The Tale of Fats…
Delicate, faint sounds of collision between metal and glass resonated throughout the open room, fading over time, until they were barely noticeable. Sounds of chatter and conversation periodically intensified, reaching a moderate apex before diminishing into a cold silence, reigning in the room over all other sounds.
The lighting was dim, only just illuminating the reflective, white surface on which the subject currently resided. A large, four-pronged metal object quickly descended from the heavens like a ferocious hawk, impaling its unsuspecting prey and slowly hoisting it back into the air. It creeped towards a dark, gaping hole, where the subject would meet a very interesting fate…
This is, of course, a dramatic recreation of the phenomenon we all know simply as “eating”, or at least the component of eating that is most well-understood. I was recently enrolled in a course where I was given the opportunity to study the processes of ingestion, digestion, absorption, and metabolism of several macromolecules (carbohydrates, proteins, lipids, and Nucleic acids).
I can tell you now, there is so much more to the story of “eating” than simply placing food into the mouth, chewing it, and swallowing it.
For this article, I want to focus specifically on the story of lipids. Lipid is an umbrella term encompassing both fats and oils, as the two categories are structurally very similar.
Fats get a lot of heat in the health community…
I will not deny that there are several health risks associated with consuming excessive quantities of certain species of lipids, but in generalizing against all fats or lipids, plethoras of complex and truly fascinating processes are completely disregarded.
Did you ever wonder how the food you eat is converted into a form of energy, allowing you to move large boulders, solve difficult puzzles, and ultimately sustain life itself?
The tale begins thus…
Once the food is ingested, it is moved through the Gastrointestinal tract (GI tract). The GI tract is a long muscular “tube” consisting of the mouth, esophagus, stomach, small intestine, large intestine and anus (in an adult, if stretched out end to end, the GI tract would be over 9 meters long!).
All food that is consumed must travel through the entire tract, where it is actively digested (broken down) and absorbed (taken in). During this process, the food is forced in one a direction along the GI tract by rhythmic contractions in the “wall” known as peristalsis, which is caused by an internal lining of smooth muscle tissue.
(There are countless more interesting details and processes that occur during digestion and absorption, but they are not the main focus now!)
After passing through the mouth, the esophagus and the stomach, food reaches the small intestine, and this is where lipid digestion begins. In the small intestine, the lipids encounter a cholesterol-based, emulsification agent called bile, which I will discuss later.
To comprehend lipid digestion/absorption, it is important to understand, first, that lipids are hydrophobic molecules.
What does that mean?
The word “hydrophobic” has two parts: “hydro” and “phobic”. They come from two Greek root words: “hudōr- (ὕδωρ)” which means “water”, and “phóbos (φόβος)” which means “aversion” or “fear”. Thus, hydrophobic essentially means “water-fearing”, so a hydrophobic molecule is a molecule that tends not to mix with water.
Why are lipids hydrophobic molecules though?
The concept is rather technical (I will definitely talk about it more in another entry!), but I will summarize the important points here.
Lipids are unique from the other macromolecules in that they are mostly nonpolar, mainly because they contain very few polar bonds. A bond can be polar because certain atoms have the capacity to hold electrons much more “tightly” compared to others. Electronegativity is the property which describes an atom’s capacity to hold electrons. If the electronegativity between atoms in a bond is unbalanced (e.g. one atom is more electronegative than the other), one atom will hold most of the electrons for the majority of the time in that bonding interaction. Electrons possess a -1 charge, so when an atom holds more electrons, it acquires a partial negative charge, leaving the other bonded atom with a partial positive charge. A bond where electrons are not equally distributed is called a polar bond, because the two ends of the bond possess different charges.
Lipids, on the other hand, have very few polar bonds, so they are considered to be nonpolar molecules. This is important because polarity has a significant effect on how a molecule interacts with water.
Water is, itself, a very polar molecule. It is composed of two hydrogen atoms bonded to one oxygen atom. The oxygen atom is very electronegative, so it will occupy most of the electrons in the molecule, forming a partial negative charge. Polar molecules interact well with other polar molecules because a partial negative charge on one molecule attracts a partial positive charge on another molecule, forming a considerably strong and stable interaction (stability is everything is thermodynamics!). This is why lipids are hydrophobic, because water molecules would much rather interact with other water molecules rather than with the nonpolar lipids!
How does this affect the digestion/absorption of lipids though?
Let me help you understand this with one question of my own: What is the best way to minimize the surface of interaction between lipids and water?
(I realize this is quite difficult to conceptualize, but see if my answer (below) makes sense.)
It is actually to push all of the lipids together in a large globule, since all of the lipid molecules in the center of the globule will only interact with each other and will not be forced to interact with the water molecules. Thus, the formation of these globules (and more generally put, the congregation of hydrophobic molecules) is referred to as the Hydrophobic Effect (as we previously discussed, hydrophobic molecules such as lipids are nonpolar, and will not interact well with water).
A practical illustration of the hydrophobic effect is when you put oil and water on a pan; you will immediately see circular formations of the oil which exclude the water molecules.
Again, why is this important for digestion/absorption of lipids?
It is because it is actually very difficult for the digestive enzymes to access the lipids embedded deep in the center of a large globule, so that is the problem… which is why bile is so important!
Remember, bile acts as an emulsifying agent, as one region of this large molecule is very polar (contains partial charges), and the other region is very nonpolar. The nonpolar regions of the bile molecules interact with the lipids, and the polar regions will face outward towards the water-based solution in the small intestine. Emulsification is the process where a large fat globule is separated into smaller micelles.
The bile eventually forms spherical capsules around small pieces of the globule (micelles) and pulls them out into the solution until there are no lipids remaining in the globule.
Now the lipids are finally accessible to the lipase enzymes which will be able to digest them!
It is about time that I discuss the actual structure of these lipids.
Lipids consist of a large variety of shapes, structures, and compositions as the only quality common to all lipids is that they are hydrophobic molecules, but we will limit our discussion to the prominent storage form of lipid called Triacylglycerol (TAG), which are composed of one glycerol backbone attached to three fatty acid tails.
Let’s re-establish the setting:
The TAGs are entering the lumen of the small intestine, and upon encountering the bile, the large globules are broken down into smaller micelles. The lipase enzymes then digest the TAGs into their basic substituents (glycerol and three fatty acids).
Once broken down, the fatty acids can be transported across the enterocyte membrane via diffusion. Enterocytes are specialized epithelial cells constituting the surface of the small intestine and allowing effective absorption of the food particles present in the lumen.
The subunits of TAGs (glycerol and three fatty acids) reunite in the cytoplasm of the enterocytes to reconstruct the TAG structure.
The reason why TAG must be disassembled into its substituents before being absorbed into the enterocyte is that, while TAG is the storage form of lipids and allows for very efficient packaging, it is also a very large molecule. This creates a problem when attempting to cross a membrane of an enterocyte.
Once re-assembled inside the cytoplasm of the enterocyte, the TAGs are incorporated into structures called chylomicrons, which will act as the transportation vehicles for all of the TAGs and other lipid molecules. These chylomicrons are essential in ensuring that the lipids successfully reach their destinations (muscle or adipose tissue cells) without causing harm to your body.
Remember how lipids are hydrophobic molecules?
Now imagine dumping tons of these lipids into the bloodstream.
(Blood is 90% water!)
What do think would happen?
The lipids would congregate, and large globules would form in your blood! This would be terrible and inconvenient as the globules would not be able to move to their proper destination, and they would interrupt the flow of other important nutrients carried through the bloodstream. This is why chylomicrons are so important!
Chylomicrons are encased in a membrane composed of another type of lipid species called phospholipids. Similar to bile, phospholipids possess a very polar region and a very nonpolar region, so they can form membranes which can interact well with the aqueous (water-based) solution of the blood, while simultaneously interacting with the lipid content within the chylomicron. The phospholipids are very prominent in biological systems as they constitute almost every membrane in every cell in the body.
Finally, the chylomicron travels through the circulatory system (bloodstream) and the lymphatic system until it reaches an adipose cell, where it can be stored until it is needed.
In this article, I went through the processes from lipid ingestion all the way to their final storage in an adipose cell.
I know this is technical and can be very challenging to understand and I used very few analogies, but the main takeaway here should be that the systems set in the human body are indescribably complex. Things that we take for granted everyday can be so inspiring if you look deeper into them.
The next part of the story is how the Lipids leave storage and are finally utilized to produce energy, and how the energy is used to activate different functions across your body!
See you next time to learn about that!
(Note: I often bold certain terms throughout the articles without defining them, as they will likely be terms I explore in more depth in the future.)
Acknowledgements:
Diagrams illustrated by Kimberly Wong
Edited by Lester Maxwell
