Activity 5.2: Molecular Models for Potatoes Growing: Biosynthesis (40 min)

Show slide 2 of the 5.2 Molecular Models for Plants Growing: 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 of the PPT.

• Remind students that plants use glucose in two ways for growth and energy. They have already learned how plants use food for energy. In this activity they will model the chemical processes involved in growth, biosynthesis.

Use slide 5 to explain how plants use the monomer they make in photosynthesis (glucose) to make all the other monomers they use to build polymers.

• Point out that ammonia is also needed to supply the nitrogen atoms to make amino acids. Note: while we use nitrogen as a mineral example, plants use many other minerals in biosynthesis (e.g., Sulfur in some amino acids, Magnesium in chlorophyll).
• Note: This step was not introduced in the tracing in Activity 5.1

Use slide 6 to explain how plants use small organic molecules (monomers) to make large organic molecules (polymers). This is how cells can grow bigger and divide.

• Explain than 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).

Use slide 7 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.

Show slide 8 to remind students of the information they learned from plant (leaves and seeds) nutritional labels: leaves are made primarily of carbohydrate (11g) and protein (2g), and seeds are made primarily of fat (50g), carbohydrate (22g), and protein (24g). This means that the cells in a plant are going to make fat, protein, and carbohydrate (starch) molecules so the 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 plants 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 cut up their monomers so each piece of paper only has one monomer molecule. 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.
• Carbohydrate: Show slide 9. Have students cut off an –H and –OH of each monomer, then tape together three glucose monomers to form one starch polymer and two water molecules. Then, watch the animation on slides 10-11.
• Protein: Show slide 12. Have students cut off an –H and –OH of each monomer, tape together four amino acid monomers to form one protein polymer and three water molecules. Then, watch the animation on slides 13-14.
• Fat: Show slide 15. Have students cut off an –H and –OH of each monomer, tape together one glycerol and three fatty acid monomers to form one fat polymer and three water molecules. Then, watch the animation on slides 16-17.

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.