Lesson 1 of 7 / Nutrition in humans
The journey through the digestive system
Where does food go, and what changes along the way?
I can locate the main digestive organs and distinguish ingestion, digestion, absorption, assimilation and egestion.
I can describe named digestive organs and ingestion, digestion, absorption, assimilation and egestion.Syllabus
Syllabus K224 / K225, Topic 4(a). Describe named digestive organs and ingestion, digestion, absorption, assimilation and egestion.
About 6 min
Make a guess. You are not marked.
The liver and pancreas both help digest your lunch. Does the food itself pass through them?
Show the answer
They send juices into the gut; food stays in the gut.
The pancreas and liver release secretions into the duodenum. Swallowed food never leaves the alimentary canal.
The key idea
Food travels along the alimentary canal. Digestive glands supply substances into it; small nutrients are absorbed and then used by body cells.
The alimentary canal is the continuous tube from mouth to anus. Accessory organs, such as the pancreas and liver, add secretions; swallowed food does not pass through them.
Explore the idea
Food route or supporting organ?
Mouth: Food is ingested. Teeth break it into smaller pieces and mix it with saliva.
Original route map, not an anatomical drawing. Solid route: food. Dashed routes: secretions. The liver also receives absorbed nutrients through blood, a route explored later.
Explanation
Ingestion means taking food into the mouth. Teeth break food into smaller pieces, which gives enzymes more surface to work on. Salivary glands release saliva. Saliva makes food wet and contains amylase, which starts to digest starch. When you swallow, food goes into the oesophagus, whose muscles push it to the stomach.
The stomach holds food for a while and churns it with gastric juice. The acid in gastric juice gives the right conditions for a stomach protease, and it kills many microorganisms in the food. Protein digestion starts here. The food then moves into the duodenum, the first part of the small intestine.
The pancreas sends digestive enzymes and an alkaline fluid into the duodenum. The liver makes bile, which is stored in the gallbladder and then released into the duodenum. These juices help digestion. Food itself never goes through the pancreas, liver or gallbladder.
Digestion makes small, soluble molecules that can be absorbed. The ileum, a later part of the small intestine, is covered in villi, which give a large surface for absorption. Digested food passes through the wall of the small intestine into the blood or lymph. The small intestine also absorbs most of the water.
The colon absorbs most of the water and mineral ions that are left, forming faeces. The rectum stores faeces until they leave through the anus. Getting rid of undigested food in this way is egestion.
After absorption, the blood carries nutrients to cells. Assimilation is when cells use the nutrients or build them into the body. For example, amino acids are used to build muscle proteins, and glucose is stored as glycogen in the liver. Digestion, absorption and assimilation are three different stages.
Step by step
- 1
Follow the canal
Mouth, oesophagus, stomach, small intestine, colon, rectum and anus form the route travelled by food.
- 2
Add the supporting organs
Salivary glands, pancreas, liver and gallbladder supply secretions. They are not stops along the food route.
- 3
Name the process precisely
Crossing the intestine wall is absorption. Building an absorbed nutrient into a body substance is assimilation.
Worked example
An amino acid has two different journeys
An amino acid crosses the small intestine wall into blood, then is used to make a muscle protein. Name each process and explain the difference.
One way to explain it
Crossing the intestinal wall into blood is absorption. Using the amino acid to synthesise muscle protein is assimilation, because it becomes part of a substance needed by the body.
Why this answer works
- Locate the boundary crossed during absorption.
- Identify the use of the absorbed molecule.
- Do not call protein synthesis digestion.
Watch out for this
A student says: "Food enters the liver and pancreas on its way through the digestive system." What is wrong with this?
Show the answer
These organs send secretions into the canal. The swallowed food itself remains in the canal until nutrients are absorbed or undigested material is egested.
Check your understanding
Glucose enters a liver cell and is converted into glycogen. Which process does this illustrate?
Show the answer
Assimilation, because an absorbed nutrient is made into a body substance.
The liver uses glucose to synthesise glycogen for storage.
Quick recall
Where does food go, and what changes along the way?