Organisms which use anaerobic respiration are yeasts, bacteria and parasites like tape worm (Taemia), Ascaris etc.

Role of saliva in digestion of Food:
(i) The saliva contains salivary amylase enzyme that breaks down starch which is a complex molecule to simple sugars.

$$\begin{gathered} \mathrm{starch} + \mathrm{salivary} \mathrm{amylase} \underset{ }{\to } \mathrm{sugars} \\ \left(\mathrm{complex}\right) \left(\mathrm{simplar} \\ \mathrm{molecule}\right) \mathrm{molecules}) \end{gathered}$$

(ii) The saliva moisten the food that help in chewing and breaking down the big pieces of food into smaller ones. So, that salivary amylase can digest the starch efficiently.

Necessary conditions for autotrophic nutrition:

(i) Presence of chlorophyll in the living cells.
(ii) Supply of water to green parts or cells of the plant either through roots or by surrounding environment.
(iii) Availability of sufficient of sunlight to provide light energy required to carry out photosynthesis.
(iv) Sufficient supply of carbon dioxide which is one of the important component for the formation of carbohydrates during photosynthesis.

Digestion of fat takes place in the small intestine.
Digestion of Fat: The fats are present in the form of large globules in the small intestine. Fats digesting enzymes are not able to act upon large globules efficiently.
Bile juice secreted by the liver poured in the intestine along with pancreatic juice. The bile salts present in the bile juice emulsify the large globules of fats. So, by emulsification large globules break down into fine globules to provide larger surface area to act upon by the enzymes
Lipase enzyme present in the pancreatic juice causes break down of emulsified fats. Glands present in the wall of small intestine secrete intestinal juice which contains lipase enzyme that converts fats into fatty acids and glycerol. Small Intestine: Region of digestion of fats:

The alveoli are designed to maximise the exchange of gases in the following way:
(i) The alveoli are thin walled and richly supplied with a network of blood vessels to facilitate exchange of gases between blood and the air filled in alveoli.
(ii) Alveoli have balloon like structure. Thus, provide maximum surface for exchange gases.