Teaching Strategies and Classroom Activities
In selecting strategies to implement this unit, I considered two things. They must demonstrate the nature of science – science is not only a body of knowledge but is also a way of doing and a way of thinking.
At the same time, the strategies must also cater to varied learning styles of my students who, as part of the net generation or millenials, are digital natives whose major way of gathering information and interacting with other people and the environment is through social media and digital devices. Millenials also prefer to learn by working collaboratively with their peers and appreciate structured activities that allow creativity.
Part I. Proteins
A discrepant event demonstration called Skewered Balloons will be used to set the tone for this sub-unit. Student volunteers will be challenged to stick a bamboo skewer that has been dipped in baby oil through a balloon without popping it. The demonstration will be used to review our past lesson on polymers and how a lot of polymers, including the latex the balloon is made from, have cross linkages. If none of the students are successful, have them push the skewer gently through the end where the knot was tied until the tip reaches the opposite end. Asking probing questions such as those listed below will help students explain why pushing the skewer through the knot and nipple end does not make it pop while puncturing it though the middle will.
- Why does the balloon stretch when inflated? (Latex is made up of intertwined polymer chains that are elastic.)
- Where is the balloon most stretched? (middle) least stretched? (ends)
- Why does the balloon pop when the skewer is pushed through the middle? (The chains are already stretched and the skewer breaks the cross linkages.)
- Why does the balloon not pop when the skewer is pushed through the ends? (The polymer chains stretch around the skewer which gets in between the molecules.)
Direct teaching using PowerPoint will be used to provide input about amino acids as the building blocks of protein and the 1 0, 2 0, 3 0, and 4 0 structures of proteins. Models of these structures using simple materials such as pop beads, chenille stems, plastic beads, slinky, folded paper and the like will be used to help students visualize the structures.
Examples of real life applications (e.g. lactase and lactose intolerance, cysteine in hair contributes to the strength of hair) will be interspersed throughout the presentation.
Working in collaborative groups, students will construct their own models using common materials of their choice to illustrate how the amino acid cysteine, an important component of keratin, the protein found in hair, can form disulfide bonds. They will also construct models to show the difference among the primary, secondary, tertiary and quartenary structures of protein in general. Each group will present and explain their model to the class.
A mini-lab activity entitled Protein in Hard Tissue will serve as closure. In this activity, students will learn the role of a structural protein in hard tissue. Working in pairs, students will observe and compare the properties of cooked chicken bone before and after it is soaked in 1 M HCl overnight. In their lab write up, students will be asked to record their observations and describe what happened to the mineral salts of the bone, explain why the soaked bone is easier to twist and bend, hypothesize what could be the insoluble protein left in the soaked bone and the possible role of this protein in the formation and maintenance of healthy bones. (The acid reacted with the minerals in the bone to dissolve them and what is left is the insoluble protein collagen. Collagen gives bones strength and resilience.)
Part II. Physical Structure and Chemical Composition of Hair and Its Properties
I will begin this sub-unit with a hands-on activity. Working in pairs, students will construct a hair hygrometer using a shoebox and a long strand of hair. Some of them may have done this in middle school so they can be tapped to share that a hygrometer measures humidity or the amount of water vapor in the air. A detailed procedure on how to make the hygrometer is found in Appendix A. Draw out from students how a hair hygrometer works by asking what happens to their hair when humidity changes. Their own personal experience of a "bad hair day" should prompt them to respond that curly hair frizzes and straight hair becomes limp when humidity increases so when humidity is high, the hair strand in the hygrometer lengthens while at a lower humidity, it contracts and shortens. This change in length moves the pointer in the hygrometer.
Later in this sub-unit, have students refer back to this activity to explain the relationship between humidity and hair behavior. Water molecules not only break the hydrogen bonds that hold the coiled chains of keratin together in the cortex of the hair shaft but also form hydrogen bonds themselves with the keratin chains. This allows water molecules to wedge themselves between the protein chains causing hair to expand.
Teacher input using PowerPoint presentations on the three layers of the hair shaft and the organization of keratin and melanin in the cortex, and how the three layers respond to pH and water will provide students with background information to help them construct models of the cross linkages in keratin chains. The model should clearly identify the three bonds (hydrogen, salt/ionic and disulfide bonds) that hold the chains together. This will be done in groups of 3. Examples of materials students could use include Chenille stems for the keratin chains and colored paper clips for the bonds. A different color is used for each of the three bonds. Students will be encouraged to use their creativity in exploring other materials they can use to construct their model.
Using the model they made and their notes from the PowerPoint, students will explain the shape (curly or straight), texture (coarse or smooth), sheen (dull or shiny) and color of their hair. The group output is a pyramid foldable TM. 13 in which each side contains an explanation of each student's hair. Instructions on how to make a foldable are included in Appendix A.
A micro scale lab activity comparing the pH of different kinds of shampoo (pH balanced, shampoo with built-in conditioner, shampoo without conditioner, anti-dandruff, clarifying, baby, etc.) will serve as closure for this sub-unit. A pH meter or pH paper will be used to measure the pH of undiluted and diluted samples of shampoo. Careful measurements of pH should show that pH balanced shampoos have a lower pH than regular shampoos while baby shampoos have a pH close to neutral (hence Johnson baby shampoo's slogan of no more tears). In their lab write up, each student will decide the best type of shampoo for their hair based on the results of the activity.
Part III. The Chemistry Behind Hair Perm, Straightening and Coloring
In the third and final sub-unit, students will have guided reading activities using articles from ChemMatters and other publications and internet access to learn how perm, straightening and coloring work. They will summarize their findings in the form of a three-tab foldable TM. 14 Their foldable must show that curling and relaxing or straightening hair involve breaking not only the hydrogen bonds between protein strands but the disulfide bonds as well and creating new cross linkages in new locations of the keratin chains. The third section of the foldable must show that permanent coloring involves altering the melanin pigments in the cortex while semi-permanent and temporary coloring do not affect the melanin. In semi-permanent coloring, the dye settles around the cortex while in temporary coloring, the dye simply coats the cuticle.
Students will also test the effects of solutions with different pH on hair. This activity will be combined with a Predict, Observe, Explain (POE) strategy. I will make arrangements with a hair salon near the school to collect cut hair from one individual with straight hair. Bundles of hair containing 10-15 strands each will be coiled around wooden splints and held in place by rubber bands. These bundles will be soaked in 3 different solutions for 15 minutes and 10 minutes for the fourth one. The first 3 solutions are vinegar with a pH of 4, clear window cleaner with a pH of 8, and permanent wave solution, while the fourth is permanent wave solution followed by neutralizer. For the fourth solution, the hair sample will be removed from the permanent wave after 10 minutes, rinsed with tap water and then soaked in the neutralizer for 5 minutes. Students will then predict which solution is the most effective in making hair curly before and after the samples get wet and which one makes the hair easier to split or become more damaged.
The 4 hair samples will be removed from the solution after 15 minutes and allowed to dry partially. After removing the rubber band carefully, students will observe and rank the degree of curling shown by each hair bundle. The strength of each hair sample will be tested by removing one strand of hair from each sample and stretching it gently from each end.
Students will then rinse with tap water, dry completely and compare the amount of curling of the 4 hair bundles after which another stress test is performed. The students' lab write ups must show each POE component. In their explanation, they should compare their observation with their prediction and explain their observation in terms of the cross linkages in keratin. Their explanation must also include that basic solutions are more damaging than acidic solutions because the former opens up the cuticle of the hair shaft.
A group project will be used to synthesize what students have learned from this unit. Working in collaborative groups of 4-5, they will create a 3-5 minute video presentation on how chemistry leads to a "good hair day."
Comments: