what food digested here carbohydrates, protein, lipids in small intestine and large intestine
Answers
By the end of this section, you will be able to:
Identify the locations and primary secretions involved in the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids
Compare and contrast absorption of the hydrophilic and hydrophobic nutrients
As you have learned, the process of mechanical digestion is relatively simple. It involves the physical breakdown of food but does not alter its chemical makeup. Chemical digestion, on the other hand, is a complex process that reduces food into its chemical building blocks, which are then absorbed to nourish the cells of the body (Figure 1). In this section, you will look more closely at the processes of chemical digestion and absorption.
Answer:
Large food molecules (for example, proteins, lipids, nucleic acids, and starches) must be broken down into subunits that are small enough to be absorbed by the lining of the alimentary canal. This is accomplished by enzymes through hydrolysis. The many enzymes involved in chemical digestion are summarized in Table 8.
*These enzymes have been activated by other substances.
The Digestive Enzymes (Table 8)
Enzyme Category Enzyme Name Source Substrate Product
Salivary Enzymes Lingual lipase Lingual glands Triglycerides Free fatty acids, and mono- and diglycerides
Salivary Enzymes Salivary amylase Salivary glands Polysaccharides Disaccharides and trisaccharides
Gastric enzymes Gastric lipase Chief cells Triglycerides Fatty acids and monoacylglycerides
Gastric enzymes Pepsin* Chief cells Proteins Peptides
Brush border enzymes α-Dextrinase Small intestine α-Dextrins Glucose
Brush border enzymes Enteropeptidase Small intestine Trypsinogen Trypsin
Brush border enzymes Lactase Small intestine Lactose Glucose and galactose
Brush border enzymes Maltase Small intestine Maltose Glucose
Brush border enzymes Nucleosidases and phosphatases Small intestine Nucleotides Phosphates, nitrogenous bases, and pentoses
Brush border enzymes Peptidases Small intestine
Aminopeptidase: amino acids at the amino end of peptides
Dipeptidase: dipeptides
Aminopeptidase: amino acids and peptides
Dipeptidase: amino acids
Brush border enzymes Sucrase Small intestine Sucrose Glucose and fructose
Pancreatic enzymes Carboxy-peptidase* Pancreatic acinar cells Amino acids at the carboxyl end of peptides Amino acids and peptides
Pancreatic enzymes Chymotrypsin* Pancreatic acinar cells Proteins Peptides
Pancreatic enzymes Elastase* Pancreatic acinar cells Proteins Peptides
Pancreatic enzymes Nucleases Pancreatic acinar cells
Ribonuclease: ribonucleic acids
Deoxyribonuclease: deoxyribonucleic acids
Nucleotides
Pancreatic enzymes Pancreatic amylase Pancreatic acinar cells Polysaccharides (starches) α-Dextrins, disaccharides (maltose), trisaccharides (maltotriose)
Pancreatic enzymes Pancreatic lipase Pancreatic acinar cells Triglycerides that have been emulsified by bile salts Fatty acids and monoacylglycerides
Pancreatic enzymes Trypsin* Pancreatic acinar cells Proteins Peptides
CARBOHYDRATE DIGESTION
The average American diet is about 50 percent carbohydrates, which may be classified according to the number of monomers they contain of simple sugars (monosaccharides and disaccharides) and/or complex sugars (polysaccharides). Glucose, galactose, and fructose are the three monosaccharides that are commonly consumed and are readily absorbed. Your digestive system is also able to break down the disaccharide sucrose (regular table sugar: glucose + fructose), lactose (milk sugar: glucose + galactose), and maltose (grain sugar: glucose + glucose), and the polysaccharides glycogen and starch (chains of monosaccharides). Your bodies do not produce enzymes that can break down most fibrous polysaccharides, such as cellulose. While indigestible polysaccharides do not provide any nutritional value, they do provide dietary fiber, which helps propel food through the alimentary canal.
The chemical digestion of starches begins in the mouth and has been reviewed above.
In the small intestine, pancreatic amylase does the ‘heavy lifting’ for starch and carbohydrate digestion (Figure 2). After amylases break down starch into smaller fragments, the brush border enzyme α-dextrinase starts working on α-dextrin, breaking off one glucose unit at a time. Three brush border enzymes hydrolyze sucrose, lactose, and maltose into monosaccharides. Sucrase splits sucrose into one molecule of fructose and one molecule of glucose; maltase breaks down maltose and maltotriose into two and three glucose molecules, respectively; and lactase breaks down lactose into one molecule of glucose and one molecule of galactose. Insufficient lactase can lead to lactose intolerance.