Sustainable Living Conditions in Space

The Cell Respiration Components on Earth

C6H12O6 + 6 O2 + 36 ADP + 36 Pi —-> 6 CO2 + 6 H2O + 36 ATP + heat

       The cell respiration equation is the single most important chemical reaction occurring in the body of any multicellular living thing on Earth. It is the equation that stores energy from glucose molecules in the cell as ATP, the main energy storage in most cells. This ATP is then broken apart into ADP and phosphate to release that energy and do cell work. Cell work is defined as any job a cell performs, whether it be creating a certain material or simply expanding and contracting repeatedly forever. It also powers mitosis, meiosis, and allows the cell to grow.

       However, the equation shown above only applies to a single cell using a single piece of glucose to produce energy. In reality, cells are storing the energy from glucose as ATP constantly, every moment of every day, in every cell in the human body. The millions of cells in a person’s body means that one must carry out the above reaction millions of times, every day. No wonder people eat so much, with all of that glucose they need!

       Even if it’s multiplied for every cell in the human body, the above equation is still a simplification. In reality there are many steps hidden beneath that equation.

  1. Glucose – The only outside product that the cell requires to do work, glucose is the most basic sugar the body can process. The C6H12O6 portion of the equation, glucose stores a lot of energy within its bonds. It undergoes a process called glycolysis within the cytoplasm of the cell that turns glucose into pyruvate that can be further processed and used to make extra ATP, turn NAD into NADH, or turn FADH into FADH2. Glycolysis by itself generates 4 ATP but requires 2 ATP to start the process, resulting in a net gain of 2 ATP. Glucose is created by the body during digestion, when it breaks down more complex starches and sugars into the more simple glucose molecule that cells can process.

  2. Oxygen – The 6O2 term of the product side of the equation, oxygen is required during the stage of respiration called oxidative phosphorylation. This stage uses a series of redox reactions to transfer electrons from electron donors within the mitochondria of the cell to the oxygen, releasing energy that is used to create ATP. This is known as the electron transport chain, as it transports electrons from the donors to the oxygen. This eventually becomes CO2 and can be found in plentiful quantities in Earth’s atmosphere. Acquiring enough oxygen for cell respiration is as simple as breathing in.

  3. ADP and Pi – ADP and Pi are two components of ATP. ADP stands for adenosine diphosphate. It is comprised of two main components, the adenosine base and the phosphate tail. As it is diphosphate, it has two phosphates in its tail. While not generally done in normal cell respiration, these phosphates can be broken off to release some energy and produce AP, or adenosine phosphate, which only has one phosphate on its tail. The Pi in the equation stands for the inorganic phosphates that are being attached to the ADP throughout the process of cell respiration. Both ADP and phosphate are recycled in the cell meaning it is never created nor destroyed.

  4. ATP – The main product of the cell respiration equation is adenosine triphosphate or ATP. The main energy carrier of the cell, ATP is made up of an adenosine base and a phosphate tail containing three phosphate groups. The third phosphate group is what is broken off when the cell requires energy to do work. Throughout the entire process of cell respiration, 38 total ATP are created and 2 ATP are used, resulting in a net gain of 36 ATP from a single molecule of glucose. Like ADP and phosphate, it is recycled in the cell until it dies.

  5. CO2 – A waste product of cell respiration, CO2 is inert by itself, but can suffocate cells in large quantities. The body needs to get rid of the CO2 generated by cell respiration, and does that whenever one breathes out.

2.     The Equation Components for Earth Orbit in the ISS

       The two issues space engineers have for designing a permanent orbital station such as the ISS is filtering of carbon dioxide, production of oxygen, and how to supply glucose. Supplying glucose to the astronauts in the space station is relatively easy, if expensive. Every shuttle that goes up to the station to change out the crew is also packed with supplies for the astronauts until the next shuttle goes up. This is much more expensive than supplying the ISS with a renewable food source, but much less complicated and leaves room for more scientific equipment and living quarters on the space station.

       However, this still leaves the somewhat more complex issues of carbon dioxide filtration and oxygen production. While these two may seem to go hand in hand, they actually require very different systems to accomplish.

  1. Carbon Dioxide Scrubbing – Carbon dioxide buildup is one of the main hazards for the International Space Station. With up to eight astronauts living and working in the station at the same time, it can happen quickly if special precautions and advanced filtration systems aren’t used to remove the waste gas.

    1. Trace Contaminant Control Assembly – Air first passes through the Trace Contaminant Control Assembly on its way to the carbon dioxide removal system. As its name suggests, the TCCA removes any trace amounts of potentially harmful substances, such as gas and vapors from lab experiments. If these were left to accumulate, it could cause permanent damage to the station or the astronauts.

    2. Temperature and Humidity Control – The Temperature and Humidity Control system’s function is also obvious. It circulates air constantly, allowing water to condense and bringing the temperature to a constant level for recirculation through the system.

    3. Carbon Dioxide Removal and Overboard Venting – After having the water removed for recirculation through the station, the dry air is sent to Carbon Dioxide Removal. The CO2removal system on the ISS involves a regenerable metal-oxide removal system, which uses a metal oxide to absorb the carbon dioxide from the air. Extremely hot air has to be pumped through the system to activate the regeneration system and regenerate the oxide. Carbon dioxide collected in this way is then vented overboard.

  2. Oxygen Generation (See Fig. 3)

    1. Carbon dioxide removal is not enough to support life, oxygen has to be present as well. The Oxygen Generation system on the ISS was created to do just that. Before the current systems were implemented, the ISS used to get oxygen from the remaining oxidizer present in the space shuttles that brought astronauts to and from the station. The new generation system allows the ISS to go longer without oxygen refueling from a shuttle. The only requirement for the stations generation system is water and electricity. The water collected earlier from the Temp and Humidity Control systems is hydrolyzed into hydrogen and oxygen. The oxygen is pumped throughout the ISS and the excess hydrogen is vented overboard.

  3. Water

    1. Water only appears in the cell respiration equation on the products side, but still has to be considered when sending astronauts into space. Water is not only important for drinking, but it generates the oxygen required for the astronauts to continue living. Water on the ISS is both recycled and generated by the systems on the station. The fuel cells of the ISS that use electrolysis to combine oxygen and hydrogen into water and electricity generates drinkable water for the astronauts to use, as well as the energy that keeps the station running. Water is also kept track of meticulously, with every ounce that the astronauts waste from their body collected and filtered. The water collected in the Temp and Humidity control system is sent directly to Potable Water Processing which sanitizes it and brings it to a constant temperature. Urine is also collected and heavily filtered throughout the station in the same system. This sanitized water is sent to the oxygen generation system, as well as any other systems that require water such as showers, hand washing, and drinking water dispensers.

    2. Electrolysis is the decomposition of water (H2O) into Oxygen (O2) and Hydrogen gas (H2) by passing an electric current through it. This is done by attaching a power source to two electrodes, or plates that are placed in water. Hydrogen appears on the cathode, or negatively charged plate, while the oxygen will appear on the anode, or the positively charged plate. The plates ‘attract’ different particles because of the two different reactions involved within Electrolysis: Reduction and Oxidation

      1. Oxidation involves the loss of electrons overall. Because of this, it takes place at the positive electrode, the anode, due to the lack of electrons on it. This half reaction is: 2 H2O(l) → O2(g) + 4 H+(aq) + 4e−.  In this reaction, the liquid water is electrolyzed resulting in Oxygen gas, and aqueous H+ ions. The electrons are put through the battery into the cathode bar.

      2. Reduction involves the gain of electrons. By gaining the electrons lost in the oxidation reaction at the anode, we have the half reaction: 2 H2O(l) + 2e− → H2(g) + 2 OH-(aq).  In this, electrons are added to liquid water to create hydrogen gas and OH-.

      3. These reactions put together simplify into the overall reaction of 2 H2O(l) → 2 H2(g) + O2(g). The H+ and OH- ions are joined back into liquid water, and put back into the equation. Then, the gaseous oxygen and hydrogen are captured and used as needed.

Simple Electrolysis demonstrationAtoms in the Chemical Equation

       All of these systems have to be specialized, protected, transported, and maintained by the countries part of the ISS program. This tends to get extremely costly, inefficient for a single-use ship. As the ISS is a more permanent fixture of the night sky, the cost becomes less and the benefits become greater.

3.     Is it Worth the Effort and Cost?

As it is so expensive to send astronauts up in the first place, is space travel really worth it anymore? Many people would argue that it isn’t without accomplishing anything new. The truth is, humanity has a lot to learn about space and space travel still and the International Space Station is perfectly equipped to help humans learn these things.

  • Effects of Staying in Space for Years

    • Travelling to far-off planets such as Mars could take up to a year of space travel. Permanent space stations such as the ISS have the systems and supplies needed to research the effects of low-gravity, cosmic radiation, and confined spaces on a human for that kind of time. To bring humanity to Mars, and eventually the stars, systems for keeping humans in space without any contact from the outside world for months or years at a time are required.

  • Spare Repairmen

    • The Global Positioning System, a huge network of satellites in orbit all across the Earth, occasionally requires maintenance that cannot be done remotely. A stable “repair crew” for satellites that lived in a space station, even one in low-Earth orbit, would be more efficient than sending shuttle after shuttle to repair them when damaged. While the initial setup would be expensive, as well as the systems required to keep them alive while in space, the cost over time would be extremely small compared to sending up repeated shuttles. The same goes for non-satellites in orbit as well, such as the Hubble Space Telescope.

  • Increased Research Potential

    • The atmosphere, the great sphere of gases that sustains life and protects us from the harmful radiation of the sun’s light, is an astrophysicist’s worst nightmare. It scatters light and radiation in all directions, making pictures of space in all spectrums of light blurry and distorted. The Hubble Space Telescope, one of the most famous telescopes in existence, was a huge leap forward as it allowed scientists to see clearly without the scattering effect of the atmosphere. Sending a crew of astrophysicists and astronauts into space in a similar fashion and leaving them there to collect data would be invaluable in understanding the universe.
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