THE BIOCHEMISTRY OF LIPIDS - Digestion and Absorption

In Biochemistry, Lipids are biological molecules that are insoluble in aqueous solution like water (are hydrophobic in nature) and soluble in polar solvents like benzene.

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They are the generic names assigned to a group of fat soluble compounds found in the tissue of plants and animals. Lipids are hydrophobic in nature. Fats are a class of lipids containing two kinds of monomers - fatty acids and glycerol. Glycerol is a 3 carbon molecule containing 3 hydroxyl groups (this means its chemical structure has 3 C and 3 -OH groups), one bonded to each carbon.

Structure of Glycerol

Triglycerides & Triacylglycerols are the simple type of lipids.

Now this is the part where we relate

Triglycerides and Triacylglycerols are what our basic school teachers referred to as Fats and Oil back in elementary school.
Elementary school was really elementary... Imagine a 7 year old trying to understand Triglycerides and Triacylglycerols.. LOL
This Triacylglycerols are made up of a backbone called glycerol and fatty acids - which is kinda our focus on this topic..

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So, sit back, grab a bottle of soda and read on!

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HOW ARE LIPIDS DIGESTED

First of, lets understand that a lot of chemical reactions go on the body - a lot more than you can imagine. The body has its way of secreting proteins called enzymes to act on the food we ingest, so that digestion doesn't have to be too long of a process. Without this enzymes, your pack of fries would take longer than a few minutes to digest!. These enzymes act by breaking down the molecule it act on at a faster rate.

Lets get an official insight of what enzymes are

Enzymes are proteins and function as biological catalysts which means they speed up specific chemical reactions within the body by providing an alternative reaction pathway and thus lowering the activation energy.

The enzymes which act on lipids are called lipases.

For how lipids are digested, lets start from when your fries get into your..

MOUTH

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While you are trying to savor the taste of your delicious fries in your mouth, your teeth do a little crushing mechanically, meanwhile chemically, enzymes are secreted to break down the food. An enzyme called lingual lipasewhich is secreted by the lingual glands on the dorsal surface of the tongue is responsible for the breakdown of fatty foods. But the pH of the mouth is between 7.0-7.4 - slightly alkaline

pH is the measure of acidity or alkalinity of a system

A pH Scale

Because the lingual lipase can only function in acidic conditions, digestion of lipids does not start in the mouth.

So your fries which must have been chewed & has undergone mastication mixing with your saliva goes down your oesophagus into your....

STOMACH

Now lets learn something about the stomach...
The secretions of the stomach are highly acidic and causes it to have a pH ranging from 1-3 (highly acidic). This secretions are known as gastric secretions. The hydrochloric acid present in the stomach acts on food that enters the stomach. But during fasting, starvation theres no food in the stomach for the acid to act on, so it begins to act on the walls of the stomach (try relating acid pouring on your skin), this causes injuries and leads to ulcer. Back to the topic...

Because the stomach is highly acidic in nature, the lingual lipase that was secreted in the mouth are now activated & acts on the chyme (chyme here refers to the food that has been mixed with salivary secretions). Also there is a secretion of a gastric lipase in the stomach - which mainly acts on milk fat. Note that in this form, the food is still a lipid because it has not been catalyzed by any enzyme. It is in the stomach that lingual lipase from the mouth acts on triacylglycerol and splits it into 1,2-diacylyglycerol and a free fatty acid. Also, the gastric lipase acts on this 1,2-diacylglcerol produced from the previous reaction to form glycerol and free fatty acids. Therefore, digestion of lipids starts in the stomach. The free fatty acids released as well as the glycerol are absorbed through the stomach wall into the enterohepatic circulation.

Enterohepatic circulation - relating to or denoting the circulation of bile salts and other secretions from the liver to the intestine, where they are reabsorbed into the blood and returned to the liver.

Alright, lets move down to the...

SMALL INTESTINE

The small intestine (also called duodenum) is the main site for absorption of nutrients and the digestion of fat. When chyme from the stomach enters the duodenum - the upper portion of the small intestine - hormones secreted signal the gallbladder to contract. These contractions push bile, which is made by the liver, out of the gallbladder and into the common bile duct, which connects the gallbladder to the small intestine. At the same time, the pancreas, located just underneath the stomach, secretes bicarbonate ions, which neutralize the pH of the chyme entering the small intestine, and gastric lipases, enzymes that break down fat.

Creation of Micelles

Fats are hydrophobic, which means they do not dissolve in water. Left to their own nature, fat molecules would clump together and form one big fat molecule that is not easily digested. Bile prevents this from happening. Bile molecules have a hydrophobic, or water-fearing, end and a hydrophilic, or water-loving, end. The hydrophobic end sticks to each fat molecule and the hydrophilic protrudes to prevent the molecules from sticking together. The combined structures of fat molecules and bile molecules are called micelles. So in other words, bile secreted in the liver comes in contact with the fat through the enterohepatic circulation and forms micelles.

Breakdown of Micelles

Once fat molecules become micelles, lipases go to work, breaking down fat molecules into fatty acids and monoglycerides, which pass through the small intestine. After they pass through the small intestine, fatty acids are converted to triglycerides, which combine with cholesterol, phospholipids and protein to form a structure called a chylomicron. The protein coating of the chylomicron makes it water-soluble so it can travel through the lymph vessels and eventually the bloodstream. This fat in the blood stream is used up for energy but excess are transported to adipose tissues for storage.

Obese patients consume lot of fat and have the excess stored in their adipose tissues..

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And there we have it...!

That little pack of fries got metabolized and used up by the body in a matter of few minutes, thanks to enzymes..
Subsequent series of the Metabolism of Lipids are coming up, so do well to follow @alisonudeme and Stay tuned.

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