Lesson Video: Metabolism | Nagwa Lesson Video: Metabolism | Nagwa

Lesson Video: Metabolism Biology • Second Year of Secondary School

In this video, we will learn how to define the terms metabolism, anabolism, and catabolism and describe examples of anabolic and catabolic reactions.

13:08

Video Transcript

In this video, we will learn about metabolism in the body and a two different types of metabolic reactions, anabolic reactions and catabolic reactions. We will go over some examples of each of these types of reactions and understand how energy is involved in these processes.

All of the systems in our body work together to provide the body with the energy, oxygen, and nutrients it needs. Each of these systems are constantly performing chemical reactions in the body to support and maintain life. For example, when you do some form of exercise like running or playing sports, you may notice that you breathe heavily for a short while afterwards. When you exercise, your body cells need energy to keep you going.

In order to release energy, cells need to perform cellular respiration. This is a chemical reaction that occurs most efficiently in the presence of oxygen. It also explains why your breathing becomes faster and heavier so that your body can acquire the oxygen that it needs. Cellular respiration is just one example, but there are actually millions of chemical reactions that happen in our body every second. Molecules are constantly being built up and broken down within our various organ systems.

We use the term metabolism to describe the set of all of these chemical reactions that take place within living organisms to maintain life. We can consider these metabolic chemical reactions to be of two broad types. In some cases, large molecules in the body are broken down into smaller ones. In other cases, small molecules are put through chemical reactions that combine them into larger molecules. We call the breakdown of large molecules into smaller ones catabolic reactions. And the production of large molecules from smaller ones are called anabolic reactions. Both anabolic and catabolic reactions involve energy in the form of a tiny molecule called ATP, which is short for adenosine triphosphate. Let’s take a look at the structure of ATP.

It consists of a sugar called ribose attached to a nitrogenous base called adenine and three phosphate groups, as we can see here. We can deduce a lot about the structure of ATP from its name. Aden- tells us that the nitrogen-containing base in this molecule is adenine. And tri- means three, telling us that there are three phosphate groups in an ATP molecule. ATP is often said to be the energy currency of the cell. It is broken down when the cell needs energy to carry out certain reactions and formed when different reactions within the cell release energy.

So how is this energy stored in an ATP molecule? The bond between the two outer phosphate groups in an ATP molecule is said to carry this chemical energy. When the cell needs energy for its reactions, this bond is broken and energy is released along with the outermost phosphate group, which we call an inorganic phosphate and represent with the initials P i. The resulting molecule only has two phosphate groups, which is why it’s called ADP, or adenosine diphosphate, as the prefix di- means two. If a cell’s reactions release energy, this energy can be used to bind ADP to an inorganic phosphate, reforming ATP. In this way, the energy that’s released by certain cellular reactions can be captured and stored in the form of ATP, ready to be broken down again whenever it’s needed to release this energy once more.

So what is the relationship between the different metabolic reactions and ATP energy? As we learned earlier, anabolic reactions construct large molecules out of smaller ones. To form these chemical bonds that make up a larger molecule, anabolic reactions need energy which they get by breaking down ATP molecules. Catabolic reactions, on the other hand, break down large molecules into smaller molecules. By breaking the chemical bonds in the larger molecules, catabolic reactions release energy which is stored as ATP molecules. In short, we say that anabolic reactions require ATP, while catabolic reactions release ATP.

Now let’s take a look at some examples of metabolic reactions that take place in the human body, starting with catabolic reactions. Catabolic reactions play a major role in breaking down the food that we eat into the nutrients that our body utilizes to function. Foods like bread, rice, and potatoes are rich in carbohydrates. One of the most common forms of carbohydrates that all these foods contain is starch. Starch is called a polymer because it’s made of multiple repeating units of a smaller sugar called glucose. We’ll represent glucose as a hexagon because it has six carbon atoms. Thousands of glucose molecules can join together to form a single starch molecule. So a portion of a starch molecule might look something like this.

Our body needs glucose for the process of cellular respiration, which we briefly mentioned earlier. But if all these glucose molecules are connected together in starch molecules, they’re not of much use to the cell. These long molecules of starch are therefore broken down into glucose through the action of enzymes in our digestive system. This is a simple example of a catabolic reaction. So now that we have glucose, this simple sugar can be used in cellular respiration, which as we mentioned earlier is another example of a catabolic reaction. In cellular respiration, glucose is broken down to release carbon dioxide, water, and energy in the form of ATP. In our body, this reaction most commonly takes place in the presence of oxygen. This is called aerobic cellular respiration.

We’ve discussed what happens to the carbohydrates that we eat in our food. But what about other molecules like proteins? Foods like lentils, nuts, and meat and dairy products like milk are usually rich in proteins. Proteins are also broken down in the body through catabolic reactions using enzymes that are present in the stomach and the small intestine. Proteins are long chains of individual units called amino acids, which are joined together and folded into a specific shape to perform a function. When enzymes in the stomach and small intestine act on these proteins, they can break them down into amino acids. These free amino acids can then be used to make other proteins that are essential in the body. As with all catabolic reactions, the breakdown of proteins into amino acids releases energy in the form of ATP.

Next let’s look at some examples of anabolic reactions in the human body. As we just mentioned, the amino acids that we get from the proteins we eat can be used to synthesize other proteins that our body needs. This is an example of an anabolic reaction as a large molecule is being constructed out of smaller units. Amino acids are joined together through peptide bonds which need ATP to form. This creates a chain of amino acids called a polypeptide chain. The polypeptide chain can then fold into a specific shape to perform a specific function in the body as we mentioned earlier. One example of a protein that can be formed in this process is hemoglobin, the protein that carries oxygen in our red blood cells.

Another example of an anabolic reaction in the body is the formation of glycogen through a process called glycogenesis. Earlier, we talked about glucose, a simple sugar molecule that we get from foods that contain carbohydrates. If there’s too much glucose in the blood, this can actually lead to problems with the heart, kidneys, or vision. To prevent this, glucose is sometimes converted into glycogen. Glycogen is a branched molecule made of repeating units of glucose. It acts as a storage molecule for glucose in the body. Since the large molecule is being constructed out of smaller ones, this is another example of anabolic reaction. Let’s apply what we’ve learned about metabolism to some practice questions.

A buildup of amino acids can be toxic to the human body. To prevent this, amino acids have their amine group removed, which is then converted to a relatively nontoxic molecule and excreted. The removal of an amine group is an example of what type of metabolic reaction, anabolism or catabolism?

Let’s start by going over some of the key terms that have been used in the question. Metabolism is the term used to describe all the chemical reactions that take place in the body to support and maintain life. Metabolic reactions can either be anabolic, in which large molecules are constructed from smaller molecules, or catabolic reactions, in which large molecules are broken down into smaller molecules. This question talks about amino acids and how their buildup can be toxic to the human body. Amino acids are the individual units that make up proteins. Here we can see the simple structure of an amino acid with the amine group circled.

As mentioned in the question, to prevent toxicity to the body, the amine group is removed from the amino acid. This process is called deamination. Since deamination involves the breakdown of amino acids to separate the amine group, this is an example of a catabolic reaction. So we’ve deduced that deamination is an example of catabolism.

Let’s have a go at another question together.

A student reads about metabolism in humans. They conclude that the cells of humans require larger amounts of energy to break down complex molecules into smaller ones than they require to synthesize complex molecules from small ones. Are they correct? (A) No, it is the synthesis of complex molecules that requires a relatively larger amount of energy. Or (B) yes, it is the breaking down of complex molecules that requires a relatively larger input of energy.

Metabolism refers to all of the chemical reactions that take place within the body to support and maintain life. There are two types of metabolic reactions, anabolic reactions and catabolic reactions. Let’s take a look at each of these types of reaction. In anabolic reactions, small molecules are linked together through chemical bonds to form larger, more complex molecules. The formation of these chemical bonds requires energy which cells obtain from a molecule called ATP. In catabolic reactions, on the other hand, large and complex molecules are broken down into smaller units. This process involves breaking chemical bonds, and so it releases energy in the form of ATP.

Let’s take another look at the conclusion that the student in our question has made. They’ve stated that larger amounts of energy is required to break down large complex molecules into smaller ones than is needed to synthesize large molecules from small ones. This is in fact the opposite of what we’ve learned about anabolic and catabolic reactions. While catabolic reactions that break down large complex molecules into multiple smaller ones release energy in the form of ATP, anabolic reactions that synthesize large complex molecules from multiple smaller ones require an input of ATP. So the correct answer is therefore “No, it is the synthesis of complex molecules that requires a relatively larger amount of energy.”

Let’s review the key points that we have learned about metabolism in this video. Metabolism describes all the chemical reactions that take place within a cell or body of a living organism that allow them to survive. Anabolism and catabolism are the two types of metabolic reactions. Anabolic reactions require an input of energy to synthesize large molecules from multiple smaller ones. The formation of polypeptides from individual amino acids is an example of an anabolic reaction. This requires an input of energy which is supplied by a molecule called ATP.

Catabolic reactions, on the other hand, break down large molecules into smaller ones, releasing energy in the process which is stored in the form of ATP. An example of a catabolic reaction is the breakdown of starch in carbohydrate-rich foods into simple sugars like glucose. The breaking of bonds in the large starch molecules is what releases energy.

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