Lesson Explainer: Metabolism Biology

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

Metabolism refers to all of the chemical reactions that occur in a cell or body. Metabolic reactions are occurring in all of your cells, all the time!

Key Term: Metabolism

Metabolism describes all the chemical reactions that occur within living organisms to maintain life.

There are two types of metabolic reactions: anabolic and catabolic.

In anabolic reactions, small molecules are built up into larger molecules. These processes require energy in the form of a molecule called ATP. In catabolic reactions, large molecules are broken down into smaller molecules. These processes release ATP.

Key Term: Anabolism

Anabolism is when metabolic reactions construct molecules from smaller units. Anabolism requires ATP.

Key Term: Catabolism

Catabolism is when metabolic reactions break down molecules into smaller units. Catabolism releases ATP.

Let’s look at some examples of catabolic reactions.

Foods like bread, pasta, and potatoes are full of large biological molecules called carbohydrates. A carbohydrate commonly found in potatoes is starch. A basic diagram outlining the structure of a section of a starch molecule is given in Figure 1.

In Figure 1, you can see 3 monomers of glucose that would be joined together to make starch. In reality, there would be hundreds (or even thousands!) of glucose monomers in a single molecule of starch. To make this more visually understandable, the central monomer in brackets is denoted with an “𝑛” to represent the number of glucose monomers that would be located between the outer two.

Our bodies cannot do much with starch when it is in this form. To be really useful for our cells, starch needs to be broken down into its subunits, or monomers. These monomers are multiple repeats of the simple sugar glucose. Figure 2 demonstrates the breakdown of starch into glucose, an example of a catabolic reaction.

A molecule of glucose produced this way can undergo further catabolic reactions. In cellular respiration, glucose is broken down to release ATP energy. This happens in the presence of oxygen (aerobically) in the mitochondria or in the cytoplasm when oxygen is not present (anaerobically).

Reaction: Aerobic Cellular Respiration

Glucose+OxygenCarbondioxide+Water(+Energy)

As we can see from these examples, catabolic reactions are those that break down molecules into smaller units.

Example 1: Classifying Metabolic Reactions as Anabolic or Catabolic

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 nontoxic molecule and excreted. What type of metabolic reaction, anabolism or catabolism, is the removal of an amine group an example of?

Answer

To help us answer this question, we first need to understand some of the key terms. Metabolism, or metabolic reactions, describes all the chemical reactions that occur in a cell or a body of an organism. Our metabolism is responsible for keeping us alive and functioning!

Metabolic reactions can be divided into two categories: anabolic reactions and catabolic reactions. Anabolic reactions are those that “build up” large molecules from smaller ones. Catabolic reactions, on the other hand, are reactions where large molecules are broken down into smaller ones.

Let’s now look back at the example given in the question.

The structure of a simple amino acid is provided in the diagram below, and the amine group is circled.

CHCNROOHHH

The question describes deamination, which is the removal of this amino group. Because we are taking a molecule and removing a component of it, we can determine that this reaction belongs to the category of metabolism concerned with breaking up molecules.

Therefore, the correct answer is catabolism.

But what about anabolic reactions? Let’s look at an example of an anabolic reaction that is continually happening in your body cells.

Proteins are biological macromolecules formed from multiple chains of many smaller units called amino acids. In our cells, some of the proteins we require can be “built” from the joining of amino acids. Figure 3 gives a brief outline of how bonds can be formed between separate amino acids to create a polypeptide chain. This is an example of an anabolic reaction—a larger molecule is being constructed from multiple smaller units.

Multiple polypeptide chains can then interact and form bonds, creating a functional protein. An example of a protein made up of many interacting polypeptides is hemoglobin—a basic outline of the structure of hemoglobin is provided in Figure 4.

Example 2: Classifying Metabolic Reactions as Anabolic or Catabolic

When blood glucose levels are too high, glucose is converted into glycogen. Glycogen is a large storage molecule made of many repeats of glucose. What type of metabolic reaction, anabolism or catabolism, is this an example of?

Answer

The term metabolism refers to all the chemical reactions that occur in the cells of a body. We can categorize different metabolic reactions into anabolic and catabolic reactions. Anabolic reactions “build” larger molecules from smaller ones, whereas catabolic reactions break down large molecules into smaller components.

In this example, we are told that glucose is converted into glycogen when blood glucose levels are high. We are also told that glycogen is a large storage molecule made of many repeats of glucose. A simple diagram outlining the structure of glycogen is provided below:

Each of the yellow hexagons represents a single glucose molecule.

We can assume from this information that the reaction to convert multiple units of glucose into a glycogen molecule is “building” a larger molecule from many repeating smaller ones. Looking back at our different categories of metabolic reactions, we need to find the term that describes this type of reaction.

Therefore, the correct answer is anabolism.

Another example of anabolism that occurs in the human body is a process called gluconeogenesis, which mostly occurs in the cells of the liver. The prefix gluco- refers to glucose, as this process occurs as part of the body’s response to blood glucose levels dropping below a normal healthy range. Neo- comes from the Greek word for new, and genesis means generation or production. This is because gluconeogenesis is how the liver cells produce new glucose from proteins and fats that have been broken down by other body cells. The metabolites of this breakdown, for example amino acids and glycerol, are bonded together with an input of energy to form glucose. This glucose is then released into the bloodstream to increase blood glucose concentration back to a normal range.

Example 3: Differentiating between Anabolic and Catabolic Reactions Concerning Proteins and Amino Acids

The diagram provided shows the conversion of amino acids into a polypeptide chain and the breakdown of a polypeptide chain into amino acids.

  1. Which reaction (X or Y) shows an anabolic reaction?
  2. Which reaction (X or Y) shows a catabolic reaction?

Answer

To help us answer this question, let’s recap what the key terms mean.

Anabolic reactions are metabolic reactions that synthesize large molecules from smaller ones. This will involve the formation of bonds between molecules or atoms to build up larger molecules or compounds. Catabolic reactions, on the other hand, are metabolic reactions that break down large molecules into smaller components by breaking the bonds between atoms or molecules.

Part 1

Looking back at our diagram, we can see that reaction X indicates that many circles—in this case, representing individual amino acids—are being joined to form a single chain, which represents a polypeptide chain. This is an example of a larger molecule being synthesized from many smaller ones, so it is an anabolic reaction.

Therefore, reaction X is showing an anabolic reaction.

Part 2

Reaction Y indicates that the polypeptide chain is being broken down into its constituent parts, which are, in this case, many single amino acids. This is an example of a catabolic reaction.

Therefore, reaction Y is showing a catabolic reaction.

Let’s look at energy and how it relates to anabolism and catabolism.

Generally, when a molecule is broken down—which we know is an example of a catabolic reaction—energy is released. When this energy is released in our body cells, it is stored in the form of ATP. ATP is a relatively small molecule that is different from other energy storage molecules like fats or carbohydrates, as it is broken down to release energy almost as fast as it is stored. The general structure of ATP is shown in Figure 5.

Key Term: ATP

ATP, or adenosine triphosphate, is the molecule that stores chemical energy in living organisms.

The breaking of the bond between the two outer phosphate groups in an ATP molecule, which is marked in red in Figure 5, releases the energy stored in this bond. ATP is easily and quickly broken down and regenerated by cells. This means that cells have a constant supply of energy to use when needed.

When molecules are “built up”—for instance, in anabolic reactions—this usually requires an input of energy. The cells can now use the molecules of ATP produced in catabolic reactions and utilize this energy stored in the bond between the two phosphate groups to form larger molecules.

The relationship between anabolism and catabolism and their association with ATP is summarized in Figure 6 below.

Example 4: Differentiating between Anabolic and Catabolic Reactions in terms of Requirements of Energy

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?

  1. No, it is the synthesis of complex molecules that requires a relatively larger amount of energy.
  2. Yes, it is the breaking down of complex molecules that requires a relatively larger input of energy.

Answer

Anabolic and catabolic reactions are the two categories of metabolic reactions that occur in living organisms. Anabolic reactions are those that synthesize large molecules from smaller ones, such as building up a polypeptide chain from the joining of multiple amino acids. You can remember this by recognizing that “anabolic” starts with the same letter as “addition,” and, in anabolic reactions, we are “adding” molecules together to make larger ones! Catabolic reactions are those that break down large molecules into smaller ones.

Anabolic and catabolic reactions have different energy requirements. When large molecules need to be synthesized, this often requires an input of energy. This energy can be used to form bonds between individual molecules. However, when molecules are broken down, energy is released. This is because energy is stored in the bonds between atoms or molecules.

Looking back at the conclusion the student made, we can see that they are incorrect. Generally, the breakdown of complex molecules will release comparatively more energy than the synthesis of new molecules requires.

So, the correct answer is option A. The student is not correct because it is the synthesis of complex molecules that requires a relatively larger amount of energy.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Metabolism describes all the chemical reactions that take place within a cell or a body.
  • Anabolism and catabolism are the two categories of metabolism.
  • Anabolic reactions use energy to synthesize large molecules from multiple smaller ones.
  • Catabolic reactions break down large molecules into smaller ones, releasing energy in the process.

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