Learn Chapter 16- Digestion and Absorption with Free Lessons & Tips

In an adult human, there are four main types of teeth, each with a specific function in the process of mastication (chewing) and digestion. These types of teeth and their respective numbers in a typical adult human mouth are as follows:

1. Incisors: Incisors are the front teeth located in the center of the mouth. They have flat, chisel-shaped edges and are used for cutting and slicing food.

   • There are eight incisors in total, four in the upper jaw (maxillary incisors) and four in the lower jaw (mandibular incisors).

2. Canines (Cuspids): Canines are the pointed teeth located next to the incisors, one on each side of the incisor teeth. They have a single pointed cusp and are used for tearing and grasping food.

   • There are four canines in total, two in the upper jaw (maxillary canines) and two in the lower jaw (mandibular canines).

3. Premolars (Bicuspids): Premolars are located behind the canines and have two cusps on their biting surface. They are used for grinding and crushing food.

   • There are eight premolars in total, four in the upper jaw (maxillary premolars) and four in the lower jaw (mandibular premolars).

4. Molars: Molars are located at the back of the mouth and have multiple cusps on their biting surface. They are the largest and strongest teeth and are used for crushing and grinding food.

   • There are twelve molars in total, including six in the upper jaw (three on each side) and six in the lower jaw (three on each side).

In summary, a typical adult human mouth contains a total of 32 teeth, consisting of eight incisors, four canines, eight premolars, and twelve molars. However, variations in tooth number and morphology can occur due to factors such as dental anomalies, genetics, and dental treatments.

Butter is primarily composed of fats, specifically triglycerides, along with small amounts of proteins and water-soluble compounds like vitamins and minerals. The digestion and absorption of butter in the body involve several steps:

1. Mouth:

   • Mechanical breakdown of food begins in the mouth through chewing (mastication). However, since butter is a fat and does not require extensive mechanical breakdown, the mouth's role in butter digestion is minimal.

2. Stomach:

   • In the stomach, the presence of fats triggers the release of the hormone gastrin, which stimulates the secretion of gastric lipase. Gastric lipase begins the digestion of fats by breaking down triglycerides into diglycerides and free fatty acids.
   • However, the main fat digestion occurs in the small intestine, so the stomach's contribution to butter digestion is relatively minor.

3. Small Intestine (Duodenum, Jejunum, and Ileum):

   • The partially digested food mixture, known as chyme, enters the duodenum from the stomach. Once in the duodenum, the presence of fats triggers the release of the hormone cholecystokinin (CCK) from the intestinal wall.
   • CCK stimulates the gallbladder to release bile into the duodenum. Bile is produced by the liver and stored in the gallbladder. Bile salts in bile help emulsify fats, breaking them down into smaller droplets and increasing their surface area for enzyme action.
   • Pancreatic lipase, secreted by the pancreas into the duodenum, hydrolyzes triglycerides into monoglycerides and free fatty acids. This is the primary enzyme responsible for fat digestion in the small intestine.
   • Additionally, pancreatic lipase works in conjunction with colipase, a protein cofactor, and bile salts to facilitate the digestion of fats.

4. Absorption:

   • Once broken down into monoglycerides, free fatty acids, and glycerol, these smaller fat molecules are absorbed across the apical membrane of enterocytes (intestinal epithelial cells) lining the small intestine.
   • Inside the enterocytes, monoglycerides and fatty acids are reassembled into triglycerides and packaged into lipid droplets called chylomicrons.
   • Chylomicrons are released from the enterocytes and enter the lymphatic system through lacteals (specialized lymphatic capillaries) in the intestinal villi. From the lymphatic system, chylomicrons are transported to the bloodstream, where they deliver fatty acids and glycerol to various tissues for energy production, storage, or other metabolic processes.

In summary, the digestion and absorption of butter in the body involve the action of enzymes like gastric lipase and pancreatic lipase, as well as bile salts to emulsify fats. Once broken down into smaller molecules, fats are absorbed by enterocytes in the small intestine and transported throughout the body via chylomicrons for use in energy metabolism and other physiological processes.

Polysaccharides and disaccharides are complex carbohydrates that are broken down into simpler sugars during the process of digestion. Here's how each of these types of carbohydrates is digested:

1. Polysaccharides: Polysaccharides are large carbohydrate molecules composed of multiple monosaccharide units linked together by glycosidic bonds. Examples include starch, glycogen, and cellulose. The digestion of polysaccharides primarily occurs in the following steps:

• Mouth: The digestion of polysaccharides begins in the mouth with the action of salivary amylase, an enzyme secreted by the salivary glands. Salivary amylase breaks down starch molecules into smaller polysaccharides, maltose (a disaccharide), and dextrins.

• Stomach: Once food enters the stomach, the acidic environment denatures salivary amylase, halting further starch digestion. Therefore, starch digestion is minimal in the stomach.

• Small Intestine: The majority of polysaccharide digestion occurs in the small intestine, specifically in the duodenum and jejunum. Pancreatic amylase, secreted by the pancreas, continues the digestion of polysaccharides by breaking down starch and glycogen into maltose, maltotriose (a trisaccharide), and dextrins.

• Brush Border Enzymes: The final stage of polysaccharide digestion occurs at the brush border of the small intestine, where enzymes known as α-glucosidases (including sucrase, maltase, and isomaltase) further break down disaccharides and trisaccharides into monosaccharides.

• Absorption: The resulting monosaccharides (such as glucose, galactose, and fructose) are absorbed across the epithelial lining of the small intestine into the bloodstream and transported to various tissues for energy or storage.

2. Disaccharides: Disaccharides are carbohydrate molecules composed of two monosaccharide units joined by a glycosidic bond. Examples include sucrose, lactose, and maltose. The digestion of disaccharides follows a similar process to polysaccharides but is simpler:

• Small Intestine: Disaccharides are primarily digested in the small intestine, specifically at the brush border membrane of enterocytes (intestinal epithelial cells). Brush border enzymes, such as sucrase, lactase, and maltase, hydrolyze disaccharides into their constituent monosaccharides.

• Hydrolysis: Sucrase breaks down sucrose into glucose and fructose, lactase breaks down lactose into glucose and galactose, and maltase breaks down maltose into two glucose molecules.

• Absorption: The resulting monosaccharides are then absorbed across the epithelial lining of the small intestine into the bloodstream and transported to various tissues for energy or storage, similar to the absorption of monosaccharides derived from polysaccharides.

In summary, both polysaccharides and disaccharides are digested into monosaccharides, which are then absorbed into the bloodstream for use by the body. The digestion of these carbohydrates involves the action of various enzymes secreted by the salivary glands, pancreas, and brush border of the small intestine.

Chymotrypsin is a digestive enzyme that plays a crucial role in the breakdown of proteins into smaller peptides during the process of digestion. It is produced and secreted by the pancreas as an inactive precursor called chymotrypsinogen. Chymotrypsinogen is activated into its active form, chymotrypsin, by the proteolytic enzyme trypsin, which is also secreted by the pancreas.

The digestive role of chymotrypsin involves the hydrolysis of peptide bonds within protein molecules, particularly those adjacent to aromatic amino acids such as tyrosine, phenylalanine, and tryptophan. Here's how chymotrypsin functions in the digestion of proteins:

1. Activation: Chymotrypsinogen is released from the pancreas into the duodenum along with other pancreatic enzymes in response to hormonal signals such as cholecystokinin (CCK). Once in the duodenum, chymotrypsinogen is activated into chymotrypsin by trypsin, which cleaves off a small peptide fragment from chymotrypsinogen, converting it into its active form.

2. Substrate Specificity: Chymotrypsin exhibits specificity for peptide bonds adjacent to aromatic amino acids in proteins. It cleaves these peptide bonds by hydrolyzing them, resulting in the formation of smaller peptide fragments.

3. Hydrolysis of Peptide Bonds: Chymotrypsin catalyzes the hydrolysis of peptide bonds within protein molecules, breaking them down into shorter peptides. This process involves the addition of a water molecule to the peptide bond, resulting in the separation of the amino acid residues on either side of the bond.

4. Formation of Peptide Fragments: As chymotrypsin cleaves peptide bonds, it generates a mixture of smaller peptide fragments with varying lengths. These peptide fragments are further broken down into individual amino acids by other digestive enzymes, such as carboxypeptidases and aminopeptidases, to facilitate their absorption by the intestinal epithelium.

In addition to chymotrypsin, the pancreas secretes two other digestive enzymes of the same category, known as serine proteases or serine endopeptidases. These enzymes are trypsin and elastase:

1. Trypsin: Like chymotrypsin, trypsin is produced as an inactive precursor called trypsinogen and is activated by enteropeptidase, an enzyme produced by the duodenal mucosa. Trypsin hydrolyzes peptide bonds adjacent to positively charged amino acids such as lysine and arginine.

2. Elastase: Elastase is another serine protease secreted by the pancreas. It is involved in the digestion of elastin, an insoluble protein found in connective tissues. Elastase cleaves peptide bonds adjacent to small amino acids such as glycine and alanine in elastin molecules.

Overall, chymotrypsin, along with trypsin and elastase, plays a critical role in the digestion of proteins by breaking them down into smaller peptides, which can then be further digested into individual amino acids for absorption in the small intestine.

Bile juice, produced by the liver and stored in the gallbladder before being released into the small intestine, plays a crucial role in digestion despite not containing any digestive enzymes. Here are several reasons why bile juice is important for digestion:

1. Emulsification of Fats: Bile contains bile salts, which act as emulsifiers. Emulsification is the process of breaking down large fat globules into smaller droplets, increasing their surface area and facilitating their digestion by lipase enzymes. Bile salts surround the fat droplets, reducing surface tension and allowing lipase enzymes to access and digest the fats more efficiently.

2. Aiding in Lipid Digestion: While bile itself does not contain digestive enzymes, its role in emulsifying fats enhances the action of pancreatic lipase, the primary enzyme responsible for digesting triglycerides into fatty acids and monoglycerides. By increasing the surface area of fat particles, bile facilitates the interaction between lipase enzymes and fats, leading to more efficient lipid digestion.

3. Facilitating Micelle Formation: Once fats are broken down into smaller droplets by bile and digested by lipase, the resulting fatty acids and monoglycerides combine with bile salts to form structures called micelles. Micelles are small, water-soluble complexes that transport lipids across the watery environment of the intestinal lumen to the surface of enterocytes (intestinal epithelial cells) for absorption.

4. Promoting Absorption of Fat-Soluble Nutrients: Bile salts also aid in the absorption of fat-soluble vitamins (A, D, E, and K) and other fat-soluble nutrients by facilitating their incorporation into micelles and transport to the surface of enterocytes for absorption into the bloodstream.

5. Regulating pH: Bile helps neutralize acidic chyme entering the small intestine from the stomach, creating a more favorable pH environment for the action of pancreatic enzymes involved in digestion. By raising the pH of the intestinal contents, bile contributes to the optimal functioning of digestive enzymes and ensures efficient digestion and absorption of nutrients.

In summary, while bile juice itself does not contain digestive enzymes, its composition, particularly bile salts, plays a critical role in the digestion and absorption of fats and fat-soluble nutrients. Bile aids in emulsifying fats, facilitating lipid digestion, promoting the formation of micelles for nutrient absorption, and regulating the pH of the intestinal environment, all of which are essential processes for efficient digestion and nutrient uptake in the small intestine.