1. Begin by asking the class, “What does skin do for us?”

Focus the discussion on skin as a protective barrier. If not mentioned, bring out other functions such as “holding us together” and heat regulation through sweating.

2. Ask the class, “What does muscle do for us?”

Students may say that muscle allows us to move and gives us strength. Emphasize that the primary function of muscle is to interact with the skeleton to produce movement.

3. Ask the class, “What does bone do for us?”

Focus the discussion on the ability of bone to provide strength, support, and protection for the body. Other functions include storing minerals and containing bone marrow.

4. Explain to the class that in each case (skin, muscle, and bone), the functions of the body system are related to its unique structure. Explain that in this activity, students will examine the relationship between structure and function in bone.

5. Remind the class that they learned in Lesson 1 that bone is a living tissue. Ask, “What is bone made of?”

Using what they learned in Lesson 1, students should respond that bone is made of cells. Some students may also recognize that bone contains calcium. If students do not mention calcium, remind them of the phrase, “Milk builds strong bones.” Ask them why they think milk builds strong bones. Bring out the idea that calcium in milk helps build strong bones.

6. Explain that in addition to cells, bone contains minerals and a protein called collagen that is found outside the cells. Ask the class if they know what minerals and collagen are.

Students likely will not have a good understanding of minerals and collagen. For the purpose of this lesson, bring out the idea that minerals are chemicals important to our health. Explain that the minerals most important to the health of bone are calcium and phosphorus. These minerals provide bone with strength. Collagen is a protein that also contributes to bone strength.

7. Display a transparency of Master 2.1, Bone Structure. Explain to students the following:

• Bone contains collagen and minerals, which form the hard structure of bone.
• Bone cells make collagen and send it outside the cells, where the collagen molecules are linked into bundles; the cells then deposit minerals around the collagen.
• There are two forms of bone: compact bone and spongy bone.
• The outside of bone is compact bone, which makes the bone strong.
• The inside of bone is spongy bone. Spongy bone provides structure to the bone. It also allows bone to slightly compress when placed under stress and return to its original shape when the stress is removed.

The master shows a section of bone from the hip joint. The inset on the left is a microscopic view of spongy bone showing the collagen-mineral framework. The inset on the upper right shows individual collagen molecules that have been linked together to form a bundle. The calcium-containing mineral is deposited around the collagen molecules (the mineral is not shown in this inset).

You can make an analogy to reinforced concrete. Imagine that you have steel rods and cement that you will use to construct a wall or a bridge. Pouring cement around steel rods placed in a suitable frame produces a material (reinforced concrete) that is stronger and more capable of withstanding movement than either steel rods or cement alone. Bone has a similar organization. The steel rods are chains of collagen, which confer resiliency, and the cement is crystals of mineral containing calcium and phosphorus, which confer strength.

8. Ask students to predict which is stronger, a group of protein molecules that are not linked together and have mineral around them or a similar number of protein molecules that are linked together and have mineral around them.

This can be done quickly with a show of hands.

9. Explain that scientists often use models to gain information about biological systems and that students will use a model to study bone strength.

10. Once again, display the transparency of Master 2.1, Bone Structure. Hold up a bundle of pasta held together with rubber bands (see Figure 2.3). Explain that it represents bone.

11. Ask students to relate the pasta model of bone to the images on Master 2.1:

• What do the pieces of pasta represent?

The pasta represents the collagen and minerals in bone.

• What do the rubber bands represent?

The rubber bands represent the links that bundle the protein molecules together.

12. Divide the class into teams of three. Give each team

• 50 pieces angel-hair of pasta
• Rubber bands: 10 (if using tiny bands intended for braces) or 5 (if using small regular bands)
• A ruler
• Two large paper clips
• Two weights such as several large washers (about 200 grams each)

Note to teachers: This modeling activity is intended to be performed by each team of students. If you do not have the time or materials for this approach, then perform the modeling as a class demonstration. If you perform a demonstration, be sure to ask students what they think will happen before you perform each test. Make sure that students explain their reasoning. Also, some students may think that this activity is trivial and obvious. Make the point that it is not enough in science to believe we know the answer to a question. Rather, it is essential to investigate and obtain the evidence on which conclusions are based.

13. Explain that students are to compare the strength of bone with minerals and bundled collagen (pasta bundled with rubber bands) to bone with minerals and unbundled collagen (loose pasta strands).

Tip from the field test: You may want to explain that the pasta model being tested here is similar to models that engineers construct to assess the strength of different bridge and building designs.

14. Give each team one copy of Master 2.2, Testing the Pasta Model, and ask teams to perform strength tests on their models (bundled pasta and unbundled pasta).

Explain that the master provides instructions for carrying out the tests to determine the relative strengths of the two models. The instructions suggest pushing two desks close together to provide support for the pasta. Alternatively, students can use stacks of books as shown in Figure 2.4. Circulate among the teams and make sure they are performing the tests correctly. Give teams about 15 minutes to complete their tests.

15. After teams have completed their tests, ask for volunteers to report their findings.

Students will report that the bundled sample of pasta bent less than the unbundled pasta.

16. Ask, “What do the results of your tests say about the contributions of bundled collagen to the strength of bone?”

Student responses will vary. Students should understand that the presence of the rubber bands strengthened the pasta and caused it to bend less than unbundled pasta when put under stress. They also should be able to relate this to bone by concluding that linking the collagen together leads to stronger bones.

17. Show students the bone that was soaked in bleach. Explain that this bone was soaked in bleach to remove the bone’s collagen but not the mineral. Ask students to predict how removing the collagen will affect the strength of the bone.

Students likely will respond that the bone will not be as strong with the collagen removed.

18. Show the class the chicken bone that was soaked in water. Demonstrate (or have a student come up and demonstrate) that the bone is hard and not flexible by holding it at each end and trying to bend it.

Do not use so much force that the bone breaks. Explain that this bone has not had either collagen or mineral removed. It is the control being used to study the effect of removing collagen from bone.

Note to teachers: Remember to place a bundle of pasta in water. It should soak for about two minutes before you perform Step 23.

19. Demonstrate (or have a student come up and demonstrate) that the bleach-treated bone is brittle and cracks easily compared with the water-treated (control) bone.

20. Ask students if this result met their expectations.

Some students will feel that the demonstration shows that the bone has less strength and confirmed their expectation (especially if it breaks!). Bring out during the discussion the fact that this bone has less collagen than normal. Note that this demonstration complements the modeling that students have done. Both the model and the chicken bones show that low levels of collagen (the bleach-treated bone) or defective collagen (no bundling; the unbundled pasta) result in bone that is less strong than normal.

21. Ask students to predict how removing calcium will affect the strength of bone.

Students likely will respond that removing calcium will weaken bones.

22. Show students a bundle of pasta that has not been soaked in water. Ask students how they could use this pasta to model the removal of calcium from bone.

If necessary, remind students that the pasta strands represent calcium around collagen molecules. If no students suggest it, mention that you have soaked a bundle of pasta in water to model the removal of calcium from bone.

23. Place both the unsoaked and soaked bundles of pasta between desks as described on Master 2.2, Testing the Pasta Model. Begin placing weights on both bundles. Ask students to observe what happens.

The soaked pasta is not capable of supporting much weight compared with the unsoaked control pasta.

24. Show students a bone that was soaked in vinegar. Explain that this bone was soaked in vinegar to remove some of its calcium. Ask students to predict, based on the modeling demonstration just performed, what effect removing calcium will have on the bone’s strength.

Students will answer that the bone soaked in vinegar will not be as strong as the control bone soaked in water.

25. As before, demonstrate (or have a student come up and demonstrate) that the bone is more flexible and somewhat rubbery.

Tip from the field test: You can soak a wishbone in vinegar and it will become flexible enough to tie into a knot!

26. Ask students if this result met their expectations.

Some students will feel that the demonstration shows that the bone has less strength and confirmed their expectation. Other students may feel that because the bone bends and does not break, it did not meet their expectation. To these students, point out that the collagen is still present and accounts for the strength observed. If necessary, remind them again of the reinforced concrete analogy. The steel rods are still present but the concrete is partially removed.

27. Explain that during this lesson, students have been collecting information about bone in the same way that scientists do—by removing the object for study (bone) from the body and subjecting it to testing and observation.

It is important for students to realize that although studying bone outside the body is helpful, it cannot tell us everything we want to know about bone. For example, a different sort of model must be used to investigate how bone behaves when inside the body.

28. Ask students to think back to the skin system. Explain that the body is continually making new skin cells. Ask, “Why is this necessary?”

Student answers will vary. Bring out during the discussion the fact that skin cells are continually dying and that they must be replaced.

29. Explain that a similar situation exists within bones: old bone material is actively destroyed and is replaced by new bone material.

30. Write the word “homeostasis” on the board. Explain that scientists use this term to describe a process in the body that is maintained in a balance.

You may need to explain that balance means keeping the level of something constant, that is, replacing a body material that is lost with an equal amount of new material.

31. Ask students why is it important that replacing old skin with new, or old bone with new, be kept in balance.

Students’ responses will vary. Make sure that they realize that if more new material is made than old is removed, too much material (skin or bone) accumulates. Likewise, if more old material is removed than new material is made, a lack of material (skin or bone) results.

32. Display an overhead transparency of Master 2.3, Bone in Balance. Explain that

• bone contains cells called osteoblasts, which make collagen and deposit calcium around the collagen;
• bone contains other cells called osteoclasts, which remove collagen and calcium from bone; and
• the activities of osteoblasts and osteoclasts are balanced to maintain healthy bones.

Do not emphasize memorizing the names of these two bone cells. Rather, mention these cells and what they do to reinforce the notion that bone contains cells and is a living tissue.

33. Continue by explaining that scientists often use a bathtub model to represent processes in the body that are kept in balance. Indicate that the water in this diagram represents the calcium and collagen in bone. The faucet represents osteoblasts, which make collagen, move it outside the cells, and deposit minerals around the collagen. The drain represents the osteoclasts, which remove calcium and collagen from bone.

34. Give each student one copy of Master 2.3, Bone in Balance, which they will use for a brief homework assignment. Instruct students to write a short paragraph that explains how the bathtub represents bone as a living tissue. Challenge them to consider what changes in the level of “water” in the bathtub mean.

A major point of the bathtub model is that bone is dynamic (ever changing). The body maintains the calcium and collagen (the water) at a constant level in healthy bone. The level of calcium and collagen (the water) may be viewed in two ways in this model. In one, changes in the level, either up or down, can represent changes leading to poor bone health if the changes are the result of imbalance between osteoblast and osteoclast activities. Viewed a second way, changes in the level of calcium and collagen, either up or down, can be used to represent the different amounts of bone associated with humans of different ages. Viewed this way, the level may be higher or lower at different ages, but the level remains in balance because osteoblast and osteoclast activities are balanced. Students will not understand how and why these changes in bone content occur. The influence of diet and exercise on bone health will be investigated in Lesson 5, Helping the Body Build Strong Bones.