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Protein is one of the primary nutrients your body uses to build cellular structures. In this lesson, you will learn how protein is broken down into amino acids within your digestive tract and how amino acids are absorbed out of the intestines and into the hepatic portal system.


Proteins are important nutrients that your body uses to build cellular structures that carry out vital functions. In fact, the basic building blocks of proteins are so important to your cells that if you did not consume protein, it would be like trying to build a house without tools or nails.In this lesson, you will learn how proteins are broken down into their basic units, called amino acids. You will then see how amino acids are absorbed through the small intestine wall and used by your body to build cellular structures and components. When we talk about digestion, we look at protein as a large molecule composed of one or more chains of amino acids.

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Proteins are large molecules made up of one or more chains of amino acids.
Protein Made of Chains

Amino Acids

When you feel hungry, you typically think of protein as a food, such as eggs, milk, meat, nuts, or beans. Regardless of which way you want to look at protein, one fact remains, and that is that amino acids are the basic building blocks of protein. The term ‘building block’ is a good way to describe amino acids because they are literally used by your body for building biological substances.

In fact, many of these biological substances are actually specialized proteins.So, you might eat an egg for breakfast, and once you swallow it, enzymes break down the protein in the egg into amino acids. Then, those free amino acids recombine in different ways to form specialized proteins. These specialized proteins become different things, such as enzymes, or antibodies, or hormones. Or, they might become structural proteins, such as muscle proteins or collagen found in connective tissue.

Protein Digestion

It’s good to keep in mind that protein digestion is not as simple as eating an egg and magically getting amino acids. A large protein molecule breaks down via a few intermediate steps, in the stomach and in the small intestine, before it becomes the tiny amino acids. So, let’s take a look at how proteins are broken down by your digestive system.Protein digestion begins in the stomach with the action of an enzyme that we previously learned about called pepsin. Pepsin is the active protein-digesting enzyme of the stomach. When pepsin acts on the protein molecule, it breaks the bonds that hold the protein molecule together, called peptide bonds.

So, you can think of pepsin as the enzyme that breaks peptide bonds. When these bonds are broken, you get chains of amino acids linked together called polypeptides. Since we know that the prefix ‘poly’ means ‘many,’ we can easily recall that a polypeptide is many amino acid units joined together. These polypeptides then move into your small intestine, where digestion will be completed by additional enzymes.

Pepsin is an enzyme in the stomach that breaks down the peptide bonds in protein.
Pepsin Breaks Peptide Bonds

In the small intestine, pancreatic enzymes that we previously learned about, called trypsin, chymotrypsin, and carboxypeptidase, really go to work breaking down the polypeptides. These enzymes enter the duodenum via the pancreatic duct. These pancreatic enzymes are helped by the brush border enzymes.

We previously learned that the brush border enzymes are special enzymes found on the microvilli of the small intestine that complete digestion.The peptide bonds holding the polypeptides together continue to be hydrolyzed, or broken down, and result in smaller units called peptides. Peptides are simply defined as two or more amino acids linked together.

Enzymes continue to break down polypeptides and peptides into amino acids. Because amino acids are very small, they are able to be absorbed through the small intestine lining and into your bloodstream.

Hepatic Portal System

It’s important to note that digested nutrients that leave the digestive tract take a detour to the liver before entering the general bloodstream. Your liver is an important organ, and this detour allows your liver to have first claim at nutrients coming from the digestive tract.

It’s almost like the liver is king and it gets first dibs at the good nutrients; then, when it takes its fill, the rest of the body has the leftovers.So, we see that capillaries in the wall of the digestive tract pick up the amino acids. These amino acids, along with other digested nutrients, then move to the liver through a unique system of veins called the hepatic portal system. This is the system of veins that are responsible for directing blood from the digestive tract to the liver. We see that the word ‘hepatic’ means ‘liver’ and the word ‘portal’ means ‘gateway,’ so you can think of the hepatic portal system as the gateway to the liver.

The liver is like a king and gets the first claim at nutrients coming from the digestive tract.
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