9. Summary of the digestive system

The parts and function of the parts of the alimentary canal and the role of gut bacteria

Some important definitions

Absorption is the movement of small food molecules and ions through the wall of the intestine into the blood.

Assimilation is the movement of digested food molecules into the cells of the body where they are used and become part of the cells and tissue.

Chemical digestion is the breakdown of large, insoluble molecules into small, soluble molecules.

Diarrhoea is the 'urgent' loss of watery faeces.

Egestion is the passing out of food that has not been digested or absorbed, as faeces, through the anus

Ingestion is the taking of substances, e.g. food and drink, into the body through the mouth.

Mechanical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules.

Peristalsis is the involuntary constriction and relaxation of the muscles of the intestine or oesophagus canals, creating a wave-like movements that pushes the contents forward (i.e. undigested or digested food).

• You need to know about the structure and functions of the  tissues and organs of the human digestive system, including adaptations to function and how the digestive system digests food using enzymes as biological catalysts.

• Through the digestive organs food and liquids travel as they are swallowed, digested, absorbed, and leave the body as faeces.

• You should also know the importance of bacteria in the human digestive system

• These organs include the mouth, pharynx (throat), oesophagus, stomach, small intestine, large intestine, rectum, and anus.

• The alimentary tract is part of the digestive system and is also called digestive tract and gastrointestinal tract.

• Know that organs are made of different tissues acting together to perform some particular function.

  • Know that one organ may contain several tissues.

• Know that the stomach is an organ that contains:

  • muscular tissue, to churn the contents and break up the food into smaller chunks to aid digestion,

  • glandular tissue, to produce digestive juices containing enzymes to break food down at the molecular level,

  • epithelial tissue, to cover the outside and the inside of the stomach.

• Know that organ systems are groups of organs that work together to perform a particular function.

  • Know that the digestive system is one example of a system in which humans and other mammals exchange substances with the environment.

  • The digestion process requires a variety of enzymes to breakdown food into soluble products we can absorb from the digestive system, and these enzymes are produced by specialised cells in the glands and gut system.

  • Large insoluble molecules like proteins, starch like carbohydrates and oils/fats cannot pass through the membranes of the cell walls of the gut system.

  • However, smaller soluble molecules like amino acids, sugars and fatty acids can pass through the walls of the digestive system.

  • Some examples of the enzymes responsible for the breakdown of large insoluble molecules into small soluble absorbable molecules are ...

    • protease enzymes like pepsin convert proteins to amino acids,

    • carbohydrase enzymes like amylase convert carbohydrates like starch to sugars such as glucose,

    • lipase enzymes convert oils/fats to fatty acids and glycerol,

    • and these smaller molecules can now pass from the small intestine into the blood capillaries.

 

There are 8 sections to the function of the alimentary canal in the human digestive system

You need to be able to identify the main regions of the alimentary canal and associated organs, including mouth, salivary glands, oesophagus, stomach, small intestine (duodenum and ileum), pancreas, liver, gall bladder and large intestine (colon, rectum and anus).

Be able to describe the functions of the regions of the alimentary canal listed above, in relation to ingestion, digestion, absorption, assimilation and egestion of food

Know that the digestive system includes, and their functions:

(1) After ingestion, in the mouth, salivary glands produce the enzyme amylase which can break down carbohydrates like starch into maltose. Further down in the intestine this continues and the enzyme maltase breaks down the maltose into glucose. The saliva also moistens the food and, together with the chewing action (mastication) of the mouth muscles, balls of food are formed that are easily swallowed. The chewing action of the teeth mashes the food and increases the surface area the enzymes can act on - mechanical digestion.

 

(2) The oesophagus (gullet) is a tube that connects the mouth to the stomach and its lined with muscles that help move the balls of food along (this action is an example of peristalsis).

 

(3) In the stomach the food is churned and broken up into smaller chunks by the muscles of the stomach wall.

The protease enzyme pepsin is secreted from the pancreas and breaks down proteins to amino acids.

At the same time hydrochloric acid is produced in the gastric juice:

(i) killing most of the bacteria present by denaturing their enzymes.

(ii) creates the right acidic pH conditions (~pH 2) for the protease enzyme pepsin, which works best in these acid conditions to break down proteins into amino acids.

This is the start of the chemical digestion process of protein.

Muscles in the stomach wall compact the food as well as churning and mixing everything up.

Other muscles in the digestive system squeeze the food along by the process of peristalsis.

The contraction and relaxation of the tissue lining muscles in the wall of the digestive system produce a wave-like movement that pushes the balls of food along the digestive system, a process known as peristalsis.

 

(4) The liver produces alkaline bile, which neutralises excess stomach acid in the mixture of churned up food and gastric juices entering the duodenum of the small intestine.

Apart from the likes of pepsin, most enzymes can't work in very acid conditions of the stomach, and also bile helps to emulsify oils/fats which speeds up their breakdown by lipase enzymes into fatty acids and glycerol.

The emulsification is essential for the efficient faster digestion of oils/fats, the oils/fats are more dispersed giving greater surface area (greater surface area - think of the oil/fat as broken down into smaller droplets/particles).

A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine.

 

(5) The gall bladder stores bile before its released into the small intestine to help with digestion.

 

(6) The pancreas gland tissue produces digestive juices containing the enzymes

(i) protease enzyme pepsin breaks down proteins to amino acids,

(ii) amylase enzyme breaks down starches to sugars.

(iii) lipase enzymes breaks down oils/fats, which are released into the small intestine.

 

(7) The small intestine is where most of the digestion process continues with the release of the enzymes from the pancreas.

The small intestine consists of two sections:

The duodenum is fed into from the stomach.

The ileum is the last and longest section of the small intestine where the walls begin to thin and most nutrients and water are absorbed here and the waste is passed on to the large intestine.

The small intestine is the longest segment of the gastrointestinal tract where most food is broken down into a fluid containing soluble small molecules e.g.

The enzyme amylase breaks down carbohydrates like starch into maltose (soluble) and the enzyme maltase breaks down the maltose into glucose (soluble) for respiration. This reaction happens on the epithelial lining of the small intestine.

The enzyme protease breaks down protein (polypeptides) into the much smaller soluble amino acids.

Lipase enzymes break down lipids (saturated animal fats or unsaturated vegetable oils) into fatty acids and glycerol.

The small intestine is where the absorption of soluble food into the blood stream occurs from the digestive system e.g. smaller molecules like amino acids, sugars, fatty acids and ions - all the products of enzyme breakdown of larger food molecules.

These smaller soluble molecules can then be absorbed by diffusion through the gut wall into the bloodstream and transported to wherever they are needed in the body i.e the cells and tissues of the organ systems.

Note that large insoluble molecules cannot be absorbed into the body.

Water is absorbed mostly in the small intestine and much less so in the colon

For more details on the structure and function of the small intestine see The small intestine and villi

 

(8) In the large intestine excess water is absorbed from the undigested food, producing faeces which are initially stored in the rectum before release through the anus - the process of egestion.

The colon is the longest part of the large intestine.

The colon receives almost completely digested food from the cecum, absorbs water and nutrients, and passes waste (stool or faeces) to the rectum which is passed out through the anus.

The large intestine technically consists of three sections - the colon, rectum and anus.

 

(9) The role of gut bacteria and fungi (known as the microbiome)

The microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gut is the main location of the microbiome in human beings.

Bacteria are unicellular organisms and there are over 100 million bacterial cells in our alimentary canal.

Most of these are in the lower end of the small intestine and in the large intestine.

These bacteria are naturally found in your body and are not usually harmful to you.

These gut bacteria ...

(i) produce enzymes that help digest food,

(ii) synthesise important vitamins such as vitamin K,

(iii) produce important useful hormones,

(iv) reduce the possibility of harmful bacteria growing in your intestine and causing illness.

 

You should be able to recognise the organs of the digestive system on a diagram and know their function where described.

e.g. salivary glands, stomach, gall bladder, liver, large intestine, pancreas, small intestine, rectum

See also Surfaces for the exchange of substances in animal organisms (includes small intestine)

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key ideas on how our digestion system works

Overview: Why Study the Digestive System?

The digestive system is essential for breaking down food into smaller, absorbable molecules that supply the body with energy, growth materials, and cellular repair.

Understanding how this system works helps explain key biological processes such as enzyme activity, homeostasis, absorption, and transport—all common exam themes.

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The Alimentary Canal – Key Structures and Their Functions

The alimentary canal is a continuous tube that runs from the mouth to the anus. Here's what happens along the way:

1. Mouth

• Mechanical digestion: Teeth break food into smaller pieces; tongue helps mix it with saliva.

• Chemical digestion: Salivary glands release amylase, which begins digestion of starch into maltose.

• Importance: Increases surface area of food for enzyme action.

2. Oesophagus

• Connects mouth to stomach via peristalsis (wave-like muscular contractions).

• No digestion occurs here.

• Importance: Ensures unidirectional movement of food.

3. Stomach

• Mechanical digestion continues via muscular churning.

• Chemical digestion:

  • Glands release protease (pepsin) to start protein breakdown into peptides.

  • Hydrochloric acid (HCl) kills bacteria and creates the low pH (≈2) needed for pepsin activity.

• Importance: Prepares proteins for further breakdown.

4. Small Intestine (divided into duodenum and ileum)

a. Duodenum

• Receives bile (from the liver/gallbladder) and enzymes (from the pancreas).

• Bile:

  • Neutralises stomach acid.

  • Emulsifies lipids into droplets (increasing surface area for lipase).

• Pancreatic enzymes:

  • Amylase → starch to maltose.

  • Protease (trypsin) → proteins to peptides.

  • Lipase → lipids to fatty acids + glycerol.

b. Ileum

• Main site of absorption.

• Inner wall lined with villi, each covered with microvilli:

  • Large surface area for efficient absorption.

  • Rich blood capillary network absorbs glucose and amino acids.

  • Lacteals (part of lymphatic system) absorb fatty acids and glycerol.

• Importance: Maximises nutrient uptake into the bloodstream.

5. Large Intestine (Colon)

• Absorbs water and some vitamins (e.g. vitamin K).

• Bacteria here help digest fibre and produce useful compounds.

• Formation of faeces (undigested food + bacteria).

6. Rectum and Anus

• Stores faeces before egestion.

• Egestion: Removal of undigested waste (not to be confused with excretion).

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Digestive Enzymes Summary Table

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Why Is This System So Important in Biology?

• Demonstrates enzyme specificity and optimal conditions (e.g. temperature, pH).

• Links to homeostasis (e.g. pH balance, nutrient availability).

• Highlights the relationship between structure and function (e.g. villi for absorption).

• Supports understanding of malnutrition, enzyme deficiencies, and conditions like lactose intolerance or coeliac disease.

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Summary of learning objectives and key words or phrases

Be able to describe, with the aid of a diagram of the alimentary canal,

how human digestive system functions, including, the small intestine, importance of gut bacteria and fungi (the microbiome),

describe the function of the oesophagus, gall bladder, liver, the absorption of digested food,

know the difference between egestion and ingestion,

know how digested food is assimilation into the blood stream. know the action of peristalsis in the alimentary canal.

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