13.  Describe the structure of stomach.

- Stomach is a large 'J' shaped organ present on the left side just below the left lung.
- It expands when food enters it.
- Movement of muscular walls of stomach helps in mixing and further crushing of food.
- Food is removed from stomach in small amounts due to controlled movements of sphincter muscle.

14.  What are the secretions secreted by various glands in stomach collectively called? Write its composition.

Secretions secreted by various glands in stomach are collectively called gastric juice. It has following components:
(i) Hydrochloric acid (HCl) that creates an acidic medium in stomach
(ii) Enzyme pepsin that helps in digestion of proteins. 
(iii) Mucus that protects the inner lining of stomach.

15.  What maintains the acidic environment in the stomach and why?

Hydrochloric acid (HCl) secreted by gastric glands maintains the acidic environment in the stomach.
Acidic environment in stomach is required for the action of protein digesting enzyme pepsin secreted in the stomach.
Acidic environment may also kill any microorganism that has entered the body along with the food.

16.  What is the role of mucus present in the gastric juice?

Mucus protects the inner lining of the stomach from the action of the hydrochloric acid under normal conditions.

17.  What is meant by acidity?  How is it caused?

Our stomach secretes HCl acid that is essential in the digestive process.  When there is excess production of acid in the stomach, it results in condition known as acidity.

18.  Name the muscle that regulates the exit of food from stomach. How and where the food is released.

Sphincter-muscle of stomach regulates the exit of food
From stomach food moves into small intestine.
Sphincter muscle of stomach releases small amounts of food into small intestine.

19.  Which is the longest part of human alimentary canal?  How it is fitted into compact space?

Small intestine is the longest part of human alimentary canal. It is fitted into a compact space due to extensive coiling.

20.  How does the length of small intestine dependent on food habit of an animal?

In herbivores the small intestine is longer where as in carnivores it is comparatively smaller.

21.  Why is the small intestine longer in herbivores as compared to carnivores?

The difference in length of small intestine is because of the food habit of these animals. Herbivores are grass/ plant eating animals.
Plant products have cellulose, which takes longer time to digest. Hence, herbivores have longer small intestine.
Carnivores are meat-eating animals. Since, meat is easier to digest hence these animals have shorter small intestine.

22.  What is the role of small intestine in the process of digestion?

- Small intestine is the longest part of the alimentary canal
- It is the site of complete digestion of food material
- It receives bile from liver, which makes the medium alkaline and helps in emulsification of fats.
- It also receives pancreatic juice that contains protein digesting enzyme trypsin and fat digesting enzyme lipase.
- the walls of small intestine secretes intestinal juice that finally convert protein to amino acid, carbohydrates to glucose and fats to fatty acid and glycerol.
- Later part of small intestine are specially modified to facilitate absorption of food.

23.   What is bile?  What is its function?

The secretion of liver that is poured in the small intestine is called bile. Role of bile is as follows
(i) Alkaline medium:  It makes the medium alkaline for the pancreatic enzymes to act.
(ii) Emulsification of fats: Bile acts on fats to break them into small globules.  This helps the fat breaking enzymes to perform their action more efficiently.

24.  Why is medium in small intestine changed from acidic to alkaline?

The medium in the small intestine is made alkaline for the pancreatic enzymes to act as these enzymes can function only in alkaline medium.

25.  What is the role of pancreas in the process of digestion?

- Pancreas is a digestive gland. It secretes pancreatic juice in the small intestine.
- Pancreatic juice contains enzymes like trypsin for digesting proteins, lipase for breakdown of fats and amylase for digestion of carbohydrates.

26.  Name the end products formed after digestion of following food items

(i) Proteins 
(ii) complex carbohydrates
(iii) Fatty acids.

(i) Proteins                -  amino acids
(ii) Carbohydrates     -  glucose
(iii) Fats                    -  fatty acids and glycerol

27.  How does small intestine helps in absorption of food?

1. The inner lining of the small intestine has numerous finger-like projections called villi. They increases the surface area for absorption of digested food. 
2. Villi are richly supplied with blood vessels, which carry the absorbed food material.

28.  What happens to the digested food absorbed in blood?

The digested food absorbed in blood is taken to each and every cell of the body where it is utilized for obtaining energy, building up of new tissues and the repair of old tissues.

29.   What happens to unabsorbed food left in small intestine?

The unabsorbed food is called the undigested food. It is sent into the large intestine. The walls of large intestine absorb water from this material. This waste is then removed out of our body from anus through the regulation of anal sphincter muscle.