Don’t let the word ‘biochemistry’ scare you away.
Easier said than done, I know – biochemistry scares me all the time. You know what else scares me? My bank account, disappearing spiders, and that feeling of dread when there’s no toilet paper left. But that’s not the point of this post. Back to biochem! I am going to try my absolute best to explain a couple processes of digestion in simple, easy-to-understand terms. After all, why say “resynthesized into triglycerides” when I can tell you that we’re “making fat again”? Here we go.
Let’s take a look at how we digest carbohydrates.
Carbohydrates are mainly found in starch. If I ate a bowl of oatmeal right now (mmm, oatmeal), digestion actually begins in my mouth. Chewing helps break the food into smaller pieces, which is good because the increased surface area means more space for enzymes to break down the oats. My spit contains salivary amylase, an enzyme that helps to break down the carbohydrates in my oatmeal. When they enter the body, they are broken down into single sugars inside our guts and on the surface of cells in our small intestine. Cells in our small intestines have a lot of surface area, so it is easy for them to make enzymes and absorb nutrients. Small intestine cells absorb these simple sugars using special cell transporters.
Fats are a bit more confusing, but still cool!
The problem with fats is that they are “hard” to digest. Fats hate water, which is unfortunate because our whole bodies are largely made of water. Fats are also compact and have little surface area, which means they must be mixed up. An enzyme called lipase interacts with the fats and water. Because our internal body temperatures are hot enough to “melt” the fats, solid fat particles become liquid. Once melted, the fats are mixed up. Our stomach squeezes together to push the mixture into our small intestine.
Our liver and gallbladder, two special organs, produce bile salts that cause the mixture to become smaller bundles. A bile salt is a molecule of cholesterol with extra polar groups, causing these bundles to love and hate water. Some parts of the bundle love water, so they are displayed on the outside, where they are exposed to water. Other parts of the bundle are scared of water, so they hide inside, and interact with the fat that we are trying to digest.
Lipase, the aforementioned enzyme, cuts each fat particle into a “head” part and three long “tails”. In scientific terms, each triacylglycerol molecule becomes a monoacylglycerol and three fatty acids. This is done because it is easier on our small intestines. An analogy of this would be moving a dresser into different room. The dresser itself is too big to fit through the door, so you have to remove the drawers and parts of the dresser itself. Then, you can pass through the door and put the dresser together again. The long tails are absorbed into the intestinal cells instantly. However, the small intestine cells need to make these “heads” whole again (even though they were just broken). Special structures made of protein surround these reformed fats. The inside of this structure is not exposed to water, because the inner membrane, as well as the fats that they are carrying, hate water. This is similar to putting the drawers back into the dresser.
Finally, we will absorb the fat with one last step. These round structures enter our lymph system for filtration and finally into our blood in a duct named the thoracic duct. Proteins and carbohydrates never enter our lymph systems, and flow straight into our bloodstreams. Fats use a different duct (not the one our regular circulation goes through) to bypass the liver, which would spend too much time breaking down the fats. The goal of this is filtration.
In the world of babies…
Babies have specialized GI tracts that help them digest milk. Their stomachs are less acidic, and there are fewer protein-cutting proteins. Mothers can transfer antibodies and proteins directly into their unborn babies through unformed gap junctions. This is often called a “leaky gut”. In the neonatal gut, there is increased activity in three different enzymes:
- Rennin: rennin turns curds out of milk protein (casein, which some people are allergic to).
- Lactase: lactase breaks down lactose (from milk) into simple sugars: glucose and galactose.
- Salivary lipase: salivary lipase helps break down fats. Breast milk is very high in fats, rather than protein.