Have you ever thought about what keeps you alive? Is it food? Is it water? Or is it the processes that occur in your body? Well, it's actually all of them! The water we drink and the food we eat are digested by the body and turned into sugars that we use for energy.
The pathways our bodies take to digest what we consume can be thought of as metabolism. We must understand these processes because they operate in our bodies all the time, keep us functioning, and cycle nutrients throughout different organisms to keep our ecosystem balanced.
So, let's dive into metabolic pathways!
- First, we will look at the definition of a metabolic pathway.
- Then, we will explore the different types of metabolic pathways.
- After, we will look at a chart showing some metabolic pathways.
- Lastly, we will look at some examples involving metabolic pathways.
Metabolic Pathway Definition
Let's start by looking at the definition of a metabolic pathway.
A metabolic pathway is a series of chemical reactions interconnected by intermediates in a living organism.
Metabolism can be thought of as all the chemical changes that occur within a living organism.
The word metabolism derives from the Greek word “metabolismos,” which means change. This change refers to all the chemical processes that occur inside the body.
Metabolic pathways generally consist of a sequence of reactions activated by enzymes where the product of the previous reaction becomes the starting point or reactant for the following reaction.
Enzymes are proteins that speed up or catalyze chemical reactions in the body.
Proteins are organic compounds that perform essential functions such as transporting materials, controlling physiological processes such as growth, speeding up chemical reactions, storing things, etc. They consist of smaller molecules called amino acids that can be linked together to form polypeptides.
Organic compounds are compounds that contain mainly carbon and can sustain life. Organic compounds are also commonly made of hydrogen, oxygen, or nitrogen.
Major metabolic pathways mostly consist of the synthesis of organic compounds that contribute to reproduction, cell growth, repair, energy uptake, etc.
Types of metabolic pathways
There are three types of metabolic pathways that you need to be familiar with: anabolic, catabolic and amphibolic pathways.
Anabolic pathways are pathways that require energy to buildup or construct molecules, as shown in Figure 1. For example, the buildup of carbohydrates is an example of an anabolic pathway.
Catabolic pathways create energy through the breakdown of molecules, as shown in Figure 1. For instance, the breakdown of carbohydrates is an example of the catabolic pathway.
Amphibolic pathways are pathways that include both anabolic and catabolic processes.
One of the most important metabolic pathways can involve the breakdown or buildup of organic compounds called carbohydrates to synthesize energy for our bodies.
A way to remember that catabolic pathways involve the breakdown of molecules is to think that the c in catabolic stands for "cutting" molecules down. In contrast, anabolic would be the opposite of catabolic.
Figure 1: Types of metabolic pathways. Daniela Lin, Vaia Originals.
Another example of an anabolic pathway is making sugar from carbon dioxide or building polypeptides from amino acids. Breaking down amino acids into their intermediates, on the other hand, is an example of a catabolic pathway.
Metabolic Pathways Chart
There are a ton of metabolic pathways, some of which are shown in the chart below (figure 2). So, let's do an overview of some of the most important metabolic pathways in humans:
| Process name | Type of pathway | Description |
| Glycogenesis | Anabolic | This process involves the formation of glycogen from glucose or sugar. |
| Glycolysis | Catabolic | Glycolysis is the process of breaking down glucose. |
| Gluconeogenesis | Anabolic | Gluconeogenesis is the formation of glucose from non-carbohydrates. This occurs when our body doesn't have enough glucose or carbohydrates. |
| Fatty Acid oxidation | Catabolic | Process of breaking down fatty acids into the product to start the citric acid cycle. |
| Citric Acid Cycle (Kreb's Cycle or Tricarboxylic Acid Cycle) | Amphibolic | Starts with the product from glycolysis and reduces it to NADH (nicotinamide adenine dinucleotide). |
| Oxidative Phosphorylation (Electron Transport Chain) | Catabolic | Process of ATP is synthesized through the breakdown of electron carriers. |
| Pentose Phosphate Pathway (PPP) | Amphibolic | Breaks down an intermediate of glycolysis to make important components for RNA and DNA. |
| Urea Cycle | Catabolic | Eliminates toxic ammonia from the body by breaking it down into urea which is then excreted as urine. |
Essential definitions to understand the processes in the table above and for the next few sections are:
Glycogen is a polysaccharide used by animals, fungi, and bacteria to store energy. Glycogen is made up of many bonded glucose molecules. In contrast, glucose, a monosaccharide, is the simplest form of sugar from which carbohydrates can be constructed.
If the chart above didn't show how complex metabolism is, figure 2 certainly does! To the eye, it just looks like a mess of lines, but these lines represent the connections of all the interconnected processes' reactants, intermediates, and products. Our body really does a lot to keep us alive!
Figure 2: Metabolic pathways in the body illustrated. Daniela Lin, Vaia Originals.
Are you overwhelmed by the big picture yet? Well, don't worry! Next, we'll look at some of the most important examples of metabolic pathways to better understand metabolism.
Examples of Metabolic Pathways
Let's take a look at two of the most vital processes that allow living organisms to gain energy and break it down for usage: photosynthesis and cellular respiration.
Photosynthesis is an Anabolic Pathway
Photosynthesis is the process plants use to make energy.
Photosynthesis is an overall anabolic process because plants get energy from the sun to convert carbon dioxide (\(CO_2\)) into glucose (\(C_6H_{12}O_6\)) or sugar.
Plants use these sugars for their own needs, but we can consume the plants to gain their energy. The overall reaction for photosynthesis is:
$$ 6CO_2+ 6H_2O + \text{solar energy} \longrightarrow C_6H_{12}O_6 + 6O_2 $$
The steps to photosynthesis are:
1. Light-dependent reactions: Solar energy is converted to chemical energy in the form of ATP and NADPH.
Protein complexes and chlorophyll molecules in the photosystems together produce chemical energy.
Since chemical energy in the form of ATP and NADPH are being formed, this process is anabolic.
2. Light-independent reactions or The Calvin cycle: Uses chemical energy from the light-dependent reactions to form glucose.
Cellular Respiration is a Catabolic Pathway
Up next, we have cellular respiration.
Cellular respiration is the process we use to break down glucose for energy, making it an overall catabolic process.
The overall reaction for cellular respiration is:
\(C_6H_{12}O_6 + 6O_2 \longrightarrow 6CO_2+ 6H_2O + \text {chemical energy}\)
The steps to cellular respiration are as follows:
1. Glycolysis: Glycolysis is the process of breaking down glucose, making it a catabolic process.
2. Pyruvate Oxidation
The conversion or oxidation of pyruvate from glycolysis to acetyl-COA, an essential cofactor.
This process is catabolic since it involves oxidizing pyruvate into acetyl-COA.
3. Citric Acid Cycle (TCA or Kreb's Cycle):
Starts with the product from pyruvate oxidation and reduces it to NADH (nicotinamide adenine dinucleotide).
This process is amphibolic or both anabolic and catabolic.
The catabolic part occurs when acetyl-COA is oxidized into carbon dioxide.
The anabolic part occurs when NADH and \(\text {FADH}_2\) are synthesized.
4. Oxidative Phosphorylation:
Take notice that the overall reaction of cellular respiration and photosynthesis are almost the opposite. This is because they work in tandem, as plants use the energy from the sun to convert carbon dioxide that other organisms release through cellular respiration into glucose, which we break down for energy.
Simultaneously, plants also release oxygen that we use to breathe and perform cellular respiration with. The broken-down glucose allows us to utilize chemical energy in the form of ATP, which can provide energy for many cellular processes. This "cycle of life" is shown in Figure 3, which is crucial for survival.
Figure 3: Metabolic processes compared. Daniela Lin, Vaia Originals.
Keep in mind that there are other metabolic processes that are not aerobic or do not involve oxygen. These processes are called anaerobic processes, such as fermentation. Anaerobic metabolism can break down carbohydrates for energy in the absence of oxygen.
Scientists think that anaerobic processes like glycolysis evolved many years ago when there was no oxygen in the atmosphere. Despite the complexity of metabolism, living organisms still share some pathways indicating our shared evolutionary history.
Metabolic Pathways - Key takeaways
- A metabolic pathway is a series of chemical reactions connected by intermediates.
- Metabolic pathways exist because cells need to perform chemical reactions to maintain bodily functions to keep you alive. These processes turn what we eat and drink into energy for our bodies to work.
- Anabolic pathways are pathways that require energy to buildup or construct molecules. For example, photosynthesis is a prominent anabolic pathway.
- Catabolic pathways are pathways that create energy through the breakdown of molecules. For instance, cellular respiration is a major catabolic pathway.
- Amphibolic pathways are pathways that include both anabolic and catabolic processes.
References
- https://openoregon.pressbooks.pub/nutritionscience/chapter/3b-photosynthesis-and-metabolism/
- http://www.metabolicpathways.teithe.gr/?part=all〈=en
- https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/metabolic-pathway
- http://homepage.ufp.pt/pedros/bq/integration.htm#:~:text=In%20humans%2C%20the%20most%20important,glycolysis%20and%20citric%20acid%20cycle.
- https://openstax.org/books/biology-2e/pages/8-3-using-light-energy-to-make-organic-molecules
- https://openstax.org/books/biology-2e/pages/6-1-energy-and-metabolism
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