In our study of the Digestive System, we have framed our discussion with the following question:
Suppose for lunch you ate the following: a fully dressed cheeseburger (lettuce, tomato, mayo), french fries, and a milkshake. Describe the digestion of this meal, including in your discussion the chemical and mechanical digestive processes that take place in the alimentary canal.
Sounds like a big question, right? Not to fear, we are breaking it down into its component parts. As we cover each section of the chapter, I will update this post. Soon, we will have a complete answer to our Cheeseburger Question! Stay tuned ....
One way that you can organize your answer is to discuss the events that happen in each segment of the alimentary canal. This is the way that your chapter is organized, and this is how we went through the Powerpoint.
Mouth - mechanical digestion = chewing - chemical digestion = saliva, salivary amylase
Stomach - mechanical digestion = churning. Chemical digestion includes lingual lipase being activated, gastric lipase acting on lipids, pepsinogen secreted by chief cells and becoming activated to pepsin in the lumen of the stomach to digest proteins, HCl secreted as H+ and Cl- by the parietal cell.
Small intestine - mechanical digestion = segmentation. Chemical digestion includes neutralization of acid by bicarbonate secreted from the pancreas in response to secretin, the pancreas also secretes enzymes like pancreatic amylase, pancreatic lipase, trypsin and chymotrypsin in response to CCK. In order for the lipase to effectively act, the lipids must be emulsified by bile salts secreted by the liver and gall bladder in response to CCK. Brush border enzymes complete the breakdown into individual subunits, and we discussed a couple of examples in lecture.
(Obviously I am summarizing the discussion from lecture sessions...)
The other way that you can organize your answer is to look at each macromolecule in sequence. In this case, you could discuss mechanical digestion first - mechanical digestion does not care what macromolecules are included in the food. Then you can discuss chemical digestion, looking at each macromolecule individually.
Carbohydrates: digestion of carbohydrates begins in the mouth with salivary amylase, so you can begin with a description of that process. Carbohydrate digestion does not continue again until the chyme reaches the small intestine, the pH is neutralized, and fresh enzymes are secreted from the pancreas. You can discuss pancreatic amylase at this point. Brush border enzymes like lactase, sucrase, and maltase finish the digestion into single monosaccharides.
Proteins: digestion of proteins begins in the stomach. You can discuss how hydrochloric acid is secreted by the parietal cell, and how pepsinogen is secreted by the chief cell and activated to pepsin in the lumen to begin to digest polypeptides. Digestion of proteins continues in the small intestine with the secretion of trypsin and chymotrypsin in the small intestine, and then the brush border enzymes finish digestion into individual amino acids or dipeptides.
Lipids: some digestion of lipids occurs in the stomach with gastric lipase. But the lipases can act most effectively on smaller lipid droplets, and so the surface area needs to be decreased. This is done by emulsification using bile salts secreted from the liver and gall bladder into the small intestine.
In lecture, we then went on to discuss the absorption of each of these macromolecular subunits. We also mentioned some disorders to help us understand the process. For example, people with cystic fibrosis that affects their pancreas must take digestive enzymes.