- A Iron, stored mainly in the bone marrow
- B Sodium, concentrated in the extracellular matrix
- C Calcium phosphate (hydroxyapatite)
- D Magnesium, bound loosely within collagen fibres
Correct answer: C. Calcium phosphate (hydroxyapatite)
Explanation: Bone hardness comes from calcium phosphate crystals (hydroxyapatite - Ca10(PO4)6(OH)2) deposited in a collagen protein matrix. Collagen provides flexibility; hydroxyapatite provides compressive strength. Bone is about 70% mineral and 30% organic matrix.
Concept context
Types of movement, muscle contraction, skeletal system, joints, and disorders. Important for Class 11 and NEET.