Digestion And Absorption - Class 11 Medical Biology - Extra Questions

• Carbohydrases: These enzymes digest carbohydrates. For example, Amylase.
• Proteinases: These enzymes digest proteins. For example, Pepsin.
• Lipases: These enzymes digest lipids. For example, Gastric lipase
• Nucleases: These enzymes digests nucleic acids.
  

• The small intestine has millions of tiny finger-like projections called villi. These villi increase the surface area for more efficient food absorption. Within these villi, are present numerous blood vessels that absorb the digested food and carry it to the bloodstream. It is hence from the bloodstream, the absorbed food is delivered to each and every cell of the body.

• The process of passing digested food from the small intestine to the blood vessels is called absorption. 
• Assimilation is the process of conversion of absorbed food complex substances such as proteins and vitamins required by the body. It is the absorption of nutrients into the living tissue.

• The digested food can be used as the building blocks such as proteins and amino acids.
  
• The digested carbohydrates and the fats are used as energy resources both for storage and utilization.
  

The small intestine is mainly responsible for the absorption process. The villi and microvilli increase the surface area of absorption. The more is the amount of absorbed food, the more would be its availability to the cells of the body. The stomach, on the other hand, is an organ that primarily stores food temporarily along with helping in digesting proteins. Hence, the presence of villi is more justified in the small intestine and not in the stomach.

• The small intestine has millions of tiny finger-like projections called villi. These villi increase the surface area for more efficient absorption of nutrients from digested food.
• Within these villi, numerous blood vessels absorb the digested food and carry it to the bloodstream, from the bloodstream, the absorbed food is delivered to each cell of the body.

• Milk after reaching the stomach,is acted upon by the proteolytic enzyme pepsin. Pepsin hydrolyzes the milk protein, Casein (soluble) into insoluble paracasein which precipitates as calcium paracaseinate to form semi solid curd. This is known as coagulation of milk. Rennin promotes the coagulation process in the stomach of infants.
• The digestion of fats begins in the small intestine. Bile juice secreted by the liver emulsifies the fat into smaller fat globules so as to increase the surface area for the lipase to act. Pancreatic lipase then acts on the globules and convert it into di- and monoglycerides. The enzymes in the succus entericus act on the end product of the above reaction to form the simple absorbable form. Lipases present in the succus entericus converts the di- and monoglycerides into fatty acids and glycerol.
• The polysaccharides(starch) is digested by various enzymes produced in the digestive tract. The polysaccharides are broken down to oligosaccharides and disaccharides which are further digested to form monosaccharides which are simple sugars.

It is an important process that breaks down the proteins, fats, carbohydrates, vitamins, minerals into simpler forms so that it can be absorbed easily into the body cells. During this process, proteins are converted into amino acids, carbohydrates are converted into simple sugars and fats are broken down into fatty acids and glycerol. Many digestive enzymes and hormones act on food, at various stages during the process of digestion. The whole process occurs in a sequential manner.

• Digestion is the process by which the complex food is broken down into simpler, soluble chemical compounds which are then used as nutrients by the body. 
• The food is digested physically, by mouth, or chemically, by enzymes. The digestion begins at mouth. 
• In mouth, the food is broken down into smaller pieces by teeth. It is then mixed with saliva. 
• Enzyme amylase breaks down starch into simple carbohydrate. 
• The food then travels to esophagus via pharynx. 
• By the peristaltic movement, the food moves from esophagus to the stomach. The acid and enzymes present in the stomach breaks down the food further. 
• Gastric juices present in stomach contains protease enzyme which breaks down proteins. Food then enters the small intestine. 
• Enzymes released by the pancreas and bile released from the liver enters in duodenum part of small intestine. 
• Bile carries out the digestion of fat. 
• Enzymes released by the pancreas break down protein, fat and carbohydrates. 
• Absorption takes place in other two parts of small intestine, namely jejunum and ileum. 
• The indigestible food enters the large intestine. 
• In large intestine, maximum absorption of water takes place. 
• The waste then enters the rectum in the form of stool which is comes out from the anus. 

$$\textbf{Solution:}$$ 

$$\textbf{Lingual frenulum:}$$ 

$$\bullet$$ The lingual frenulum is the elongated mucous membrane that connects the ventral tongue to the floor of the mouth. 

$$\bullet$$ It helps in the movement of the tongue in different directions while it is connected to the lower base of the mouth. 

• Small intestine is the main part of the digestive system, where maximum absorption of nutrients occurs.
• It is made up of simple columnar epithelium and have finger like projections called villi, which increase surface area for absorption of nutrients into the blood capillaries. 
• Also, the intestinal walls of small intestine are made of blood vessels which help in transporting food molecules to various parts of the body. 

• HCl converts the inactive enzyme pepsinogen into its active form pepsin. Pepsin digests proteins and breaks them into amino acids. 
• HCl also kills harmful pathogens or particles in our food when it reaches the stomach through the gastrointestinal duct.
  

• Digestive enzymes main function is to target a specific nutrient, splitting it up into a form that can eventually be absorbed by the body. The most significant digestive enzymes are: Amylase, Maltase, Lactase, Lipase and Protease.
• For example, Amylase breakdown carbohydrates into simple sugars. Maltase breakdown maltose into glucose. Lipases convert fats into fatty acids and glycerol. And Proteases convert proteins into amino acids.

1. Digestion begins in the oral cavity by the action of saliva which contains salivary amylase that breaks down starch into disaccharide.
2. Food is reached to stomach and HCl is mixed which activates pepsinogen into pepsin that breaks down proteins into dipeptides and converts into chyme with pH 2-3.
3. Chyme is reached to the duodenum of the small intestine and pancreatic enzyme acts on it. Enterokinase activates trypsinogen, chymotrypsinogen, and procarboxypeptidase into trypsin, chymotrypsin and carboxypeptidase and digest rest of the protein to dipeptidase.
4. Starch is breakdown into disaccharides by the action of pancreatic amylase.
5. Fat is emulsified by bile juice and further convert into the micelle.
6. The succus entericus acts on rest of dipeptides, disaccharides and on micelles to form glucose, galactose, fructose, amino acid, and fatty acids.
7. Nucleic acids are converted into nucleotides and then into nucleosides by nucleases.
8. These are absorbed by intestinal wall and release to the bloodstream.

$$\textbf{Solution: }$$

$$\textbf{Assimilation of Food}$$ 

$$\bullet$$ Assimilation of food is the process of accumulation of the nutrients inside the cell where the food molecules are metabolized or stored to provide energy to the cell. 

$$\bullet$$ Digested food from the intestine is transported into the blood vessels or the lymph vessels. This acts as a transporter to move the absorbed nutrients from the intestine to the cells where the nutrient is stored. 

$$\bullet$$ $$\textbf{Assimilation of Glucose:}$$ Glucose is the primary source of energy in the cell. 

$$\bullet$$ Excess glucose gets stored in the liver and the muscle cells. It is stored in the form of glycogen which can be used for energy requirements when the level of glucose gets depleted in the body. 

$$\bullet$$ Fatty acids are stored in the adipocyte cells and based on the requirements they can be converted to glucose using gluconeogenesis. 

• Most absorbed materials cross the mucosa into the blood and carried off in the bloodstream to other parts.
  
• Digested molecules of food as well as water and minerals from the diet are absorbed by small intestine.
  

Simple sugars are far and away the predominant carbohydrate absorbed in the digestive tract and in many animals the most important source of energy. Monosaccharides, however, are only rarely found in normal diets. Rather, they are derived by enzymatic digestion of more complex carbohydrates within the digestive tube.

Particularly important dietary carbohydrates include starch and disaccharides such as lactose and sucrose. None of these molecules can be absorbed by small intestine for the simple reason that they cannot cross cell membranes unaided and, unlike the situation for monosaccharides, there are no transporters to carry them. 

• Tunica adventitia is the outer layer of the oesophagus.
• The stomach has thick muscles because it contracts in order to churn the food during digestion.
• Brunner's glands, also called sub mucosal glands, are located in the duodenum.
• Since the rectum has the thinnest muscular layer, it has minimum peristalsis.

• Papillae are the raised protrusions of the tongue surface on which taste buds of the tongue sit.
• The dorsal surface of the mammalian tongue is covered with four kinds of papillae, fungiform, circumvallate, foliate and filiform papilla.
• The filiform papillae don't have any taste buds.
• Fungiform papillae-these are slightly mushroom-shaped if looked at in longitudinal section. These are present mostly at the dorsal surface of the tongue, as well as at the sides. Innervated by the facial nerve.
• Foliate papillae- these are ridges and grooves towards the posterior part of the tongue found at the lateral borders. Innervated by facial nerve (anterior papillae) and glossopharyngeal nerve (posterior papillae).
• Circumvallate papillae- they are present at the back of the oral part of the tongue and are innervated by the glossopharyngeal nerve.
• Filiform papillae- filiform papillae do not contain taste buds. They cover most of the front two-thirds of the tongue's surface.

Villi are very tiny, tentacle-like structure present in the inner surface of the small intestine. They are covered by a brush-like structure calle microvilli.

Digestion occurns in the stomach and the large intestine. Further, some enzymatic secretion takes placed in the villi which completely breaks down the carbohydrates, proteins, and lipids into their respective micromolecules to be absorbed by the villi.

• It makes the medium of gastric juice acidic so that the enzyme pepsin can digest the food properly. Pepsin can work only in an acidic medium if acidic pH is not provided, then the pepsin remains inactive. 
• It kills any bacteria which may enter into the stomach with food.

• The absorption of  digested food from the ileum into the blood occurs with the help of active transport.


• The villi provide an increased surface for the absorption. The Villikinin hormone enhances absorption by increasing villi movements.


• There about 45 lakh villi in the ileum of human beings.


• The monosaccharides (glucose, galactose, fructose, mannose etc.) are absorbed directly into the blood by active transport in the presence of water.


• The amino acids are also absorbed directly into the blood by active transport.


• Alcohol is absorbed by diffusion in the whole alimentary canal.


• The fatty acids before the absorption form an emulsion called micelles.


• These micelles are taken up by the epithelial cells with the help of pinocytosis.


• The epithelial cells pack these fatty acids in small packets which are called chylomicrons.


• The diameter of the chylomicron is about 1µ. The chylomicrons are released into the lacteals by exocytosis.


• The chylomicrons from the lymph reach into the blood at the junction between subclavian & jugular veins.


• The water-soluble vitamins are absorbed directly into the blood by diffusion.


• Absorption of the vitamin $$B_{12}$$ needs the presence of a castle factor.


• Fat-soluble vitamins are absorbed with the help of bile salts like the absorption of the fatty acids.


• About 20 litres of water can be absorbed in human intestine per day.
  
  

• Ingestion is taking food into the mouth for example.
• Digestion is when the ingested food is now broken down into smaller building blocks. This may be achieved by mechanical means (chewing, churning) and chemical means (digestive enzymes of stomach and intestines).
• Absorption is when the digested building blocks are now absorbed into the bloodstream or lymphatic system through the intestines and then transported to various cells, tissues, organs in the body.
• Elimination is when waste products produced during digestion and absorption or left over after use by cells, or undigested remains is now expelled from the body either via the urinary system as urine, digestive system as faces, respiratory system as gases or integumentary system as perspiration.

Dysentery is an intestinal infection that causes severe diarrhoea with blood in the stool. In some cases, mucus may also be found in the stool. This usually lasts for 3 to 7 days. Diarrhoea causes loss of body fluids which in turn results in loss of salts and minerals such as sodium, chloride, and potassium from the body. This leads to weakness in the body.

• The intestinal juice or succus entericus is secreted by crypts of Lieberkuhn.
  
• The intestinal juice is secreted by the glands present in the walls of duodenum in small quantities.
• It is alkaline in nature. 
• it consists of water, mucoproteins and hydrogencarbonate ions and digestive enzymes such as enteropeptidase, lactase, sucrase, lipase.
• The alkaline nature of intestinal juice protects duodenum against damage by the acidic chyme entering into it from the stomach.

$$\textbf{Solution:}$$

$$\textbf{Digestion:}$$ 

$$\bullet$$ Digestion is the physicochemical process in which complex organic macromolecules are converted to simple organic micromolecules that can be easily absorbed by the system. 

$$\bullet$$ This involves the action of specific enzymes which can break down the macromolecules into smaller molecules. 

$$\textbf{Digestion in buccal cavity:}$$ 

$$\bullet$$ In the buccal cavity digestion of carbohydrates takes place in the presence of the invention salivary amylase. 

$$\bullet$$ After ingestion of food into the buccal cavity, teeth are used for chewing the food particles and breaking them down.

$$\bullet$$ Small food particles mix with the saliva with the help of the tongue and the amylase present in the saliva acts on the carbohydrates. 

$$\bullet$$ The mixing of the saliva with the food helps in the formation of the food bolus. This food bolus can easily move from the buccal cavity into the stomach through the oesophagus.