Assimilation is the process by which absorbed nutrients are used by body cells to build new materials, release energy or be stored. It is the stage after absorption, when nutrients actually become part of the body or are put to work.
Uses of the main nutrients
- Glucose — oxidised in cell respiration to release energy (ATP); extra glucose is stored.
- Amino acids — used to build proteins for growth, repair, enzymes and hormones.
- Fatty acids and glycerol — used for energy or stored as fat under the skin and around organs.
Key idea
Absorption brings nutrients into the blood; assimilation is the use of those nutrients by cells. They are different stages.
The role of the liver
The liver is the main organ of assimilation:
- Regulates blood glucose — excess glucose is converted to glycogen and stored; it is changed back to glucose when needed.
- Deamination — excess amino acids cannot be stored, so their nitrogen part is removed to form urea, which is excreted by the kidneys.
- Stores iron and some vitamins (A, D and B12).
Building the body
Amino acids that reach the cells are joined together to make specific proteins. This allows children to grow and allows worn-out tissues to be repaired throughout life.
The liver keeps blood glucose steady between meals. Soon after eating, glucose is high, so the liver stores the excess as glycogen. Between meals, when glucose falls, glycogen is broken back down into glucose and released into the blood. This constant adjustment is an example of how assimilation supports homeostasis.
Example
After a high-protein meal, amino acids are assimilated by muscle cells to make new muscle proteins. Any surplus amino acids are deaminated in the liver, and the resulting urea is removed in urine.
Remember
Excess glucose → glycogen (stored). Excess amino acids → deaminated to urea (they cannot be stored).