Intracellular Digestion vs. Extracellular Digestion

Lo and behold! Welcome our holy grail – glucose! While glucose’s essence as the holy grail may be debatable, glucose undeniably serves as an essential energy source for our bodies. In order to utilize glucose as our energy, we must first digest food that contains glucose. There are two main types of digestion: intracellular and extracellular.

Our Human Digestive System: Wonderful, isn’t it? 🙂

Let’s first dissect the ‘anatomy’ of our word “intracellular.” The prefix intra means “on the inside” or “within,” and the suffix cellular literally refers to our cells. Intracellular digestion thus means digestion that occurs inside our cells. As for the word “extracellular,” with the prefix extra meaning “on the outside,” we can thus infer that extracellular digestion means digestion that occurs outside our cells.

Intracellular Digestion = Digestion that occurs inside our cells (literally in the cytoplasm)

Extracellular Digestion = Digestion that occurs outside our cells

While it may not seem intuitive, areas such as the opening of our mouth or the alimentary canal of our intestines are referred to as the “outside” of our body. Anything that is not inside the cell or anything outside the cell borders is known as the outside. Let’s apply this. Our sandwich entered the stomach and is simply churning around inside, bumping against the stomach’s rugae. Pepsin is breaking down the peptide bonds of proteins, but no nutrients have been absorbed by any cells yet. Which type of digestion is this? Extracellular!

1280px-stomach_with_prominent_parietal_cells_-_intermed_mag
Pretty Pink: A view of our stomach’s parietal cells

To elaborate more, extracellular digestion in our human digestive system occurs in the alimentary canal. The alimentary canal runs from the starting point of digestion, our mouths, to the end of the digestive system, our anuses. Releasing enzymes, such as pepsinogen and pepsin, to the outside of our cells also means extracellular digestion.

 
For digestion to be considered intracellular, the nutrients – glucose, fatty acids, proteins, etc – must be inside our cells, not the lumen or opening of our digestive system.

L0037817 Anatomy of the small intestine in ancient Chinese medicine
The alimentary canal of our intestines was also observed in this fascinating ancient Chinese scripture for medicine.

For example, if the chyme (partially digested food ball) was being emulsified by bile from our liver in the small intestine, this emulsification is occurring in the inside of the lumen. This is extracellular digestion. However, if the lipid molecules were being taken up by the lacteals inside the villi and later transported into a small intestine’s cell for oxidation, then yes, this is considered intracellular digestion. The act of oxidation of fatty acids inside the cytoplasm of a cell is where intracellular digestion starts.

 
And, just for kicks, single-celled organisms such as paramecium and amoeba undergo intracellular digestion to get their nutrients. This relates back to the intracellular digestion in us humans because in order for intracellular digestion to occur, it must be performed physically inside the individual cell.

 
Can you guess if the photograph below is demonstrating extracellular or intracellular digestion? Hint: This is fungi releasing its enzymes to digest a decaying leaf.*

White hyphae of fungi digesting a browning leaf: Yum for nutrients!

Sources:

  1. Intracellular and Extracellular Digestion. http://study.com/academy/lesson/intracellular-extracellular-digestion.html
  2. The Digestive System by Alexander Stone Macnow, MD. MCAT Kaplan Biology Review.

*Answer: Extracellular! This fungi (the white hyphae) produces extracellular enzymes to digest the leaf, which is outside of its body and outside of its cells.

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