Organic Compounds

In the natural world, organic compounds are essential. Lipid is one of the diverse groups of organic compounds living things cannot live without. Lipids are generally a big biomolecule; however, they are one of the biomolecules that are not a formation of repeating monomers (Mckinley, O'Loughlin and Bidle). This group of natural biomolecules is hydrophobic. Lipid's general functions are storage of nutrients, acting as components of cellular membranes, and structures of hormones (Mckinley, O'Loughlin and Bidle). As lipids are a group of diverse biomolecule, they are subdivided into four major classes: triglycerides, phospholipids, steroids, and eicosanoids (Mckinley, O'Loughlin and Bidle).

The most common form out of the four classes of lipids is triglycerides (Mckinley, O'Loughlin and Bidle). The main functions of triglycerides are the long-term adipose connective tissue storage, insulation of the body, structural support for the body, and the cushioning to the body (Mckinley, O'Loughlin and Bidle). One example for the function in cushioning can be seen in the human body. As triglyceride is long-term energy storage for adipose tissue, adipose connective tissue is actually one of the cushions located within and posteriorly to the eye's orbit (Mckinley, O'Loughlin and Bidle).

The second class of lipids, phospholipids, can be seen in majority of the cell's plasma membrane (Mckinley, O'Loughlin and Bidle). Phospholipids are extremely essential in the microcellular level. This class forms a barrier between the inside and outside for a cell (Mckinley, O'Loughlin and Bidle). Without phospholipids, there will be no formation of the phospholipid bilayer, which prevents unwanted molecules inside the cells. In other words, phospholipids helps providing a structural wall around the cell to protect the inside structure from the outside.

Steroids, the third class of lipids, contain three types of subclasses. The structure of steroids generally contains arrangement of four cycloalkane rings (Stanczyk). The subclasses of steroids are cholesterol, steroid hormones, and bile salts (Mckinley, O'Loughlin and Bidle). Cholesterol functions as the major component in the cellular membranes and it also act as a precursor biomolecule for synthesis of other steroids (Mckinley, O'Loughlin and Bidle). Steroid hormones are molecules acting just like hormones. They regulate endocrine glands' hormone molecules to the rest of the body (Mckinley, O'Loughlin and Bidle). The last subclass of steroids is called eicosanoids. Eicosanoids are also divided into prostaglandins, prostacylins, thromboxanes, and leukotrienes (Mckinley, O'Loughlin and Bidle). The collective functions of all four eicosanoid molecules are local signaling molecules to the body, primary inflammatory response in the immune system of the body, and the communication between the parts of the body with the nervous system (Mckinley, O'Loughlin and Bidle).

Aside from the four major classes of lipids there are other lipids that are essential in the body. These lipids are glycolipids and fat-soluble vitamins (Mckinley, O'Loughlin and Bidle). Glycolipids are basically just lipid molecules joined by carbohydrate (Mckinley, O'Loughlin and Bidle). This type of lipid works along side with phospholipids in the association of plasma membranes (Mckinley, O'Loughlin and Bidle). Not only that glycolipids actually helps in formation of lipids by functioning as cellular binding (Mckinley, O'Loughlin and Bidle). As for fat-soluble vitamins, they are essential in our dietary needs. They can be seen in the form of vitamin A, vitamin E, and vitamin K (Mckinley, O'Loughlin and Bidle).

In human body, lipid is an essential component from microcellular level and molecular level. To obtain lipid, we, as humans, generally eat it. The flow of digestion in the body is from the oral cavity, stomach, then small intestine. Let's use the most common class of lipid as the molecule being digested. In the oral cavity, there is an enzyme called lingual lipase (Mckinley, O'Loughlin and Bidle). Lingual lipase will only be secreted/activated if there is a low pH level in the stomach (Mckinley, O'Loughlin and Bidle). As the triglyceride travels to the stomach, there are two routes the triglycerides can take to be broken down as monoglyceride and fatty acids (Mckinley, O'Loughlin and Bidle). The first route is by the breakdown of lingual lipase (Mckinley, O'Loughlin and Bidle). The second route is by the breakdown of pancreatic lipase (Mckinley, O'Loughlin and Bidle). Even both enzymes breakdown triglycerides into monoglyceride and fatty acids, they only produce limited amount of fatty acids (Mckinley, O'Loughlin and Bidle). In fact, both lingual lipase and pancreatic lipase do not require bile salts (Mckinley, O'Loughlin and Bidle). As the triglyceride travel down to the small intestine, it is being broken down into monoglyceride and two fatty acids by pancreatic lipase produced by the pancreas (Mckinley, O'Loughlin and Bidle). However, breaking down triglyceride is not just a one step deal. Due to the molecular size of triglyceride, bile salts comes in to emulsify triglyceride into smaller droplet form (Mckinley, O'Loughlin and Bidle). Bile salts are a structure of micelle (Mckinley, O'Loughlin and Bidle). By emulsifying the lipid, the pancreatic lipase has better success in digesting the molecule (Mckinley, O'Loughlin and Bidle).

As enzymes digesting lipids, lipids are also being absorbed into the body either as storage or energy. To be able to transport digested lipids, they need to be contained within micelles (Mckinley, O'Loughlin and Bidle). This structure helps the ease of transport of lipids into small intestine of the simple columnar epithelial lining (Mckinley, O'Loughlin and Bidle). As the lipids enter the epithelial lining, monoglyceride are attached back to the fatty acids and form triglycerides (Mckinley, O'Loughlin and Bidle). This results in the combination of lipids with protein in the formation of chylomicron (Mckinley, O'Loughlin and Bidle). Chylomicron is then being packaged by Golgi apparatus and released by exocytosis (Mckinley, O'Loughlin and Bidle). Due to the size of the chylomicron, it is excreted into the lymphatic capillaries (Mckinley, O'Loughlin and Bidle). In the end, chylomicrons enter the blood stream and deliver the lipids to adipose connective tissue, skeletal muscle tissue, cardiac muscle and the liver (Mckinley, O'Loughlin and Bidle).

With any functions in the body, there will always be malfunctions. The defective digestion and absorption in lipids can cause steatorrhea (Binder). Steatorrhea is basically a condition where there are excess fat in feces (Binder). Many situations steatorrhea is responsible for diarrhea (Binder). If there is any abnormality in any of the phases in the process, they can cause steatorrhea (Binder). In digestive phase, if there is a decreased lipase secretion and decreased intradudenal bile acids, it can lead to chronic pancreatitis (Binder). In absorptive phase, if there is any mucosal dysfunction, it can lead to celiac disease (Binder). In postabsorptive phase, if there is absent betalipoproteins or abnormal lymphatics, they can lead to Abetalipoproteinemia or Intestinal lymphangiectasia (Binder). Majority of the cases, steatorrhea can be linked with the deficiency of fat-soluble vitamin (Binder).

Lipids are essential in the human body like proteins, carbohydrates, and other biomolecules. To be useful in the body, lipids need to be digested then be absorbed into the system. However, in any body process, there will always be defects. This is why understanding and acknowledging the any process is essential in figure out the main point of the problem. In Henry J. Binder's article, in Harrison's Principles of Internal Medicine, he concluded the best, 'the knowledge of the mechanism of the digestion and absorption of carbohydrates, proteins, and other minerals and vitamins is useful in the evaluation of patients with altered intestinal nutrient absorption' (Binder).

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