Although a small amount of carbohydrate digestion takes place in the mouth (through the action of salivary amylase) and protein digestion is initiated in the stomach (by the action of pepsins) the vast majority of chemical digestion takes place in the small intestine.
As we have already established, most of the substances that contribute to the processes of chemical digestion are not derived form the small intestine itself, but from the accessory organs that secrete substances into it. As a result of segmentation the pancreatic enzymes and bile salts are thoroughly mixed with dietary proteins, carbohydrates and fats and it doesn’t take long for these substances to get to work on them.
Lets have a look at the processes that take place in the small intestine with reference to each of the major food groups:
A. CARBOHYDRATES
The carbohydrates are found in a wide variety of foods such as bread, pasta, sugar, milk and vegetables. With the exception of milk sugar (lactose) the bulk of ingested carbohydrate is in the form of polysaccharides or disaccharides derived from plants. The building blocks of dietary carbohydrates are the monosaccharides galactose, glucose and fructose.
Note that although most vegetables that we eat contain a lot of the polysaccharide cellulose our digestive system is unable to break it down. As a result it passes through our digestive system largely unaltered (but it does help keep food moving by providing fibrous bulk that stimulates peristalsis).
Through the combined actions of the salivary amylase, pancreatic amylases and the brush border enzymes of the small intestine, dietary polysaccharides and disacharides are quickly broken down into monosaccharides that are small enough to be absorbed (see opposite).
B. PROTEINS
Proteins are found in varying amounts in eggs, milk, meat, legumes, nuts and cereals and are essential for the normal function of virtually every physiological system. Dietary protein is also supplemented by the exocrine secretions of the salivary, gastric and intestinal glands, the pancreas, liver as well as epithelial cells that are sloughed-off as part of the cyclical regeneration of the mucosal layer.
The molecular building blocks of the proteins are the amino acids and whilst the body can synthesise some of these there are 8 essential amino acids that can only be sourced from dietary intake.
Protein digestion begins in the stomach under the influence of the pepsins and then is massively accelerated by the proteolytic pancreatic enzymes and assisted by the brush border enzymes associated with the microvilli. In general the pepsins, trypsin and chymotrypsin cleave whole proteins and larger polypeptides and then carboxypeptidase and brush border enzymes act on the smaller peptides to progressively release single amino acids suitable for absorption (see opposite.
C. LIPIDS
The most abundant dietary lipids are the neutral fats that are the bodies most concentrated usable energy source. Neutral fats are made up of long chains of triglycerides (formed from a single molecule of glycerol and three fatty acid chains). The length of these chains and the nature of the bond between the molecules determines whether the fats are unsaturated (many vegetable oils) or saturated (milk and meat fats). Unlike carbohydrate and proteins, the small intestine is the only part of the digestive tract where lipid digestion occurs.
Because triglycerides are hydrophobic they tend to collate into globules of fat within the aqueous environment of the digestive tract 
. The size of the globules makes them fairly resistant to enzymatic degradation.
To overcome this problem the combined action of chewing, gastric motility and the squirting of food through the pyloric sphincter is used to break up the fat globules into small droplets (around 1 micron in diameter). These small droplets in aqueous solution form an emulsion. (Examine a bottle of salad dressing and shake it vigorously if you are unsure what an emulsion is).
Pancreatic lipases then begin to digest the exposed surface triglycerides of these emulsion droplets by removing two fatty acid chains from each triglyceride molecule to yield free fatty acids and monoglyceride (glycerol with one fatty acid chain attached) .
The products of lipid digestion are also hydrophobic and quickly coalesce to form tiny vesicles consisting of a core of monoglycerides and fatty acids that is surrounded by a shell of bile salts and lecithin . These structures are known as micelles and in this form are made available for absorption.