Absorption moves digested food molecules from the gut into the blood or lymph. Assimilation is what cells do with them afterwards: they use the molecules to release energy or to build new substances. Exams test both terms together, often in the same question.
This lesson follows explaining absorption using villus structure and belongs to human nutrition and digestion.
What happens to each nutrient after absorption?
Absorption ends when the molecule is in the blood or lymph. Everything after that, in body cells, is assimilation.
| Absorbed molecule | Example of assimilation |
|---|---|
| Glucose | Used in respiration to release energy; stored as glycogen in the liver and muscles |
| Amino acids | Built into new proteins, such as enzymes and muscle protein |
| Fatty acids and glycerol | Used to build cell membranes or stored as fat; used in respiration |
Blood from the small intestine passes through the liver first. The liver adjusts what reaches the rest of the body, for example by taking up extra glucose and storing it as glycogen.
Worked example
The example is a model. Follow one amino acid from a protein meal to muscle.
Step 1, digestion: proteases in the stomach and small intestine break the protein in the meal into amino acids.
Step 2, absorption: an amino acid diffuses or is moved across the wall of a villus into a blood capillary. This is the end of absorption.
Step 3, transport: the blood carries it through the liver and on around the body.
Step 4, assimilation: a muscle cell takes the amino acid in and joins it to others in a new protein.
The word test is simple. If the question says “from the gut into the blood”, write absorption. If it says “used by the cell”, write assimilation.
The mistake to watch for
A common slip is to swap the definitions.
Mistaken answer: “Assimilation is when glucose moves from the small intestine into the blood.”
This describes absorption. Assimilation comes after, when the body uses the glucose.
The correction is to place the molecule on a map. Gut lumen, then wall, then blood = absorption. Blood, then cell and used = assimilation.
A second slip is to call egestion “excretion”. Egestion removes undigested food that never entered cells.
Check yourself
Try these without notes, then open each answer.
1. Is each statement absorption or assimilation? (a) Glucose enters a blood capillary in a villus. (b) A muscle cell joins amino acids into a protein.
Show answer
(a) Absorption. (b) Assimilation.
2. Give one way glucose may be assimilated.
Show answer
It may be used in respiration to release energy, or stored as glycogen in the liver or muscles. Either answer earns the idea.
3. Undigested plant fibre leaves the body as faeces. Is this absorption, assimilation or egestion, and why?
Show answer
It is egestion. The material was never absorbed into the body’s cells, so it is removed from the gut as faeces.
Where this leads next
Next, apply these ideas to numbers in interpreting a dietary dataset without personal diet prescriptions, or go straight to the practice set. The assimilation of glucose in respiration is developed further in gas exchange and respiration.
A teacher can trace one molecule with you in online one-to-one Biology tuition. The probability tree and counting board and inheritance model board are separate tools for modelling tasks in other topics.