The body’s metabolism

Everything the body consists of, and all the food we eat, is made up of molecules. Everything that happens in the body can be explained by chemistry. It is a sea of chemical reactions that make the body work. Human metabolism is a complex network of reactions. To illustrate this vast network of reactions, only small parts of the metabolism are studied at a time. However, it should be emphasized that the individual pathways do not stand alone, but are part of a huge network of many pathways that affect each other.

This section on the body’s metabolism focuses on the biochemical understanding of diet and energy metabolism.

ATP

ATP (adenosine triphosphate) is the body’s energy source. It is a molecule that contains a lot of chemical energy that the body can use to perform work, for example to move a muscle.

The ATP molecule consists of a relatively large molecule called adenosine and of three phosphate groups. The phosphate groups are made up of the elements phosphorus and oxygen. ADP is a molecule that has the same structure as ATP, but unlike ATP, ADP has only two phosphate groups.

ATP acts like a rechargeable battery. When charged, the battery has the shape of ATP, and when the battery is flat, it has the shape of ADP.

When ATP is used for energy-intensive work, one phosphate group is decomposed, leaving ADP. This is shown in Figures 2 and 3.

Figure 2. ATP and ADP

When ADP is to be “recharged” to ATP, energy must be supplied in order for ADP to bind to a phosphate group. This process is shown in Figure 2.

Figure 3. ATP is the cells’ currency for energy. ATP can be formed from ADP, by the supply of energy, which is extracted, for example, by the burning of nutrients.

Mitochondria – ATP Factories

Mitochondria are found inside cells. The mitochondria are the body’s combustion plant, where important chemical processes such as the metabolism of fat and sugar take place. One of the mitochondria’s most important tasks is to convert ADP into ATP.

Figure 4 shows an illustration of a mitochondria. A mitochondria has two membranes, an inner and an outer one. These membranes act as membranes in the same way as cell membranes.

The membranes are intended, among other things, to separate molecules on each side of the membranes. In this way, the different molecules do not mix. The inner membrane cannot be penetrated even by very small molecules. The inner membrane separates the matrix from everything else in the cell. There is therefore a difference between which enzymes and molecules are present in the liquid inside the matrix and what is present in the liquid outside the matrix.

Figure 4. Mitochondria are organelles found inside our cells. They are often said to be the cell’s powerhouse, and have the role of forming large amounts of ATP.

Lack of clean drinking water can be solved with the help of membranes

Since membranes, made up of phospholipids, have a unique ability to keep molecules separate, they can be used to purify water. Special water transport proteins that are intended to transport water can be inserted into a phospholipid membrane, and you can thereby obtain completely pure water by “seeping” the water through the membrane.

Formation of ATP

The majority of ATP production occurs in the mitochondria. ATP synthesis is divided into two parts: the respiratory chain and ATP synthesis. There are five advanced enzymes in the innermost mitochondrial membrane. The enzymes make sure to “recharge” the ATP batteries. Four of them are active in the electron transport chain and one in ATP synthesis.

The respiratory chain – the proton pump

In the respiratory chain, electrons (e^–) from the molecules NADH and FADH₂, which are found inside the matrix, are passed through the first four enzymes located in the membrane. In connection with the transport of the electrons, the enzymes pump protons (H⁺ ions) through the inner mitochondrial membrane. In other words, protons are transported from the inside to the outside of the inner mitochondrial membrane. This means that there are more protons on the outside of the membrane than on the inside.

When the electrons have passed through all four enzymes, they are transferred to_O2 (oxygen) and water is formed. This process is shown in Figure 5.

If oxygen is not present, the entire respiratory chain will come to a standstill, and ATP will therefore not be formed. Since ATP is necessary for all the body’s energy-demanding processes, including for movement of muscles or for brain activity, it is important that you get sufficient amounts of oxygen.

Figure 5. The respiratory chain is the series of processes in which electrons from FADH₂ and NADH are used to form a proton gradient across the inner mitochondrial membrane. The process consumes oxygen, and forms water. This gradient can be used for the production of ATP.

ATP – synthesis

Only when the four enzymes have worked enough for there to be a sufficiently large difference in the number of protons on the two sides of the inner mitochondrial membrane does the fifth enzyme start working. The fifth enzyme is called ATP synthase.

Figure 6 shows an illustration of ATP synthase, which is located in the inner membrane of the mitochondria. ATP synthase is the last step in charging ADP to ATP. This is done by binding a phosphate group to ADP. This process requires energy. ATP synthase is energized by shutting down protons back from the outside of the membrane to the matrix.

Figure 6. ATP synthase is an enzyme that uses the proton gradient built up by the respiratory chain to form ATP from ADP.

Molecules like to be distributed evenly. For example, if you open a window in a room, the oxygen molecules (_O2 molecules) will eventually be distributed so that there is the same concentration of oxygen molecules outside and inside. If this were not the case, there would be a risk that there were parts of the room where you could not breathe.

Like the oxygen molecules, the protons prefer to be distributed so that there is the same concentration everywhere. When the protons are closed back into the matrix, energy is released, which the ATP synthase uses to convert ADP into ATP.

Respiration

Respiration is the process in which the body uses oxygen (O₂) to burn sugar. Respiration is often described as follows: the chemical reaction, in which sugar (C₆, H_12, O₆) reacts with oxygen and becomes water (H₂O) and carbon dioxide (CO₂).

The energy that you get out of respiration is in the form of ATP molecules. Respiration does not take place in one step, as it otherwise seems to do if you take the reaction equation above as a starting point. A number of chemical processes are required before sugar and oxygen are converted into water and carbon dioxide. These processes are called the glycolysis, the citric acid cycle, and the respiratory chain.

Glycolysis – breakdown of sugar

Glycolysis is a reaction pathway in the body’s metabolism that uses sugar to make ATP. Three important molecules that you get out of glycolysis are ATP, NADH and pyruvate.

Glycolysis is the first step in the body’s sugar metabolism and is followed by further energy metabolism.

Figure 7. Glycolysis is a series of reactions that convert glucose (sugar) into 2 molecules of pyruvate, 2 ATP and 2 molecules of NADH.

Pyruvate

Pyruvate is a product of glycolysis. Pyruvate, like sugar, is converted in the body. What pyruvate is made into depends on whether oxygen is present.

If oxygen is present, pyruvate can be used to form more ATP via the reaction pathway called the citric acid cycle. Here, ATP and more NADH are formed.

If there is no oxygen present, pyruvate is converted to lactic acid. In the conversion of pyruvate to lactic acid, there is no energy yield. Without oxygen, you only get the ATP produced by glycolysis. If you want to make the best use of sugar, it is necessary that oxygen is present.

In the past, lactic acid has been suspected to be the cause of soreness in connection with exercise. From there comes the expression “that the muscles acidify” in connection with exercise. However, research suggests that it is just a myth that lactic acid is the cause of the soreness.

During high physical activity, the body needs more oxygen than when the body is at rest. This is because the body’s consumption of ATP is very high when the muscles are working. Therefore, more oxygen is needed so that pyruvate can be used to form more ATP molecules.

Citric acid cycle and respiration

The citric acid cycle, like glycolysis, is a reaction pathway in the body’s metabolism. The purpose of the citric acid cycle is to use acetyl-CoA to convert ADP molecules into ATP molecules.

The molecule to be used in the citric acid cycle is called acetyl-CoA. Acetyl-CoA can be formed from pyruvate, which is the product of glycolysis, but acetyl-CoA can also be formed from fat.

When acetyl-CoA reacts in the citric acid cycle, ATP, NADH, FADH₂ and_CO2 are formed

Figure 8. In the citric acid cycle, pyruvate is first converted to Acetyl-coenzyme A (Acetyl-CoA), which is then converted to_CO2. The cycle also forms NADH, FADH₂ and ATP.

Respiration

Figure 9 shows that CO₂ is formed in the citric acid cycle. The_CO2 molecules that are formed in the citric acid cycle are the same ones that appear in the respiration reaction equation.

If you take a closer look at the reaction pathway for sugar, you can trace where all the molecules from the respiration equation are formed. In Figure 9, the molecules from the respiratory equation are also colour-marked:

Figure 9. Respiration is the term for the processes that convert sugar into ATP,_CO2 and water when consuming oxygen. The conversion takes place via glycolysis, the citric acid cycle and the respiratory chain.

Lack of oxygen causes the electron transport chain to be blocked. If this happens, NADH and FADH₂ accumulate, which also inhibits the citric acid cycle. Therefore, you can only get a limited energy yield from sugar if the body is in a deficit of oxygen. The ability to transport oxygen around the body is therefore crucial for how quickly you can metabolize energy.

Burning fat

Fats are molecules that the body can use to convert ADP into ATP. When fat is burned in the body, you get a lot of NADH and FADH₂ molecules, which the body uses to make ATP. In addition, you get some molecules, acetyl-CoA, which can make ATP by entering into the reaction pathway called the citric acid cycle. In the citric acid cycle, even more molecules are made that can convert ADP into ATP.

However, it takes much longer for the body to burn fat than it does to burn sugar. Therefore, in order to get energy quickly, the body always burns sugar before burning fat. Much of the fat you eat settles in fat stores in the body and is only used in cases where there is not enough sugar to form the number of ATP molecules that the body needs.

Energy-rich food

Diet is the body’s source of energy. The diet contains various molecules that the body can use to make ATP. The diet includes carbohydrates and fat, which are two types of molecules that the body can use to “recharge” its ADP molecules into ATP.

Figure 10. Fat is broken down into Acetyl-CoA, which is metabolized in the citric acid cycle.

Carbohydrates

Carbohydrates are different types of sugar. These are very energy-rich molecules. There are large amounts of carbohydrates in fruit, among other things. Grain products, i.e. products made from flour such as bread and pasta, also contain large amounts of carbohydrates.

There is a difference between carbohydrates. Some types of carbohydrates are faster to absorb than others. Sugar from, for example, cola can be absorbed very quickly by the body. You therefore get very large fluctuations in your blood sugar if you drink a cola. Carbohydrates from wholemeal bread cannot be absorbed as quickly by the body, and the fluctuations in blood sugar are therefore smaller when you eat rye bread than when you drink cola.

Figure 11. Foods with a high glycemic index have a greater effect on one’s blood sugar. A high glycemic index means that the food greatly increases blood sugar after consumption.

A glycemic index illustrates the food’s effect on blood sugar for 2 hours. A high glycemic index means that the food causes blood sugar to rise and fall again quickly.

Fat

Fat also contains large amounts of energy, even more than carbohydrates. Fat is burned more slowly in the body than carbohydrates. Therefore, fat is burned only in cases where the amount of carbohydrates in the body is not sufficient to meet the energy needs. This means that you burn fat when you use more ATP than what can be produced by the carbohydrates in the body.

When fat is not burned, it is stored in depots that the body can use in situations of low blood sugar. You have low blood sugar when, for example, a long time has passed after a meal. Fat burning is often high during sleep periods and after physical activity, where you have burned a lot of carbohydrates.

Proteins

In addition to the fact that the diet contains energy-rich molecules such as carbohydrates and fat, the diet also contains proteins, vitamins, minerals and salts that are also important for the body’s maintenance.

What are proteins?

Proteins are large molecules that are made up of long chains of different amino acids. Some proteins contain over 100000 amino acids that twist in long chains. There are 20 different amino acids that are needed for protein building. Amino acids consist of a skeleton that is the same for all amino acids. The skeleton consists of the elements carbon (C), nitrogen (N), oxygen (O) and hydrogen (H). In addition to the fact that amino acids consist of an amino acid skeleton, all amino acids have their own side chain. These side chains have very different chemical properties. Some have excess or missing charges, some are water-soluble, and others are fat-soluble.

The amino acid skeletons fit together like Lego bricks, and the body builds its own proteins by making different combinations of amino acids.

The proteins are amino acid chains that fold together spontaneously. The side chains of the amino acids affect each other, so that the protein folds in a very specific way. The water-soluble and fat-soluble side chains are separated from each other as far as possible, while positively charged side chains lie close to negatively charged side chains.

Figure 12. Proteins are long chains of amino acids. Proteins can fold together in different ways, depending on which amino acids they consist of.

As a result of the different chemical properties of the side chains, all proteins acquire a special three-dimensional structure. This structure is determined by the order in which the amino acids are put together.

Proteins in the diet

It is important to get proteins through the diet. Unlike carbohydrates and fats, proteins are not used as an energy source. Instead, they are important building blocks for the body. The body needs proteins to make new cells and to build muscle. It is also important to get proteins in the diet, as the body needs them to build enzymes. Enzymes are special proteins that make a number of processes run faster, see below.

When the body gets proteins from the diet, they are broken down into single amino acids. The body can then put together its own proteins from the various amino acids.

Vitamins and minerals

Vitamins and minerals are necessary for the metabolism to work optimally. Some of the body’s processes do not work if you are deficient in vitamins and minerals. It can cause diseases, and in the end, the body cannot survive without the necessary vitamins and minerals.

Fruits and vegetables are important sources of many vitamins. That’s why you should eat both fruits and vegetables.

The element iron (Fe) is an important mineral. Found especially in whole grain products and meat. Iron is important for many processes in the body. Iron is important in the transport of oxygen, among other things. Iron turns red when it comes into contact with oxygen. It is the presence of iron that turns the blood red.

Iron is also important for the respiratory chain. Several of the enzymes that are part of the respiratory chain contain centres that are made up of iron, among other things. These centers are important for electrons to be transported through the enzymes.

The diet pyramid

The Diet Pyramid is a tool that describes guidelines for an optimal ratio between different sources of nutrition. A diet pyramid is built up of several layers, where the lower layers are what you should eat most of.

Figure 13. The diet pyramid describes how healthy eating should be composed. You should eat most of the foods that are at the bottom of the pyramid and least of the foods that are at the top of the pyramid.