Where does mechanical and chemical breakdown start in the digestive system?
a. stomach
b. liver
c. small intestines
d. mouth/teeth

The answer is D.

Both mechanical and chemical digestion of food begin in the mouth.

Mechanical digestion involves physically breaking down food into smaller pieces. This happens in the mouth as the food is chewed. Adult humans have 32 specialized teeth which can tear, grind and chew different kinds of food.

Chemical digestion also begins in the mouth. It involves breaking down food into simpler nutrients that can be used by the cells. In the mouth, food is mixed with saliva. Saliva contains an enzyme  called salivary amylase that begins the breakdown of carbohydrates. Salivary glands beneath and in back of the tongue secrete the saliva.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.