Physiology of digestion Questions in English

(C) The alimentary canal exhibits rhythmic,wave-like muscular contractions and relaxations that help move food through the digestive tract. This specific process is known as $Peristalsis$.

(C) Parietal cells (also known as oxyntic cells) in the stomach lining are responsible for secreting $HCl$ (hydrochloric acid) and intrinsic factor.
$HCl$ is essential for converting the inactive proenzyme pepsinogen into the active enzyme pepsin.
Pepsin is the primary enzyme responsible for the digestion of proteins into proteoses and peptones in the acidic environment of the stomach.
If parietal cells become non-functional,$HCl$ secretion decreases,leading to a higher (less acidic) $pH$ in the stomach.
Consequently,pepsinogen will not be activated into pepsin,and protein digestion will be severely impaired.

(A) The parietal cells (also known as oxyntic cells) of the gastric glands are responsible for the secretion of $HCl$ (hydrochloric acid) and Castle's intrinsic factor.
$HCl$ plays a crucial role in the activation of the proenzyme pepsinogen into its active form,pepsin,by providing an acidic $pH$ (approximately $1.8$).
If the secretion of $HCl$ is blocked by an inhibitor,the conversion of inactive pepsinogen to active pepsin will not occur.
Therefore,the digestion of proteins in the stomach will be severely affected.

(C) The digestion of proteins begins in the stomach,where pepsin acts on proteins to convert them into proteoses and peptones. However,the final breakdown of proteins into amino acids occurs in the small intestine. In the small intestine,pancreatic enzymes like trypsin,chymotrypsin,and carboxypeptidase,along with intestinal peptidases (aminopeptidases and dipeptidases),act on the protein fragments to complete the process of digestion,resulting in the formation of amino acids.

(B) Bile juice,secreted by the liver and stored in the gallbladder,contains bile salts (such as sodium glycocholate and sodium taurocholate).
These bile salts play a crucial role in the digestion of lipids by the process of emulsification.
Emulsification is the breakdown of large fat globules into smaller droplets,which increases the surface area for the action of the enzyme lipase.
Therefore,bile juice specifically acts on fats.

(A) The rate of gastric emptying depends on the chemical composition and physical state of the food.
Liquids empty faster than solids.
Among liquids,those with lower caloric density and simpler chemical structures empty more rapidly.
Beer,being a liquid with a relatively low caloric density compared to complex nutrients like proteins,fats,or carbohydrates,passes through the pyloric sphincter into the duodenum much faster than solid foods or nutrient-dense liquids.

(B) Bile,secreted by the liver and stored in the gallbladder,does not contain any digestive enzymes. Its primary function is the emulsification of fats. Emulsification is the process of breaking down large fat globules into smaller droplets,which increases the surface area for the action of the enzyme lipase. This significantly facilitates the digestion of fats in the small intestine.

(A) The digestion of carbohydrates is primarily carried out by the enzyme $Amylase$.
$1$. $Salivary \ amylase$ (ptyalin) acts on starch in the mouth.
$2$. $Pancreatic \ amylase$ acts on starch in the small intestine.
$Lipase$ is responsible for the digestion of fats,$Pepsin$ for proteins,and $Erepsin$ for peptides.
Therefore,the correct option is $A$.

(D) The stomach secretes gastric juice,which contains $HCl$ (hydrochloric acid).
$HCl$ creates an acidic environment with a low $pH$ (approximately $1.8$).
This highly acidic environment is essential for the activation of pepsinogen into pepsin and also acts as a potent bactericidal agent,killing most bacteria and pathogens present in the ingested food.

(C) In herbivores like rabbits, the diet consists mainly of cellulose.
Cellulose is a complex carbohydrate that requires microbial fermentation for digestion.
The $Caecum$ is a large, sac-like structure located at the junction of the small and large intestine.
It contains symbiotic microorganisms that produce the enzyme cellulase, which breaks down cellulose into simpler absorbable forms.
Therefore, the digestion of cellulose primarily occurs in the $Caecum$.

(B) The stomach of a rabbit,like many mammals,contains gastric glands that secrete hydrochloric acid $(HCl)$.
This acid creates an acidic environment in the stomach,which is essential for the activation of pepsinogen into pepsin and for the digestion of proteins.
The $pH$ of the rabbit's stomach typically ranges between $1$ and $3$,making $3$ the most appropriate choice among the given options.

(A) Gram (chickpeas) primarily consists of starch (carbohydrates) and proteins. Digestion of carbohydrates begins in the mouth with the action of salivary amylase (ptyalin),which breaks down starch into simpler sugars like maltose. Therefore,the digestion of gram starts in the mouth.

(A) Bile contains bile pigments ($bilirubin$ and $biliverdin$),bile salts,cholesterol,and phospholipids,but no enzymes.
Bile salts (sodium glycocholate and sodium taurocholate) are essential for the emulsification of fats,which breaks down large fat globules into smaller droplets,thereby increasing the surface area for the action of lipase enzymes.
Bile pigments are excretory products and do not participate in digestion.
Therefore,bile salts are the only component of bile that is useful in the process of digestion.

(C) The small intestine is the site where the final stages of digestion occur for all major nutrients.
In the small intestine,the pancreatic juice and intestinal juice (succus entericus) contain various enzymes like pancreatic amylase,trypsin,chymotrypsin,carboxypeptidase,and lipases.
These enzymes break down complex carbohydrates into monosaccharides,proteins into amino acids,and fats into fatty acids and glycerol.
Therefore,the complete digestion of carbohydrates,proteins,and fats takes place in the small intestine.

(D) Protein digestion begins in the stomach with the action of pepsin.
However,the process of protein digestion is completed in the small intestine,specifically in the duodenum and the ileum.
In the duodenum,pancreatic enzymes like trypsin,chymotrypsin,and carboxypeptidase break down proteins into smaller peptides.
Finally,in the ileum,intestinal enzymes (aminopeptidases and dipeptidases) break down these peptides into amino acids,which are then absorbed.

(A) The process of taking in food into the body is known as $Ingestion$.
$Egestion$ is the process of eliminating undigested food waste from the body.
$Digestion$ is the process of breaking down complex food substances into simpler absorbable forms.
$Nutrition$ is the overall process of obtaining and utilizing food for growth and energy.

(D) The process of digestion in humans is accomplished by both mechanical and chemical processes.
Mechanical processes include mastication of food in the mouth and churning in the stomach.
Chemical processes involve the action of various enzymes and digestive juices on food components to break them down into simpler absorbable forms.

(A) Pancreatic juice contains a variety of enzymes that are essential for the digestion of all major food components.
$1$. Proteolytic enzymes (like trypsin,chymotrypsin,and carboxypeptidase) digest proteins into peptides and amino acids.
$2$. Pancreatic amylase digests carbohydrates (starches) into disaccharides.
$3$. Pancreatic lipase (steapsin) digests fats (triglycerides) into fatty acids and glycerol.
Therefore,pancreatic juice acts on proteins,carbohydrates,and fats.

(D) Trypsin is a proteolytic enzyme secreted by the pancreas as an inactive proenzyme,trypsinogen.
It is activated by enterokinase in the duodenum.
Unlike pepsin,which functions in the acidic environment of the stomach $(pH \approx 1.8)$,trypsin functions in the alkaline environment of the small intestine (duodenum) where the $pH$ is approximately $7.8$ to $8.4$.
Therefore,trypsin digests proteins in an alkaline medium in the duodenum.

(B) Bile,produced by the liver and stored in the gallbladder,contains bile salts (such as sodium glycocholate and sodium taurocholate). These salts are essential for the emulsification of fats. Emulsification is the process of breaking down large fat globules into smaller droplets,which significantly increases the surface area for the action of the enzyme lipase. Therefore,bile primarily induces the breakdown (emulsification) of fats.

(B) The small intestine is the primary site for the digestion and absorption of food.
Within the small intestine,the $Duodenum$ is the initial part where most of the chemical digestion occurs due to the action of bile,pancreatic juices,and intestinal enzymes.
However,the $Jejunum$ is the middle part of the small intestine where the majority of the chemical digestion is completed and the absorption of nutrients is most efficient.
Therefore,the maximum digestion and absorption of food occur in the small intestine,specifically within the $Jejunum$ and $Duodenum$ regions,but $Jejunum$ is often cited as the site where the process is most extensive.

(B) The digestion of fats occurs primarily in the small intestine.
Bile salts emulsify large fat globules into smaller droplets,increasing the surface area for enzyme action.
The enzyme $Lipase$ (specifically pancreatic lipase) then acts on these emulsified fats to break them down into fatty acids and glycerol.
$Pepsin$ is involved in protein digestion,$Amylase$ in carbohydrate digestion,and $Sucrase$ in the breakdown of sucrose.

(A) In mammals,milk digestion is primarily facilitated by the enzyme $Rennin$ (also known as $Chymosin$).
This enzyme is secreted by the gastric glands in the stomach of young mammals (infants).
$Rennin$ acts on the milk protein $Casein$ and converts it into insoluble $Paracasein$ (calcium paracaseinate),which is then further broken down by $Pepsin$.
Therefore,the correct option is $A$.

(D) In $Hydra$,digestion is a two-step process.
First,extracellular digestion occurs in the gastrovascular cavity where enzymes are secreted onto the food.
Second,intracellular digestion occurs within the food vacuoles of the gastrodermal cells after the partially digested food is engulfed.
Therefore,$Hydra$ exhibits both extracellular and intracellular digestion.

(D) Milk contains a disaccharide sugar called lactose.
Lactose is composed of two monosaccharide units: glucose and galactose.
The enzyme lactase,present in the small intestine,hydrolyzes lactose into its constituent monosaccharides,which are glucose and galactose.
Therefore,the end products of milk digestion are glucose and galactose.

(B) The correct answer is $B$. The $Sphincter$ \text{ of } $Oddi$ is a muscular valve that regulates the flow of digestive juices, specifically bile and pancreatic juice, from the hepatopancreatic duct into the duodenum. It ensures that these secretions enter the small intestine only when required for digestion.

(C) The correct answer is $C$.
$Rennin$ (also known as $chymosin$) is a proteolytic enzyme secreted by the peptic cells (chief cells) present in the epithelium of gastric glands.
It is primarily found in the gastric juice of human infants and calves to facilitate the digestion of milk.
In adults, the secretion of $rennin$ is absent.
$Rennin$ acts on the milk protein $casein$ and converts it into $paracasein$ (calcium paracaseinate), which leads to the coagulation or curdling of milk, allowing it to stay in the stomach longer for further digestion by $pepsin$.

(C) The correct answer is $C$. Baked potatoes consist of starch, which is a polysaccharide.
$1$. In the oral cavity, food is mixed with saliva. Saliva contains an enzyme called salivary amylase (ptyalin), which converts starch into maltose, isomaltose, and small dextrins.
$Starch \xrightarrow[\text{Salivary amylase}]{} \text{Maltose} + \text{Isomaltose} + \alpha\text{-Dextrins}$.
$2$. The pancreatic juice, which enters the small intestine, contains a starch-digesting enzyme called pancreatic amylase. This enzyme further breaks down any remaining starch into maltose, isomaltose, and $\alpha$-dextrins.
$Starch \xrightarrow[\text{Pancreatic } \alpha\text{-amylase}]{} \text{Maltose} + \text{Isomaltose} + \alpha\text{-Dextrins}$.
$3$. Finally, disaccharidases such as maltase, present in the intestinal juice (succus entericus) of the small intestine, break down disaccharides like maltose into simpler monosaccharides (glucose).

(A) The yellow colour of the stool is due to the presence of bile pigments,specifically bilirubin,which is yellow in colour.
Bile pigments are excretory products formed from the breakdown of hemoglobin.
Bile is a bitter-tasting,greenish-yellow alkaline fluid produced by the liver,stored in the gall bladder,and secreted into the duodenum.
Even in infants feeding solely on mother's milk,these pigments are processed and excreted,resulting in the characteristic yellowish colour of the stool.

(B) The correct answer is $B$.
$1$. Saliva contains salivary amylase,which begins the digestion of starch in the mouth. Therefore,starch does not reach the stomach completely undigested.
$2$. Saliva does not contain any lipase,and the stomach lacks fat-emulsifying agents (bile salts),so fat remains largely undigested until it reaches the small intestine.
$3$. Humans lack the enzyme cellulase required to digest cellulose; therefore,cellulose passes through the digestive tract completely undigested.
$4$. Proteins begin their digestion in the stomach due to the action of pepsin and hydrochloric acid.
Thus,fat and cellulose reach the stomach in an undigested state.

(D) The correct option is $D$.
In the small intestine,pancreatic juice contains pancreatic $\alpha$-amylase.
This enzyme hydrolyzes starch and glycogen into disaccharides like maltose.
The reaction is: $\text{Starch/Glycogen} \xrightarrow{\text{Pancreatic } \alpha\text{-Amylase}} \text{Maltose} + \text{Isomaltose} + \text{Limit Dextrins}$.
Option $A$ is incorrect because $Pepsin$ acts in the stomach.
Option $B$ is incorrect because $Lipase$ acts primarily in the small intestine.
Option $C$ is incorrect because $Trypsin$ acts on proteins,not triglycerides.

(A) The parietal cells (also known as oxyntic cells) of the gastric glands secrete $HCl$ (hydrochloric acid).
In the presence of $HCl$,pepsinogen (a proenzyme),which is an inactive precursor of the enzyme pepsin,is converted into its active form,$i.e.$,pepsin.
The activated pepsin then acts through autocatalysis to convert more pepsinogen into pepsin.
This pepsin enzyme is the primary protease or proteolytic enzyme of the stomach.
$\text{Pepsinogen (Inactive form)} \xrightarrow{HCl} \text{Pepsin (Active form)}$
Therefore,if the secretion of $HCl$ is blocked,inactive pepsinogen will not be converted into the active enzyme pepsin.

(C) In the small intestine,the enzyme lipase acts on fats.
Lipases break down triglycerides into diglycerides and monoglycerides.
Further,the enzymes present in the intestinal juice,such as lipases,act on di- and monoglycerides to break them down into their final absorbable forms.
The final products of the digestion of di- and monoglycerides are fatty acids and glycerol.

(B) The small intestine is the primary site for both the completion of digestion and the absorption of nutrients.
After the food is converted into chyme in the stomach,it enters the small intestine.
In the small intestine,bile,pancreatic juice,and intestinal juice (succus entericus) act upon the chyme to complete the digestion of carbohydrates,proteins,and fats.
The digested end products are then absorbed into the blood and lymph through the intestinal mucosa,primarily in the jejunum and ileum.

(B) The correct matching is as follows:
$(a)$ Nuclease acts on nucleic acids to break them down into nucleotides. Thus, $(a - iii)$.
$(b)$ Nucleotidases act on nucleotides to break them down into nucleosides and inorganic phosphate. Thus, $(b - iv)$.
$(c)$ Nucleosidases act on nucleosides to break them down into sugars and nitrogenous bases. Thus, $(c - i)$.
$(d)$ Lipases act on fats to break them down into monoglycerides and fatty acids. Thus, $(d - ii)$.
Therefore, the correct sequence is $(a - iii), (b - iv), (c - i), (d - ii)$.

(C) Option $A$ is incorrect because fructose is absorbed by facilitated diffusion,not by $Na^+$ dependent transport.
Option $B$ is incorrect because chylomicrons are transported into lacteals (lymph vessels),not directly into blood capillaries.
Option $C$ is correct because approximately $30\%$ of starch is hydrolyzed into maltose by salivary amylase in the oral cavity.
Option $D$ is incorrect because oxyntic cells (parietal cells) secrete $HCl$ and intrinsic factor,while chief cells (peptic cells) secrete the proenzyme pepsinogen.

(C) The stomach is a primary site for the absorption of certain substances.
Absorption of water,simple sugars,and alcohol takes place in the stomach lining.
While the small intestine is the principal organ for the absorption of nutrients,the stomach specifically facilitates the absorption of these substances due to its mucosal structure.

(C) The digestion of carbohydrates begins in the mouth. The salivary glands secrete saliva,which contains the enzyme salivary amylase (ptyalin). This enzyme breaks down starch into maltose.
After passing through the stomach (where no significant carbohydrate digestion occurs),the food enters the small intestine. In the small intestine,pancreatic amylase continues the digestion of starch,and intestinal enzymes (disaccharidases like maltase,lactase,and sucrase) break down disaccharides into monosaccharides like glucose,galactose,and fructose. Therefore,the mouth and small intestine are the primary sites for carbohydrate digestion.

(D) In the stomach, the gastric glands secrete gastric juice which contains the proenzyme pepsinogen.
This pepsinogen is converted into the active enzyme pepsin in the presence of hydrochloric acid $(HCl)$ at an acidic $pH$ of approximately $1.8$.
Pepsin acts on proteins to convert them into proteoses and peptones.
Therefore, the reaction $Protein \xrightarrow[{pH\, 1.8}]{{Pepsin}} Proteoses + Peptones$ is correct for digestion in the stomach.

(C) Succus entericus,also known as intestinal juice,contains various enzymes including intestinal lipase. Lipase acts on emulsified fats (lipids) to break them down into simpler forms. Specifically,intestinal lipase converts triglycerides into diglycerides,monoglycerides,and eventually into fatty acids and glycerol. Among the given options,the conversion of lipids into di- and monoglycerides represents the initial stage of lipid digestion mediated by lipases in the small intestine.

(C) Emulsification is the process of breaking down large fat globules into smaller droplets,which increases the surface area for the action of enzymes. This process is primarily facilitated by bile salts (such as sodium glycocholate and sodium taurocholate) present in the bile juice. In the absence of bile salts,fats cannot be emulsified,which significantly hinders their digestion. Therefore,the correct answer is $C$.

(A) Protein digestion begins in the stomach,where the enzyme $pepsin$ acts on proteins to break them down into proteoses and peptones in the presence of $HCl$.
While digestion continues in the duodenum due to pancreatic enzymes like $trypsin$,$chymotrypsin$,and $carboxypeptidase$,the initial and significant site for protein digestion is the stomach.

(D) The digestion of fats begins in the stomach to a very small extent,but the majority of fat digestion occurs in the $Small \ intestine$.
Specifically,the $Duodenum$ (the first part of the small intestine) receives bile from the liver,which emulsifies fats,and pancreatic lipase,which breaks down the emulsified fats into fatty acids and glycerol.
Therefore,the small intestine is the primary site for fat digestion.

(D) Rennin (also known as chymosin) is a proteolytic enzyme found in the gastric juice of infants.
It acts on the milk protein casein.
In the presence of calcium ions,it converts soluble casein into insoluble calcium paracaseinate (curdling of milk).
This reaction occurs in the acidic environment of the stomach,typically at a $pH$ range of $1-3$.

(B) The epithelial cells of the intestinal mucosa,specifically the enterocytes,are responsible for the absorption of nutrients.
To increase the surface area for efficient absorption,these cells possess numerous microscopic projections on their apical surface known as $Microvilli$.
These $Microvilli$ form a brush border,which significantly enhances the absorptive capacity of the small intestine.

(C) The parietal cells (also known as oxyntic cells) of the gastric glands are responsible for the secretion of $HCl$ (hydrochloric acid) and Castle's intrinsic factor.
$HCl$ plays a crucial role in the stomach by providing an acidic medium ($pH$ $1.8$) which is necessary for the activation of the inactive proenzyme pepsinogen into the active proteolytic enzyme pepsin.
If the secretion of parietal cells is blocked,$HCl$ will not be produced.
Consequently,the conversion of inactive pepsinogen to active pepsin will be inhibited,leading to impaired protein digestion in the stomach.

(A) In the duodenum,the enzyme $Lipase$ acts on $Triglycerides$ to break them down into $Monoglycerides$ and fatty acids.
Option $A$ correctly identifies the site (Duodenum),the substrate (Triglycerides),and the end product (Monoglycerides).
Option $B$ is incorrect because starch digestion by amylase primarily occurs in the mouth and duodenum,not just the small intestine generally.
Option $C$ is incorrect because $Pepsin$ acts in the stomach,not the small intestine.
Option $D$ is incorrect because $Micelles$ are formed in the small intestine with the help of bile salts,not in the stomach.

(B) In humans,the process of digestion begins in the mouth (buccal cavity).
Salivary amylase (ptyalin) acts on starch in the mouth,breaking it down into maltose.
Therefore,starch is partially digested in the mouth and does not reach the stomach completely undigested.
Fats and cellulose are not chemically digested in the mouth.
Fats require bile salts and lipase (which act in the small intestine) for digestion.
Cellulose is a complex carbohydrate that cannot be digested by human enzymes.
Thus,both fats and cellulose reach the stomach in an undigested state.

(A) The yellow color of the stool in a newborn baby is primarily due to the presence of bile pigments,specifically bilirubin and biliverdin.
Even though the baby consumes white milk,the liver secretes bile into the digestive tract.
These bile pigments are metabolic waste products of the breakdown of red blood cells.
As they pass through the gastrointestinal tract,they undergo chemical changes by intestinal bacteria,resulting in the characteristic yellow color of the stool.