Absorption of Nutrients

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    All of digestion boils down to absorption.   Without absorption, we cannot access any of the nutrients we ingest.  And the more absorption we can do, the more nutrients we can obtain.  Absorption is essential!

    How does absorption work?  This question will take much of the page to answer.  But, to start off, you need to keep in mind that we need to absorb smaller pieces of food-stuff.  absorb.jpg (30513 bytes)Much of absorption works through either endocytosis of food chunks or transport of tiny monomers across the membrane of the epithelial cells lining the alimentary canal lumen.

    I have tried to illustrate what absorption of food stuff would look like in this diagram.  The big green circles are the food chunks, already broken down and rather small (but still "large bits").  The little green dots are the tinier bits, like monomers.

    When either the larger or the smaller bits get into the cells, that is absorption.  For either to do that, they must encounter and interact with the cell membrane.  Therefore, the more apical membrane that exists, the more interaction it can make and the more absorption that can occur.

    In order to understand this better, you have to understand what is being absorbed as well as how it happens.

What are we absorbing & how?

What is being absorbed? Example of that type of material How does it occur?
broken food pieces tiny pieces of food that broke off the original piece... these can include more than one type of macromolecule Endocytosis-- because these tiny pieces are still very large to a cell.
macromolecules protein
carbohydrate
nucleic acid
or broken off pieces of these molecules

lipid

Endocytosis-- of proteins, carbohydrates, or nucleic acids... these molecules, even if broken a bit, are still too large to go through the membrane.

Simple Diffusion-- of lipid, only. (note: lipids tend to go into the lacteal rather than the blood)

monomers amino acids
monosaccharides (like glucose & fructose)
nucleotides

lipids don't have monomers, but some smaller pieces of them, like fatty acids or monoglycerides can sort of fit this category.

Facilitated Diffusion-- of amino acids, monosaccharides, and nucleotides enables them to cross the membrane.

Simple Diffusion-- of the lipid.  (note: lipids tend to go into the lacteal rather than the blood)

    You may have noticed that it depends on what the food stuff is as to how we pick it up as absorption...

    Simple diffusion and facilitated diffusion are both types of passive transport.  Remember that?  Passive transport gives materials a method to cross the membrane.

    Endocytosis requires energy and is a way to bring larger material into the cell using vesicles.

Where is absorption occurring?

    Mainly in the small intestines.  A teeny bit in the stomach and a teeny bit in the large intestines, but mainly in the small intestines.

    Because of this, the small intestines are nicely designed for absorption.  They have tons and tons of apical membrane on their epithelium.   The epithelial cells have microvilli, kind of like I tried to show you in the diagram at the top.

    The small intestines are also highly vascularized, to facilitate transport of the nutrients from the epithelium to the blood.  They also have lymphatic vessels throughout... these pick up the lipid nutrients in the specific lymphatic capillary called a lacteal.  They also tend to pick up infectious agents that may be in our chyme, so that these agents can be destroyed by our immune systems.  See Figure 17.37 for a good picture on this.

How can we increase absorption?

    Since absorption depends on the amount epithelial membrane exposed to the chyme, we can increase absorption by increasing the surface area of our membranes.   This is done in a few ways:

bullet1.  Have the lumen surface of the canal wall be folded rather than flat.  This is done with plicae circulares in the small intestines.
bullet2.  Have the epithelium on the lumen edge of the canal wall be folded more on top.   This is done with the villi in the small intestines.  It is within the mucosa of each villus that the lacteal and blood capillaries are found.
bullet3.  Have the apical membrane of the epithelial cells themselves get folded up.  This is done with microvilli in the small intestines.

These three things are shown in these diagrams (numbered and in order).  Note that the first and second images have black rectangles in them... these rectangles point out the region that was magnified for the second and third drawings, respectively.

plicae.jpg (7672 bytes) villi.jpg (10102 bytes) microvilli.jpg (11561 bytes)

Absorbing fat

    When lipid (fats) are taken in, they can diffuse pretty readily across the membranes of our cells if the fats are small enough.  When they do that, the fats run into the smooth ER within our cells, and are converted there into chylomicrons.  You had learned in the blood unit that chylomicrons are the newly absorbed fats.  Well, this is how it happens.  They enter the cell, are combined with some protein in the sER, released from the cell, travel into the lacteals, run through the lymphatic system, and are added to the blood via the collecting ducts.  A lot more detail about this is offered to you in Figure 17.44, but you don't need to know all of it.

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.