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Name: Jim C.
Status: Educator
Age: 40s
Location: N/A
Country: N/A
Date: February 2003


Question:
I have come across a question posed by my students that gives me pause to answer.... in light of our textbook claims I always thought that the process of ABSORPTION in the small intestine was a passive process (that is a diffusional process) for glucose, amino acids, a fatty acids and glycerol,and nucleotides . Our textbook seems to suggest that the process is an ACTIVE TRANSPORT phenomena........ this shakes the foundations of my University experience! Q .....Is the process of human digestional ABSORPTION a passive or active process?



Replies:
I am not a biologist, but I would think both take place. And there is a semantic issue too. Even apparently diffusion controlled processes may be "active transport" if one looks at the molecular level. In the case of small molecules things get even trickier. Example: Is the transport of ethanol active or passive? It is known that ethanol absorption begins at the lips and rapidly distributes to all water bearing parts of the body very quickly. That's the basis for breath tests, blood tests, and urine analyses used in law enforcement. If is strictly diffusion controlled, it must have one big Fick's Law diffusion coefficient!! I do not think that distinguishing ACTIVE vs. PASSIVE is a very good distinction to make. I say this because there are many chemical reactions that have "simple" zero (rate independent of concentration), or first order (rate is proportional to concentration) kinetics that have complex underlying mechanisms even though on a "macro" scale they appear to have "simple" kinetics. In the case of digestion, the absorption may appear to be diffusional, but what is going on the molecular scale could be quite complicated.

Vince Calder



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