Final answer:
Proteins are the main organic molecules digested in the stomach, with enzymes such as pepsin breaking them down into amino acids. Carbohydrates start digesting in the mouth but are mainly digested in the small intestine, while lipids are digested in the small intestine with the help of bile and pancreatic lipase.
Step-by-step explanation:
The main organic molecule digested in the stomach is proteins. Protein digestion begins in the stomach where enzymes such as pepsin, which is activated by hydrochloric acid (HCl), helps break down proteins into smaller chains called peptides and eventually into amino acids. This process is assisted by the acidic pH of the stomach, which also helps to kill microbes present in the food.
Carbohydrate digestion starts in the mouth with the aid of the enzyme salivary amylase, which begins breaking down complex carbohydrates into simpler sugars. However, carbohydrate digestion is halted in the stomach because the acidic environment inactivates salivary amylase. Most carbohydrate digestion occurs later in the small intestine.
Lipids mainly begin their digestion in the small intestine, where bile salts emulsify fats, and enzymes such as pancreatic lipase break them down into fatty acids and glycerol. Contrary to what was stated, fats are not primarily digested in the large intestine; only a small portion of lipid digestion may occur there through the action of intestinal bacteria.
However, it's important to note that digestion of digested carbohydrate, fat, and protein molecules is completed in the small intestine where they are absorbed by the villi.