed in the solution after the heating process then the solution will be identified as a monosaccharide.
- Station #2: Iodine Test - Students use a well plate to arrange/label three known carbohydrate solutions and 5 common household carbohydrate solutions. Once the eight solutions have been prepared on the well plate, students place two drops of the iodine solution in each well. Students use a toothpick to mix the solution to determine if a color change has occurred. If the solution changes to a deep violet color then the solution is identified as a polysaccharide.
- Process of Elimination: How to Identify A Disaccharide - If any of the carbohydrate solutions did not change colors with either the Benedict's Test (test tubes) or the Iodine Test (well plate), then the solution, through the process of elimination, is identified as a disaccharide.
This lab activity includes multiple steps at each station, so I place an emphasis on following the directions from the handout, teacher demonstration, and relying on support from their collaborative lab partners. Students record their laboratory observations and data on the Identifying Carbohydrates - Student Answer Sheet. As the students are progressing through the activity, I rotate to each of the student-lab groups to ensure that the steps of the procedure are being followed and that the students have a strong understanding of how to analyze the outcome from both tests, the Benedict's and Iodine tests.
Students are reminded of the safety precautions for this lab activity and are required to wear their safety goggles throughout the duration of the experiment.
The video clip demonstrates student-group collaborative effort to analyze data and determine the identification of the unknown sugars.
In an effort to build a class culture that emphasizes student responsibility and lab safety, each group member is responsible for assisting in the lab clean-up procedure. The group is not excused from the lab area until their lab supplies have been cleaned and put away for the next group of students to use later in the day. The supportive class culture allows the classroom lab supplies to be well taken care of, as well as extends to the team effort in accomplishing our academic tasks in our daily activities.
Once the students have completed the data collection, they will return to their seats to collaboratively complete the conclusion questions from the Student Answer Sheet. Students are instructed to develop a conclusion statement on the back of their worksheet that summarizes what they did (procedure) and what they learned (data analysis).
Close - A Final Review
With five minutes remaining in the class period, the students turn their attention to the front board to view a sample structural diagram of two carbohydrates. They use the diagrams to identify which type of carbohydrates are represented, as well as describe the process that allowed the sample carbohydrate molecule to form. Both examples, Sucrose and Lactose, are diasaccahrides that were formed through the process of dehydration synthesis/condensation.
Student volunteers share their responses and review the process of condensation/dehydration synthesis and hydrolysis.