During digestion, four different groups of molecules are commonly encountered. Each is broken down into its molecular components by specific enzymes:
- Complex carbohydrates, or polysaccharides, (such as starches) are broken down into oligosaccharides (consisting of two to ten linked monosaccharides), disaccharaides (such as maltose), or individual monosaccharides (such as glucose or fructose). Enzymes called amylases break down starch.
- Proteins are broken down into short chains of amino acids (peptides) or individual amino acids by enzymes called proteases.
- Fats (lipids) are broken down into glycerol and fatty acids by enzymes called lipases.
- Nucleic acids are broken down into nucleotides by enzymes called nucleases.
A summary of enzymes and their substrate (substances upon which enzymes operate) appear in Table 1 .
|TABLE 1||Enzymes and Their Substrates|
|Enzyme||Substrate||Products of Enzyme Activity|
|salivary amylase||starches||maltose, oligosaccharides|
|Gastric juice (chief cells of stomach)|
|Pancreatic juice (acinar cells of pancreas)|
|pancreatic amylase||starches||maltose, oligosaccharides|
|Carboxypeptidase||proteins||peptides, amino acids|
|pancreatic lipase||fats||fatty acids, monoglycerides|
|Nucleases||RNA & DNA||nucleotides|
|Brush border (absorptive cells of small intestine)|
|Sucrase||sucrose||glucose & fructose|
|Lactase||lactose||glucose & galactose|
|Aminopeptidase||peptides||peptides, amino acids|
|nucleosidases||nucleotides||nitrogen bases, ribose, deoxyribose & phosphates|
|Phosphatases||nucleotides||nitrogen bases, ribose, deoxyribose & phosphates|