My book says

glucose , galactose and amino acids are transported into cells of villi via secondary active transport(sat). Fructose is transported by facilitated diffusion(FD) .short chain fatty acids , vitamins diffuse into cells of villi. From there all these are transported to blood by FD.

This site says the reason for using SAT: http://www.phschool.com/science/biology_place/biocoach/biomembrane1/glucose.html

But I want to know why fructose, vitamins and short chain fatty acids do not need S.A.T. and travel via FD (or just diffusion)?dont they get reabsorbed into small intestine ?


1 Answer 1


In general:

  • Hydrophilic/polar/charged molecules require some kind of assisted transport across a membrane
  • Hydrophobic/nonpolar molecules can easily diffuse across a membrane

Glucose, galactose, and amino acids are hydrophilic. Fructose, fat-soluble vitamins, and short-chain fatty acids are hydrophobic.

Glucose and galactose have similar structures (aldose/pyranose) and are both hydrophilic. Fructose has a slightly different structure (ketose/furanose) which affects the way it is transported. Basically, it has a ketone (rather than an aldehyde) so it is relatively less reactive/polar. Because it is hydrophobic, it can diffuse across a membrane more easily.


"Since the concentration of fructose is higher in the lumen, fructose is able to flow down a concentration gradient into the enterocytes, assisted by transport proteins." (Wikipedia) Based on this statement, it seems that the lumen maintains a higher [fructose], relative to the enterocytes.

As explained in your link, glucose is mediated via SAT to ensure one direction of transport: we want glucose going only INTO our bloodstream. The [glucose] in lumen decreases after a meal, but we want glucose to keep being absorbed (hence SAT).. Also, unlike fructose, glucose and galactose are highly regulated for use in glycolysis, so it is important to keep glucose in the appropriate cells. In particular, because of its structure, fructose enters glycolysis after the "rate-limiting" phosphofructokinase step in glycolysis.

Quick notes on fat-soluble vitamin and short-chain fatty acids: they're both relatively small and hydrophobic. Short-chain fatty acids can easily diffuse across a membrane, which is also made of lipids. (I recommend reading about hydrophobic effect if you want to know more about this).. Larger fatty acids require shuttles because of their size.

Regarding "reabsorption" into the small intestine: Diffusion is the result of the random motion of particles. The more collisions in a higher concentration region eventually leads to a net movement "down a gradient." Using this logic: at some point, yes, the molecule may go back out into the lumen or go back in ("reabsorb") because it basically follows the gradient.

(I may come back to this post after learning more about it in a few months.. I used most of my answer based on what I learned in nutrition, o-chem, biochem)

Read more here about fructose metabolism


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