When we eat, food passes through many organs before it is effectively digested and transformed into nutrients for our body.
This is because we humans have the complete digestive system. However, there are animals that do not have all the structures of this complex, they are beings that have incomplete digestive system.
In the midst of this, there are still those who do not have any organ responsible for digestion and therefore performs it in the breakdown of food through cells.
These differences in the digestive process, makes biology divide the types of digestion in two: intracellular and extracellular.
Explaining how each works and citing living beings that perform them will be our task today.
Intracellular digestion: what is it, how does it occur and in which animals is it performed?
In the case of intracellular digestion, the food is absolved by some cavity of a certain animal and within the body there are some processes that are called phagocytosis and pinocytosis.
When cells encompass solid food, there are enzymes present in these particles that break down food and digest it, this is called phagocytosis.
Now, when cells ingest liquids or elements of smaller sizes, we say that pinocytosis has occurred.
Both intracellular digestive processes occur exclusively in protozoan and poriferous phylum organisms, as examples we can mention amoebae and sea sponges, respectively.
The latter, fixed in the sea, manages to acquire food through the water that travels and enters its pores carrying particles that, when they are encompassed by the cells, became nutrients.
Therefore, it is possible to affirm that in these beings the digestion does not occur inside a digestive cavity, but directly in the cells.
Extracellular digestion: what is it, how does it occur and in which animals is it performed?
Both animals that have a complete digestive system and those that have an incomplete digestion perform extracellular digestion. This is due to the passage that the food makes until it is digested.
Unlike what occurs in intracellular digestion, in extracellular everything that is ingested by beings is fragmented within organs, so cells do not play this role directly.
This digestive process emerged as living beings evolved and became more complex. The first animals to present this new system were the coelenterates (from the Greek koilos = hollow + enteron = intestine).
However, these beings do not have the complete digestive system, as they lack anus. In this way, with just one opening, the mouth, they receive and eliminate waste.
Examples of celenterates are freshwater hydras, jellyfish and marine corals.
An example of a living being that presents the complete digestive system, we are human beings. We have mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus.
All of these organs have a specific function, thus ensuring a proper functioning of the digestive system.
Human digestive system
Starting with the mouth, where there are teeth and tongue, the food is already being digested. In this first region, it is crushed and a food cake is formed, which is carried by the tongue to the pharynx.
The pharynx serves as a channel for the bolus to reach the esophagus. In this part, peristalsis begins, which are the wave movements produced with the intention of taking food to the stomach.
The bolus when it reaches the stomach receives the action of several agents present in that organ. Initially, it is mixed with gastric juice, this element is composed of hydrochloric acid and enzymes, such as pepsin and renin.
All of these items work with a single objective, to perform the chemification of the bolus, which consists of softening the food, which in this sphere is called chyme.
Once the process is finished in the stomach, the chyme is sent, again through peristaltic movements, to the duodenum, where the liver and pancreas release their substances.
The first organ launches bile, responsible for emulsifying fats. The second produces pancreatic juice, capable of neutralizing the acidity of the chyme. In addition, glands located in the intestinal mucosa provide a secretion that has digestive enzymes.
With all these processes, the chyme turns into a kilo, or in other words, the food cake turned into water and minerals.
After being transformed, they go through the absorption and distribution process through the small intestine, leaving only a part of the water and residues that are already considered feces and therefore sent to the large intestine and eliminated through the anus.
Extracellular and intracellular digestion
The cnidarians and in most of the flatworms a particular case occurs, that is to say the two processes. This is because the food ingested by these beings is made, initially in the light of the tube, thus being extracellular.
However, then, there is the inclusion of micromolecules for the end of digestion, causing intracellular digestion. As an example we can mention the planaria.