What are the Different Types of Metabolism Pathways?

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  • Written By: Greg Caramenico
  • Edited By: Daniel Lindley
  • Last Modified Date: 14 October 2019
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Metabolism pathways include the basic chemical reactions that provide cells with the energy to remain alive and repair themselves. Cellular respiration is the central metabolic activity, and it operates through three different pathways — glycolysis, the Krebs cycle, and oxidative phosphorylation — that make energy-rich molecules that fuel cells. Different metabolism pathways are specialized for manufacturing proteins, nucleic acids, and other essential molecules. Toxic byproducts of metabolism are disposed of in the urea cycle pathway.

There are two classes of metabolism reactions: anabolism, which uses energy to build proteins and other cellular components, and catabolism, which generates energy by breaking down food into energy-rich compounds. Most metabolic pathways fall into one of these categories. Pathways are catalyzed by enzymes that regulate each individual chemical reaction, usually producing many intermediate compounds and chain reactions in the process. At many points in the evolution of life, the same molecules have been used to meet different metabolic needs, so the same enzymes direct metabolism in many different organisms.


Cellular respiration converts food to a high-energy molecule, adenosine triphosphate (ATP), that cells use for energy. It comprises three metabolic pathways. The first of these, glycolysis, is the splitting of glucose, a six-carbon sugar, into two molecules of a three-carbon sugar and into acetyl coenzyme A. Glycolysis generates two molecules of ATP along with other energy-rich molecules. In anaerobic metabolism, such as occurs in some bacteria and yeast, glycolysis is called fermentation and is a single step-form of cellular respiration.

In animals, glycolysis is just the first step of cellular respiration. Its second metabolism pathway is the citric acid cycle, also known as the Krebs or TCA cycle. This begins when the acetyl coenzyme A from glycolysis is converted into several energy-carrying chemicals, and into two ATP molecules. The last of the metabolism pathways in cellular respiration is oxidative phosphorylation, which requires oxygen and a series of electron-carrying molecules to get started. This pathway uses the transfer of electrons from the energy-rich chemicals to oxygen to power the production of ATP.

Beta-oxidation is a metabolic pathway that converts fatty acids into ATP. The metabolism of fats involves both catabolism to form ATP and anabolism to produce phospholipids. Proteins are broken up into their constituent amino acids in many different pathways, beginning with digestion and continuing with processing at the cellular level. Two other common metabolism pathways are the urea cycle, which removes the toxins formed by nitrogen metabolism from the body, and glycogenesis, which converts glucose into starch for long-term storage.


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Post 3

@irontoenail - Well, I know that yeast will often choose anaerobic metabolism pathways over other pathways even if oxygen is present, so maybe they can figure out a way to make it do that more consistently in order to bring the cost down.

Of course, if they bio-engineered bacteria or yeasts which did that they might be playing with fire.

I guess if they weren't limited by the presence of oxygen like they are now they might take off in unexpected ways.

Although, as I understand it, yeast which is used for fermentation only continues while sugar is present, so maybe they could have a different limiting factor, aside from oxygen.

Or maybe they could add sugar to the manure?

Post 2

@Mor - I actually found quite a bit about the biochemical pathways we learned in biology to be useful in everyday life.

For example, the fact that yeast and bacteria use a different method for metabolism is used in all kinds of situations.

When I went on a field trip to a dairy farm, we saw the way they used anaerobic metabolism to break down some of the compounds in the cow manure so it could be processed quickly for fertilizer.

Using an anaerobic digestion method meant that there wasn't much of the smell you might expect from that kind of process and they could use the gases produced for energy as well.

It's the perfect way to run a farm

really, using a substance that would otherwise be poisoning the rivers in order to create energy and renew your fields.

Unfortunately, it costs quite a bit to set up, since you need an anaerobic environment for the bacteria and yeasts to work.

Post 1

One of the products when body fat is broken down into energy is water, so I've always found it quite encouraging when I've had to urinate more after exercising. Although it gets released as sweat as well.

Of course, that might also come from drinking more water because of the exercise. I'm sure the amount of water released by the amount of fat I've burned isn't actually that much when it comes down to it.

Still, it was something I learned in biology class which I still remember, since it applied to every day life.

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