Activity 5.2 - 5.2: Molecular Models for Cows Growing: Digestion and Biosynthesis (40 min)

Show slide 2 of the 5.2 Molecular Models for Cows Growing: Digestion and Biosynthesis PPT.

Display slide 3 in the PPT.

• Revisit the driving questions first seen in Activity 5.1. Tell students that today’s activity is focused at the atomic-molecular scale.

Display Slide 4 to review that animals use digested food in two ways.

Tell students that these large organic molecules in food are broken down into small organic molecules during digestion

Display slide 5 to show students large organic molecules are broken down into small organic molecules during digestion. Tell students large organic molecules are called polymers and small organic molecules are called monomers. It may help students to remember these words by explaining the meaning of the words’ prefixes (poly means many and mono means one).

• Tell students that they’ll be using molecular models to model the process of digestion, which will help them answer several of their unanswered questions.

Use slide 6 to remind students how atoms bond to make molecules.

• Oxygen atoms bond to carbon or hydrogen (not other oxygen atoms) whenever possible. This will help students decide which monomer will bond to an –OH and which will bond to an –H.
• Nitrogen forms three bonds.
• Point out that digestion will not make or break "high energy" C-C or C-H bonds. Students can use this information to determine where to attach the –H vs. –OH in the activity.

Give each pair of students a Molecular Models 11 x 17 Placemat, one set of Forms of Energy Cards, one pair of scissors, a removable tape dispenser, and one protein molecule, one carbohydrate molecule, one fat molecule, and eight water molecules (from the 5.2 Polymers for Cutting Handout).

• Have students place a “chemical energy card” on the reactants side of their placemat, along with their water, protein, carbohydrate, and fat molecules.
• Coach students to simulate the process of hydrolysis by cutting a water molecule each time they make a cut in the polymer. This helps show that each time a bond between two monomers is broken, the chemical reaction requires water and new bonds form.
• Protein:Show slide 7. Have students cut one protein polymer into amino acid monomers. Then, cut the water molecules and attach an –H and an –OH to each amino acid. Watch the animation on slides 8-9.
• Carbohydrate: Show slide 10. Have students cut one starch polymer (a type of carbohydrate) into glucose monomers. Then cut the water molecules and attach an –H and an –OH to each glucose. Watch the animation on slides 11-12.
• Fat: Show slide 13. Have students cut one fat polymer into glycerol and fatty acid monomers. Then, cut the water molecules and attach an –H and an –OH to each monomer. Watch the animation on slides 14-15.
• Have students move the new molecules with the energy card to the products side of their placemat. When watching the slides, ask students what is happening to energy. Listen to see if they notice that chemical potential energy is conserved through digestion.
• Show the students the Digestion and Biosynthesis 11 x 17 Posters as a visual of the process.

Use slide 16 to remind students that blood carries digested monomers to all parts of animals’ bodies.

• Because the molecules are small they can pass through the intestinal walls and into the blood.

Use slides 17 and 18 in the PPT to transition to biosynthesis.

• Ask students what they remember about biosynthesis from Activity 5.1

Show slide 19 to remind students of the information they learned from beef nutritional labels: cow muscles are made primarily of protein (18g) and fat (21g). This means that the cells in the animal’s leg muscle are going to make fat and protein molecules so the muscle cells can grow bigger and divide.

• Tell students that they will use the placemat and molecules to model the process of biosynthesis, which is what happens when animals build polymers from monomers
• Point out that when they are modeling, they should remember that during biosynthesis, no "high energy" C-C or C-H bonds will be made or broken. The chemical energy is conserved!
• Refer to the Digestion and Biosynthesis 11 x 17 Posters in your classroom to help students visualize the biosynthesis of monomers to polymers.

Have students place a “chemical energy card” on the reactants side of their placemat, along with their amino acids, fatty acids, glycerol and glucose molecules. Coach students to simulate the actual process of dehydration synthesis by making a water molecule each time they tape two monomers together. This helps show that each time a bond is broken a chemical reaction takes place and new bonds form.

• Protein: Show slide 20. Have students tape together four amino acid monomers to form one protein polymer and three water molecules. Then, watch the animation on slides 21-22.
• Fat: Show slide 23. Have students tape together one glycerol and three fatty acid monomers to form one fat polymer and three water molecules. Then, watch the animation on slides 24-25.
• Carbohydrate: Tell students that animals don’t make starch (though they make other carbohydrates)!
• Have students move the new molecules with the energy card to the products side of their placemat. Ask students what is happening to energy during biosynthesis. Listen to see if they notice that chemical potential energy is conserved through the chemical change.
• Make the connection to cell division: cells have to both get bigger and also divide in order for animals to grow. This is why we perform biosynthesis: to make our cells get bigger (growth) so they can divide.

Use slide 26 to have students consider the composition of materials that enter, stay in, and leave a cow.

• Students may point out that there is fat in the cow, but not in the grass. Ask them “Where did the fat in the cow come from?” After the lesson, they should be able to answer that question as they “built” fat molecules in their model biosynthesis.