NCERT Class 11 Biology Digestion and Absorption Important Notes

Food:

Food is a basic requirement of all living beings. Food provides energy for different activities in the body. Food also provides organic materials for growth and repair. Carbohydrates, proteins and fats are the major components of food and hence these are called macronutrients. Vitamins and minerals are required in small quantities and hence are called micronutrients. Water plays an important role in various metabolic processes. Water also prevents dehydration of the body.

Digestion:

The macromolecules in food cannot be utilised by our body in their original form. They need to be broken down and converted into simple substances so that they could be absorbed. The process of converting complex food into absorbable forms is called digestion. Various mechanical and biochemical methods are involved in digestion of food.

DIGESTIVE SYSTEM

The human digestive system consists of the alimentary canal and the associated glands.

Alimentary Canal: The alimentary canal begins with the mouth and ends in the anus. Mouth is situated at the anterior part and anus is situated at the posterior part of the body. The alimentary canal can be divided into five main parts, viz. mouth, oesophagus, stomach, small intestine and large intestine.

Buccal Cavity: The mouth opens into the buccal cavity or oral cavity. A muscular tongue and a number of teeth are present in the buccal cavity.

Thecodont: Each tooth is embedded in a socket of jaw bone. This type of attachment is called thecodont.

Dentition: The arrangement of different types of teeth in the jaws of an animal is called dentition. Two sets of teeth form during the lifetime of a human being. A set of temporary milk teeth or deciduous teeth are formed when the child is about 6 months old. These teeth are replaced by a set of permanent teeth when the child is about 10 years of age.

Diphyodont: The type of dentition in which an animal gets two sets of teeth during its lifetime is called diphyodont. This type of dentition is present in most of the mammals.

Heterodont Dentition: When different types of teeth are present, this arrangement is called heterodont dentition.

Dentition in Humans: An adult human has four different types of teeth, viz. incisors (I), canine (C), premolars (PM) and molars (M). There are 32 permanent teeth in an adult human being.

Dental Formula: In human beings, each half of the upper and lower jaw has teeth in the order I, C, PM, M as represented by following dental formula:

Tongue: Tongue is a freely movable muscular organ. It is attached to the floor of the buccal cavity by the fernulum. Small projections; called papillae are present on the upper surface of the tongue. Some of the papillae bear the taste buds.

Pharynx: The oral cavity leads into the pharynx. The pharynx serves as a common passage for food and air.

Oesophagus: The oesophagus is a long tube which connects the buccal cavity to the stomach. A muscular sphincter; called gastrooesophageal sphincter regulates the opening of oesophagus into the stomach. A cartilaginous flap; called epiglottis; prevents the entry of food into the glottis during swallowing.

Stomach: Stomach is a J-shaped bag-like structure. It is situated in the upper left portion of the abdominal cavity. There are three major parts in the stomach, viz. the cardiac, fundic and pyloric regions. The oesophagus opens into the cardiac region of stomach. The pyloric region opens into the first part of small intestine.

Small Intestine: Small intestine is a long and highly coiled structure. It is divided into three regions, viz. duodenum, jejunum and ileum. The duodenum is U-shaped, jejunum is somewhat coiled and the ileum is highly coiled. The opening of the stomach into the duodenum is guarded by the pyloric sphincter.

Large Intestine: Ileum opens into the large intestine. The large intestine is somewhat shorter than the small intestine but has larger diameter. The large intestine consists of caecum, colon and rectum. Caecum is a blind sac which hosts some symbiotic microorganisms.

Appendix: This is a narrow finger-like tubular projection situated at the junction of small and large intestines. The vermiform appendix arises from caecum. Appendix is a vestigial organ.

Colon: Colon is divided into three parts, i.e. an ascending, a transverse and a descending part. The descending colon opens into the rectum. The rectum; in turn; opens out through the anus.

Wall of Alimentary Canal: The wall of the alimentary canal is made of four layers, viz. serosa muscularis, sub-mucosa and mucosa. Serosa is the outermost layer and is made up of a thin mesothelium with some connective tissues. The epithelium of visceral organs is called mesothelium. Muscularis is formed by smooth muscles which are usually arranged into an inner circular and outer longitudinal layer. An oblique muscle layer may be present in some regions. The sub-mucosal layer is formed of loose connective tissues.

DIGESTIVE GLANDS

Salivary Glands: There are three pairs of salivary glands, viz. parotids, sub-maxillary/submandibular and sbublinguals. The parotids are present in the cheek. The sub-maxillary/sub-madnibular is present in the jaws. The sublinguals are present below the tongue. The salivary glands secrete salivary juice into the buccal cavity.

Liver: Liver is the largest gland in the human body. It weighs about 1.2 to 1.5 kg in adult human. Liver is situated in the abdominal cavity, just below the diaphragm. There are two lobes in the liver. The functional and structural units of liver are the hepatic lobules. Hepatic cells are arranged in the form of cords in the hepatic lobules. Each lobule is covered by a thin connective tissue; called the Glisson’s capsule. Hepatic cells produce bile. The bile passes through the hepatic ducts and is stored and concentrated in the gall bladder. The gall bladder is a thin muscular sac, situated above the liver. The duct of gall bladder (cystic duct); along with the hepatic duct forms the common bile duct.

The bile duct and the pancreatic duct open into the duodenum as the common hepato-pancreatic duct. The hepato-pancreatic duct is guarded by a sphincter called the sphincter of Oddi.

Pancreas:

Pancreas is a compound gland, i.e. both exocrine and endocrine. It is an elongated organ. This is situated between the limbs of the U-shaped duodenum. The exocrine portion of the pancreas secretes and alkaline pancreatic juice which contains various enzymes. The endocrine portion of the pancreas secretes hormones; insulin and glucagon.

NCERT Solution for NCERT Class 11 Biology Digestion and Absorption

Question – 1- Gastric juice contains

a. Pepsin, lipase and rennin

b. Trypsin, lipase and rennin

c. Trypsin, pepsin and lipase

d. Trypsin, pepsin and rennin 

Answer: (a) Pepsin, lipase and rennin

Question – 2 - Succus entericus is the name given to

a. A junction between ileum and large intestine

b. Intestinal juice

c. Swelling in the gut

d. Appendix

Answer: (b) Intestinal juice

Question – 3 - Match column I with column II

Question – 4 - Why are villi present in the intestine and not in the stomach?

Answer: Major portion of absorption of food takes place in small intestine. Absorption in small intestine is also important because digestion is almost complete by the time food reaches ileum. Presence of villi in the stomach is not relevant because digestion is only partial in the stomach.

Question – 5 - How does pepsinogen change into its active form?

Answer: Pepsinogen changes into its active form by the action of hydrochloric acid.

Question – 6 - What are the basic layers of the wall of alimentary canal?

Answer: The wall of the alimentary canal is made of four layers, viz. serosa muscularis, sub-mucosa and mucosa. Serosa is the outermost layer and is made up of a thin mesothelium with some connective tissues. The epithelium of visceral organs is called mesothelium. Muscularis is formed by smooth muscles which are usually arranged into an inner circular and outer longitudinal layer. An oblique muscle layer may be present in some regions. The sub-mucosal layer is formed of loose connective tissues.

Question – 7 - How does bile help in the digestion of fats?

Answer: Bile helps in emulsification of fats and also activates lipases. Thus, bile helps in digestion of fats.

Question – 8 - State the role of pancreatic juice in digestion of proteins.

Answer: Proteins, proteoses and peptones are acted upon by the proteolytic enzymes of pancreatic juices; as given below:

Question – 9 - Describe the process of digestion of protein in stomach.

Answer: The proenzyme pepsinogen gets converted into the active enzyme pepsin; on exposure to hydrochloric acid. Pepsin converts proteins into proteoses and peptones (peptides).

Question – 10 - Give the dental formula of human beings.

Answer: In human beings, each half of the upper and lower jaw has teeth in the order I, C, PM, M as represented by following dental formula:

Question – 11 - Bile juice contains no digestive enzymes, yet it is important for digestion. Why?

Answer: Bile juice plays two important roles, i.e. of changing the medium of food from acidic to alkaline and of emulsifying the fat. Alkaline medium is necessary for action of enzymes in the succus entericus. Thus, bile is important for digestion.

Question – 12 - Describe the digestive role of chymotrypsin. Which two other digestive enzymes of the same category are secreted by its source gland?

Answer: Chymotrypsin completes the digestion of protein. Trypsin and carboxypeptidases are the two other enzymes of the same category secreted by its source gland.

Question – 13 How are polysaccharides and disaccharides digested?

Answer: Polysaccharides are hydrolysed by pancreatic amylase into disaccharides.

Disaccharides are then digested by the enzymes in succus entericus into various monosachharides.

Question – 14 - What would happen if HCl were not secreted in the stomach?

Answer: Acidic medium is necessary for the action of gastric enzymes. Moreover, it also kills various microbes which may be present in food. These actions would not be possible in the absence of hydrochloric acid.

Question – 15 - How does butter in your food get digested and absorbed in the body?

Answer: Butter is mainly composed of fat. Fat is digested by lipases.

Di and monoglycerides are then digested into fatty acids and glycerol.

Question – 16 - Discuss the main steps in the digestion of proteins as the food passes through different parts of the alimentary canal.

Answer: The proenzyme pepsinogen; in stomach; gets converted into the active enzyme pepsin; on exposure to hydrochloric acid. Pepsin converts proteins into proteoses and peptones (peptides).

Proteins, proteoses and peptones are acted upon by the proteolytic enzymes of pancreatic juices (in small intestine); as given below:

The enzymes in the intestinal juice act on dipeptides to make amino acids.

Question – 17 - Explain the term thecodont and diphyodont.

Answer: Thecodont: Each tooth is embedded in a socket of jaw bone. This type of attachment is called thecodont.

Diphyodont: The type of dentition in which an animal gets two sets of teeth during its lifetime is called diphyodont. This type of dentition is present in most of the mammals.

Question – 18 - Name different types of teeth and their number in an adult human.

Answer: Different types of teeth and their number in an adult human are as follows:

Incisors: 8

Canine: 4

Premolars: 8

Molars: 12

Question – 19 - What are the functions of liver?

Answer: Function of liver can be broadly categorized into two types, viz. synthesis and breakdown.

Synthesis:

• A large part of amino acid synthesis happens in liver.

• Liver has many roles in carbohydrate metabolism and lipid metabolism.

• Main part of protein metabolism, synthesis and degradation takes place in liver.

• Liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, X and XI. It also produces protein C, protein S and antithrombin.

• Liver is the main site of RBC production during the first trimester of foetus.

• Liver produces bile. 

Breakdown

• Breakdown of insulin and other hormones, haemogloin, some toxic substances and conversion of ammonia into urea takes place in liver.