Fats and lipids

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Fats and lipids


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USMLE® Step 1 questions

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High Yield Notes

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Fats and lipids

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USMLE® Step 1 style questions USMLE

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A study is done on the processes of fat digestion and absorption in order to develop novel pharmacologics that can help prevent diseases associated with increased levels of fat and lipids in the blood. Which of the following is true regarding the digestion and absorption of fats and lipids?  


Fats are an essential part of a healthy diet. They contribute to the taste and texture of foods, like the smoothness of guacamole and the flakiness of a croissant. Fats are also a major source of energy and a critical component of cells and tissues, and they also help absorb essential vitamins, and can be converted into other molecules like prostaglandins which help cells communicate with each other.

Fats have a three-carbon backbone called glycerol, as well as fatty acid chains. The fatty acid chain is basically a string of carbon and hydrogen atoms. When an “OH” group from the glycerol molecule binds to a Hydrogen from the fatty acid, an “H20” or a water molecule - gets released, and the two molecules link up.

If this happens once, the result is a monoglyceride. If it happens twice, it’s a diglyceride, and three times makes a triglyceride.

Now, there are various types of fatty acid chains, and one way to categorize them is by their length, in other words, how many carbons they have. Short chain fatty acids have 2 to 5 carbons, medium chain fatty acids have 6 to 12 carbons, and long chain fatty acids have 13 or more carbons.

Fatty acid chains are also categorized by the bonds connecting the carbons in the chain. A single bond is just one bond between the carbon atoms, and when a fatty acid chain has only single bonds, it’s called a saturated fatty acid - because it has as many hydrogen atoms as possible or it’s saturated with them.

Triglycerides with saturated fatty acids are nice and straight so they pack together really well, and as a result they’re usually solid at room temperature. And the longer the saturated fatty acid chain, the more likely it will be solid at room temperature.

Carbons can also have double bonds between them though, and when a fatty acid has one or more double bonds, it’s called an unsaturated fatty acid because it’s not saturated with hydrogen atoms - for every double bond there are two fewer hydrogen atoms. Also, a double bond causes a kink in the molecule so the unsaturated fats don’t pack together as nicely as saturated fats. As a result, unsaturated fats are usually liquid at room temperature.


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