Physiology of digestion Questions in English

(C) The bile duct and the pancreatic duct open together into the duodenum as the common hepatopancreatic duct.
This duct is guarded by a muscular valve called the $Sphincter$ $of$ $Oddi$.
$A)$ The $Ileocaecal$ $valve$ prevents the backflow of faecal matter from the large intestine into the small intestine.
$B)$ The $Pyloric$ $sphincter$ regulates the passage of food from the stomach into the duodenum.
$D)$ The $Semilunar$ $valve$ is found in the heart and prevents the backflow of blood from the arteries into the ventricles.

(D) Parietal cells (also known as oxyntic cells) in the stomach lining are responsible for secreting hydrochloric acid $(HCl)$ and intrinsic factor.
$HCl$ is essential for converting the inactive enzyme pepsinogen into its active form,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 cannot be activated efficiently,and proteins will not be adequately hydrolyzed by pepsin.

(N/A) Pancreatic juice contains inactive enzymes such as $Trypsinogen$, $Chymotrypsinogen$, and $Procarboxypeptidases$. These enzymes are essential for protein digestion.
$1$. $Enterokinase$, secreted by the intestinal mucosa, activates $Trypsinogen$ into $Trypsin$:
$Trypsinogen \xrightarrow{Enterokinase} Trypsin + \text{Inactive peptide}$
$2$. $Trypsin$ then activates $Chymotrypsinogen$ and $Procarboxypeptidase$:
$\text{Chymotrypsinogen} \xrightarrow{Trypsin} \text{Chymotrypsin}$
$\text{Procarboxypeptidase} \xrightarrow{Trypsin} \text{Carboxypeptidase}$
$3$. $Chymotrypsin$ converts proteins into peptides:
$\text{Proteins} \xrightarrow{Chymotrypsin} \text{Peptides}$
$4$. $Carboxypeptidase$ acts on the carboxyl end of the peptide chain, releasing the terminal amino acids:
$\text{Peptides} \xrightarrow{Carboxypeptidase} \text{Smaller peptide chains} + \text{Amino acids}$
In summary, partially hydrolyzed proteins (peptones and proteoses) in the chyme are acted upon by these proteolytic enzymes for complete digestion into dipeptides and amino acids:
$\text{Proteins, Peptones, Proteoses} \xrightarrow[Carboxypeptidase]{Trypsin/Chymotrypsin} \text{Dipeptides}$

(N/A) The digestion of proteins begins in the stomach and is completed in the small intestine.
The digestive juice secreted by the gastric glands present in the stomach wall is called gastric juice. The food that enters the stomach becomes acidic upon mixing with this gastric juice.
The main components of gastric juice are hydrochloric acid $(HCl)$, pepsinogen, mucus, and rennin (in infants). Hydrochloric acid creates an acidic medium $(pH \approx 1.8)$ so that the inactive enzyme pepsinogen is converted into its active form, pepsin.
$Pepsinogen \xrightarrow{HCl} Pepsin$
Pepsin is a proteolytic enzyme that converts proteins into proteoses and peptones.
$Proteins \xrightarrow{Pepsin} Proteoses + Peptones$
Rennin is a proteolytic enzyme found in gastric juice of infants, which helps in the digestion of milk proteins. It is released in an inactive form called prorennin, which is activated by $HCl$.
$Prorennin \xrightarrow{HCl} Rennin$
$Casein (milk protein) \xrightarrow{Rennin} Paracasein \to Calcium \ Paracaseinate$

(N/A) Bile is a digestive juice secreted by the liver. Although it does not contain any digestive enzymes,it plays an important role in the digestion of fats. Bile juice contains bile salts,bilirubin,and biliverdin. These break down large fat globules into smaller globules so that the pancreatic enzymes can easily act on them. This process is known as emulsification of fats. Bile juice also makes the medium alkaline and activates lipase.

(N/A) The digestion of carbohydrates occurs in the mouth and the small intestine of the alimentary canal. Enzymes acting on carbohydrates are collectively known as carbohydrases.
Digestion in the mouth:
As food enters the mouth,it mixes with saliva. Saliva,secreted by salivary glands,contains salivary amylase. This enzyme breaks down starch into sugar at $pH\, 6.8$.
$Starch \xrightarrow[pH\, 6.8]{\text{Salivary amylase}} \text{Maltose} + \text{Isomaltose} + \text{Limit dextrins}$
Salivary amylase continues to act in the oesophagus,but its action stops in the stomach due to the acidic environment.
Digestion in the small intestine:
Carbohydrate digestion resumes in the small intestine. Pancreatic juice contains pancreatic amylase,which hydrolyses polysaccharides into disaccharides.
$Starch \xrightarrow{\text{Pancreatic amylase}} \text{Disaccharides}$
Intestinal juice contains disaccharidases (maltase,lactase,sucrase,etc.) which complete the digestion of disaccharides into monosaccharides:
$Maltose \xrightarrow{\text{Maltase}} 2 \text{ Glucose}$
$Lactose \xrightarrow{\text{Lactase}} \text{Glucose} + \text{Galactose}$
$Sucrose \xrightarrow{\text{Sucrase}} \text{Glucose} + \text{Fructose}$

(A) Hydrochloric acid $(HCl)$ is secreted by the oxyntic or parietal cells present in the stomach lining. It performs two main functions: it creates an acidic medium (pH $1.8$) and kills bacteria present in the food.
The acidic medium is essential for the activation of the enzyme pepsinogen into its active form,pepsin. Pepsin is a proteolytic enzyme responsible for the digestion of proteins into peptides.
If $HCl$ were not secreted,the stomach environment would not become acidic. Consequently,pepsinogen would not be converted into active pepsin,which would severely impair the digestion of proteins in the stomach.

(N/A) The digestion of proteins begins in the stomach and is completed in the small intestine.
The enzymes that act on proteins are known as proteases.
Digestion in the stomach:
The gastric glands in the stomach wall secrete gastric juice, which contains $HCl$, pepsinogen, and rennin. The acidic medium converts inactive pepsinogen into active pepsin. Active pepsin converts proteins into proteases and peptides.
$Proteins \xrightarrow{Pepsin} Proteases + Peptides$
Rennin is involved in the coagulation of milk proteins.
Digestion in the small intestine:
In the small intestine, pancreatic juice and intestinal juice act on the food.
Action of pancreatic juice:
Pancreatic juice contains inactive enzymes: trypsinogen, chymotrypsinogen, and procarboxypeptidases. Enterokinase, secreted by the intestinal mucosa, activates trypsinogen into trypsin.
$Trypsinogen \xrightarrow{Enterokinase} Trypsin + \text{Inactive peptide}$
Activated trypsin then activates chymotrypsinogen into chymotrypsin and procarboxypeptidases into carboxypeptidases.
$Proteins \xrightarrow{Chymotrypsin} Peptides$
$Peptides \xrightarrow{Carboxypeptidase} \text{Smaller peptide chains} + \text{Amino acids}$
Action of intestinal juice:
Intestinal juice (succus entericus) contains dipeptidases that complete the digestion of proteins.
$Dipeptides \xrightarrow{Dipeptidases} \text{Amino acids}$

(N/A) Pepsinogen is an inactive proenzyme (zymogen) secreted by the chief cells of the gastric glands in the stomach. It is converted into its active form,pepsin,by the action of hydrochloric acid $(HCl)$. The reaction is as follows:
$\text{Pepsinogen (Inactive)} \xrightarrow{HCl} \text{Pepsin (Active)} + \text{Inactive peptide}$

(N/A) Bile is a digestive juice secreted by the liver and stored in the gall bladder. Bile contains bile salts (such as bilirubin and biliverdin). These salts break down large fat globules into smaller globules,increasing the surface area for pancreatic enzymes to act upon. This process is known as the emulsification of fats. Additionally,bile makes the medium alkaline,which is necessary for the activation of lipase.

(N/A) The oral cavity performs two major functions: mastication of food and facilitation of swallowing.
Teeth and tongue,with the help of saliva,masticate and mix the food thoroughly.
Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.
The bolus is then conveyed into the pharynx and subsequently into the oesophagus by the process of swallowing or deglutition.
The bolus passes down through the oesophagus by successive waves of muscular contractions known as peristalsis.
The gastro-oesophageal sphincter controls the passage of food into the stomach.
Saliva secreted into the oral cavity contains electrolytes $(Na^{+}, K^{+}, Cl^{-}, HCO_{3}^{-})$ and enzymes,specifically salivary amylase and lysozyme.
The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate-splitting enzyme,salivary amylase.
Approximately $30\%$ of starch is hydrolysed here by this enzyme into the disaccharide maltose.
$\text{Starch} \xrightarrow{\text{Salivary Amylase}, \text{pH } 6.8} \text{Maltose}$
Lysozyme present in saliva acts as an antibacterial agent that prevents infections.

(N/A) The stomach mucosa contains gastric glands. Gastric glands have three major types of cells:
$(1)$ Mucous neck cells which secrete mucus.
$(2)$ Peptic or chief cells which secrete the proenzyme pepsinogen.
$(3)$ Parietal or oxyntic cells which secrete $HCl$ and intrinsic factor (essential for the absorption of vitamin $B_{12}$).
When food enters the stomach,the hormone gastrin is secreted into the blood by the pyloric region,which stimulates the secretion of gastric juice from the gastric glands.
The stomach stores food for $4-5$ hours. The food mixes thoroughly with the acidic gastric juice through the churning movements of the muscular wall,forming a semi-digested acidic mass called chyme.
The proenzyme pepsinogen,upon exposure to hydrochloric acid,is converted into the active enzyme pepsin. Pepsin converts proteins into proteoses and peptones.
$\text{Pepsinogen} \xrightarrow{HCl} \text{Pepsin} \rightarrow \text{Proteins} \rightarrow \text{Proteoses} + \text{Peptones}$.
The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated $HCl$.
$HCl$ provides the acidic pH optimal for pepsin activity. Rennin is a proteolytic enzyme found in the gastric juice of infants,which helps in the digestion of milk proteins.
Small amounts of lipases are also secreted by gastric glands.

(N/A) The mucosa of the stomach contains gastric glands. Gastric glands consist of three major types of cells:
$(1)$ Mucous neck cells which secrete mucus.
$(2)$ Peptic or chief cells which secrete the proenzyme pepsinogen.
$(3)$ Parietal or oxyntic cells which secrete $HCl$ and intrinsic factor (essential for the absorption of vitamin $B_{12}$).
When food enters the stomach,the hormone gastrin is secreted into the blood by the pyloric region,which stimulates the secretion of gastric juice from the gastric glands.
The stomach stores food for $4-5$ hours. The food mixes thoroughly with the acidic gastric juice through the churning movements of the muscular wall,forming a mass called chyme. The proenzyme pepsinogen,upon exposure to hydrochloric acid,is converted into the active enzyme pepsin. Pepsin converts proteins into proteoses and peptones.
$\text{Pepsinogen} \xrightarrow{HCl} \text{Pepsin} \rightarrow \text{Proteins} \rightarrow \text{Proteoses} + \text{Peptones}$
The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated $HCl$.
$HCl$ provides the acidic $pH$ optimal for pepsin activity. Rennin is a proteolytic enzyme found in the gastric juice of infants,which helps in the digestion of milk proteins. Small amounts of lipases are also secreted by gastric glands.

(N/A) The semi-digested food,known as chyme,enters the small intestine from the stomach through the pyloric sphincter. In the small intestine,chyme comes into contact with three major secretions: pancreatic juice,bile juice,and intestinal juice.
$1$. Mechanical Digestion: The muscularis layer of the small intestine generates various movements (peristalsis and segmentation) that facilitate the thorough mixing of food with these secretions.
$2$. Pancreatic Juice Action: Pancreatic juice contains inactive enzymes: trypsinogen,chymotrypsinogen,and procarboxypeptidases,along with amylases,lipases,and nucleases. Trypsinogen is activated by enterokinase (secreted by the intestinal mucosa) into active trypsin,which then activates other pancreatic enzymes.
- Proteins,peptones,and proteoses are acted upon by trypsin,chymotrypsin,and carboxypeptidase to form dipeptides.
- Carbohydrates (starch) are hydrolyzed by pancreatic amylase into disaccharides.
- Fats are broken down by lipases into diglycerides and monoglycerides.
- Nucleic acids are acted upon by nucleases to form nucleotides and nucleosides.
$3$. Bile Action: Bile contains bile pigments (bilirubin and biliverdin),bile salts,cholesterol,and phospholipids,but no enzymes. It aids in the emulsification of fats,breaking them down into small micelles,which increases the surface area for lipase action.
$4$. Intestinal Juice (Succus Entericus) Action: The final steps of digestion occur in the jejunum and ileum,where enzymes in the intestinal juice convert complex molecules into absorbable simple forms:
- Dipeptides $\xrightarrow{\text{Dipeptidases}}$ Amino acids
- Maltose $\xrightarrow{\text{Maltase}}$ Glucose + Glucose
- Lactose $\xrightarrow{\text{Lactase}}$ Glucose + Galactose
- Sucrose $\xrightarrow{\text{Sucrase}}$ Glucose + Fructose
- Nucleotides $\xrightarrow{\text{Nucleotidases}}$ Nucleosides $\xrightarrow{\text{Nucleosidases}}$ Sugars + Nitrogenous bases
- Di and Monoglycerides $\xrightarrow{\text{Lipases}}$ Fatty acids + Glycerol
The resulting simple components are absorbed in the jejunum and ileum,while undigested and unabsorbed substances pass into the large intestine.

(A) The process of conversion of complex food substances into simple absorbable forms is called digestion. This process is carried out by our digestive system by mechanical and biochemical methods.

(B) The proenzyme pepsinogen is secreted by the chief cells of the stomach.
When pepsinogen comes in contact with hydrochloric acid $(HCl)$ secreted by the parietal cells,it undergoes a conformational change.
This process converts the inactive proenzyme pepsinogen into the active proteolytic enzyme pepsin.
This active pepsin then helps in the digestion of proteins into proteoses and peptones.

(N/A) Pancreatic juice contains inactive enzymes: $Trypsinogen$, $Chymotrypsinogen$, and $Procarboxypeptidases$, along with $Amylase$, $Lipase$, and $Nuclease$.
$Trypsinogen$ is activated by $Enterokinase$ (secreted by the intestinal mucosa) into active $Trypsin$, which subsequently activates the other proteolytic enzymes in the pancreatic juice.
$Trypsin$ acts on proteins, $Proteoses$, and $Peptones$ to convert them into $Polypeptides$.
$Chymotrypsin$ converts $Proteoses$ and $Polypeptides$ into smaller $Peptides$.
$Carboxypeptidase$ acts on the terminal ends of $Polypeptides$ to release individual $Amino acids$ and smaller $Peptides$.

(N/A) Chyme is the semi-digested,acidic,and pulpy mass of food that passes from the stomach into the small intestine.
It is formed in the stomach through the mechanical churning of food and its chemical breakdown by gastric juices,which include $HCl$ and enzymes like pepsin.
The acidic nature of chyme helps in the activation of pepsinogen into pepsin and kills bacteria present in the food.
Once the food is converted into chyme,the pyloric sphincter relaxes to allow it to enter the duodenum for further digestion and absorption.

(B) Bile salts present in bile help in the emulsification of fats,which is the process of breaking down large fat globules into very small micelles.
This process increases the surface area of the fats,allowing lipases to act more efficiently on them.
The breakdown process is as follows:
$1.$ Emulsification: Large fat globules $\longrightarrow$ Small micelles.
$2.$ Enzymatic action: Fats $\xrightarrow{\text{Lipases}}$ Diglycerides $\longrightarrow$ Monoglycerides.
Additionally,nucleases in the pancreatic juice act on nucleic acids to form nucleotides and nucleosides:
Nucleic acids $\xrightarrow{\text{Nucleases}}$ Nucleotides $\longrightarrow$ Nucleosides.

(N/A)

Chyme	Completely digested food
$(1)$ It is the semi-solid and acidic food present in the stomach.	$(1)$ It is the completely digested,liquid,and alkaline (basic) food.
$(2)$ It represents the partial digestion of food.	$(2)$ It represents the conversion of food into simple absorbable components.
$(3)$ It is acidic in nature.	$(3)$ It is alkaline (basic) in nature.
$(4)$ Proteins are converted into peptones and proteoses; these are not absorbed.	$(4)$ Simple compounds like amino acids,glucose,fructose,and monoglycerides are formed; these are absorbed.

(N/A) $(1)$ The process of converting complex food components into simple and absorbable forms with the help of enzymes is called digestion.
$(2)$ The rhythmic contraction and relaxation of the muscular walls of the digestive tract that pushes food forward,particularly in the oesophagus,is called peristalsis.

(N/A) Emulsification is the process in which bile salts,present in bile juice,break down large fat globules into smaller fat droplets,thereby increasing the surface area for the action of lipase enzymes and facilitating the digestion of fats.

(DUODENUM) The breakdown of complex food molecules like dipeptides,disaccharides,and glycerides into simpler absorbable forms (amino acids,monosaccharides,and fatty acids/glycerol) occurs primarily in the duodenum. The pancreatic enzymes and bile salts released into the duodenum facilitate these chemical digestion processes.

(B) The intestinal mucosa is protected from the highly acidic chyme entering from the stomach primarily by the alkaline secretions.
$1$. The pancreas secretes pancreatic juice containing bicarbonate ions $(HCO_3^-)$,which neutralizes the acidic chyme.
$2$. Brunner's glands (submucosal glands) present in the duodenum secrete an alkaline,mucus-rich fluid that further neutralizes the acid and protects the intestinal wall.
$3$. Goblet cells in the intestinal mucosa secrete mucus,which provides a physical barrier and lubrication for the lining.

(N/A) In the duodenum,fats are first emulsified by bile salts,which increases the surface area for enzyme action.
The enzyme pancreatic lipase,secreted by the pancreas,acts on these emulsified fats.
Fats are broken down into diglycerides and subsequently into monoglycerides.
Finally,these are broken down into fatty acids and glycerol.
The process is represented as:
$1. \text{Fats} \xrightarrow{\text{Lipases}} \text{Diglycerides} \rightarrow \text{Monoglycerides}$
$2. \text{Diglycerides/Monoglycerides} \xrightarrow{\text{Lipases}} \text{Fatty acids} + \text{Glycerol}$

(N/A) The hepato-pancreatic complex consists of the liver and the pancreas,which release their secretions into the duodenum via the hepato-pancreatic duct.
$1$. Bile (from the liver): Bile contains bile salts (bilirubin and biliverdin) which do not contain enzymes. Its primary role is the emulsification of fats,breaking down large fat globules into smaller micelles,thereby increasing the surface area for lipase action.
$2$. Pancreatic Juice (from the pancreas): This contains inactive enzymes like trypsinogen,chymotrypsinogen,and procarboxypeptidases,along with active enzymes like pancreatic amylase,lipases,and nucleases.
- Carbohydrate digestion: Pancreatic amylase breaks down polysaccharides into disaccharides.
- Protein digestion: Trypsinogen is activated by enterokinase to trypsin,which then activates other proteases. These enzymes break down proteins,proteoses,and peptones into dipeptides.
- Fat digestion: Pancreatic lipases,aided by bile,break down emulsified fats into diglycerides and monoglycerides.

(N/A) The digestion of carbohydrates begins in the mouth,where food mixes with saliva.
This makes the food soft and moist. Food components are broken down into smaller pieces due to the process of chewing (mastication).
Saliva contains the enzyme salivary amylase. This enzyme converts starch into maltose,isomaltose,and $\alpha$-dextrin. Approximately $30\%$ of the starch in food is hydrolyzed in the mouth.
$\text{Starch} \xrightarrow{\text{Salivary Amylase}} \text{Maltose} + \text{Isomaltose} + \alpha\text{-Dextrin}$
Saliva does not contain any enzymes for the digestion of proteins or fats,so they are not digested in the mouth.
In the mouth,teeth are embedded in the sockets of the jawbone. This type of dental arrangement is called thecodont.
There are four types of teeth arranged in the order of $2, 1, 2, 3$ in each half of each jaw. These are incisors,canines,premolars,and molars,respectively. They form the dental formula: $\frac{2123}{2123}$.

(B) Trypsinogen is an inactive enzyme secreted by the pancreas into the small intestine. It is activated into its active form,trypsin,by an enzyme called enterokinase (also known as enteropeptidase),which is secreted by the intestinal mucosa.

(D) The correct answer is option $D$ because $Rennin$ is a proteolytic enzyme found in the gastric juice of infants,which helps in the digestion of milk proteins.
Option $A$ is incorrect as the proteolytic enzymes found in pancreatic juice are $Trypsin$,$Chymotrypsin$,$Carboxypeptidase$,etc.
Option $B$ is incorrect as the proteolytic enzymes found in intestinal juice are $Dipeptidases$.
Option $C$ is incorrect as no enzymes are present in bile juice.

(C) Digestion is the process in which non-diffusible food substances are converted into diffusible forms with the help of digestive enzymes.
Digestion is primarily carried out by the process of hydrolysis,which involves the use of water molecules to break chemical bonds.
Therefore,digestive enzymes are classified as hydrolases.

(B) In human beings,the digestion process starts in the mouth,continues in the stomach,and is completed in the small intestine of the alimentary canal.
About $25-30 \%$ of carbohydrates (polysaccharides like starch) are digested or converted into disaccharides (maltose) in the buccal cavity by the action of salivary amylase.
The remaining digestion of carbohydrates is completed in the small intestine.

(A) The secretions of the intestinal mucosal goblet cells and the secretions of the brush border cells of the mucosa constitute the intestinal juice,which is also known as succus entericus.
This juice contains a variety of enzymes like disaccharidases (e.g.,maltase),dipeptidases,lipases,and nucleosidases,which complete the final stages of digestion.

(C) Pancreatic $\alpha$-amylase is secreted into the small intestine. It acts on starch and converts it into disaccharides like maltose,isomaltose,and dextrins.
Equation $(a)$ is incorrect because the stomach does not contain significant fat-emulsifying agents or lipase for effective fat digestion.
Equation $(b)$ is incorrect because pepsin acts in the stomach,not the small intestine,and it converts proteins into proteoses and peptones,not directly into amino acids.
Equation $(d)$ is incorrect because triglycerides are broken down into monoglycerides and fatty acids by the enzyme lipase,not trypsin (which acts on proteins).

(A) Polysaccharides (starch) are partially digested in the buccal cavity and stomach.
They are completely digested in the small intestine through the action of various enzymes,primarily pancreatic amylase.
The digestion of starch mainly occurs in the proximal part of the small intestine,known as the duodenum.

(A) Digestion involves both mechanical and biochemical processing of food.
Basically,it is the breakdown of complex organic substances of food like carbohydrates,proteins,and fats (macronutrients) into simple,soluble,and absorbable forms.
Therefore,it is defined as the conversion of insoluble polymers of food into their soluble monomers.

(A) The enzyme enterokinase (also known as enteropeptidase) is secreted by the intestinal mucosa.
It acts on the inactive proenzyme trypsinogen,which is secreted by the pancreas into the duodenum.
Enterokinase activates trypsinogen by converting it into its active form,trypsin.
Trypsin then further activates other pancreatic enzymes like chymotrypsinogen and procarboxypeptidase.

(A) The process of digestion is carried out by both mechanical and chemical means.
Mechanical digestion involves the mastication (chewing) of food in the buccal cavity and the churning movements in the stomach.
Chemical digestion involves the action of various enzymes (like salivary amylase,pepsin,etc.) that break down complex food substances into simpler absorbable forms.
Therefore,digestion is a combined process of mechanical and chemical breakdown.

(B) When food in the form of $Bolus$ reaches the stomach through the involuntary movements of the muscular coat of the oesophagus,it mixes thoroughly with the gastric juices present in the stomach.
This thoroughly mixed food is called $Chyme$.

(B) The digestion of protein begins in the stomach with the action of pepsin and is completed in the small intestine. In the small intestine,pancreatic enzymes like trypsin,chymotrypsin,and carboxypeptidase,along with intestinal peptidases,break down polypeptides into amino acids.

(B) Protein-digesting enzymes are known as proteases.
In the stomach,the gastric glands secrete gastric juice which contains the proenzyme pepsinogen.
This proenzyme is converted into its active form,pepsin,in the presence of $HCl$.
Pepsin is the primary enzyme responsible for the digestion of proteins into proteoses and peptones within the stomach.
Trypsin is secreted by the pancreas into the small intestine,and erepsin is found in the intestinal juice.

(D) Bile does not contain any digestive enzymes,yet it plays a crucial role in fat digestion.
$1$. Emulsification: Bile salts reduce the surface tension of large fat globules and break them down into smaller droplets,increasing the surface area for lipase action.
$2$. Alkalinity: Bile salts provide an alkaline medium to the chyme,which is necessary for the optimal activity of pancreatic enzymes and inactivates gastric pepsin.
$3$. Micelle formation: Bile salts combine with fatty acids and monoglycerides to form micelles,which are essential for the transport of fat digestion products from the intestinal lumen to the absorptive cells of the intestinal mucosa.

(C) In the small intestine,the enzyme $Enterokinase$ (also known as $Enteropeptidase$) is secreted by the intestinal mucosa.
It plays a crucial role in activating $Trypsinogen$ into its active form,$Trypsin$.
$Trypsin$ is a proteolytic enzyme that further activates other pancreatic zymogens like $Chymotrypsinogen$ and $Procarboxypeptidase$.
Since $Albumin$ is a protein,its digestion relies on the action of these pancreatic proteases.
Therefore,in the absence of $Enterokinase$,the activation of these enzymes is inhibited,and the digestion of proteins like $Albumin$ is significantly affected.

(B) Pepsinogen is an inactive proenzyme (zymogen) secreted by the chief cells or zymogenic cells of the gastric glands.
It is converted into its active form,pepsin,by the action of hydrochloric acid $(HCl)$.
$HCl$ is secreted by the oxyntic or parietal cells of the gastric glands,which provides the acidic medium necessary for the activation of pepsinogen.

(A) Digestion of starch (polysaccharides) begins in the mouth due to the action of salivary amylase.
However, the stomach is the primary site for the digestion of proteins, where enzymes like pepsin break down proteins into peptides.
Therefore, $A$ is the stomach and $B$ is protein.

(A) In the small intestine,the partially hydrolysed food (chyme) coming from the stomach is acted upon by three main secretions:
$1$. Bile: Secreted by the liver and stored in the gallbladder,it helps in the emulsification of fats.
$2$. Pancreatic juice: Secreted by the exocrine part of the pancreas,it contains enzymes like trypsinogen,chymotrypsinogen,procarboxypeptidases,amylases,lipases,and nucleases.
$3$. Intestinal juice (Succus entericus): Secreted by the goblet cells of the mucosa and the brush border cells of the intestinal mucosa,it contains various enzymes like disaccharidases,dipeptidases,lipases,and nucleosidases.
These three secretions mix with the chyme to complete the process of digestion.

(A) In human beings,the process of digestion begins in the mouth.
$1$. Carbohydrate digestion initiates in the mouth,where salivary amylase breaks down polysaccharides into disaccharides like maltose.
$2$. Protein digestion initiates in the stomach,where pepsin acts on proteins to convert them into proteoses and peptones.
$3$. Fat digestion initiates in the small intestine,primarily through the action of bile salts (emulsification) and pancreatic lipase.
Therefore,the correct sequence for the initiation of digestion of proteins,fats,and carbohydrates is stomach,intestine,and mouth,respectively.

(A) The process of swallowing is known as deglutition.
Through deglutition,the bolus (masticated food) is moved from the mouth into the pharynx and subsequently into the oesophagus.
Peristalsis is the wave-like muscular contraction that moves food through the alimentary canal.
Ingestion is the intake of food.
Succus entericus refers to the intestinal juice secreted by the glands of the small intestine.

(C) The food remains in the stomach for a duration of $4-5$ hours.
During this time,the food is mixed thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall.
The resulting acidic semi-digested food is called chyme.

(C) Saliva contains a starch-splitting enzyme called salivary amylase or ptyalin.
When food reaches the stomach,the highly acidic environment created by $HCl$ (hydrochloric acid) in the gastric juice denatures and inactivates the ptyalin,as it functions optimally at a neutral $pH$.