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.
Proteins are large molecules made up of one or more chains of 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.
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.
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.
This nutrient-rich blood that drains out of your digestive tract travels to the liver via the hepatic portal vein. This vein is defined as a vein that transports blood from the digestive tract to the liver. This is a unique vein; in fact, some would argue that it's not a true vein because it conducts blood to the capillary beds of the liver. We know that 'true veins' carry blood back to the heart, and arteries are typically the blood vessels that travel toward the capillary beds. So, the hepatic portal system has many unique circulatory facts to consider.
After the liver takes its share of the nutrients, the blood enters the general circulation through the hepatic veins, which drain the liver. The remaining amino acids can now circulate to your body cells. Your cells remove amino acids from the blood and use them to build the specialized proteins that we talked about at the beginning of this lesson, such as enzymes, antibodies, hormones, muscle proteins, or collagen.
Let's review. Proteins are defined as large molecules composed of one or more chains of amino acids. So, we see that amino acids are the basic building blocks of protein. Amino acids can be used by your body to form important cellular structures, such as enzymes, antibodies, hormones, muscle proteins, and collagen.
Protein digestion begins with the action of an enzyme called pepsin. Pepsin is the active protein-digesting enzyme of the stomach. Pepsin acts on protein molecules by breaking the peptide bonds that hold the molecules together. Digestion of protein is completed in the small intestine by the pancreatic enzymes trypsin, chymotrypsin, and carboxypeptidase. These enzymes also get some help from the brush border enzymes found in the small intestine. When proteins are broken down, they create polypeptides, which are chains of amino acids linked together, and peptides, which are two or more amino acids linked together.
Before entering the general circulation, digested nutrients that leave the digestive tract take a detour to the liver. The hepatic portal system is the system of veins that are responsible for directing blood from the digestive tract to the liver. The hepatic portal vein is the vein that transports blood from the digestive tract to the liver. After the liver takes its share of the nutrients, the blood enters the general circulation through the hepatic veins that drain the liver.
After this video, you'll be able to:
- Describe the structure of protein
- Explain the importance of amino acids
- Summarize the process of protein digestion
- Describe the function of pepsin
- List the pancreatic enzymes and enzymes from the small intestine that aid in protein digestion
- Understand the importance of the hepatic portal system in the process of protein digestion