Difference Between Paracellular and Transcellular Diffusion

Key Difference – Paracellular vs Transcellular Diffusion
 

Transportation of substances within a living system takes place according to different pathways. In context of diffusion, it is defined as the tendency of molecules to move out with the intention of achieving available space. In other terms, diffusion is the movement of molecules from a place of higher concentration to a place of lower concentration. Diffusion could be active or passive. In passive diffusion, no energy is utilized on movement of substances, whilst in active diffusion, energy is utilized. During the transportation of molecules across the epithelial layer of gut, two types of diffusion mechanisms are described. They are paracellular diffusion and transcellular diffusion. Paracellular diffusion occurs across the epithelium where substances are passed through the intercellular space between the cells whilst in transcellular diffusion, the transportation of substances occurs through both apical and basolateral membrane. This is the key difference between paracellular and transcellular diffusion.

CONTENTS

1. Overview and Key Difference
2. What is Paracellular Diffusion
3. What is Transcellular Diffusion
4. Similarities Between Paracellular and Transcellular Diffusion
5. Side by Side Comparison – Paracellular vs Transcellular Diffusion in Tabular Form
6. Summary

What is Paracellular Diffusion?

Paracellular transport is the movement of substances across the epithelium through the intercellular space between the cells. This movement is a type of passive diffusion. Therefore, there is no requirement of energy as the diffusion occurs through a concentration gradient. In paracellular diffusion, the absorption rate is matched to load. This occurs as a result of the mechanism of diffusion does not contain any transporters that have the ability to be saturated. Paracellular diffusion has a minor role in the absorption of nutrients in the intestine.

Figure 01: Paracellular Diffusion

Nevertheless, it has been found out that, the functioning of the paracellular diffusion pathways become more prominent when high amounts of nutrients are available in the intestinal lumen. In the context of absorption of drugs in the gastrointestinal tract, paracellular pathway plays a major role. With regards to hydrophilic pharmaceuticals, the paracellular pathway is an important aspect since it facilitates the movement of these compounds across the lipid membrane. Also, pharmaceutical compounds that have no affinity towards transporters in the cell membrane are transported through paracellular diffusion. Paracellular transportation does not take place across the blood-brain barrier. Therefore, the capillaries of the blood-brain barrier only contain transcellular pathway.

What is Transcellular Diffusion?

Transcellular diffusion is the transport of molecules through  both apical and basolateral membrane. In other terms, transcellular diffusion is the process of movement of solutes by a cell through a cell itself. The most common example that could be provided for transcellular diffusion is the transportation of glucose. This occurs from the lumen of the intestine into the extracellular fluids by the epithelial cells. Transcellular movement is a type of active diffusion. Therefore, this type of diffusion requires energy.

The epithelial cells utilize active transports of primary and secondary where they regularly work in parallel with passive diffusion through ion channels. This produces the transcellular transporting system across the epithelial tissues. The transcellular pathway is considered as an important aspect in the context of absorption of nutrients in the gastrointestinal tract.  The whole transcellular pathway that occurs within a living system comprises of three subsets that include transcellular diffusion, transcytosis and active carrier-mediated transportation. Transcellular diffusion occurs through a concentration gradient that facilitates the transportation of solutes from a region of high concentration to a region of low concentration.

What are the Similarities Between Paracellular and Transcellular Diffusion?

• Both Paracellular and Transcellular Diffusion occur in most of the capillaries of the body.
• Both Paracellular and Transcellular Diffusion are types of diffusion that occurs from a region of higher concentration to a region of lower concentration.

What is the Difference Between Paracellular and Transcellular Diffusion?

Summary – Paracellular vs Transcellular Diffusion

Diffusion is the movement of molecules from a place of higher concentration to a place of lower concentration. Paracellular transport is the movement of substances across the epithelium through the intercellular spaces between the cells. Paracellular movement is a type of passive diffusion. Transcellular diffusion is the transport of molecules through  both apical and basolateral membrane. Transcellular movement is a type of active diffusion. Therefore, this type of diffusion requires energy. This is the difference between paracellular and transcellular diffusion.

Reference:

1.Peña, José Carlos, and Richard L. Malvin. “Transcellular Diffusion of Non-Electrolytes across the Renal Tubular Epithelium.” The Journal of General Physiology, The Rockefeller University Press, 1 Mar. 1962. Available here
2.Tang, Vivian W., and Daniel A. Goodenough. “Paracellular Ion Channel at the Tight Junction.” Biophysical Journal, vol. 84, no. 3, 2003, pp. 1660–1673., doi:10.1016/s0006-3495(03)74975-3

Image Courtesy:

1.’Protein digestion’By Pdeitiker – Own work, (Public Domain) via Commons Wikimedia

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