Digestion in small intestine - class-XI

(A) Enterokinin is the hormone secreted by Brunner's gland activates the inactive enzyme of the pancreas and convert procarboxypeptidase into carboxypeptidase.

pH scale is not a linear scale like a centimetre or inch scale (in which two adjacent values have the same difference). It is a logarithmic scale in which two adjacent values increase or decrease by a factor of 10. As pH 7.5 and 5.5 differ by 2, the difference in H$^{+}$ ion concentration between them would be 10$^{2}$ = 100.

In the stomach, food is mixed with gastric juices and hydrochloric acid, churning it into a semi-fluid state known as chyme. This acidic mixture is subsequently released in small portions into the duodenum through the pyloric sphincter. Bolus refers to chewed food mixed with saliva before swallowing, while chyle refers to milky lymphatic fluid containing emulsified fats found in the intestine.

Secretin is a hormone released by the duodenum in response to low pH (acidic chyme). Its primary function is to stimulate the pancreas to release bicarbonate-rich fluid to neutralize the acid. If the pH is not low, the stimulus for secretin release is absent.

Glucose is absorbed via active transport (specifically secondary active transport coupled with sodium) in the small intestine. Alcohol is absorbed via passive diffusion, and water is absorbed via osmosis.

Matching the anatomical terms correctly reveals that the duodenum is a U-shaped structure emerging from the stomach (A-iii). The epiglottis is a cartilaginous flap that prevents food from entering the trachea (B-i). The glottis is the opening of the windpipe (C-iv), and the caecum is a small blind sac that hosts symbiotic microorganisms (D-ii).

The folds of Kerkring (plicae circulares) are permanent folds of the mucosa and submucosa, most prominent in the duodenum and jejunum, which increase the surface area for absorption.

The pyloric sphincter is a band of smooth muscle at the junction between the pylorus of the stomach and the duodenum of the small intestine. It acts as a valve to control the flow of partially digested food (chyme) into the small intestine. The cardiac sphincter guards the opening from the esophagus into the stomach.

The entire small intestine has circular folds of the mucous membrane, the valves' of kerkring These folds are more prominent in the jejunum.

The sphincter of Oddi (or hepatopancreatic sphincter) controls the flow of bile and pancreatic juice into the duodenum. When closed, bile is diverted back to the gallbladder.

Intrinsic factor is a glycoprotein produced by the parietal cells of the stomach, which is essential for the absorption of Vitamin B12 in the ileum.

Peyer's patches are aggregated lymphoid nodules found specifically in the submucosa of the ileum, which is part of the small intestine.

The mucosa of the duodenum contains absorptive cells (enterocytes), goblet cells (secreting mucus), and argentaffin (enteroendocrine) cells, which produce hormones like secretin and CCK.

The ileum is characterized by the presence of Peyer's patches and club-shaped villi. Brunner's glands are a characteristic feature of the duodenum, not the ileum.

Bile salts are synthesized in the liver, stored in the gall bladder, and aid in fat digestion in the small intestine before being actively reabsorbed specifically in the terminal ileum. This process is known as the enterohepatic circulation of bile salts. Other nutrients like glucose and fats are mostly absorbed in the duodenum and jejunum.

The semi-digested, acidic food mass that moves from the stomach into the duodenum is called chyme.

The duodenum is an endocrine organ that produces hormones such as secretin, cholecystokinin (CCK), and enterogastrone to regulate digestion.

Bile salts are amphipathic molecules that emulsify large fat globules into smaller droplets, increasing the surface area for lipase enzymes to act upon.

Cholecystokinin (CCK) and secretin are produced by enteroendocrine cells located in the mucosa of the duodenum.

The stomach is divided into the cardia, fundus, body, and the distal pylorus, which connects to the duodenum.

Active absorption in the small intestine involves energy-dependent transport mechanisms for various nutrients, including glucose, amino acids, and sodium ions.

Secretin is produced by the S-cells in the mucosa of the small intestine, specifically the duodenum.

Both secretin and cholecystokinin are hormones secreted by the duodenal mucosa in response to chyme entering the small intestine.

The hepatopancreatic duct (formed by the common bile duct and pancreatic duct) opens into the duodenum at the major duodenal papilla.

Meckel's diverticulum is a common congenital anomaly resulting from the incomplete closure of the vitelline duct, occurring in the distal ileum.

The small intestine is the primary site for both the chemical digestion of food and the absorption of nutrients into the bloodstream. While the stomach begins protein digestion, the vast majority of nutrient uptake occurs in the small intestine.

Vomiting is a reflex action typically triggered by irritation or distension of the stomach or the upper small intestine (duodenum). It is controlled by the vomiting center in the medulla oblongata, not the hypothalamus.

Secretin is a hormone released by the S cells of the duodenum in response to low pH (acidic chyme) entering from the stomach. It stimulates the pancreas to release bicarbonate-rich fluid to neutralize the acid.

Peyer's patches are organized lymphoid follicles found in the submucosa of the ileum, which is the distal portion of the small intestine. They play a critical role in immune surveillance of the intestinal lumen.

Enterokinase (also known as enteropeptidase) is an enzyme that activates trypsinogen into trypsin. Without trypsin, other pancreatic proteases cannot be activated, leading to a severe failure in protein digestion.

The small intestine is divided into three parts in order: the duodenum (first), the jejunum (middle), and the ileum (last). The duodenum receives chyme from the stomach.

Bile salts are actively reabsorbed in the final part of the small intestine, known as the terminal ileum, returning to the liver via enterohepatic circulation. Most other basic nutrients like glucose and fats are primarily absorbed earlier in the duodenum and jejunum.

The small intestine consists of the duodenum, jejunum, and ileum. The jejunum is the middle segment located between the duodenum and the ileum.

The hepatopancreatic duct (ampulla of Vater) enters the duodenum and its opening is controlled by the sphincter of Oddi, which regulates the flow of bile and pancreatic juice.

Peptidases are the intestinal proteases, which act on peptides as follows:
$Polypeptide\xrightarrow[Alkaline ,medium]{Polypeptidase} Dipeptides$

Statement A is incorrect because the small intestine is actually much longer than the large intestine, measuring about 6 to 7 meters in humans compared to the large intestine's 1.5 meters. Statement B is also incorrect because the small intestine is narrower in diameter than the large intestine, which is why they are named small and large based on width rather than length.

The correct answer is (c).

Various types of movements are generated by the 'muscular' layer of the small intestine.

The correct answer is (c).

(A) Duodenum - 'U' shaped structure emerging from the stomach

The correct answer is (a).

Starch is broken down by alpha-amylase into disaccharides like maltose in the small intestine. Option A is wrong because pepsin works in the stomach, and Option C is wrong because trypsin acts on proteins, not triglycerides.

Isomaltase is an enzyme that breaks the bonds linking saccharides, which cannot be broken by amylase or maltase. It digests polysaccharides at the alpha 1-6 linkages

The end product of carbohydrate digestion is mainly glucose together with some fructose and galactose (monosaccharide). 

• Fatty acids and glycerol being insoluble, cannot be absorbed into the blood. So, they are first incorporated into small droplets called micelles which move into the intestinal mucosa. 
  
• They are then re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the bloodstream. Hence, chylomicrons are fat droplets coated with cholesterol and proteins.

• Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides. However, fats are completely digested in the small intestine.
• Fat is completely digested in the small intestine because it gets bile from the liver through gallbladder which is responsible for fat digestion.
  
• Hence Fat is completely digested in the Small intestine.
  
• So, the correct answer is 'Small intestine'.

The
chyme produced by digestion in the stomach is passed on to the small intestine
for further digestion and absorption where the enzyme pancreatic amylase of the
pancreatic juice converts the carbohydrates present in the chyme into
disaccharides and monosaccharides. 

• During the mastication process, the food is positioned by the cheek and tongue between the teeth for grinding. As chewing continues, the food is made softer and warmer, and the enzymes in saliva begin to break down carbohydrates in the food. After chewing, the food (now called a bolus) is swallowed.

•  The salivary glands produce two different types of secretions: thin, watery saliva and thick mucus. This mucus is an excellent lubricant and makes swallowing food easier.
• Secretion of bicarbonate into the adherent layer of mucus gel creates a pH gradient with a near-neutral pH at the epithelial surfaces in stomach and duodenum, providing the first line of mucosal protection against luminal acid.
• Bile contains bile acids, which are critical for digestion and absorption of fats and fat-soluble vitamins in the small intestine. The function of bile is to bring ingested fats into the emulsion to facilitate the work of the lipases.

The stomach is the site for the digestion of Protein. Pepsin present in duodenum breaks down the food proteins into polypeptides which are then broken down by various exopeptidases and dipeptidases into amino acids.

Chylomicrons are lipoprotein particles that consist of triglycerides (85–92%), phospholipids (6–12%), cholesterol (1–3%), and proteins (1–2%).They transport dietary lipids from the intestines to other locations in the body. However, they are not produced in the lumen of the small intestine rather in the epithelial cells of it. The chylomicrons produced in the epithelial cells are released into the lacteals and then get transported into the lymphatic vessels.

Amylopsin is a pancreatic enzyme that converts starch to sugar. It is most active in an alkaline pH. Rennin functions best in an acidic environment and loses its activity when pH is higher than 7. So, the correct option is 'Alkaline and acidic'.

Fats are broken down into fatty acids and glycerol, which are absorbed into the intestinal villi. Because they are large, they enter the lymphatic vessels called lacteals rather than the blood capillaries.

Invertase is an enzyme present in succus entericus. It is also called as sucrase. It hydrolyzes sucrose into its components glucose and fructose.

Salivary juice contains starch splitting enzyme amylase. The amylase of saliva is called as ptyalin. Pepsin is the major proteolytic enzyme of stomach. Pepsin is secreted as a proenzyme pepsinogen and is activated to pepsin on exposure to acid in stomach. Trypsin is a component of pancreatic juice where it is present as trypsinogen. Trypsinogen is activated to trypsin on exposure to enterokinase. The intestinal juice contains a number of disaccharidases like maltase, lactase, sucrase for splitting disaccharides into monosaccharides, which can be readily absorbed.

Excess carbohydrates in diet are converted into glycogen and stored in liver and muscles to be used during starvation or in-between meals to maintain a constant blood glucose level. Still excess carbohydrates are converted into adipose tissue of fat and stored around visceral organs and under the skin.

Trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, nuclease are the enzymes secreted by the pancreas.

Secretion and synthesis of bile is the main function of liver. Bile is yellowish-green, alkaline fluid. In bile juice, bile salts, sodium carbonate, glycolate, taurocholate, bile pigments, cholesterol, lecithin etc. are present. Bile salts help in emulsification of fats in small intestine. It mainly act at the duodenum part of the small intestine.

Bile salts (of bile) plays a very important role in the digestion of fats. As fats are insoluble in water, they enter the intestine as drops within the watery chyme. The bile salts, which are partly lipid-soluble and partly water-soluble, emulsify the fat particles and, as a result, the fat-drops acquire a greater surface area. The enzyme lipase, then acts upon these fat-drops and allows its digestion into glycerol and fatty acids. 

Carbohydrates are hydrolysed by concerning enzymes and converted into monosaccharides like glucose and fructose. Monosaccharides are absorbed via the capillary blood with in the villus to finally reach into portal vein. Absorption of glucose molecules occurs along with, Na+ by active import. Fructose is absorbed passively through facilitated diffusion.

In herbivores like rabbit, the symbiotic bacteria and protozoans present in the caecum, help in digestion of cellulose into glucose. So, the digestion of cellulose takes place in caecum by the process of decomposition. This decomposition process is very slow. So, very less amount of cellulose is digested at a time in caecum.

Chyme is a thick semifluid mass of partially digested food and digestive secretions that is formed in the stomach and intestine during digestion.

Lipases are enzymes found in the small intestine of humans that helps in the breakdown of fats. it is not involved in the digestion of starch.

Amylase digests complex starch (polysaccharide) molecules into smaller, simpler maltose (disaccharide) molecules. 

Lactase is an enzyme produced by the small intestine, found in the digestive system of humans. It is essential to complete the digestion of whole milk. Lactase breaks down lactose, a simple sugar which gives milk its sweetness. 

•  In the digestive environment, proteins, carbohydrates are initially attacked by enzymes produced by your salivary glands and stomach. And then it attacks fats.
• However, effective enzymatic breakdown of fats requires emulsification by bile acids, a process that does not occur until fats leave your stomach and enter your small intestine. Hence, Fats in the stomach is inhibited. This inhibition is due to the slow digestion of fat. So, the correct answer is 'Slow
  digestion of fat'.

In small intestine Pancreatic amylase converts starch and dextrose into maltoses and small intestine is main site for digestion of carbohydrates.

Starch is hydrolyzed by pancreatic amylase to maltose, isomaltose and dextrose sucrose $\overset{suctase}{\rightarrow}$ glucose and fructose.

Glucose and gala dose are absorbed by active transport. Fructoseis absorbed by facilitated diffusion.

Generally, fatty acids upto a chain length of 10 carbon atom are primarily absorbed through the blood capillaries, but those with higher chain length through lymphatic route (lymph vessels).

Gastric juice contains hydrochloric acid (HCl), which gives it a very low pH, making it highly acidic. This acidity is necessary to activate pepsinogen and kill bacteria.

Pancreatic juice and bile both empty into the duodenum. They typically join to form the common bile duct or hepatopancreatic duct before entering the duodenum at the major duodenal papilla.

Chymotrypsin is a proteolytic enzyme secreted by the pancreas in an inactive form (chymotrypsinogen) as part of the pancreatic juice.

Gastric lipase is an enzyme that is particularly important in infants for the digestion of milk fats, as their pancreatic lipase levels may not yet be fully developed.

Succus entericus is the technical term for intestinal juice, which is secreted by the glands of the small intestine (crypts of Lieberkuhn and Brunner's glands).

The exocrine portion of the pancreas consists of acinar cells, which synthesize and secrete digestive enzymes into the pancreatic ducts.

Digestion of fat begins in duodenum with the help of pancreatic lipase but cannot be completed there because of its small size. Further it is component of small intestine. In infants some digestion occurs in stomach.

Peyer's patches are lymphoid aggregates found in the submucosa of the ileum. They are a key component of the gut-associated lymphoid tissue (GALT).

Succus entericus, also known as intestinal juice, contains a wide variety of enzymes including disaccharidases, dipeptidases, lipases, and nucleosidases that complete the final stages of carbohydrate, protein, and fat digestion. Thus, it is referred to as a complete digestive juice. Pancreatic juice contains many enzymes too, but succus entericus finalizes all nutrient breakdowns.

• Most of the fat in the human diet is in the form of triacylglycerol, which consists of three fatty acids linked to glycerol.
• In the digestive tract,  triacylglycerol is hydrolyzed by the enzyme pancreatic lipase, to release free fatty acids and monoglycerides. Hence, If you have just digested triglyceride molecules, you would expect to find increased amounts of fatty acids and glycerol.

• Most lipids that you consume in your diet are fats. Some digestion occurs in your mouth and the stomach but mostly takes place in the small intestine. The purpose of the small intestine is to finish digestion and absorb nutrients. 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. Hence, most of the digestion of lipids in humans takes place in the small intestine.
  

• Trypsin is produced in the pancreas as an inactive protease trypsinogen. It is a protease found in the digestive system of many vertebrates, where it hydrolyses protein.

CCK is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gallbladder. This hormone is secreted in response to the fat in the chyme

In the intestinal mucosal cells, after absorption, the triglycerides and cholesterol are re-esterified to form chylomicrons. The chylomicrons in turn are secreted into intestinal lymph, enter the blood stream through the thoracic duct and bind to the wall of capillaries in adipose and skeletal tissue.

• Digestion is the process where complex material in the food is broken down to simple nutrients and molecules so that they can be absorbed and circulated in the body. 
• The digestion of the food starts in the buccal cavity of the organism where the enzyme amylase is secreted. This enzyme is used for the digestion of the starch present in the food. Though much of the starch is digested in the mouth it is divided in disaccharide units of carbohydrates which will be further digested in intestine. 
• The food from the buccal cavity is taken to the stomach via oesophagus where the food is stored for about 3 to 4 hours and mixed with the hydrochloric acid which is secreted by the oxyntic cell present in the mucosa wall of the stomach. This will give the food a acidic pH which is necessary for the activation of the pepsin enzyme which are there for digestion of proteins to peptides. 
• This acidic chyme of the stomach is to be transported for further digestion in the intestine where it is mixed with the alkaline secretion released from the liver and pancreas. 
• In intestine the food components such as carbohydrates, proteins, lipids etc are digested to small absorbable molecules. 
• The carbohydrates are digested to monosaccharides glucose, fructose, galactose, proteins are broken down to amino acids and lipids are reduced to small fat globules called microns. 
• Some of these components are already present in the cell fo the body surrounding the digestive track. So for the transport of molecules ion transporters are used sach as sodium ion transporter, etc. 
• Glucose is easily digested compared to other molecules of monosaccharides as it is easily soluble in water. Monosaccharides such as fructose and amino acid units are transported to the blood stream along with the sodium ions that are transportedby the sodium ion transporter. 
• Fat globules are transported by the lacteal cells that are present in the intestine mucosa of the intestine. Chylomicrons are also used to transport fat to liver and adipose tissue where it can be stored. 
• Therfore for the following question option A and C are correct. 

Trypsinogen is the inactive zymogen form of trypsin secreted by the pancreas. It is activated in the duodenum by enterokinase.

(A) Lipase - Fats

Rennin is an aspartic endopeptidase belonging to MEROPS A1 family. It is produced by newborn ruminant animals in the lining of the abomasum to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. It is widely used in the production of cheese. Lactase is an enzyme produced by many organisms.  Lactase can be purchased as a food supplement, and is added to milk to produce "lactose-free" milk products. Lactase is the enzymes that splits lactose (milk sugar) into glucose and galactose. The enzymes necessary to break down and digest milk (rennin and lactase) are all gone by the age of three in most humans. 

The correct answer is (c).

The correct answer is (c).

Starvation mode is a state in which the body is responding to prolonged periods of low energy intake levels. The body responds to reduced energy intake by burning fat reserves and consuming muscle and other tissues. Specifically, the body burns fat after first exhausting the contents of the digestive tract along with glycogen reserves stored in muscle and liver cells. After prolonged periods of starvation, the body will utilize the proteins within muscle tissue as a fuel source.

Hindgut fermentation is a digestive process seen in monogastric herbivores, animals with a simple, single-chambered stomach. Cellulose is digested with the aid of symbiotic bacteria. The microbial fermentation occurs in the digestive organs that follow the small intestine: the large intestine and caecum.

The correct answer is (b).

Sucrase enzymes are located on the brush border of the small intestine. The enzymes catalyze the hydrolysis of sucrose to fructose and glucose. The sucrase enzyme invertase, which occurs more commonly in plants, also hydrolyzes sucrose but by a different mechanism