Lesson Explainer: Metabolism and the Liver Biology

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

There are many misconceptions surrounding metabolism in humans. Perhaps you have heard that slim people have really high metabolism. This is actually false. Metabolism is not how fast someone digests food or how little weight they gain, but it is the sum of all chemical reactions that occur in a cell or body. Metabolic reactions are occurring in all of your cells, all the time. Our metabolism describes all the chemical reactions that occur within us that help all of us sustain life. A sunflower in a field or a bacterium on the floor also have metabolic reactions occurring within them.

Metabolism is vitally important, partly as it allows us to release the energy that is stored within molecules into a usable form. This release of energy provides us with the energy our cells need to carry out our essential life processes such as growth, reproduction, and responding to our environment to sustain our lives and our species.

Key Term: Metabolism

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

Example 1: Defining Metabolism

Which of the following best defines metabolism?

  1. Metabolism is the sum of all the energy lost from an organism.
  2. Metabolism is the sum of the chemical reactions that occur in each cell of an organism.
  3. Metabolism is how quickly the body digests a meal.
  4. Metabolism is how quickly a person can lose weight.
  5. Metabolism is the total energy input divided by the total energy output of an organism.

Answer

Metabolic reactions are all of the reactions that occur inside the cells of living organisms. An organism’s metabolism is the sum of all of these reactions that are occurring within it that help it sustain life.

Metabolism is not how well someone loses weight or digests their food, but instead their capability to conduct chemical reactions within their cells. For example, adults actually tend to have higher metabolism than children, as they are larger and have more cells, and therefore more reactions occur within them.

Therefore, the correct answer is B: metabolism is the sum of the chemical reactions that occur in each cell of an organism.

There are two types of metabolic reactions: anabolic and catabolic reactions. Take a look at Figure 1 to visualize the main differences between the two. 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.

Definition: Anabolism

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

Definition: Catabolism

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

Example 2: Describing Catabolic and Anabolic Reactions

Which of the following statements correctly relates energy to catabolic and anabolic reactions?

  1. Anabolic reactions require energy to form new molecules, whereas catabolic reactions release energy as molecules are broken down.
  2. Catabolic reactions require energy to form new molecules, whereas anabolic reactions release energy as molecules are broken down.

Answer

Anabolic and catabolic reactions are both types of metabolic reactions occurring within the cells of living organisms to help them survive. While anabolic reactions occur to form larger molecules from smaller units, catabolic reactions break down larger molecules into smaller units. Anabolic reactions require energy in the form of ATP to build up these larger molecules, while catabolic reactions release ATP when the larger molecules are broken down.

Therefore, the correct answer is that anabolic reactions require energy to form new molecules, whereas catabolic reactions release energy as molecules are broken down.

Let’s look at some examples of anabolic and catabolic reactions to understand these processes better.

Amino acids and glucose are small molecules that are broken down from proteins and carbohydrates respectively. The process of these larger molecules being broken down in our digestive system are examples of catabolic reactions, releasing energy. Another example of a catabolic reaction, shown in Figure 2, is aerobic respiration within our cells. Glucose is broken down by mitochondria to release ATP molecules. Liver cells conduct a huge range of metabolic reactions. One catabolic reaction in liver cells aids the conversion of excess proteins into urea. The process of breaking down protein releases energy in the form of ATP. Some of this energy is then used to form urea, which is then removed from the body in urine.

Once amino acids and glucose are in our cells, they can be used to build up a variety of different proteins and carbohydrates as needed by the cells. These processes, shown in Figure 3, are examples of anabolic reactions requiring ATP energy.

Example 3: Defining Metabolism

In some metabolic reactions, large molecules are broken down into smaller ones.

  1. What biological reaction breaks down glucose in cells to release energy?
  2. Which biological molecules in the body break down to form urea?

Answer

Metabolic reactions that occur in living organisms can either be catabolic or anabolic. In anabolic reactions, small molecules are joined together using ATP energy to form larger molecules. In catabolic reactions, large molecules are broken down into smaller units, releasing energy in the form of ATP.

Part 1

Respiration is an example of a catabolic reaction that occurs in eukaryotic cells. Glucose is broken down within the cells into carbon dioxide and water, and the reaction releases energy in the form of ATP in the process.

Therefore, the biological reaction that breaks down glucose in cells to release energy is respiration.

Part 2

The liver carries out a number of metabolic reactions involved in detoxification. One example is a reaction in which excess amino acids are broken down into urea, a substance that will be removed from the body as part of urine.

Therefore, the biological molecules that break down to form urea are amino acids.

Another example of a metabolic reaction that occurs in the liver is the breaking down and removing of a substance called lactic acid. This is just one example among many metabolic reactions that involve removing waste. Lactic acid fermentation, the production of lactic acid, occurs as a side effect of a process called anaerobic respiration.

Definition: Anaerobic Respiration

Anaerobic respiration is the process by which energy is released in cells in the absence of oxygen.

Usually, our cells carry out aerobic respiration to release energy. This process occurs when we have enough oxygen in our cells. However, when human body cells are not receiving enough oxygen, they carry out anaerobic respiration. 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. Though this does provide the cells with much-needed energy, it also produces lactic acid, a substance that is toxic to humans. In high concentrations, lactic acid causes muscle pain, which is why exercise with insufficient oxygen causes muscle cramps. This lactic acid must be broken down into a less-toxic substance. The organ that detoxifies lactic acid, among many other toxic substances, is the liver.

The liver has two options: it can either convert lactic acid into glucose or break it down into carbon dioxide and water, the usual products of aerobic respiration. Anaerobic respiration is said to result in oxygen debt, as oxygen is required by the liver to convert lactic acid into carbon dioxide and water. This is a catabolic reaction. If the liver converts lactic acid into glucose, this is an anabolic reaction and requires energy. This process is shown in Figure 4 below.

Example 4: Explaining the Importance of Lactic Acid Removal

Why is it important that lactic acid be removed from the body after exercise?

  1. Lactic acid is toxic at high concentrations and can break down cell walls.
  2. In high concentrations, lactic acid can build up in arteries and clog them.
  3. In high concentrations, lactic acid can interact with stomach acid and lower the pH of the body.
  4. Lactic acid is toxic at high concentrations and can cause muscle pain.
  5. Lactic acid is poisonous and can cause blood poisoning.

Answer

Lactic acid is a product of a process called anaerobic respiration, or lactic acid fermentation. This occurs when cells are in a low-oxygen environment and are therefore unable to carry out aerobic respiration that releases comparatively more energy. In high concentrations, lactic acid buildup can cause muscle pain and cramps, which is why they occur when carrying out intense exercise where muscle cells are not receiving the oxygen they require for aerobic respiration. Therefore, lactic acid must be converted into a less-toxic substance. The liver can reform glucose from lactic acid if it has a sufficient energy input. Alternatively, with an input of oxygen, the liver can break lactic acid down into carbon dioxide and water.

Therefore, our correct answer is that lactic acid is toxic at high concentrations and can cause muscle pain.

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

Key Points

  • Metabolic reactions occur in every living organism to help it survive.
  • Reactions can either be anabolic, using energy to build larger molecules from smaller ones, or catabolic, releasing energy from breaking down large molecules into smaller ones.
  • The liver is an example of an organ that carries out many metabolic reactions involved in waste removal, such as the detoxification of lactic acid from anaerobic respiration.

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