Biology Class 11 Science Digestion in the Small Intestine
Class 11 Biology : Digestion in the Small Intestine
The length of the small intestine can vary greatly, from as short as 2.75 m (9.0 ft) to as long as 10.49 m. The average length of a living person is 3m-5m. The length depends both on how tall the person is and how the length is measured. Taller people generally have a longer small intestine. The partly digested food (chyme), from the stomach, comes to the duodenum (the first part of the small intestine.) Here it is mixed with the intestinal juice, bile juice produced by the liver and pancreatic juice produced by the pancreas.
The intestinal juice is also known as Succus Entericus is secreted by two types of glands;
- The duodenum contains Brunner’s glands, which are present in the submucosa of the duodenum region only. They produce a mucus-rich alkaline secretion containing bicarbonate and weak proteolytic enzyme. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids present in the gastric
- The crypts of Lieberkuhn (intestinal glands) are distributed throughout the small intestine. These are tubular glands which open into the spaces between the neighboring Intestinal villi (singular: villus) are small, finger-like projections that extend into the lumen of the small intestine. Villi increase the internal surface area of the intestinal walls making available a greater surface area for absorption.
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Composition of Intestinal Juice/ Succus Entericus
The intestinal juice produced by the intestinal glands has a pH of about 7.6. Only 1.6% of the juice is solid matter and rest is liquid. Of the solid portion, 1% is made up of inorganic component and 0.6% is an organic component. The organic components are the various enzymes and proteins present in the intestinal juice.
Enzymes that help in digestion in the small intestine
The small intestine is where most chemical digestion takes place. Many of the digestive enzymes that act in the small intestine are secreted by the pancreas and liver and enter the small intestine via the pancreatic duct. Pancreatic enzymes and bile from the gallbladder enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. Secretin, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach.
The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates:
Proteins: Chemical breakdown of proteins begins in the stomach and continues in the small intestine. They are degraded into small peptides and amino acids before absorption. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products.
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Lipids (fats): They are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the bile secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help emulsify them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.
Carbohydrates: They are degraded into simple sugars, or monosaccharide (e.g., glucose). The enzyme Sucrase hydrolyzes sucrose into glucose and fructose. The enzyme maltase helps in the breakdown of maltose into two glucose units. While lactase breaks lactose into glucose and galactose. Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine.
Nucleic Acid: The nucleic acids are broken down by the action of nucleases, nucleotidases, and nucleosidases into a mixture of purine and pyrimidine bases, phosphoric acid, and pentose sugars.
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