CELL DIGESTION: WHAT IT IS AND CLASSIFICATION - BIOLOGY - 2025

The cell digestion includes a series of processes by which a cell is capable of transforming food substances, which thanks to complex enzyme reactions. There are two basic categories to classify cellular digestion: intracellular and extracellular.

Intracellular digestion refers to the digestive phenomenon that occurs inside the cell as a result of phagocytosis and is typical in simple organisms. It occurs due to the expulsion of enzymes to the extracellular medium, followed by the absorption of the transported material. The latter occurs in more complex animals with complete digestive systems.

Source: pixabay.com

What is cellular digestion?

One of the crucial functions of heterotrophic organisms is to nourish themselves through the incorporation of macromolecules essential for growth and maintenance. The processes that allow the absorption of these molecules are collectively called cellular digestion.

In small, single-celled organisms such as amoebae and paramecia, the exchange of substances with the environment can be carried out simply by diffusion.

As complexity increases in the animal kingdom, the existence of structures strictly dedicated to the absorption of substances becomes necessary. In the world of multicellular, most foods cannot pass through the membrane due to their size.

For this reason, a prior disintegration must occur for absorption to occur, mediated by enzymes. The most complex animals have a whole set of organs and structures that orchestrate this process.

Classification

Digestion is classified into two main types: extracellular and intracellular. Between the two there is an intermediate category called contact digestion. Below we will describe the most relevant characteristics of the types of nutrition:

Intracellular digestion

This first type of nutrition is characteristic of protozoa, sea sponges (porifers), and other simple animals. Food particles can enter by two energy-requiring pathways: pinocytosis or phagocytosis.

In both processes, a portion of the plasma membrane is responsible for encapsulating the food particles, which enter the cell in the form of a vesicle - that is, covered with lipids.

Inside the cell there are organelles (or organelles) specialized in digestion called lysosomes. These vesicles contain a large amount of digestive enzymes inside them.

After the initial vesicle with the particles enters the cell, it begins to fuse with the lysosomes, which release the enzymatic battery contained within and promote the degradation of the compounds. This fusion of the lysosomes results in the formation of a secondary lysosome, also known as a phagolysosome.

It is worth mentioning that lysosomes not only digest material that has entered from the extracellular environment, they are also capable of digesting material that exists inside the same cell. These organelles are called an autolysosome.

Once the digestive process is complete, the waste is expelled to the outside by a mechanism of excretion of products called exocytosis.

Contact digestion

In the spectrum of digestive phenomena, contact digestion connects the extremes: the extracellular and the intracellular. This type is present in sea anemones and is considered a model of digestive transition.

When the animal consumes a large prey or particle, digestion occurs in the same gastrovascular cavity. The enzymes that are present in this space are negatively affected by the presence of seawater. To overcome this drawback, anemones have developed a contact system.

In this process, the filaments of the endothelial cells are found as a lining of this cavity, they are located close to the location of the particle to be digested, and once the particle enters the enzyme secretion for digestion begins.

As the particle comes into contact with the enzymes, a gradual disintegration begins and the cells themselves can absorb the newly formed product. However, when the particles to be digested are small, intracellular digestion can occur, as mentioned in the previous section.

Extracellular digestion

The last type of digestion is extracellular, typical of animals with complete digestive tracts. The process begins with the secretion of digestive enzymes into the digestive tract and the muscular movements contribute to the mixing of the food material with the enzymes.

As a result of this disintegration, the particles can pass through different pathways and be effectively absorbed.

Enzymes involved in extracellular digestion

The most prominent enzymes involved in extracellular digestion are the following:

Mouth

The degradation of food begins in the mouth, with the action of salivary amylase, responsible for splitting starch into simpler compounds.

Stomach

The particles that have already started an enzymatic degradation continue their way to the stomach, where they will find pepsin, responsible for the hydrolysis of proteins, and renin, whose substrate is the protein found in milk.

Pancreas

In the pancreas, digestive enzymes are trypsin, chymotrypsin, and carboxypeptidase, each responsible for the hydrolysis of specific peptides and proteins.

In addition, another version of amylase is present, which breaks down leftover starch.

Regarding the degradation of nucleic acids that are consumed in the diet, we have two enzymes, ribonucleases and deoxyribonucleases, which are responsible for the hydrolysis of RNA and DNA, respectively.

Small intestine

In the small intestine, the enzymatic composition is dominated by maltase, responsible for the breakdown of maltose, lactase for lactose and sucrase for sucrose.

For peptide breakdown, the small intestine relies on dipeptidases. In turn, for nucleic acids there are polynucleotidases and nucleosidases.

For a certain type of food, the enzymatic degradation of the nutrient must be assisted by the presence of microorganisms that inhabit the interior of the digestive tract, mainly in the colon, establishing symbiotic relationships with the host.

References

1. Arderiu, XF (1998). Clinical biochemistry and molecular pathology. Reverte.
2. Audesirk, T., Audesirk, G., & Byers, BE (2003). Biology: Life on Earth. Pearson education.
3. Freeman, S. (2016). Biological science. Pearson.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2007). Integrated Principles of Zoology. McGraw-Hill.
5. Hill, RW, Wyse, GA, Anderson, M., & Anderson, M. (2004). Animal physiology. Sinauer Associates.
6. Junqueira, LC, Carneiro, J., & Kelley, RO (2003). Basic histology: text & atlas. McGraw-Hill.
7. Kaiser, CA, Krieger, M., Lodish, H., & Berk, A. (2007). Molecular cell biology. WH Freeman.
8. Randall, D., Burggren, W., French, K., & Eckert, R. (2002). Eckert animal physiology. Macmillan.
9. Rastogi SC (2007). Essentials of Animal Physiology. New Age International Publishers.
10. Rodríguez, MH, & Gallego, AS (1999). Nutrition treaty. Díaz de Santos editions.
11. Ross, MH, & Pawlina, W. (2006). Histology. Lippincott Williams & Wilkins.