Understand how bile is stored, released and recycled during digestion.
Know how bile salts and lipase work together to aid fat digestion.
In my last post, I wrote about how bile from the liver gets stored in the gallbladder, and is released when CCK causes the gallbladder to contract. I didn't get around to explaining what bile does and what happens to it later. Well, time to make up for it!
Bile salts are amphipathic, meaning that they have a hydrophilic (water-loving) and a hydrophobic (water-fearing) part. The hydrophobic part dissolves fats, while the hydrophilic part dissolves water. Hence the bile salts help fat molecules to break up and dissolve in water. Breaking up of larger droplets into smaller droplets is called "emulsification," and it increases the total surface area available for lipases to come in and attack. (Lipases are enzymes that break fats up into free fatty acids and monoglycerides.)
Bile salts are precious, so the body wants to conserve them as much as possible. This occurs by reabsorption, a phenomenon also known as "enterohepatic circulation." (Enterohepatic circulation also occurs in some drugs, as I mentioned in an early post on PHAR2210.) Essentially the bile salts that are secreted in the duodenum are reabsorbed later down the digestive tract, usually in the ileum (second half of the small intestine).
Be familiar with the structures within the small intestine that increase absorptive surface area.
Be familiar with the basic roles of the jejunum and the ileum.
I feel like I've covered this in my ANHB2212 post on tubes. Perhaps the main thing that I didn't cover so well was the differences between jejunum and ileum. The jejunum is wider and thicker than the ileum, and the patterns of arteries that supply the two regions are different (though I don't really think it's necessary to go into detail here). The ileum also has "Peyer's patches," made up of lymphoid tissue, that protect from bacteria of the colon.
Most absorption takes place in the jejunum- the ileum is just a "reserve absorptive area" in case your body needs a bit more than what's absorbed in the jejunum. However, bile salts and B12 are only absorbed in the ileum.
It's important to note, though, that the boundary between jejunum and ileum isn't clearly defined.
Understand how fats, carbohydrates, and proteins, are digested and absorbed in the small intestine
The basic idea to understand here is that stuff gets broken down and then transported through the enterocytes of the epithelial layer lining the intestines (except in the case of fats, where the fats can simply diffuse straight through the membrane). Nevertheless, let's have a closer look:
Fats
Fats, as I mentioned above, are broken down into monoglycerides and free fatty acids with the help of bile salts. These smaller molecules can diffuse into the enterocytes much more easily than large fat molecules. Once inside the enterocytes, they reform into triglycerides and are coated with protein. This fat and protein structure is called a "chylomicron," and it is soluble in water. The chylomicron is then exocytosed into the lymph system, which eventually drains into the subclavian veins.
Carbohydrates
Breakdown of carbohydrates happens all throughout the digestive system, thanks to α-amylase from the saliva and pancreas. α-amylase breaks α-1,4-linkages (i.e. straight chain linkages, NOT branch points- for more information on terminology, see here) in carbohydrate chains, such as starch. This produces maltose (two glucose molecules joined together), maltotriose (three glucose molecules joined together) and α-limit dextrans which are short chains of glucose that include a branch point.
In the intestines, even more enzymes come into play to digest carbohydrates. You don't have to know their names, but here they are just for completeness:
- Lactase- splits lactose into glucose and galactose.
- Glucoamylase- breaks maltose and maltotriose down into individual glucose monomers
- Sucrase-isomaltase- has two parts. The sucrase part breaks down sucrose, maltose and maltotriose. The isomaltase part breaks the branches of the α-limit dextrans.
Proteins
Proteins are a bit simpler. Some protein digestion occurs earlier in the digestive tract through enzymes such as pepsin and trypsin. In the intestines, peptidases on the surface of enterocytes can break peptides down even further. Any peptide chain less than four amino acids long (i.e. 1-3 amino acids) can go through one of many amino acid or peptide transporters into the cell. (Some of these amino acid transporters also rely on Na+ moving down its concentration gradient.) Within the cell, di- and tripeptides are broken down further by dipeptidases and tripeptidases, respectively. Amino acids are transported out the other end by more amino acid transporters.
Know how salts and water are absorbed in the small intestine.
This is pretty simple, as it's quite similar to how salts and water are reabsorbed in the kidney. (If you need a refresher, see here.) Essentially the basolateral membrane has Na+/K+ pumps that create a diffusion gradient for Na+, allowing Na+ to diffuse through Na+ channels on the luminal membrane. Cl- then follows because of the electrochemical gradient created by movement of Na+, and water follows because of the osmotic gradient that has been produced by the movement of all of those salt particles.
Be familiar with the basic role of the colon.
The colon (large intestine) has three main purposes: absorbing even more salt and water, digesting dietary fibre (gut bacteria help out here) and storing fecal matter before it gets excreted. By the looks of things, we don't need to know about any of these functions in great detail.
And that's it for PHYL2001! Good luck with exams everyone :)
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