Complete digestion takes place in the small intestine, but pepsin is present in the stomach where it breaks down proteins to peptides. The pH in the stomach is very low, and pepsin, like all enzymes, has an optimal temperature and pH at which it functions best. For pepsin, this pH happens to be very low. The other enzymes mentioned that function in the small intestine have optimal pH's close to the pH of the small intestine (which is slightly alkaline due to the pancreas secreting bicarbonate).

Amylase is the enzyme that breaks down starches and carbohydrates. There are two types, salivary amylase and pancreatic amylase. Salivary amylase is produced in the mouth by the salivary glands and it is one of the first enzymes that begins the process of food digestion.

The pancreas is responsible for secreting proteolytic zymogens, such as trypsinogen, into the small intestine. This is triggered by the presence of undigested food (chyme) in the duodenum.

Bile is synthesized in the liver and secreted by the gallbladder into the small intestine upon the appearance of acidic chyme in the duodenum. Bile contains salts that emulsify (solubilize) fats, which is a necessary step in their digestion. If the gallbladder is removed, the liver can take over both roles.

Salivary amylase in the mouth targets large starch molecules and begins the digestion process to a monosaccharide (i.e. glucose). Salivary amylase secretion is stimulated by the recognition of starch on sweet taste receptors, and is carried out by the salivary glands (parotid, submandibular, and sublingual). 

Zymogens are inactive forms of enzymes that are secreted in order to prevent the accidental digestion of the body's important proteins (i.e. other enzymes, structural proteins). These zymogens are then activated through further processing in the compartments in which they are meant to work.

Trypsin and chymotrypsin belong to a family of enzymes that are "proteolytic". This means that they target proteins and break them down into their building blocks called amino acids.

The correct answer is carbohydrates because amylase is the enzyme that is able to break these macromolecules down into smaller parts. There are two kinds of amylase found in the body, salivary amylase in the mouth and pancreatic amylase in the pancreas. Both of these are involved in the breakdown of carbohydrates in digestion.

Lipase is synthesized and secreted from the pancreas into the duodenum of the small intestine. Lipase plays a key role in the digestion of lipids.

The alveoli are lined with a single layer of squamous epithelial cells, which allow for easy diffusion of vital gases. Basal and apical cells refer to cells located at the bottom and top of structures, respectively. Endothelial cells line the circulatory system and blood vessels. There is no formal class of cells known as "respiratory cells."