The Human Digestive System: Structure and Function

• Incisors: The four front teeth in each jaw, used for biting and cutting.
• Canines: Pointed teeth located next to the incisors, used for tearing and holding food.
• Premolars: Located behind the canines, these are used for crushing and grinding.
• Molars: The largest teeth at the back of the mouth, used for heavy grinding and crushing.

Exam Tip: If a question asks about the function of "pointed" teeth, they are almost certainly referring to the canines, which are specialized for tearing.

The Structure of a Tooth

A tooth is not a single uniform block of material but is composed of several distinct layers:

• Enamel: The hard, white, protective outer layer.
• Dentine: The layer situated directly beneath the enamel.
• Pulp Cavity: The central region that contains the tooth's nerves and blood vessels.
• Cementum: The specialized layer that covers the root of the tooth.

Chemical Digestion: Salivary Glands and Saliva

To assist the mechanical work of the teeth, the mouth utilizes chemical digestion via saliva. This liquid is produced by three pairs of salivary glands:

1. Parotid glands: Located in front of the ears.
2. Submaxillary glands: Located under the lower jaw.
3. Sublingual glands: Located under the tongue.

Saliva serves two vital roles: it moistens food to make swallowing easier, and it introduces digestive enzymes—most notably amylase—which begin the chemical breakdown of starches into simpler sugars like maltose.

Digestion in the Small and Large Intestine

While the mouth starts the process, the most intensive work happens in the small intestine. This organ is supported by accessory organs like the liver (which produces bile), the gallbladder (which stores bile), and the pancreas (which produces pancreatic juice).

The small intestine features millions of tiny, finger-like projections called villi. These structures vastly increase the surface area of the intestinal wall, allowing for the maximum absorption of digested nutrients into the bloodstream.

Following the absorption of nutrients in the small intestine, the remaining undigested material enters the large intestine. Here, the body absorbs water and salts, transforming the liquid residue into solid waste, known as faeces, which is eventually passed through the rectum and expelled via the anus.

Assimilation: Putting Nutrients to Use

Once nutrients are absorbed through the intestinal walls, they undergo assimilation. This is the process where the absorbed molecules are distributed to the body's cells to be used for specific biological functions:

• Glucose: Transported to cells to undergo respiration and produce energy.
• Amino Acids: Used as building blocks to construct new proteins for growth and tissue repair.
• Fats: Utilized for energy storage and providing insulation for the body.

Summary Table of Chemical Breakdown

To understand the chemistry of digestion, remember the relationship between the substrate and the enzyme:

$\text{Starch} \xrightarrow{\text{Amylase}} \text{Maltose}$ $\text{Protein} \xrightarrow{\text{Trypsin}} \text{Peptides}$ $\text{Fats} \xrightarrow{\text{Lipase}} \text{Fatty acids} + \text{Glycerol}$