Digestion and Absorption
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.
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.
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.
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 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.
Digestion and Absorption - class eleven biology - Digestion of Food
Digestion and Absorption - class eleven biology - NCERT Solution