A person had roti and dal for his lunch. Trace the changes in those during its passage through the alimentary canal.

The following changes takes place during the passage of roti and dal through the alimentary canal:

i. The human alimentary canal begins with mouth i.e., buccal cavity or oral cavity. Here the roti and dal is chewed finely changing the food to semisolid paste and mixed with saliva. This makes the food easy to swallow and saliva has salivary amylase which helps in digestion of starch to Maltose.

Starch Maltose

ii. The oral cavity then leads into short pharynx followed by oesophagus and stomach. The semisolid food with salivary amylase passes through this path without any reaction and reaches the stomach.

iii. Next in stomach food is further churned into finer paste. The stomach has HCl in it. The churned food is mixed with the HCl. Mixing of HCl kills the germs left in the food. HCl also make the pH of food to acidic in nature so that pepsin can act optimally.

The roti has carbohydrates and fibres in it and dal has protein and little fat in it.

In stomach partial digestion of protein takes place. The following reaction takes place in stomach.

Protein + pepsin → Peptones + Proteoses

iv. Stomach is followed by small intestine. The pancreas releases pancreatic juice on the way. The pancreatic juice contains pancreatic amylase.

The pancreatic amylase converts polysaccharides into disaccharides. This is processing of carbohydrates present in the roti, i.e.,

Polysaccharides (starch) Disaccharides

Pancreatic juice also contains Chymotrypsin which helps in digestion of peptones and proteoses, i.e.,

Lipase helps in digestion of fats as follows:

Fats Diglycerides → Monoglycerides

Intestinal juice contains various enzymes which help in digestion of all the nutrients.

After all the nutrients are converted into simple substances, these are absorbed by small intestine walls.

The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine and finally to rectum.