Question

As we respond to the questions in this thread we will all begin to learn the "whys" of how food passes through the digestive system.

Let's start with a ham and cheese sandwich with lettuce and pickles. As I bite into my sandwich the teeth are present for mastication to begin the process of mechanical digestion and the saliva begins to mix with my food where salivary amylase is present and starts the digestion of the starchy bread. What happens next? (Think about the carbohydrates, proteins and fat in our sandwich)

Let's begin the exploration of the digestive tract.....

Answer 1

After the mastication process by teeth the ham and cheese sandwich with pickle and lettuce are chewed. The chewed food materials are mixed with the salivary juice that contains ptyaline enzyme which acts on the starch molecules of the bread in sandwich. This ptyaline enzyme acts on this starch and produce moltose and dextrin as the product. This action of ptyaline in the buccal cavity makes the food more palatable and produce bolus. This bolus is soft and easy to digest in the next digestive tract. The soft bolus then swallowed through the oesophagus by its peristaltic movements. This peristaltic movements of oesophagus helps the food bolus to enter into the stomach.

Stomach: In the stomach the protein molecules (mainly ham of sandwich) is digested by pepsin. Pepsin is a proteolytic enzyme which acts on protein and produce peptone as the product. The fat molecules of ham is digested by gastric lipase. This gastric lipase acts on the lipid and produce fatty acids and glycerol by hydrolysed the ester bond. Others carbohydrate like sucrose is digested in the stomach by HCl, this hydrochloric acid digest the sucrose molecules in food bolus and produce glucose and fructose as byproduct which can easily be digested.

Liver: The bile salt from the liver bile duct mixed with the food materials in the duodenum part and makes the fats soluble for digestion and activates the lipase enzyme for lipid digestion.

Pancreas: The carbohydrate molecules ( in bread, lettuce and in some amount in pickle) are digested by pancreatic amylase enzyme. This amylase acts on starch (polysaccharide) and produce moltose for the digestion of carbohydrates. This pancreas also secrets pancreatic lipase for the digestion of lipid molecules in the ham sandwich. This pancreatic lipase produce fatty acids and glycerol from the lipid for easy digestion. The protein part of the sandwich also digested by others pancreatic enzyme like aminopeptidase, carboxypeptidase which brakes the polypeptide chain of the protein and produce free amino acid for easy digestion. Trypsin also a pancreatic enzyme which also helps in the protein digestion. These all pancreatic enzymes are secreted in the duodenum part and mix with the food materials coming from the stomach.

Intestine: The all digested food materials then enter into the small intestine from the duodenum part. This small intestine and larger intestine is the large area for proper digestion and ultimately absorption of digested food materials into the body and excretion of undigested material as dedication. The main enzymes for carbohydrate digestion in the intestine are moltase, lactase and sucrase. The moltose carbohydrate which is previously produced by pancreatic amylase, is digested fully by moltase enzyme in this part of digestive system. The moltose is broken down into glucose by moltase, this glucose then absorbed through the ciliary epithelium of intestine into body. The remaining fat or lipid molecules of ham sandwich is digested by intestinal lipase into fatty acids and glycerol. The protein part of the sandwich (ham, lettuce, cheese ) is digested by erepsin, nuclease enzymes and produce free amino acids and purine, pyrimidine for easy digestion.

So after mechanical digestion through mastication of sandwich the food materials mix with salivary juice for making it more soft and palatable. The food bolus then digested through the digestive system (stomach, liver, pancreas, intestine). The food materials after mastication enters the next part of the digestive tract that is oesophagus. The food materials are then enter into stomach by peristaltic movements and digestion procedure is started for carbohydrate, protein and fat molecules present inthe food bolus. This is the procedure of digestion and passing of food through digestive system.