Human teeth: types, structure and care – Heterotrophic Nutrition (Senior 3 Biology) | Ganzaa

Heterotrophic Nutrition

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Senior 3 · Biology

Heterotrophic Nutrition

5 lessons

Lesson 2 of 5

Human teeth: types, structure and care

25 min

A roasted maize cob

Watch someone eat a roasted maize cob. The front teeth bite the grains off the cob, the pointed teeth at the corners help grip it, and the grains are then pushed back and crushed between the broad teeth at the back. Ingestion, the first step of holozoic nutrition, needs different tools for different jobs, and your teeth are exactly that. Chewing (mastication) also breaks food into small pieces with a large surface area, so digestive enzymes can work on it faster.

Definition

Dentition: the number, types and arrangement of teeth in an animal's mouth.

Dental formula: a short way of writing the number of each type of tooth in one half of the upper jaw (above the line) and one half of the lower jaw (below the line). Multiply the total by 2 to get all the teeth.

Enamel: the hard, non-living outer layer of the crown; the hardest substance in the body, made mainly of calcium phosphate.

Dentine: the living, bone-like layer under the enamel, with fine channels of cytoplasm running through it.

Pulp cavity: the soft centre of the tooth, containing blood vessels and nerves.

Four types of teeth

Figure 1: The four types of human teeth, seen from the side (drawn as lower teeth).

Type	Shape	Roots	Job	Number in an adult
Incisor	Chisel-shaped, sharp edge	1	Biting off and cutting food	8
Canine	Pointed	1, long	Gripping and tearing	4
Premolar	Two cusps (points)	1 or 2	Crushing and grinding	8
Molar	Broad, four or five cusps	2 or 3	Grinding	12

A young child has 20 milk teeth (deciduous teeth), with no premolars. From about age six they fall out one by one and are replaced by 32 permanent teeth, whose last molars, the wisdom teeth, may not appear until the late teens or twenties, if at all.

The adult human dental formula is

i\,\frac{2}{2}\;\; c\,\frac{1}{1}\;\; pm\,\frac{2}{2}\;\; m\,\frac{3}{3} = 2 \times (8 + 8) = 32

where $i$ = incisors, $c$ = canines, $p m$ = premolars and $m$ = molars.

Worked example 1: using a dental formula

(a) A child's milk teeth have the formula $i\,\frac{2}{2}\; c\,\frac{1}{1}\; m\,\frac{2}{2}$ . How many milk teeth are there? (b) A dog has $i\,\frac{3}{3}\; c\,\frac{1}{1}\; pm\,\frac{4}{4}\; m\,\frac{2}{3}$ . How many teeth does it have? (c) A goat has $i\,\frac{0}{3}\; c\,\frac{0}{1}\; pm\,\frac{3}{3}\; m\,\frac{3}{3}$ . What does the 0 tell you?

(a) Upper half: $2 + 1 + 2 = 5$ ; lower half: $5$ . Total $= 2 \times (5 + 5) = 20$ .
(b) Upper: $3 + 1 + 4 + 2 = 10$ ; lower: $3 + 1 + 4 + 3 = 11$ . Total $= 2 \times (10 + 11) = 42$ .
(c) The goat has no upper incisors or upper canines. It grips grass between its lower incisors and a hard pad of gum on the upper jaw, then tears it off by jerking its head.

Inside a tooth

Figure 2: A human molar cut from top to bottom. Enamel covers only the crown; cement covers the roots and, with fibres, holds the tooth in its socket in the jaw bone.

Enamel is hard but not alive: once it is damaged, the body cannot repair it. Dentine is softer. The pulp keeps the tooth alive, and its nerve is what you feel when a tooth aches.

How teeth decay

A thin, sticky film of bacteria, saliva and food called plaque builds up on teeth every day. When you eat sugar, the bacteria in plaque use it for respiration and release acid. When the pH at the tooth surface falls below about 5.5, the acid starts to dissolve enamel. Repeated attacks make a hole, a cavity. If the hole reaches the dentine it spreads faster, and when it reaches the pulp, bacteria infect it: the tooth aches and an abscess (a pocket of pus) may form. This is dental caries.

Plaque left along the gum line also causes gum disease. The gums become red, swollen and bleed when brushed (gingivitis). Left untreated, the fibres holding the tooth are destroyed and the tooth becomes loose. Hardened plaque, called tartar, can be removed only by a dental worker.

Teeth after chewing a plaque-disclosing tablet, a harmless dye that stains plaque purple. The purple patches show plaque left along the gum line and between the teeth, the places a toothbrush most often misses. Photo: Ajeverett, CC BY-SA 4.0, via Wikimedia Commons.

Worked example 2: how often, not how much

Figure 3: The pH at the surface of a tooth after a sugary snack (illustrative data).

(a) Use Figure 3 to estimate how long one sugary snack keeps the tooth surface below pH 5.5. (b) Keza has sugar with her three meals only. Mugisha has sugar with his three meals and also sucks four sweets between meals. Estimate each one's daily time of enamel attack, and say who is more at risk.

(a) The curve crosses pH 5.5 on the way down at about 10 minutes and on the way up at about 30 minutes: about $30 - 10 = 20$ minutes.
(b) Keza: $3 \times 20 = 60$ minutes a day. Mugisha: $(3 + 4) \times 20 = 140$ minutes a day.
Conclusion: Mugisha's enamel is under acid attack for more than twice as long, even if the total sugar is similar. How often you eat sugar matters as much as how much.

Caring for your teeth

Brush twice a day with a fluoride toothpaste, as the World Health Organization advises, for about two minutes, reaching the gum line and the backs of the teeth. Fluoride makes enamel more resistant to acid.
Clean between the teeth with dental floss, where a brush cannot reach.
Cut down sugary snacks and drinks, especially between meals; drink water instead of sweet drinks.
Eat food rich in calcium and vitamin D (Senior 1), which teeth need to grow strong.
Visit a dental clinic for a check-up, and at once if a tooth aches or the gums bleed.
Don't use teeth as tools, for example to open bottles: enamel can crack.

Warning

Common mistakes. (1) Saying sugar itself dissolves the teeth: bacteria in plaque turn the sugar into acid, and the acid dissolves the enamel. (2) Thinking a damaged tooth heals: enamel is not living and cannot repair itself. (3) Forgetting to double the dental formula: it shows only one side of the mouth.

What causes cavities?, Mel Rosenberg, TED-Ed. Watch how bacteria in plaque turn sugar into acid, and why saliva helps the enamel recover between meals.

Tip

Memory hook: I C P M = 2 1 2 3. In each quarter of an adult mouth, from the front: 2 incisors, 1 canine, 2 premolars, 3 molars: 8 teeth, and $4 \times 8 = 32$ .

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Practice: human teeth

Name the type of tooth used for (a) biting off a piece of mango, (b) grinding beans.
State two ways in which chewing helps digestion.
Name the layer of a tooth that (a) is the hardest substance in the body, (b) contains nerves and blood vessels.
A cat has the dental formula $i\,\frac{3}{3}\; c\,\frac{1}{1}\; pm\,\frac{3}{2}\; m\,\frac{1}{1}$ . How many teeth does it have?
Why does an adult have 12 more teeth than a young child?
Explain how eating sweets leads to a cavity.
A pupil's gums bleed when she brushes. Name the likely condition and give its cause.
Using Figure 3, explain why a school that bans sweet drinks at break time might see fewer cavities, even if pupils still have sugar with meals.

Show answersHide answers

(a) Incisors; (b) molars (and premolars).
It breaks food into small pieces with a larger surface area for enzymes, and mixes it with saliva so it is easy to swallow.
(a) Enamel; (b) the pulp (pulp cavity).
Upper: $3 + 1 + 3 + 1 = 8$ ; lower: $3 + 1 + 2 + 1 = 7$ ; total $= 2 \times (8 + 7) = 30$ .
The 20 milk teeth have no premolars and only 8 molars; the 32 permanent teeth add 8 premolars and 4 more molars: $32 - 20 = 12$ .
Bacteria in plaque respire the sugar and make acid; below about pH 5.5 the acid dissolves enamel; repeated attacks make a hole, which spreads into the dentine.
Gum disease (gingivitis), caused by plaque left along the gum line.
Each sugary drink causes a separate acid attack of about 20 minutes; removing the break-time drinks removes those attacks, so the enamel spends less time below pH 5.5.

Key points

Incisors cut, canines grip and tear, premolars and molars crush and grind. Adult formula: $i\,\frac{2}{2}\; c\,\frac{1}{1}\; pm\,\frac{2}{2}\; m\,\frac{3}{3} = 32$ .
A tooth: enamel over dentine over pulp; cement holds the root in the jaw bone.
Plaque bacteria turn sugar into acid, which dissolves enamel below about pH 5.5. Brush twice daily with fluoride toothpaste and cut down how often you eat sugar.

Sources and credits

Tooth care advice: World Health Organization, Oral health, fact sheet (accessed 2026-09-30).
Photo: Post plaque disclosing, Ajeverett, CC BY-SA 4.0, via Wikimedia Commons.
Video: What causes cavities? - Mel Rosenberg, TED-Ed, embedded from YouTube.

Written by Ganzaa: the text, Figures 1 to 3 (Figure 3 from illustrative data), the tables, the worked examples and the practice questions.

1A roasted maize cob

2Four types of teeth

3Inside a tooth

4How teeth decay

5Caring for your teeth

A roasted maize cob

Four types of teeth

Inside a tooth

How teeth decay

Caring for your teeth