Digestion and Small Intestine
Name: Jim C.
Date: February 2003
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
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.
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Update: June 2012