Biochemical cycle — General explanation of our metabolism
We have already learned about the digestion of individual nutrients in our digestive tract — but what happens to the individual nutrients that have been absorbed from food? The actual utilization and energy production takes place where it is no longer accessible to our unaided eye: at the cellular level. All the nutrients that have been broken down and absorbed into the smallest particles in our digestive tract are fed into circuits that mesh like cogs and end in a single, special chain that you will learn about in today’s article.
Short on time, but don’t want to miss anything? Here are the facts at a glance !
- Nature and its living beings function on the basis of precisely coordinated processes. A cycle is a give and take, and only runs as long as all participants in the cycle adhere to this principle.
- There are cells that can only obtain their energy as glucose — such as red blood cells. They have no cell organelles, which is why most biochemical metabolic processes cannot take place in red blood cells. Thus, they are dependent on glucose as an energy supplier, whereas other tissues can also obtain energy from fatty tissue.
- In principle, there is always a “juxtaposition” or “overlapping” of the individual metabolic processes of energy provision, and not a “succession”.
- In glycolysis, energy is provided from carbohydrates in food (glucose) and from the body’s own glycogen reserves in the liver and muscles.
- If lactate accumulates in the tissues, metabolic acidosis (“hyperacidity”) occurs. Lactate is not a “waste product”, but serves both for energy storage and as an energy supplier
- Lactate causes brief muscle burning, but has nothing to do with the development of “muscle soreness,” as some still believe
- β-Oxidation is used by the body to obtain energy from fat reserves during prolonged periods of starvation
- The citrate cycle is the most central metabolic pathway in our body
- ATP is the fuel of all our cellular functions
- All metabolic pathways ultimately ensure that the flow of electrons in the respiratory chain is maintained and that the formation of ATP is ensured
- Approximately 80–100 kg of ATP is synthesized (and consumed) per day in the adult body
What is actually a cycle?
Nature and its living beings function on the basis of precisely coordinated processes. Here is a small example: when taking up carbon dioxide (CO2) from air and water (H2O) from the earth, plants can form glucose (grape sugar) in their chloroplasts with the help of chlorophyll and sunlight. This process occurs with the release of oxygen (O2), which in turn is needed by humans to live. If humans ingest carbohydrates from plants, they are broken down again into carbon dioxide and water while absorbing oxygen. And the cycle begins again. A cycle is a give and take, and only runs as long as all participants in the cycle adhere to this principle.
What happens to the sugar?
As we have already discussed, carbohydrates are absorbed through food and broken down into their monosaccharides. These travel through the blood to the various organs. In a nutshell, glucose is broken down to carbon dioxide and water, and the energy released in the process is used by the cells of the brain or muscle. But more on this later. There are cells that can only obtain their energy as glucose — such as red blood cells. They have no cell organelles, which is why most biochemical metabolic processes cannot take place in red blood cells. Thus, they depend on glucose as an energy supplier, whereas other tissues can also obtain energy from adipose tissue.
What are the biochemical metabolic cycles?
Even though the following metabolic pathways and their names seem very complex and complicated at first glance, they are actually quite simple. They all — glycolysis (“burning” of carbohydrates), β-oxidation (“burning” of fats) and ketogenesis (provision of ketone bodies when not enough carbohydrates are available) lead to a central metabolic circuit — citrate cycle — whose products keep the vital respiratory chain running. But beware: in principle, there is always a “juxtaposition” or “overlap” of the individual metabolic processes of energy provision and not a “succession”, as is often believed.
What happens during glycolysis?
Threw glycolysis, energy is provided from carbohydrates in food (glucose) and from the body’s own glycogen reserves in the liver and muscles. It takes place in the cytoplasm (cell fluid) of each body cell. The end product of glycolysis is pyruvate. The breakdown of glucose to pyruvate can occur in the organism under aerobic and anaerobic conditions (i.e., in the presence of oxygen or without). Pyruvate is converted to acetyl-CoA under aerobic conditions (with oxygen), and to lactate under anaerobic conditions. It can only be further utilized if it is converted back to pyruvate. If lactate accumulates in the tissues, metabolic acidosis (“overacidification”) occurs, which is not only painful but ultimately performance-limiting, since in the acidic environment (the limit is a pH of 7) many functions — such as in muscle, for example — muscle contraction is inhibited by enzyme inhibition.
Lactate is not a “waste product”, but serves both for energy storage and as an energy supplier. On the one hand, the lactate flushed out into the bloodstream is used by the heart muscles and the brain to produce energy; on the other hand, it is built up into glycogen via glucose in the liver and in unloaded muscles. By the way: Lactate causes brief muscle burning, but has nothing to do with the development of “muscle soreness”, as some still think. Therefore, after an intense anaerobic effort, it is important to let it fade out slowly for several minutes (coasting, cycling out…), since the lactate or acid degradation and thus the muscular recovery is accomplished much more quickly than in the case of physical rest (active recovery).
And what happens during β-oxidation?
The β-oxidation serves the body to gain energy from fat reserves during longer periods of hunger. If the carbohydrate stores are depleted or too few rapidly available carbohydrates are supplied, the metabolism resorts to the utilization of fats. “Fats” are generally stored as triglycerides in adipose tissue. In starvation metabolism or under prolonged physical stress, fatty acids are cleaved from the triglycerides by the enzymes “lipases” and fed to β-oxidation. This process takes place in the mitochondria, the power plants of the cells. Since red blood cells do not have mitochondria, they cannot obtain energy from fats in states of starvation. Also whilst β-oxidation, as in aerobic glycolysis, for every shortening of the fatty acid (by 2 C atoms), a molecule of acetyl-CoA is produced, which flows on to the citrate cycle.
All roads lead to the citrate cycle
The citrate cycle is the most central metabolic pathway in our body. It is a complex system consisting of degradation and synthesis reactions for carbohydrates, lipids and proteins, as well as other substances in the human organism. This mechanism takes place in the mitochondria. At the beginning of the cycle, acetly-CoA is introduced into the cycle, whereupon electrons are released in several complicated redox reactions (reduction and oxidation).
What is the purpose of all this? Simply put, the flow of electrons results in a flow of energy in the respiratory chain and thus in the synthesis of ATP — the “energy currency” of our body. All metabolic pathways therefore ultimately ensure that the flow of electrons in the respiratory chain is maintained and that the production of ATP is ensured. After all, ATP is the fuel of all our cellular functions. For example, glycolysis under normal oxygen conditions produces 38 moles of ATP from 1 mole of glucose. In anaerobic glycolysis, on the other hand, only 2 moles of ATP are obtained from the breakdown of 1 mole of glucose.
What happens if we stop eating carbohydrates?
If the body can no longer rely on carbohydrates as an energy source, little acetyl-CoA is produced. Alternative forms of energy production must be enacted. By breaking down stored fat in the organism via β-oxidation, substitute acetyl-CoA is released. Under the influence of hunger hormones (such as glucagon), ketone bodies such as acetoacetate, β-hydroxybutyrate and acetone are formed in the liver by means of ketogenesis. Thus, at least for a certain time, energy can continue to be provided for vital organs such as the heart and brain.
What is the ominous ATP?
When it comes to dealing with energy, we make very high demands on our organism. We want to be able to use energy in different forms and for different purposes. Biosynthetic processes require chemical energy, transport processes of charged particles across the cell membrane require electrical energy, the pursuit of an athletic hobby requires mechanical energy. Since we can neither generate nor destroy energy, the only way out is to convert the various forms of energy into one another. A very important role is played by the chemical form of energy already mentioned several times, which can be used for transformation processes — adenosine triphosphate (ATP) is formed.
ATP can store energy in chemical form for a short time, and through its participation in a variety of reactions in the organism, it can transfer its energy to metabolic events. In the process, ATP is split into ADP and phosphate. The energy released in this process is about 33 kJ per mole of ATP (about 500 g), and it can be used for various purposes (e.g., muscle contraction, i.e., the chemical energy of ATP is transformed into mechanical energy). Adenosine triphosphate is the central substance and the most common transport form of energy in our body, through which energy is transferred in the cell (a kind of energy credit card accepted throughout the body). Approximately 80–100 kg of ATP is synthesized (and also consumed) per day in the adult body.
Text Sources:
(1) Hahn A, Ströhle A, Wolters M (2015) Ernährung — Physiologische Grundlagen, Prävention, Therapie, 3. Aufl., Wissenschaftliche Verlagsgesellschaft, Stuttgart
(2) Kapser H (2009) Ernährungsmedizin und Diätetik, 11. Aufl., Elsevier, München
(3) Rehner G, Daniel H (2010) Biochemie der Ernährung, 3. Aufl., Spektrum Verlag, Heidelberg
Image Sources:
(4) Martin W. Reichel
(5) Hahn A. (2016) Lebensmittel und Ernährung. In: Lebensmittelchemie. Springer Spektrum, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-47112-8_1
