Enzymes, as you know, catalyze chemical reactions by reducing the activation energy. In your digestive tract, special organs produce enzymes that help to break down the complex, insoluble, polymeric forms of the substances you ingest.
Large food molecules are broken down via the process of hydrolysis. In hydrolysis, the bonds of larger molecules are broken by the addition of water molecules, and it is catalyzed by digestive enzymes. There are three main categories of digestive enzymes:
Carbohydrases, which break down carbohydrates/polysaccharides and disaccharides into monosaccharides.
Proteases, which break down proteins into amino acids
Lipases, which break down lipids/fats into fatty acids and glycerol
Chemical digestion begins in the mouth. As you chew, the salivary glands secrete saliva into your mouth to lubricate the food and make it easier to chew and swallow. Saliva is composed of water, mucus and salivary amylase, a digestive enzyme (as you ca tell by the -ase ending). Salivary amylase begins the breakdown of starch to maltose.
Continuing chemical digestion in the stomach, the cells in the stomach wall secret a mixture of enzymes and acid known as gastric juice. Gastric juice is composed of hydrochloric acid, rennin and pepsin. Hydrochloric acid has several functions within the stomach. Firstly, it has an antibacterial purpose as bacteria cannot survive in very acidic environments. It also inactivates maltase from the saliva (as maltase has an ideal pH of 7 in the mouth). Finally, it creates an ideal environment for the action of the enzymes pepsin and rennin in the stomach, which have an optimum pH of 2.
Pepsin begins to break down protein into polypeptides.
Rennin is an enzyme present in infants that clots soluble proteins in breastmilk so that the protein is retained in the stomach.
Chemical digestion actually finishes in the small intestine. You've probably noticed that none of the previously mentioned organs secrete enzymes which digest fats. The digestion of fats occurs in the duodenum, which contains a mixture of bile (produced by the liver and secreted into the duodenum via the bile duct) and pancreatic juice (produced by the pancreas and secreted into the duodenum via the pancreatic duct).
Bile contains bile pigments (like bilirubin) and bile salts. The bile salts emulsify lipids. That is, lipids are insoluble in water (hydrophobic), and therefore, they clump together and form large globules/micelles. Bile salts have a hydrophobic (insoluble) part that interacts with the lipids and a hydrophilic (soluble) part that interacts with the water. This allows the bile to break the large globules up into smaller droplets, giving the enzymes a larger surface area to act on.
Pancreatic juice consists of pancreatic amylase, trypsin and pancreatic lipase. Pancreatic amylase continues to break down starch into maltose, trypsin continues to break down protein into polypeptides and pancreatic lipase breaks down lipids into fatty acids and glycerol.
The environment of the duodenum is basic (high pH), in order to inactivate pespin and neutralize the acid from the stomach. It also creates the optimum pH for the enzymes.
The small intestine also secretes its own mixture of enzymes known as intestinal juice. Intestinal juice is comprised of maltase, sucrase, lactase and peptidase (also called erepsin). Maltase catalyzes the hydrolysis of maltose into glucose; sucrase breaks down sucrose into glucose and fructose; lactase breaks down lactose into glucose and galactose; and peptidase breaks down polypeptides into amino acids.
Now that you understand chemical digestion, you should note the differences between chemical digestion and mechanical digestion:
Involves physically breaking down the food into smaller pieces.
Occurs between the mouth and the stomach
Mostly occurs in the mouth
Driven by teeth
Works to increase the surface area for the enzymatic reactions of chemical digestion
A piece of bread would be mechanically digested
Involves the chemical breakdown of compounds with higher molecular weights into compounds with lower molecular weights that can be absorbed by the body.
Occurs between the mouth and the intestine
Mostly occurs in the stomach
Driven by enzymes
Contributes to the absorption of the nutrients by breaking them down into their smaller, soluble molecular forms
The polysaccharides and starches in the bread would be chemically digested