Difference Between Absorption and Assimilation

Absorption vs Assimilation

Heterotrophic animals obtain their energy by utilizing carbonic food synthesized by other autotrophic organisms. Human is also categorized as heterotroph. There are five steps in the process of heterotrophic nutrition. They are ingestion, digestion, absorption, assimilation, and ejection. Digestive system of humans (and other animals) is adapted to carry out these five processes. There are certain variations and specific adaptations along the alimentary canal to facilitate the different steps of the heteromorphic nutrition.


When food is travelling through the digestive system, they are broken into smaller molecules. This is done by mechanical and chemical digestion. Mechanical digestion is primarily taking place in the mouth, and the chemical digestion is taking place along the digestive system. Chemical digestion is carried out by the enzymes secreted by the system. For instance, carbohydrate breakdown starts at the mouth by the action of Ptyalin enzyme and the mechanical action of teeth. Likewise, proteins, lipids, and other macro molecules are digested into simple and small molecules by enzymes. Because of this breakdown, absorption is facilitated. So after digestion, the simpler molecules like monosaccharides, amino acids, fatty acids, etc. pass into the blood stream or lymph from the digestive system. This process is known as absorption. Without absorption, the ingested food won’t be available for use inside our bodies. Absorption is mainly taking place in our small intestine. Small intestine has special physiological features to increase the absorption and efficiency. It has villi and microvilli, so has a large surface area for the absorption. And also it is highly vascularized. Different molecules are absorbed by various mechanisms in our bodies. Monosaccharides (glucose, fructose, etc.), amino acids, water, ions, are directly absorbed from the intestinal cavity to the blood capillaries. But fatty acids, cholesterol are passed into the lymphatic vessels which are located nearby intestine, and later they are added into the blood circulation. Further, absorption can be done by active transport or passive transport.


The absorbed simple molecules are now in the blood stream, and they are distributed to all the cells in the body. Assimilation is converting these molecules and integrating them with the living tissues. In simple words, it is the process of synthesizing biological compounds / macro molecules from the absorbed simple molecules through constructive metabolism. It is because of this process the body can synthesize essential compounds (enzymes, hormones, nucleic acids, etc.) it needs to function properly. Assimilation helps cell growth and development as well as new cell production.

What is the difference between Absorption and Assimilation?

• Absorption is taking simple molecules, which were produced as a result of digestion into the body (blood stream/lymph) from the intestinal cavity. Assimilation is making new compounds from the absorbed molecules, which are necessary for normal cell functioning or to produce energy.

• Absorption is mainly taking place at the small intestine. Assimilation is taking place in the liver or any other cell.

• In absorption, nutrients are added to the blood stream or lymph, but in assimilation, nutrients are taken out from the blood stream/ lymph.