The human digestive system

Breaks down large, insoluble food molecules into smaller, soluble ones that can be absorbed into the bloodstream.

Made up of several organs working together for digestion and absorption.

The digestive process

Mouth:

• Food is physically broken down by teeth.

• Saliva contains amylase (carbohydrase) to start breaking down starch.

Oesophagus (Gullet):

• Transports food to the stomach.

Stomach:

• Produces hydrochloric acid to kill pathogens and create the optimum pH for protease enzymes.

• Protease enzymes break down proteins into amino acids.

Small Intestine (made up of the duodenum and ileum):

• Digestive enzymes (amylase, protease, lipase) complete the breakdown of food.

• Bile (from the liver) neutralises stomach acid and emulsifies fats for easier breakdown by lipase.

• Digested nutrients are absorbed into the bloodstream.

Large Intestine:

• Absorbs excess water.

Rectum and Anus:

• Undigested material is expelled as faeces.

Organs in the human digestive system

Enzymes in the digestive system

Carbohydrases:

• Break down carbohydrates into simple sugars.

• Example: Amylase (produced in the salivary glands, pancreas, and small intestine) breaks down starch into maltose.

Proteases:

• Break down proteins into amino acids.

• Produced in the stomach, pancreas, and small intestine.

Lipases:

• Break down lipids (fats) into glycerol and fatty acids.

• Produced in the pancreas and small intestine.

Bile:

• Produced in the liver, stored in the gall bladder, and released into the small intestine.

• Neutralises stomach acid and emulsifies fats to increase the surface area for lipase activity.

Enzyme mode of action

Enzymes are biological catalysts that speed up chemical reactions without being used up.

They work on specific molecules (substrates) using a "lock and key" model:

• The enzyme’s active site has a specific shape.

• Only one substrate fits into the active site.

Optimum Conditions:

• Temperature:

  • Low temperature slows reactions (enzymes and substrates move slower).

  • High temperature denatures enzymes (active site changes shape, stopping function).

• pH:

  • Too high or too low pH denatures enzymes, stopping them from working.

The lock and key model of enzyme action

Additional maths skills