Lesson Video: Metabolism and the Liver Biology

In this video, we will learn how to define metabolism, and describe the metabolic functions of the liver.


Video Transcript

In this video, we will learn what metabolism is and how metabolic reactions are essential to our survival. We will also look at the difference between the key terms anabolism and catabolism and some examples of each reaction in the human body. One specific example that we will focus on is the role of the liver in removing toxic lactic acid.

There are many misconceptions surrounding metabolism in humans. For example, you might have heard that slim people have a really high metabolism. This is not necessarily correct, as metabolism is not how fast someone digests food nor how little weight they will gain. Instead, metabolism is the sum of all chemical reactions that occur in a cell or body.

Metabolic reactions occur in all living organisms. This sunflower has metabolic reactions occurring within it, as does a single-celled bacterium, and as do humans like you and I. Metabolic reactions are occurring in all of your cells all of the time. And your metabolism describes all the chemical reactions that occur within you that help you to maintain your life.

There are two types of metabolic reactions: anabolic reactions and catabolic reactions. Let’s look at anabolic reactions first. Anabolism is when metabolic reactions construct large molecules from smaller units. Anabolic reactions require energy that is stored in the form of a molecule called ATP. When certain bonds in an ATP molecule are broken, this releases the energy needed for anabolic reactions to occur. Let’s explore catabolism next. Catabolism is when metabolic reactions break down large molecules into smaller units. Catabolic reactions release energy, some of which is stored in cells in the form of ATP molecules. This means that the energy stored in the bonds of the ATP molecules is readily available for anabolic reactions.

Let’s look at some of the examples of anabolic and catabolic reactions to understand these processes better, starting with catabolic reactions. When we consume food like this chicken sandwich, for example, that is rich in proteins and carbohydrates, these large molecules are digested by enzymes in our digestive system and are broken down into smaller molecules.

By zooming in on the reactions occurring in the small intestine, we can see that these are examples of catabolic reactions as large proteins are broken down into smaller amino acid subunits and large carbohydrates, such as the starch in bread, are broken down into smaller, simple sugars, like glucose. Both of these processes release energy. And the glucose, specifically can either be stored, or it can be further metabolized in a process called aerobic respiration.

This diagram is of a mitochondrion, the site of aerobic respiration in human cells. Aerobic respiration breaks down glucose to release energy, which is stored as ATP. So, this process is another example of a catabolic reaction. This diagram is of the liver, an organ which carries out a vast number of metabolic functions. One catabolic reaction that’s carried out by the liver is converting excess proteins into a substance called urea. This process of breaking down proteins releases energy, some of which will be used to convert the product into urea, which can then be excreted from cells to exit the body as part of urine.

Let’s look at some examples of anabolic reactions next. If we think back to our first catabolism example where proteins and starch were broken down into amino acids and glucose, respectively, you may have wondered what happens to these smaller molecules once they have been formed. Simple molecules like these can be used to build up a variety of other larger protein and carbohydrate molecules that have a function within an organism’s body. For example, these amino acids can be combined in diverse sequences to form multitudes of different proteins, while the glucose molecules can be reformed into starch in plant cells or into glycogen in animal cells. As they are anabolic, both of these processes require an input of energy, which will be provided by the breaking of certain bonds in ATP.

As the liver is such an important organ when it comes to discussing metabolism, let’s explore some other metabolic functions that it carries out, one of which is metabolism of lactic acid, sometimes called lactate. Lactic acid is produced by lactic acid fermentation, which is sometimes called anaerobic respiration. Humans are an example of aerobic organisms, as our cells usually carry out the oxygen-fueled aerobic respiration to release energy. Anaerobic respiration is the process by which anaerobic organisms that live in low-oxygen environments release energy as it can occur even when oxygen is absent.

Sometimes human cells can also carry out anaerobic processes, such as lactic acid fermentation. For example, if someone was sprinting in a race and their muscle cells demanded more energy than the oxygen-fueled aerobic respiration can deliver, anaerobic respiration occurs. Lactic acid fermentation provides the muscle cells with energy, and it also produces lactic acid. Lactic acid is toxic to humans, so it must be broken down into a less toxic substance. It is commonly believed that a buildup of lactic acid is linked to the immediate muscle pain you may feel while doing intense exercise.

The organ that detoxifies lactic acid, among many other toxic substances, is the liver. The liver has two options, one of which is converting lactic acid back into glucose. You can see from its molecular formula that lactic acid is effectively half a molecule of glucose, which means that the conversion of lactic acid to glucose would be building up a larger molecule from a smaller one. This reaction is therefore anabolic and would require ATP. The other option the liver has is to break down lactic acid by reacting it with oxygen to form carbon dioxide and water, which are the same as the product of aerobic respiration. Carbon dioxide and water are both smaller molecules than lactic acid. So, this reaction is catabolic, as it involves breaking down lactic acid to release ATP.

Let’s see how much we can remember about metabolism and the liver by having a go at a practice question.

Which of the following statements correctly relates energy to catabolic and anabolic reactions? Anabolic reactions require energy to form new molecules, whereas catabolic reactions release energy as molecules are broken down. Or catabolic reactions require energy to form new molecules, whereas anabolic reactions release energy as molecules are broken down.

Let’s investigate some of the key terms in the question to help us figure out how to answer it. Catabolic and anabolic reactions are both types of metabolic reactions that occur within the cells of living organisms to help them survive. Let’s take a look at catabolic reactions first. Catabolic reactions break down large molecules into smaller units. The breaking of bonds in the large molecule releases energy, which can be used in the cell for other reactions. For example, proteins are broken down into amino acids in our digestive system. This is an example of a catabolic reaction, as a large molecule is forming several smaller molecules and the process overall releases energy.

Anabolic reactions construct larger molecules from individual units, and this process requires energy to form the bonds between these subunits. For example, these amino acids are being joined together to form a large protein, and it requires an input of energy. The energy is used to form these bonds between the individual amino acids. So, this is an anabolic reaction. Therefore, our correct answer is that anabolic reactions require energy to form new molecules, whereas catabolic reactions release energy as molecules are broken down.

Let’s have a look at some of the key points that we have addressed in this video. Metabolism describes all the reactions occurring in the cells of a single living organism. And these metabolic reactions help us all to survive. Metabolic reactions can either be anabolic, where small molecules are built up into larger molecules using an input of energy. Or the reactions can be catabolic, where large molecules are broken down into smaller ones releasing energy. The liver is a major organ that carries out various metabolic reactions involved in detoxification and waste removal. For example, the liver cells are responsible for detoxifying the lactic acid produced in lactic acid fermentation, an example of an anaerobic process which occurs in human muscle cells.

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