AP Biology class blog for discussing current research in Biology

Tag: carbon dioxide

Researchers Discover an Ancient Metabolic Process Ruled by the Concentration of Carbon Dioxide

Summary of the Krebs Cycle:

In AP Biology, we are currently learning about how the Krebs cycle (also known as the citric acid cycle) is one of the most important metabolic pathways for sustaining eukaryotic life and generating ATP. 

In the matrix of the mitochondria, the citric acid cycle begins by the acetyl group from acetyl CoA attaching to an oxaloacetate molecule to form a citrate molecule. Citrate is then oxidized and in the process, releases two carbon dioxide molecules for each acetyl group used in the cycle. Three NAD+ molecules are converted to NADH, one FAD molecule is converted to FADH2, and a single ATP molecule is created. This pathway should occur a second time, as the oxidation of every two pyruvates creates two acetyl CoA, which begins the Krebs cycle.

The citric acid cycle also functions similarly in bacteria and eukaryotic systems, generally speaking. 

Citric Acid Cycle Diagram

There’s a “Reverse” to the Krebs Cycle Now?

In the absence of oxygen, some bacteria can perform the “reverse” of the Krebs cycle. This process results in the construction of biomass within bacteria that perform this process.

A key distinction between the citric acid cycle and the “reversed’ citric acid cycle, is that citrate synthase normally found in the Krebs cycle is replaced by ATP-citrate lyase in the “reversed” process. This is crucial because ATP-citrate lyase consumes ATP to split citrate instead of forming it. Another variation of this process requires no energy and has stumped researchers as to why organisms often utilize the energy costing pathway instead of the “easier” pathway to acquire biomass.

Researchers at the University of Münster decided to investigate potential factors that trigger this “easier” pathway in two kinds of anaerobic bacteria: Hippea maritima and Desulfurella acetivorans. These organisms thrive in oxygenless hot springs, with a carbon dioxide concentration of over 90 percent in their environment.

The team later found that the bacteria’s unique habitat proved to be an important factor responsible for the growth of the organisms’ biomass. After cultivating the bacteria under a diverse range of conditions, the research team discovered that the high concentration of carbon dioxide is responsible for enabling the “reversed” citric acid cycle in both anaerobic bacteria.

“It was mysterious why this ‘expensive’ version of the pathway exists if an energetically much cheaper alternative through the backwards reaction of citrate synthase is feasible. Now we know that this is due to the low carbon dioxide concentrations in many environments. The cheap alternative doesn’t work there.” researcher Wolfgang Eisenreich says.

What the Emphasis on the Concentration of Carbon Dioxide Means for this Pathway:

Carbon dioxide’s crucial role in this metabolic pathway suggests that it could stem from life on primordial earth. This is based on their theory that this “reversed” pathway was a widespread mechanism in organisms due to the surplus of carbon dioxide in the air during the primordial life timeline and the pathway’s unique dependence on high levels of carbon dioxide. Another supporting aspect to their theory is that 2.7 billion years ago, an estimated 25 to 50 percent of Earth’s atmosphere was comprised of carbon dioxide in contrast to today’s levels of carbon dioxide, which is about 0.04 percent.

The researchers have realized that the ATP-expending pathway exists due to the significant decrease of carbon dioxide in the air because the alternative pathway that doesn’t require energy isn’t possible without the surplus of carbon dioxide in the atmosphere. 


Do you believe they have truly discovered an ancient metabolic pathway? Or do you think these bacteria adapted to function this way?

Would you think that this process would be highly conserved in evolution if the carbon dioxide levels in the atmosphere didn’t dip?

COVID-19 New Target: The Environment

The deadly COVID-19 virus has changed our way of living greatly, including individual human behavior as well as behavior on a larger scale regarding businesses and factories.

National Geographic published an article written by Beth Gardiner surrounding the misconception on how the environment has been impacted by this widespread virus. It is noted that many people assume the environment is in a thriving state due to a major decrease of time humans spent outside of their home. Ultimately this is not the case, the question is what’s really happening to our earth in this time of uncertainty?

The only way to answer this question is to look back on the beginning of the worldwide lockdown. In April 2020, people stayed inside, there was limited traveling occurring, and businesses and factories closed, with this information it imperative to see how this vast change impacted our surroundings. It was found that “daily global carbon emissions were down by 17 percent”. Although seemingly positive, this number is not much higher than that of previous years around a similar time. This means that with a complete lifestyle change from every single person and cooperation in the world, we still are unable to show a substantial amount of beneficial actions towards the environment to save it.

Now we all may know that carbon is released into the air in a variety of ways, however it is important to distinguish the differences in these ways. One of the most known, harmless ways is how living organisms release or interact with carbon. As we breathe we inhale oxygen and exhale carbon dioxide, releasing it into the atmosphere, however plants and trees can use this CO2 to preform necessary tasks such as photosynthesis. Photosynthesis is the process where “plants use the energy from sunlight to produce glucose from carbon dioxide and water”. This process is crucial to support the life of a plant and provides their “food” to keep them thriving. Once the glucose is produced in the plant, pyruvate can be created. Pyruvic acid provides energy, ultimately allowing the increase of ATP production during the cellular respiration process.

ATP is energy used to power different processes such as forms of active transport allowing substances to move from a low to high concentration, unlike passive transport. ATP is not required when passive transport is occurring. As ATP is produced, it can be stored to be used later for processes such as cellular respiration and photosynthesis which are crucial in maintaining healthy plant cells, however, ATP can not be stored in its usual form, it must be in the form of storage molecules such as the carbohydrate glycogen. Carbohydrates function to store and release energy, once ATP is needed, it will be transformed out of it’s storage form back to ATP.

Now why is this background information important? Now that we see the good natural carbon dioxide does, we need to focus on how a certain type is damaging our planet. Carbon dioxide is emitted through the usage of gas from cars and factory productions, things so normalized on a daily basis. When these machines and vehicles release carbon, it has no where to go besides the atmosphere and plants can only take in so much carbon, ultimately its just pollution. This pollution now sits in our atmosphere and builds up as more time goes on. Carbon is needed to regulate and take in the inferred energy the earth releases, otherwise known as heat. Although carbon absorbs this energy, it still needs to go somewhere and one of those places is back into the earth’s environment. The excess amount of carbon in the atmosphere leads to something called climate change ultimately the more carbon released and built up, the hotter the earth will get which can make the earth inhospitable if we make no change. Another negative of the carbon build up in the atmosphere, is the effect is has on marine life. Carbon can make water acidic which damages the habitats and living conditions of underwater life.

Now that Carbon emission is fully explained and exemplified, lets answer our initial question. How has COVID-19 played apart in environmental issues. As mentioned there is evidence in a decrease in carbon emissions when human behavior was significantly changed, however the decrease barely surpassed that of previous years when life was ‘normal’. As things began to open up and manufacturing continued, it was found that the amount of carbon emissions went right back up to where there initially were. “In China, traffic is back to pre-pandemic levels”, and “factories pushed to make up for lost time, pollution returned in early May to pre-coronavirus levels, and in some places surpassed them”, disproving the idea that COVID-19 has been beneficial to our environment. Ultimately we have shown no progress in improving our environment even when almost every aspect of typical life was shut down. COVID-19 instilled panic in everyone including factories that are now just working to pollute the atmosphere more while they still can.

Forests and their Carbon Dioxide Intake

According to a study conducted by NASA, different types of forests have different intakes of carbon dioxide. The study concludes that of the 2.5 billion metric tons of carbon dioxide absorbed annually, tropical forests absorb around 1.4 billion metric tons of that amount. This is a greater amount absorbed than the boreal forests of the northern hemisphere. NASA scientist David Schimel characterized these findings as “good news” because the boreal forest absorption rate of carbon dioxide has been declining. However, scientists believe that tropical forests can continue to intake large amounts of carbon dioxide for many more years. Therefore, the fact that tropical forests play a large role in the absorption of carbon dioxide provides hope that nature will continue to limit the net carbon dioxide emissions that humans pour into the atmosphere.

Scientists have concluded that if the rate of absorption of carbon dioxide by forests slows down, then global warming could occur much faster. Previous studies had suggested that boreal forests might absorb more carbon dioxide than their tropical counterparts; however, this study finds that carbon dioxide absorption occurs more frequently at higher temperatures, indicating that tropical forests have the highest intake. The increased human emission of carbon dioxide gives the forests more fuel to grow, a process called carbon fertilization. Conversely, the climate change brought about by increased carbon dioxide emissions has caused water shortages in some areas as well as an increase in temperature, both factors facilitating the spread of wildfires. The burning of wood caused by these fires further increases the carbon dioxide emissions.

Despite the continual problems caused by increased carbon dioxide emissions, this study still provides hope for the near future. Not only can tropical forests continue to absorb high levels of carbon dioxide for many more years, but also this study might serve as a stepping stone for more complex research of emissions on a global scale that could facilitate the discovery a solution to the problem of global warming.


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