Calcium, Phosphate and Bone Chemistry

Your skeleton is a chemical composite: a mineral, a protein and water, arranged so cleverly that bone is stronger than concrete for its weight and far tougher than chalk, even though its mineral is chemically similar to chalk-like rocks. An adult body contains roughly 1 kilogram of calcium, and about 99% of it is in bones and teeth. This article explains what that mineral is, how it’s built and constantly rebuilt, and why the chemistry of acid and fluoride matters so much for your teeth.

What bone is made of

By mass, adult bone is roughly:

• 60–70% mineral: mainly a form of calcium phosphate;
• 20–30% organic matter: mostly the protein collagen;
• about 10% water.

The mineral: hydroxyapatite

The mineral in bone is a form of hydroxyapatite:

Ca₁₀(PO₄)₆(OH)₂

It’s an ionic solid (see ionic vs covalent bonds) made of calcium ions (Ca²⁺), phosphate ions (PO₄³⁻) and hydroxide ions (OH⁻). The charges balance: 10 × (+2) = +20, and 6 × (−3) + 2 × (−1) = −20.

Bone mineral isn’t pure hydroxyapatite. It’s a slightly impure, calcium-deficient form containing some carbonate ions (in place of some phosphate), plus traces of sodium, magnesium and fluoride. The crystals are tiny, platelets only a few nanometres thick, which gives them an enormous surface area for exchanging ions with the blood.

The protein: collagen

Collagen forms long fibres made of three protein chains wound into a triple helix (see proteins: chains of amino acids). The mineral crystals grow in and around the collagen fibres, in regular gaps along them.

Why the combination works

• Hydroxyapatite alone is hard and stiff but brittle, like chalk or porcelain.
• Collagen alone is flexible and tough but soft, like leather.
• Together, they form a composite: the mineral resists compression and bending, while the collagen fibres stop cracks spreading and let bone absorb impacts. It’s the same principle as reinforced concrete or fibreglass.

You can see this at home. Soak a chicken bone in vinegar for several days: the acid dissolves the mineral, leaving a rubbery bone you can tie in a knot. Bake a bone in a very hot oven instead, and the collagen burns away, leaving a brittle bone that crumbles.

Bone is alive: remodelling

Bone isn’t inert. It’s constantly broken down and rebuilt:

• Osteoclasts dissolve old bone. They seal themselves onto the bone surface and pump out acid (H⁺ ions) and enzymes. The acid dissolves hydroxyapatite, which is a basic salt, releasing calcium and phosphate; the enzymes digest the collagen.
• Osteoblasts build new bone. They lay down collagen, then help calcium phosphate crystallise onto it.

In adults, roughly 10% of the skeleton is replaced each year. Remodelling repairs tiny cracks, reshapes bones in response to the forces placed on them (which is why exercise strengthens bones and bed rest or spaceflight weakens them), and provides a store of calcium that can be drawn on when needed.

Controlling blood calcium

Blood calcium is kept within a very narrow range, about 2.2–2.6 mmol L⁻¹, because calcium ions are essential for nerve signals, muscle contraction (including the heartbeat) and blood clotting. The skeleton acts as a huge calcium reservoir, and three hormones control the flow:

• Parathyroid hormone (PTH): released when blood calcium falls. It increases calcium release from bone, reduces calcium loss in urine and increases the production of active vitamin D.
• Calcitriol (active vitamin D): increases calcium absorption from food in the intestine (see vitamin D chemistry).
• Calcitonin: released when blood calcium is high; it reduces bone breakdown (its role in adult humans is minor).

If dietary calcium or vitamin D is short, the body keeps blood calcium normal by taking it from bone, at the expense of the skeleton.

Teeth: enamel and acid

The outer layer of teeth, enamel, is the hardest substance in the body. It’s about 96% mineral, mostly hydroxyapatite in long, tightly packed crystals, with very little protein.

Why sugar causes tooth decay

1. Bacteria in dental plaque feed on sugars and ferment them into acids, mainly lactic acid.

2. The pH at the tooth surface drops. Below about pH 5.5, enamel starts to dissolve (demineralisation):

   Ca₁₀(PO₄)₆(OH)₂ + 8H⁺ → 10Ca²⁺ + 6HPO₄²⁻ + 2H₂O

   Acid reacts with the phosphate and hydroxide ions, pulling the equilibrium towards dissolving (see Le Chatelier’s principle).

3. Between meals, saliva neutralises the acid and supplies calcium and phosphate, so some mineral is redeposited (remineralisation).

4. Frequent sugary snacks or acidic drinks keep the pH low for long periods, tipping the balance towards decay.

Acidic drinks such as fizzy drinks and fruit juices can also dissolve enamel directly, a process called erosion (see acidic foods and teeth).

How fluoride helps

Fluoride ions (F⁻) can replace hydroxide ions in the crystal, forming fluorapatite:

Ca₁₀(PO₄)₆F₂

Fluorapatite is less soluble in acid than hydroxyapatite: it starts to dissolve only at a lower pH, around 4.5. Fluoride in toothpaste and drinking water also speeds up remineralisation and interferes with the bacteria’s acid production. Fluoride is one of the main reasons tooth decay has fallen so dramatically in many countries over the past half century.

Too much fluoride during childhood, while teeth are forming, can cause dental fluorosis: white or brown marks on the enamel. That’s why children are advised to use only a small smear or pea-sized amount of toothpaste.

Calcium in the diet

Adults typically need about 700–1,000 mg of calcium a day, with more recommended during adolescence, when bones are growing fastest. Peak bone mass is reached in the late twenties, so building strong bones early in life helps protect against fractures later. Dairy foods are the richest common source, but calcium also comes from fortified plant milks, tofu made with calcium sulfate, canned fish with soft bones, almonds and some leafy greens such as kale. Spinach contains plenty of calcium, but much of it is bound up as insoluble calcium oxalate, so little is absorbed. Absorption also depends on vitamin D, which is why the two nutrients are often discussed together.

Osteoporosis

In osteoporosis, bone breakdown outpaces rebuilding, and bones become porous and fragile, breaking easily. It’s especially common in older women, because falling oestrogen levels after menopause speed up bone loss.

Prevention and treatment focus on:

• enough calcium and vitamin D;
• weight-bearing exercise, which stimulates bone building;
• avoiding smoking and excessive alcohol;
• medicines such as bisphosphonates, which bind strongly to hydroxyapatite and slow down osteoclasts.

Key takeaways

• About 99% of the body’s calcium is in bones and teeth, as hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂.
• Bone is a composite of hard mineral and flexible collagen, giving strength and toughness.
• Osteoclasts dissolve bone with acid and osteoblasts rebuild it; about 10% of the skeleton is replaced each year.
• PTH, calcitriol and calcitonin keep blood calcium at about 2.2–2.6 mmol L⁻¹, using bone as a reservoir.
• Acid from bacteria dissolves enamel below about pH 5.5; fluoride forms more acid-resistant fluorapatite. For calcium’s properties, see the calcium element page.