The alimentary canal is nearly 9 m long. It includes -
1. Mouth and Buccal Cavity: It is the upper most opening of human digestive system which gives passage if for ingestion of food. Buccal cavity takes in the food cuts the food into small pieces and chews them to make it soft or easy swallowing. Buccal cavity also has three pairs of salivary glands. These glands secrete saliva
which contains water, salts, mucin and an enzyme ptyalin. Ptyalin is salivary amylase which splits starch and glycogen into maltose.
2. Pharynx: It acts as a passage for the food from buccal cavity to oesophagus.
3. Oesophagus: It is a long and tubular structure which pushes the food rhythmically forward.
4. Stomach: It is a large organ which expands when food enters it. It serves four functions:
(i) Storage of food - Carbohydrates gets digested till the salivary enzyme ptyalin is destroyed by HCI, secreted in the stomach.
(ii) Churning of food - The stomach wall produces muscular contraction, for the food to get churned and to mix it thoroughly with gastric juice.
(iii) Partial digestion - There are three types of gastric juice in stomach namely HCI, protein digesting enzyme (pepsin) and mucus.
(iv) Exit of food - The stomach releases food into the small intestine in a controlled and regulated manner.
Functions of the Gastric Juices
(i) HCI - Makes the tnedium acidic, kills the germs present in the food.
(ii) Pepsin - Acts in acidic medium, breaks proteins into peptone.
(iii) Mucus - Protects the inner lining of the stomach from the action of the acid under normal conditions.
(iv) Gastric juice - Also contains some gastric lipase which partially breaks down lipids.
(v) Regulation of flow of food - Stomach regulates the flow of partially digested food into the small intestine.
5. Small intestine: It is the longest part of digestive system. It is a narrow tube divided into three parts 
(i) duodenum
(ii) jejunum and 
(iii) ileum. It is the site of the complete digestion of proteins, carbohydrate and fats. Duodenum receives the secretion of bile from bile duct and pancreatic juice from pancreatic duct.
Bile juice
(i) It is secreted by liver and stored temporarily in the gall bladder.
(ii) It has bile pigments and bile salts.
(iii) It has no enzymes and thus has no chemical action on food, but breaks down the fat molecules into small globules.
(iv) This process is called emulsification of fats. Pancreatic juice
(i) It includes pancreatic amylase which breaks down starch, pancreatic lipase which breakdown lipids and trypsin for digesting proteins.
(ii) The secretions of pancreatic duct make the medium alkaline, which is favourable for the action of pancreatic enzymes.
Pancreas secretes hormones insulin and glucagons. The walls of small intestine contains glands which secrete intestinal juice called succus entericus which helps in digesting protein A into amino acids, carbohydrates into glucose, fats into fatty acids and glycerol. The digested food is taken up by the walls of the intestine which has numerous fingers like projection called villi. Villi are richly supplied with blood vessels which the absorbed food to obtain energy. The presence of villi and
blood capillaries increases the capacity of absorption by the wall of intestine
6. Large intestine: It includes three parts namely caecum, colon and rectum. It is shorter but wider than small intestine. It secretes mucus for lubrication. The undigested food is collected as faeces egested out through anus which is regulated by anal sphincter.

The mechanism of exchange of gases takes place between the blood and body tissues during respiration. The air enters the lungs through nostrils. As the blood passes
through the body tissues, the oxygen present in it diffuses into the cells due to its higher concentration in the blood. A red pigment called haemoglobin in the blood carries the oxygen. This oxygen combines with the digested food to release energy, which takes place in mitochondria of the cells. The carbon dioxide produced during the respiration diffuses into the blood stream from where it is carried to lungs and expelled out. The blood which comes from the lungs has a higher concentration of oxygen but the body tissues have a higher concentration of carbon dioxide. Due to the difference in the concentrations, the exchange of gases between the blood and the body tissues takes place. Thus, in this way the process of gaseous exchange is completed in the human respiratory system. The total area for gaseous exchange provided by 300 million alveoli in 2 IU,ngsis 400-800 square feet.

(i)Anchoring the plant
(ii)Source of water and minerals
(iii)Availability of oxygen for respiration of root cells
(iv)Symbiotic association with microbes.

The salivary glands secrete saliva. The saliva contains water, salts, mucus and an enzyme ptyalin. The enzyme ptyalin, also glycogen and convert them into maltose. The mucus and water makes the food soft and slippery for the formation of bolus and also protects soft lining of alimentary canal. Thus the digestion of starch i.e., carbohydrate begins in the mouth. Since,the food remains in the mouth only for a short time and so, the digestion of food remains incomplete.
\(Starch(Carbohydrates)+Glycogen\overset { Ptyalin }{ \longrightarrow } Maltose\)
The partially digested food is further digested in the stomach. The food is broken down into smaller pieces and churned to form a semi-solid paste. The wall of the stomach contains three tubular glands. These glands secrete gastric juice. The gastric juice contains hydrochloric acid, mucus, pepsin, gastric lipase and rennin.
Hydrochloric acid:
It softens the food
It makes food acidic for the function of pepsin
It stops the action of salivary amylase
It kills germs and bacteria to disinfect the food
It converts inactive pepsinogen and prorennin into active pepsin and rennin.
\(InactivePepsinogen\overset { HCI }{ \longrightarrow } ActivePepsin\\ InactivePro-rennin\overset { HCI }{ \longrightarrow } Activerenin\)
Pepsin: It hydrolyses proteins into soluble proteoses and peptones only in presence of hydrochloric acid.
\(Protein\overset { Pepsin }{ \longrightarrow } Peptones+Proteoses\)
Rennin: It is active only in infants. The main function of rennin is that it converts soluble milk protein casein into insoluble protein called paracasein. Due to this, milk stays in stomach for longer duration.
\(\underset { (solublemilkprotein) }{ Casein } \overset { Rennin }{ \longrightarrow } \underset { (Insoluble) }{ Paracasein } \)
Mucus: The main function of mucus is to protect the inner lining of stomach from corroding action of hydrochloric acid as well as from pepsin enzyme.
Small Intestine: It is the seat of major digestion and absorption. Small intestine receives the secretion of two glands namely,
(i) Liver, (ii) Pancreas
Liver: It is the largest gland of the body. It is reddish brown in colour. It has soft pear - shaped sac called gall bladder. The gall bladder secretes bile juice. Bile is alkaline containing bile pigment. Thus, bile is stored in the gall bladder. It helps to emulsify or break the fats or lipids present in the food.
The bile performs the following functions:
(i) It breaks the larger globules of fat into small globules to make the enzyme to act and digest them.
(ii) It prevents putrefaction of food.
(iii) The bile creates a medium for the pancreatic juice to act on food by neutralizing the acidity in the food.
Pancreas: Pancreas secretes a digestive fluid called pancreatic juice. This juice contains digestive enzymes such as trypsin, nucleases, pancreatic lipase and pancreatic amylase. The enzyme trypsin digests the proteins, the enzyme pancreatic amylase breaks down the starch and the enzyme lipase breaks down the emulsified
facts. Pancreas produces two important hormones namely,
(i) Insulin, (ii) Glucagon
Insulin is essential for using glucose and storing it into glycogen. Glucagon is essential for conversion of glycogen into glucose. The alkaline medium is favourable
for the action of pancreatic enzymes.
\(Starch\xrightarrow [ ]{ Pancreaticamylase } Maltose\\ Fat\xrightarrow [ ]{ Lipase } Fattyacids+Glycerol\\ Proteins\xrightarrow [ ]{ Trypsin } Peptides\)
Secretion of intestinal juice by liver and pancreatic glands
The intestinal juice contains a number of enzymes which complete the digestion process. Here, the complex carbohydrates are converted into glucose, proteins are converted into amino acids and fats are converted into fatty acids and glycerol. All these end products i.e., amino acids, glucose, fatty acids and glycerol are small
and water soluble molecules. This complete digestion of food takes place by biological catalysts called enzymes. The intestinal juice includes - maltase, invertase, lactase etc.
\(Peptides\xrightarrow [ ]{ Trypsin } Aminoacids\\ Sucrose\xrightarrow [ ]{ Invertase } Glucose+Fructose\\ Maltose\xrightarrow [ ]{ Maltose } Glucose+Glucose\\ Lactose\xrightarrow [ ]{ lactose } Glucose+Galactose\)

Food is prepared in the green leaves of the plant by combining carbon dioxide and water in presence of sunlight and chlorophyll. The carbon dioxide required for photosynthesis is taken by the plants from air. The carbon dioxide enters the leaves through tiny pores called stomata. The water required is taken from soil. The sunlight provides the energy required to carry out the preparation of food. The green pigment called chlorophyll present in green leaves help to absorb energy from sunlight. The oxygen produced as a by-product goes into the air.
The food prepared by the green leaves is in the form of glucose. This glucose is sent to different parts of the plant. The excess glucose is changed into starch and is stored in the leaves of the plant. The food prepared by photosynthesis provides all the energy to a plant which it needs to grow. Thus, it is clear that the green plants convert sunlight energy into chemical energy. by making food. When we eat plant foods such as fruits, vegetables, food grains etc., the chemical energy stored in them is released in our body during respiration.
During the process of photosynthesis, the following three steps take place:
(i) Absorption of sunlight energy by chlorophyll.
(ii) Conversion of sunlight energy into chemical energy and splitting of water into hydrogen and oxygen using sunlight energy.
(iii) Reduction of carbon dioxide to form carbohydrates using the chemical energy.