The Cheeseburger Question

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 ....

By Organ:

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...)

By Macromolecule:

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.

Secondary Active Transport

By the time we finish talking about the types of transport in Chapter 3, most students' heads are swimming.  Usually we have discussed several types of transport; Diffusion, Facilitated Diffusion, Osmosis, and Active Transport.  Our example of Active Transport usually involves the Sodium-Potassium pump, which is quite a discussion topic itself.

So when we finish those four types of transport, and students are fighting to try to make sense of the similarities, differences, driving forces, involvement of proteins and energy .... then the book throws on top of that a mention of Secondary Active Transport.  The discussion is limited to three paragraphs and a figure, so this type of transport just gets a brief mention at the end of a long discussion about types of transport.

Fast-forward to the end of A&P II when we discuss the Digestive and Urinary systems.  Figure 24.21 mentions that glucose transport depends upon secondary active transport using sodium.  Figure 26.12 and 26.13 show the reabsorption of several molecules and ions using secondary active transport mechanisms.  But your book doesn't include much of a review of secondary active transport - instead it assumes that you remember it from the end of the long discussion on transport from Chapter 3.

So let's take a second to review secondary active transport in this post.

Secondary Active Transport is called secondary, because it uses the sodium ion gradient - rather than ATP directly - as the driving force.

The sodium/potassium pump moves three sodium ions out of the cell for every two potassium ions moved into the cell.  So it creates a high concentration of sodium outside the cell.

Other molecules can take advantage of the sodium gradient to move across the cell membrane.  If a transport moves sodium down its concentration gradient, and moves the other ion/molecule as well, the other ion/molecule can simply ride along as sodium moves down its concentration gradient.

One example is the Na/glucose symporter.  Glucose needs to get into the cell in order to undergo the chemical reactions that make ATP.  So ... glucose needs into the cell.  Sodium wants to go down its concentration gradient to get into the cell.  And we have a transporter that can carry both sodium and glucose.  They both move into the cell, and everyone is happy.

The energy used to move glucose across the membrane was the potential energy stored in the sodium concentration gradient.  That sodium concentration gradient was set up by active transport.  Thus, this type of transport that uses the sodium gradient is called "secondary" active transport.