Category: Biology Second Paper Lecture Sheet

The thick, cylindrical and grooved part of the small intestine extending from the ileum to the anus is called the large intestine. The function of the human colon is described.
1. Absorption and Reabsorption: Water, mineral salts, sodium, chloride etc. are absorbed by the colon. 70-80% of the water present in the digestive waste is reabsorbed by the colon.
2. Reservoir of microorganisms: The appendix of the colon is said to be a protected reservoir of beneficial microorganisms. 500 species of bacteria live in the human colon. Bacteria in the appendix reestablish the intestinal bacterial flora when beneficial gut bacteria are destroyed. These bacteria break down non-nutritive food ingredients to produce short-chain fatty acids.
3. Fermentation and Digestion: There are 500 species of mitotic bacteria in the large intestine. These bacteria break down raw food ingredients in a fermentation process to produce short-chain fatty acids and release carbon dioxide, hydrogen and methane gases. Small fatty acids (acetic acid, propionic acid and butyric acid) provide energy to the cells of the colon wall.
4. Faecal production: Liquid chyme or mucus from the small intestine enters the large intestine and is absorbed into moist and soft stools. Usually 135 grams of feces are produced from 350 grams of feces.

The process by which digested food enters the blood and lymph from the alimentary canal is called absorption or absorption of food. The small intestine is the main site of absorption. Small intestine absorbs about 90% of food. 10% is absorbed by the stomach and colon. The lining of the small intestine folds to form a finger-like projection. It is called Vilai. Vilai increases the absorption capacity of the intestine by 40 times. There are about 500,000 villi in the human small intestine. The fine projections in villi cells are called microvilli. Microvilli join together to form a brush border. Controls the brush border absorption process. Microvilli increase the absorption capacity of the intestine by 60 times. Food is absorbed by endocytosis along with vitamins, minerals and water.
1. Protein Absorption: Proteins are digested to produce amino acids. Protein is absorbed in the small intestine as amino acids. However, semi-complex proteins like peptones, proteoses, peptides etc. are also absorbed in very small amounts. Egg whites, shrimp, crab etc. are absorbed unchanged. Amino acids are absorbed by the villi of the duodenum and jejunum through the process of active absorption, diffusion and phagocytosis. Normally, L-amino acids are absorbed by the active pathway and D-amino acids by absorption. The cytoplasmic tubule of Vilai functions in the absorption of proteins. Thyroxine hormone regulates protein absorption.
2. Absorption of sugars: Sugars are digested into monosaccharides like glucose, fructose, galactose, lactose, sucrose, xylose, mannose etc. The small intestine absorbs sugars as monosaccharides. However, some sugars are also absorbed as disaccharides. Of the monosaccharides, 80% are absorbed as glucose and 20% as fructose and galactose. With the help of Na+ and ATP, the tubules of the villi of the jejunum absorb glucose. Fructose is absorbed in the digestive process. Sucrose and lactose are absorbed in the duodenum and small intestine.
3. Lipid Absorption: Lipids are digested into fatty acids, glycerol, monoglycerides and cholesterol. Fatty acid absorption is a complex process. Fatty acids, monoglycerides and bile salts bind together to form micelles. Micelle particles are absorbed by the duodenum and mid-small intestine. Under the influence of thiokinase enzyme, fatty acids are converted into active fatty acids. Active fatty acids, glycerol and proteins join together to form triglycerides. These are called chylomicron particles. Chylomicron particles enter the lymphatic vessels of the small intestine. If the branches of the lymphatic duct are white, it is called lactial. Chylomicrons enter the blood plasma through lymphatic vessels. Enzymes located in the plasma break down chylomicrons or lipoproteins to form fatty acids and glycerol.
4. Water Absorption: Water is absorbed by the epithelial cells of the villi of the small intestine. Normally 200-400 ml of water is absorbed per hour.
5. Mineral Salt Absorption: Mineral salts are absorbed by the epithelial cells of the villi of the colon.
6. Absorption of Vitamins: Vitamins are absorbed when dissolved in water or fat by the epithelial cells of the villi of the small intestine.
Absorption of nutrients is a very important process for the body. Digestion produces amino acids from proteins, glucose and fructose from sugars and fatty acids and glycerol from lipids. The small intestine absorbs all the digested food. Then it reaches different parts of the body through the blood. Body cells take all these nutrients and stay healthy, strong and fresh.

If the food digestion process is controlled by the nervous system, it is called nervous control. The neural network in the wall of the human colon plays a major role in food intake and digestion. There is a hunger center in the hypothalamus of this brain. It regulates human appetite and food intake. Two nerves control digestion in humans.
1. Intrinsic plexuses: The nervous system that controls digestion by receiving stimuli from inside the alimentary canal is called intrinsic plexus. It is called enteric or enteric nervous system. The enteric nervous system is called the second brain of humans. It extends like a web in the walls of esophagus, stomach, small intestine and colon and receives stimulation from there. Two types of intrinsic neural networks control short reflexes.
(i) Myenteric plexus: It controls the contraction or peristalsis action of the smooth muscles of the digestive tract.
(ii) Submucosal plexus: It controls the secretion of chemicals in the digestive system.
2. Extrinsic plexuses: The nervous system that controls digestion by receiving stimuli from outside the alimentary canal is called extrinsic plexus. It consists of sympathetic and parasympathetic. It regulates the long reflex action of the digestive system. They release acetylcholine and adrenaline. Acetylcholine powers the flow of food and drink through the alimentary canal. Besides, it helps to secrete more digestive juices from the stomach and pancreas. Adrenaline relaxes stomach and intestinal muscles and reduces blood flow.

1. Gastrin: It is produced by the G-cells of the stomach and helps in the secretion of gastric juice. The hormone gastrin regulates HCl secretion.
2. Somatostatin: This hormone is released from G-cells and hypothalamus and inhibits gastrin secretion. It also reduces gastric secretion and pancreatic juice secretion.
3. Kallikrein: It is secreted from the salivary glands and dilates the blood vessels of the salivary glands. It increases blood circulation. As a result, the amount of salivary secretion also increases.
4. Secretin: Secretin hormone helps in the production of bicarbonate ions in pancreatic and liver cells. Pancreatic juice and bile are more alkaline due to bicarbonate ions. As a result, an alkaline environment is created in the digestive system.
5. Cholecystokinin: It helps in the secretion of bile in the duodenum. In 1928 scientists Ivy and Oldberg discovered the hormone cholecystokinin.
6. Villikinin: Villikinin hormone increases the absorption capacity by strengthening the villi of the small intestine.
7. Testosterone and Estrogen: Testosterone and estrogen regulate protein metabolism.
8. Pancreasin: This hormone helps in the secretion of pancreatic juice. In 1943, Harper and Rapper discovered the hormone pancreozynin.
9. Enterocrinein: This hormone helps in the secretion of intestinal juice. Intestinal juice participates in digestion.
10. Diocranin: It is produced by the wall of duodenum and helps in the secretion of intestinal juice.
11. Peptide YY: Peptide YY is secreted from the wall of the ileum. It makes the food flow slowly. It completes food digestion and absorption.
12. Gastric inhibitory peptide: Gastric inhibitory peptide is secreted from the wall of the duodenum. It controls the passage of food from the stomach into the intestine. It initiates insulin secretion.
13. Vasoactive intestinal peptide: This hormone is secreted from the epithelial wall of the small intestine. It dilates the blood vessels of the intestinal wall. Stops gastric acid secretion.
14. Hormones that regulate hunger and satiety: The hypothalamus of the brain has the hunger center. It regulates human appetite and food intake. Peptide YY, gastric inhibitory peptide (GIP), vasoactive intestinal peptide (VIP), glucagon like peptide-1 (GLP-1), pancreatic polypeptide (PP) and ghrelin act as neurotransmitters in the brain. Among them, peptide YY, pancreatic polypeptide and ghrelin regulate food intake. When the level of ghrelin hormone increases in the blood, hunger is stimulated. When the levels of pancreatic polypeptide and peptide YY hormones increase in the blood during food intake, food satisfaction comes.

Mechanical digestion of food
1. Segmentation of food occurs in the small intestine. Food mixes well with intestinal juices and mucus. Food moves equally everywhere from front to back.
2. Chyme moves slowly through the circulation by peristalsis.
3. Intestinal mucin mixes with food to lubricate it and move it from one place to another.
4. Mucus secreted by Brunner’s glands and goblet cells protects the small intestinal wall from enzyme action.
5. Bile reduces the action of bacteria and increases the speed of bile digestion.
6. The hormone cholecystokinin causes the gallbladder to contract. As a result, bile reaches the small intestine.
7. The brass emulsifies the lubricant into tiny particles like soap suds.
Chemical digestion of food
1. Protein digestion
(i) Amino peptidase enzyme breaks down polypeptides into amino acids.
(ii) Prolidase enzyme cleaves the peptide into proline.
(iii) Dipeptide and amino acids are produced from tripeptide under the action of tripeptidase enzyme.
(iv) Dipeptidase enzyme breaks down dipeptide into amino acids.
2. Carbohydrate digestion
(i) Amylase enzyme breaks down starch and dextin into maltose, maltotriose and minor dextin.
(ii) Maltose and glucose are produced from isomaltose under the action of isomaltase enzyme.
(iii) Maltotriase enzyme breaks down maltotriose into glucose.
(iv) Glucose is produced from maltose under the action of maltase enzyme.
(v) Sucrase enzyme breaks down sucrose into glucose and fructose.
(vi) Glucose and galactose are produced from lactose under the action of lactase enzyme.
3. Lipid digestion
(i) The enzyme lipase breaks down triglycerides and diglycerides into fatty acids, glycerol and monoglycerides.
(ii) Fatty acids and glycerol are produced from monoglycerides under the action of monoglyceridase enzyme.

1. Protein digestion
(i) Inactive trypsinogen is converted to active trypsin under the action of enterokinase enzyme. Trypsin converts peptone into dipeptides and polypeptides.
(ii) Inactive chymotrypsinogen is converted to active chymotrypsin with the help of trypsin enzyme. Chymotrypsin breaks down milk casein into paracasein.
(iii) Elastase enzyme breaks down elastin into peptides.
(R) Peptides are produced from collagen by the action of collagenase enzyme.
2. Carbohydrate digestion
(i) Enzyme amylase breaks down starch and glycogen into maltose, maltotriose and dextin.
(ii) Maltose is converted to glucose in the presence of maltase enzyme.
3. Lipid digestion
(i) The enzyme lipase breaks down lipids into fatty acids and glycerol.
(ii) Phospholipids are converted into fatty acids and monoglycerides in the presence of phospholipase enzymes.

1. The biliary system of bile emulsifies the substance and makes it suitable for the action of lipase.
2. Acts as an activator of bile lipase.
3. Bile creates an alkaline environment by neutralizing the acidity of the digestive tract.
4. Bile helps in the absorption of fat soluble vitamins.
5. Gallbladder increases the peristalsis of the colon and aids in the evacuation of stool.
6. Toxins, cholesterol, bile acids, zinc, copper, mercury etc. are released through bile.

1. Mechanical digestion of food
(i) Crushed red food passes through the cardiac orifice into the stomach. At this time cardiac and pyloric sphincter openings are closed.
(ii) Peristalsis waves are initiated in the stomach. The peristalsis wave begins at the fundus of the stomach and ends at the pylori.
(iii) Peristalsis occurs due to contraction of circular muscles and longitudinal muscles. At the top of the esophagus are the circular muscles and at the bottom are the longitudinal muscles.
(iv) Cardiac sphincter dilates when food reaches end of esophagus and food is expelled into stomach.
(v) Gastric juice and hydrochloric acid mix with food inside the stomach to form chyme.
(vi) HCl secreted from the stomach destroys food microbes. Creates an acidic environment for digestion.
2. Chemical digestion of food
(i) Inactive pepsinogen is converted to active pepsin under the influence of HCl. Pepsin breaks down proteins into proteases and peptones.
(ii) Gelatinase enzyme breaks down gelatin to produce peptones and polypeptides.
(iii) Lipase enzyme breaks down lipids to produce fatty acids, glycerol and monoglycerides.

Mechanical digestion
1. Saliva moistens and softens food. Water moistens the mouth and tastes. Saliva helps in chewing and swallowing food.
2. The taste buds of the tongue receive sensations from wet food. Then it is possible to understand the taste of food.
3. Mucin converts food into lubricators.
4. Saliva helps in chewing and swallowing food. Helps to neutralize acid and alkali.
5. Chloride present in saliva activates amylase.
6. Salivary bicarbonate acts as a buffer. It reduces the strength of mouth acid and prevents erosion of tooth enamel.
7. Salivary immunoglobulin acts as antibacterial.
8. Saliva cleans food debris from the teeth and maintains the sensitivity of the soft parts of the mouth.
Chemical digestion
1. The enzyme tyalin breaks down starch and glycogen into maltose, maltotriose and isomaltose.
2. Maltose is converted into glucose under the action of maltase enzyme.

1. Digested in the mouth
There are no enzymes that digest lipids in the mouth. The mucin of the salivary glands makes the food slippery.
2. Digestion in the stomach
The enzyme lipase breaks down lipids to produce fatty acids, glycerol and monoglycerides.
3. Digestion in the pancreatic juice of the small intestine
(i) The enzyme lipase breaks down lipids into fatty acids and glycerol.
(ii) Phospholipids are converted into fatty acids and monoglycerides in the presence of phospholipase enzymes.
4. Digestion in the intestinal juice of the small intestine
(i) The enzyme lipase breaks down triglycerides and diglycerides into fatty acids, glycerol and monoglycerides.
(ii) Fatty acids and glycerol are produced from monoglycerides under the action of monoglyceridase enzyme.
Lipids are complex biochemical substances. It is complex, insoluble and non-absorbable for the organism. It is of no benefit to the organism. Lipids are digested into fatty acids and glycerol by the action of various enzymes. Fatty acids and glycerol are simple, soluble and absorbable by the body. Our body cells absorb fatty acids and glycerol. These fatty acids and glycerol provide energy to our body.