Physiology of digestion Questions in English

(B) Symbiotic bacteria, such as those found in the human gut (e.g., $Escherichia$ $coli$), play a crucial role in the digestive process.
They assist in the breakdown of complex carbohydrates and fibers that human enzymes cannot digest on their own.
Additionally, they contribute to the synthesis of essential vitamins like Vitamin $K$ and Vitamin $B_{12}$, which are absorbed by the host.

(D) Starch is a polysaccharide composed of long chains of glucose units.
During the process of digestion,starch is broken down by enzymes such as amylase into smaller disaccharides like maltose.
Finally,the enzyme maltase breaks down maltose into two molecules of glucose.
Therefore,glucose is the final product of starch digestion that can be absorbed by the body.

(B) Pepsin is a proteolytic enzyme secreted by the gastric glands in the stomach. It acts on proteins in an acidic medium ($pH$ $1.8-2.0$) and converts them into proteoses and peptones. Therefore,the correct option is $B$.

(A) Enterokinase is an enzyme secreted by the intestinal mucosa.
It acts on the inactive proenzyme trypsinogen and converts it into its active form,trypsin.
Trypsin then activates other pancreatic enzymes like chymotrypsinogen and procarboxypeptidase.
Pepsinogen is converted to pepsin by hydrochloric acid $(HCl)$ in the stomach,not by enterokinase.

(D) Lipase is a digestive enzyme that catalyzes the hydrolysis of fats (lipids).
In the presence of water and bile salts (which emulsify the fats),lipase breaks down triglycerides into fatty acids and glycerol.
The reaction is: $\text{Fat} + H_2O \xrightarrow{\text{Lipase}} \text{Fatty acids} + \text{Glycerol}$.

(B) Peristalsis is a series of wave-like involuntary muscular contractions and relaxations that occur in the digestive tract.
It helps in the movement of food through the esophagus,stomach,and intestines.
This process is essential for the transport of food bolus towards the stomach and for mixing it with digestive enzymes.

(A) In mammals,the digestion of starch begins in the mouth. The mouth contains $3$ pairs of salivary glands that secrete saliva. Saliva contains an enzyme called salivary amylase (ptyalin),which acts on cooked starch and converts it into maltose,isomaltose,and limit dextrin.

(A) $Rennin$ (also known as $chymosin$) is a proteolytic enzyme found in the gastric juice of infants.
It acts specifically on $casein$, which is the primary phosphoprotein found in milk.
The site of action for $rennin$ is the $stomach$.
Therefore, the correct pairing is $Stomach-Casein$.

(B) The process of protein digestion begins in the $Stomach$.
In the $Stomach$, the proenzyme $pepsinogen$ is converted into the active enzyme $pepsin$ by $HCl$.
$Pepsin$ acts on proteins and converts them into $proteoses$ and $peptones$.
While digestion continues in the $Small \text{ } intestine$ via pancreatic enzymes like $trypsin$, $chymotrypsin$, and $carboxypeptidase$, the $Stomach$ is the primary site where protein digestion is initiated.

(D) The secretion of hydrochloric acid $(HCl)$ in the stomach is regulated by multiple factors:
$1$. $Gastrin$: This hormone is released by $G$-cells in the stomach and stimulates parietal cells to secrete $HCl$.
$2$. $Histamine$: Released by enterochromaffin-like $(ECL)$ cells,it acts on $H_2$ receptors on parietal cells to strongly stimulate acid secretion.
$3$. $Vagal$ $discharge$: The parasympathetic nervous system,via the vagus nerve,releases acetylcholine which stimulates parietal cells to secrete acid during the cephalic and gastric phases of digestion.
Therefore,all these factors contribute to the stimulation of acid secretion.

(A) The stomach is the primary site for protein digestion in vertebrates.
Gastric glands secrete gastric juice,which contains $HCl$ and proenzymes like pepsinogen.
$HCl$ converts inactive pepsinogen into active pepsin.
Pepsin breaks down proteins into proteoses and peptones.
While some minor digestion of fats occurs due to gastric lipase,the primary and most significant chemical digestion occurring in the stomach is that of proteins.

(B) The gastric glands in the stomach secrete mucus,which plays a crucial role in protecting the stomach lining (gastric mucosa) from the highly acidic environment created by $HCl$ and the digestive action of the enzyme pepsin. Without this protective layer,the stomach wall would be damaged by its own digestive secretions.

(B) The main nutritional component of gram is protein. The digestion of protein begins in the stomach,where the enzyme pepsin acts on it in the presence of hydrochloric acid $(HCl)$.

(C) The food in the small intestine is referred to as 'chyme' (after leaving the stomach) and is further processed by three digestive secretions:
$1$. Bile: Produced by the liver,it emulsifies fats.
$2$. Pancreatic juice: Contains enzymes for the digestion of carbohydrates,proteins,and fats.
$3$. Intestinal juice (succus entericus): Contains enzymes that complete the final stages of digestion.
Therefore,the correct combination is Bile,pancreatic juice,and intestinal juice.

(B) Erepsin is a proteolytic enzyme complex found in the intestinal juice (succus entericus). It acts on peptones and proteoses to break them down into amino acids. This process primarily occurs in the ileum,which is the final part of the small intestine where the final stages of protein digestion take place.

(C) The muscular contraction in the alimentary canal is known as $Peristalsis$.
This rhythmic wave-like movement of the muscles in the wall of the alimentary canal helps in the movement of food from the anterior to the posterior direction through the lumen.

(C) The digestion of cellulose in rabbits occurs in the $Caecum$.
In the $Caecum$,the cellulose present in plant-based food is broken down by the action of symbiotic bacteria.

(A) Bile juice contains bile salts which emulsify fats,breaking them down into smaller droplets. This increases the surface area for the action of the enzyme lipase,which is present in pancreatic juice,to complete the digestion of fats into fatty acids and glycerol.

(A) Succus entericus,also known as intestinal juice,is a clear,yellow fluid secreted by the glands of the small intestine.
It has a slightly alkaline nature with a $pH$ typically ranging between $7.6$ and $7.8$.
It contains water,mucus,and various enzymes such as disaccharidases,dipeptidases,and lipases,which are essential for the final stages of digestion.
Therefore,among the given options,$7.8$ is the correct value.

(B) Succus entericus is the term used for intestinal juice.
It is secreted by the glands of the small intestine,specifically the crypts of Lieberkühn and Brunner's glands.
It contains various enzymes like disaccharidases,dipeptidases,lipases,and nucleosidases that help in the final digestion of food components.
Therefore,the correct option is $B$.

(A) The $pH$ of the small intestine is slightly alkaline to facilitate the action of pancreatic and intestinal enzymes.
In the duodenum,the $pH$ is approximately $7.6$ due to the presence of bile and pancreatic juice.
As the chyme moves further into the jejunum and ileum,the $pH$ gradually increases to a slightly alkaline range,typically around $7.5$ to $8.5$.

(B) The chief function of bile is the emulsification of fats.
Bile salts break down large fat globules into smaller droplets,which increases the surface area for the action of the enzyme lipase.
This process makes the digestion of fats by enzymes much more efficient.

(B) Carnivorous animals consume a diet that is significantly higher in proteins compared to herbivores. The metabolism of proteins results in the production of nitrogenous waste products,such as ammonia,which is toxic. The liver is the primary site for the urea cycle,where ammonia is converted into urea for excretion. Because carnivores ingest a higher protein load,their liver must process a larger amount of nitrogenous waste,necessitating a larger liver size to handle the increased metabolic demand.

(C) Bile contains bile salts (such as sodium glycocholate and sodium taurocholate) which break down large fat globules into smaller droplets. This process is known as emulsification of fats. Emulsification increases the surface area of fats,allowing lipase enzymes to act more efficiently during digestion.

(A) The gall bladder stores and concentrates bile juice produced by the liver.
Bile juice contains bile salts which are essential for the emulsification of fats.
Emulsification breaks down large fat globules into smaller droplets,increasing the surface area for the action of the enzyme lipase.
If the gall bladder is surgically removed (cholecystectomy),the bile is no longer stored or concentrated; instead,it flows directly into the small intestine.
While the liver continues to produce bile,the lack of concentrated bile storage leads to a reduced efficiency in the digestion and absorption of dietary fats,particularly after a high-fat meal.

(D) Lactase is a brush border enzyme found in the intestinal juice (succus entericus).
It catalyzes the hydrolysis of lactose into glucose and galactose.
Other enzymes present in intestinal juice include maltase,sucrase,dipeptidases,and lipases.

(C) The $pH$ values of various digestive juices are as follows:
$1$. Bile: $7.6 - 8.6$ (Alkaline)
$2$. Saliva: $6.2 - 7.6$ (Slightly acidic to slightly alkaline)
$3$. Gastric juice: $1.5 - 3.5$ (Highly acidic)
$4$. Pancreatic juice: $7.5 - 8.8$ (Alkaline)
Among the given options,gastric juice has the lowest $pH$ due to the presence of $HCl$.

(C) Pepsinogen is an inactive proenzyme secreted by the chief cells of the stomach.
In the presence of hydrochloric acid $(HCl)$,pepsinogen is converted into its active form,pepsin.
Pepsin is a proteolytic enzyme that breaks down proteins into proteoses and peptones.
Therefore,the correct answer is $HCl$.

(C) Pancreatic juice contains a variety of enzymes that act on different food components.
It contains pancreatic amylase (amylopsin),which digests starch into maltose.
It also contains inactive proteases like trypsinogen,chymotrypsinogen,and procarboxypeptidases,which are activated to digest proteins into peptides and amino acids.
Therefore,pancreatic juice is responsible for the digestion of both starch and protein.

(C) Nucleic acids ($DNA$ and $RNA$) are broken down by nucleases into nucleotides.
Nucleotides are further broken down by nucleotidases and nucleosidases into nitrogenous bases (purines and pyrimidines),pentose sugars,and phosphoric acid.
Therefore,the final products of nucleic acid digestion are nitrogenous bases,sugars,and phosphates.

(A) Emulsified fat is broken down into fatty acids and glycerol by the enzyme $Lipase$.
$Lipase$ is primarily secreted in the pancreatic juice (pancreatic lipase) and the intestinal juice (intestinal lipase).
Bile salts are responsible for the emulsification of fats (breaking large fat globules into smaller droplets),but they do not digest the fat chemically.
Therefore,the correct answer is $Lipase$.

(C) Digestion is a catabolic process in which complex,large,and insoluble food molecules are converted into simple,small,and soluble forms.
This process is primarily facilitated by the action of digestive enzymes,which act as biological catalysts to break down macromolecules like carbohydrates,proteins,and fats into their respective absorbable units (monomers).
Therefore,the correct answer is $C$.

(A) The food in the stomach is mixed with gastric juices through the churning action of the stomach walls,which is a type of peristaltic movement.
This movement not only helps in mixing the food to form chyme but also facilitates its propulsion through the pyloric sphincter into the duodenum.
Therefore,peristaltic movement is responsible for the movement of chyme from the stomach to the duodenum.

(C) Digestion is the process of converting complex,non-diffusable food substances into simple,absorbable (diffusable) forms. This process involves both mechanical and chemical breakdown of food so that it can be absorbed by the blood and utilized by the body cells.

(B) The digestion of proteins occurs in a stepwise manner through the action of various proteolytic enzymes.
$1$. Proteins are first broken down into smaller intermediate products known as proteoses and peptones by enzymes like pepsin in the stomach and trypsin/chymotrypsin in the small intestine.
$2$. These proteoses and peptones are further broken down into smaller chains called peptides by carboxypeptidases and aminopeptidases.
$3$. Finally,these peptides are hydrolyzed into their simplest absorbable units,which are amino acids,by dipeptidases and other brush border enzymes.
Therefore,the correct sequence is: Proteins $\to$ proteoses and peptones $\to$ peptides $\to$ amino acids.

(B) The emulsification of fats is the process of breaking down large fat droplets into smaller,fine droplets to increase the surface area for the action of enzymes like lipase.
This process is carried out by bile salts (such as sodium glycocholate and sodium taurocholate) present in the bile juice secreted by the liver.
Bile pigments (bilirubin and biliverdin) do not participate in the emulsification of fats; they are merely excretory products.

(C) The digestion of fats (lipids) is facilitated by enzymes like lipases.
After the emulsification of fats by bile salts, lipases break them down into their simpler components.
The major end products of fat digestion are $Fatty acids$, $monoglycerides$, and $glycerol$.

(D) $Ptyalin$ (also known as salivary amylase) is a carbohydrate-digesting enzyme present in saliva.
It acts on starch and breaks it down into simpler sugars like maltose and dextrin.
Therefore,the correct option is $D$.

(B) $Pepsin$ is an endopeptidase enzyme that hydrolyzes proteins into peptones and proteoses.

(D) Bile does not produce enzymes,nor does it perform esterification.
Bile salts are primarily responsible for the emulsification of fats,which breaks down large fat globules into smaller droplets to increase the surface area for lipase action.
Additionally,bile salts help in the absorption of fats and fat-soluble vitamins $(A, D, E, K)$ by forming micelles.
Therefore,the correct answer is $(d)$ None of these.

(B) The digestion of starch begins in the buccal cavity,where salivary amylase (ptyalin) acts on starch to convert it into maltose.
Further digestion of starch occurs in the duodenum,where pancreatic amylase breaks down the remaining starch into disaccharides.
Therefore,the correct sites for starch digestion are the buccal cavity and the duodenum.

(D) The $HCl$ (hydrochloric acid) secreted by the oxyntic or parietal cells in the stomach serves two primary functions:
$1$. It creates an acidic medium ($pH$ $1.8$) which is essential for the activation of the inactive proenzyme pepsinogen into the active proteolytic enzyme pepsin.
$2$. It kills harmful microorganisms that may be ingested along with the food,thereby protecting the body from potential infections.

(C) Saliva contains the enzyme salivary amylase (also known as ptyalin).
This enzyme acts on starch,which is a complex carbohydrate,and breaks it down into maltose,a disaccharide.
This process begins in the oral cavity as part of the chemical digestion of food.

(C) Carbohydrates are broken down by various enzymes in the digestive system into their simplest absorbable forms,known as monosaccharides.
During the process of digestion,complex carbohydrates like starch and disaccharides (sucrose,lactose,maltose) are hydrolyzed.
The final end products of carbohydrate digestion that are absorbed by the intestinal mucosa are glucose,galactose,and fructose.
Therefore,the correct group of end products is glucose,galactose,and fructose.

(B) Rennin (also known as chymosin) is a proteolytic enzyme found in the gastric juice of infants.
It acts on the milk protein casein (a soluble protein) and converts it into insoluble calcium paracaseinate.
This process is known as the curdling of milk,which allows the protein to remain in the stomach for a longer duration for effective digestion by pepsin.

(C) The digestion of proteins is most adversely affected if the $pH$ of the stomach is made neutral.
In the stomach,the enzyme $Pepsin$ is responsible for the digestion of proteins.
$Pepsin$ is secreted in its inactive form,$Pepsinogen$,which is activated by $HCl$ (hydrochloric acid).
$Pepsin$ functions optimally in a highly acidic environment (optimum $pH$ of $1.8$ to $2.0$).
If the $pH$ of the stomach becomes neutral,$Pepsin$ becomes inactive,thereby halting the digestion of proteins.

(D) In humans,cellulose is not digested at all. This is because the human alimentary canal lacks the enzyme cellulase,which is required to break down the complex polysaccharide cellulose. Furthermore,humans do not possess the symbiotic micro-organisms in their digestive tract that are capable of fermenting cellulose,unlike ruminant animals.

(A) Proteins are large,complex macromolecules that cannot pass through the intestinal wall in their intact form.
To be absorbed into the bloodstream,they must be broken down into their simplest units,which are amino acids.
Therefore,digestion is essential to convert these complex proteins into absorbable amino acids.

(B) The enzyme $Erepsin$ is a component of the intestinal juice (succus entericus).
It acts on the final products of protein digestion,specifically peptones and proteoses,and converts them into $Amino \ acids$.
Therefore,the correct function of erepsin is to convert proteins (in the form of peptones) into amino acids.