The small intestine is the major site of absorption in the digestive tract and the only part where absorption of nutrients occurs.
Because the epithelial cells of the mucosal layer form a continuous barrier between the lumen and the lamina propria and have tight junctions between them, virtually all substances have to pass through their cytoplasm.
This involves a two-step process where substances are transported into the epithelial cell across the apical membrane and then out of the cell across its basolateral membrane.
Not surprisingly these cells play a very important regulatory role in absorption.
The substances that are absorbed include the products of digestion as well as a number of dietary requirements that are not broken down before being transferred across the wall of the digestive tract. We will look at the absorption of each of these classes of substance in turn.
A.CARBOHYDRATES
The monosacchrides that are the end result of carbohydrate digestion move across the epithelial layer by two-step process:
Glucose and galactose are transported into the epithelial cells against their concentration gradient by active transport that is driven by the binding and simultaneous transport of Na+ down its concentration gradient.
Fructose is transported into the cell down its concentration gradient by facilitated diffusion.
All three monosaccharides then move across the basolateral membrane by facilitated diffusion.
Once across the epithelium these monosaccharides diffuse into the capillaries of the underlying lamina propria.
B. PROTEIN
The products of protein digestion are the amino acids and these are transported across the apical membrane by specific membrane proteins. Because this movement is also against the concentration gradient these transporters use co-transport of Na+ down its concentration to drive this process.
Movement of amino acids across the basolateral membrane is enabled by facilitated diffusion.
Recently a membrane protein on the apical membrane has been identified that can transport small peptides (mainly di- and tri-peptides) into the epithelial cells. It is believed that these peptides are then cleaved by intracellular enzymes enabling the amino acids products to diffuse across the basolateral membrane.
Once into the lamina propria amino acids quickly move into the adjacent capillaries.
C. FATS
The products of lipid digestion are the fatty acids and monoglycerides that are collected together in micelles 
. When these micelles collide with the apical membrane of the epithelial cells the fatty acids and monoglycerides encounter an environment much more suited to their hydrophobic nature and so are released and waste no time moving into the cell down their concentration gradient by simple diffusion .
Once in the cytoplasm of the
epithelial cells the fatty acids and monoglycerides are reassembled into triglycerides by the smooth endoplasmic reticulum and covered in a layer of protein to form tiny (less than 1 micron in diameter) intracellular droplets known as chylomicrons. These chylomicrons are then packed into membrane-bound vesicles by the Golgi apparatus and transported across the basolateral membrane by exocytosis .
Although the exocytosed chylomicrons are very small they are still too large to move into the capillaries so are unable to go directly into cardiovascular system. However the spaces between the cells than line the walls of the lymphatic vessels (lacteals) are much bigger and so the chylomicrons diffuse into these lymphatic vessels that eventually drain into the bloodstream. In the bloodstream the chylomicrons are broken down by lipoprotein lipase and the fatty acids and monoglycerides released.
D. VITAMINS
Although vitamins are only required in very small amount they are vital for many physiological process. Most vitamins function by acting as cofactors that are required to catalyse the reactions of some enzymes. With a few exceptions the body cannot synthesis vitamins so they are obtain through dietary intake.
Vitamins are found in many foods but no one food contains them all. (Yet another reason to have a varied and balanced diet.) Vitamins are identified using letters but they also have a more descriptive name that is seeing increasing use.
There are two major classes of vitamins:
(i) Fat Soluble Vitamins
The fat soluble vitamins (such as vitamins A, D, E & K) generally associate with dietary fats and are incorporated into micelles and move across the apical and basolateral membranes in the same way as monoglycerides and fatty acids. Consequently anything that interferes with fat absorption has a detrimental effect on absorption of these vitamins.
(ii) Water Soluble Vitamins
Although most water soluble vitamins (the B vitamins & vitamin C) cross the apical membrane by simple or facilitated diffusion a few require active transport:
Before Vitamin B12 can be absorbed it has to bind to intrinsic factor (secreted by the parietal cells of the gastric glands). This complex is required for binding to a specific carrier protein that enables transport of the vitamin into the epithelial cell.
Vitamin C on the other hand binds to a specific carrier protein on the apical membrane. As for both monosaccharides and amino acids this protein uses contransport of Na+ down its concentration to drive this process.
For the most part, the mechanisms by which water-soluble vitamins cross the basolateral membrane remain to be established.
E. ELECTROLYTES
You really cannot have got to this stage of the semester without realising just how important electrolytes are physiologically. Sodium and potassium are required for the electrical signals used by excitable tissues for communication, calcium is a prerequisite for synaptic transmission and muscle contraction and phosphate is essential for the strength of teeth and bone.
The absorption of electrolytes is mediated by a number of different mechanisms:
Sodium
As we have already seen, the absorption of sodium across the apical membrane is linked to the transport of other nutrients such as monosaccharides, amino acids and vitamin C. On the basolateral membrane sodium is pumped out of the cell by the sodium-potassium exchange pump and enters the bloodstream by diffusion.
Potassium
The movement of potassium through the epithelial cells lining the small intestine occurs by simple diffusion that is linked to the movement of water. When water is absorbed, the luminal potassium concentration increases and this concentration gradient provides all the motivation that potassium needs to move across the epithelium by simple diffusion.
Calcium
Most calcium is absorbed by the duodenum and moves across the apical membrane down its concentration gradient by diffusion through calcium channels. The basolateral membrane contains specific proteins that pump the calcium out by active transport.
The number of calcium pumps expressed by the basolateral membrane of epithelial cells of the small intestine is regulated by vitamin D levels. Vitamin D deficiency therefore seriously attenuates the rate of calcium absorption.
F. WATER
We ingest approximately 2 litres of water each day and the exocrine secretions of the digestive tract and accessory organ contribute another 8 litres. As the chyme moves along the length of the small intestine about 9 litres of this fluid is absorbed.
The absorption of water is a completely passive affair and the driving force for the process is osmotic pressure. The epithelial cells lining the small intestine are freely permeable to water and it can also move through the intercellular spaces between the tight junctions. As a consequnce of this high permeabilty of water, the osmolarity of chyme in the small intestine will always be same as the extracellular fluid in the lamina propria.
However as nutrients are absorbed by all the mechanisms outlined above it follows that the osmolarity of the chyme will decrease. As the osmolarity decreases water will move out of lumen across the epithelium by osmosis. So the absorption of water is intimately linked to the absorption of nutrients.