Bone Composition

Bone is a kind of specialized connective tissue providing a supportive framework for the body. Bones are continually being remodeled during the life of the body so they can be used to determine the age of a body.

About 70% of bone is made up of calcium hydroxyapatite, 3Ca₃(PO₄)₂.Ca(OH)₂.
The crystals of calcium hydroxyapatite in bone are very small, about 5nm x 5 nm x 40nm.
The total surface area of calcium hydroxyapatite in bone is extremely large, about 40 hectares in the skeleton of a 70kg man.
About 10% of bone is made up of water, and the other 20% is made up of organic compounds such as collagen. The term collagen refers to a group of proteins, and is the most abundant protein found in mammals, about 30% of all the protein in the body is collagen.

The bone of very young people, less than 4 years of age, is called woven bone because the collagen fibres are interwoven and run in all directions just like a piece of felt, and the bone contains large holes like Swiss cheese.

The bones of bones of adults include lamellar bone which has collagen fibres organized into layers or sheets known as lamellae. These are usually built up around blood vessels like the layers of an onion. This produces compact bone.
Adult bones also include trabecular (cancellous, spongy) bone. The bone itself is in the form of little beams (trabeculae) which are about 200μm thick. The spaces in the bone can contain marrow.

Osteoporosis refers to a decrease in the amount of bone tissue which results in an increased tendency for the bone to fracture. The shape and size of the bone doesn't change, but the little beams, the trabeculae, become thinner and some may disappear altogether.
Astronauts in space can also display a decrease in the amount of bone tissue because there is less load, and therefore less stress, on their bones in space.

Aerobic and resistance training increase the density. Bone is deposited when there is load, and stress, applied to the bone.

Scientists at the Rensselaer Polytechnic Institute have developed a new technique to analyze the bone matrix. This technique provides information about the concentration of different proteins in the bones, which can be used to determine how the bone was formed, how it has been modified over time, and, if the bone is prone to fracture.

Reference
G. E. Sroga, L. Karim, W. Colon, D. Vashishth. Biochemical characterization of major bone-matrix proteins using nanoscale-size bone samples and proteomics methodology. Molecular & Cellular Proteomics, 2011; DOI: 10.1074/mcp.M110.006718

Further Reading
Molecular Mass (Formula Weight)
Percentage Composition
Unit Conversions
Density
Mass-Mole Calculations

Study Questions

1. Calculate the molecular mass (formula weight) of calcium hydroxyapatite.
2. Calculate the percentage composition of calcium hydroxyapatite.
3. 1 hectare is 1,000m², what is the surface area in m² of calcium hydroxyapatite crystals in a 70kg man?
4. If you laid out all the calcium hydroxyapatite crystals in the bones of the 70kg man above onto a football field, how many football fields would you need? (Assume a football field is about 6,000m²)
5. The little beams in trabecular bone are 200μm thick. Convert this to a thickness in
   
   • meters
   • millimeters
   • nanometers
   • angstroms
6. The density of a bone in the leg is about 2g/cm³. 70% of bone is made up of calcium hydroxyapatite. Calculate:
   • the mass of calcium hydroxyapatite present in a 500cm³ bone sample
   • the moles of calcium hydroxyapatite present in the same bone sample
   • the percentage of calcium present in the same bone sample
   • the mass of calcium present in the same bone sample
   • the mass of water present in the same bone sample
   • the mass of organic compounds present in the same bone sample
     
7. Which type of bone, woven bone or lamellar bone, has the greatest density? Explain your answer.
8. Will the bones of a person suffering from osteoporosis be more or less dense than the bones of an athletic young adult? Explain your answer.