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The challenge of any life support system is to create a closed system that includes the metabolism of astronauts to recycle the requirements for life. Apollo carried bottled oxygen and used lithium-hydroxide to remove carbon dioxide. The space shuttle uses the same system. This is fine for a few days or a couple weeks, but a trip lasting months or years would require too much to make the trip practical. The Mir space station used a water electrolysis system to extract oxygen from water, and recycled water from the dehumidifier and urine collection tube. Carbon dioxide was removed with a reusable sorption bed, and the CO2 dumped. The International Space Station uses the same system, but will improve water recycling to include all sources. NASA hopes to achieve 97% water recycling closure.
However, only half of the oxygen breathed to metabolise carbohydrates ends up as water; the other half is incorporated into carbon dioxide. Photosynthesis does the reverse: it releases all of the oxygen from water and half of the oxygen from carbon dioxide, and combines the hydrogen from water with the carbon and the other half of the oxygen from carbon dioxide to form sugar. An electrolysis system will only release oxygen from water. This means half of the oxygen breathed by astronauts is not recycled, but dumped as carbon dioxide. Water has to be delivered to the ISS to replace the lost oxygen. The water recycling system does not have to be perfect since even dehydrated food contains a fair amount of water, but a trip to Mars will require a lot of water to replace lost oxygen.
Adding a sabatier reactor to the electrolysis system would combine all of the hydrogen with half of the carbon dioxide to form methane. The formula is 4 H2 + CO2 → CH4 + 2 H2O. The resulting water would go back into the electrolysis tank. This doubles the amount of water that must be split by electrolysis, but all oxygen breathed is recycled.
Currently, the life support system must produce enough oxygen for the astronauts to breathe, so the electrolysis tank already splits twice as much water as human metabolism produces.
The Russian system just lets it consume more water than is produced, and relies on shipping water from Earth with each load of supplies.
A sabatier reactor would produce water to replace what is consumed by electrolysis.
This system could be used for a mission to Mars, but it relies entirely on food transported from Earth. Colonisation will require creating food on Mars itself. Depending on greenhouse farms could be tricky, and it would be nice to have a food recycling system for the interplanetary trip. The obvious candidate is to replicate nature's photosynthesis cycle to create sugar along with oxygen. Sugar could be eaten directly, or fed to yeast cultures for a more complex food supplement. The question then is how to do it?
The idea is to harvest natural chloroplasts from leaf cells.
An artificial leaf would be a thin, flat plastic bag filled with water and chloroplasts. A very loose fibre fill would hold the chloroplasts. The fibres would be transparent, and the chloroplasts would be fixed with an adhesive.
The plastic would also be transparent, and permeable to oxygen. Water would be circulated with a small pump and carbohydrates filtered out.
Several artificial leaves would be arranged in a duct with a fan to blow air across them and lights shining on them.
Starting simply, the overall chemical process of photosynthesis is:
6 H2O + 6 CO2 → 6 O2 + C6H12O6
That is, water and carbon dioxide become oxygen and sugar. Human metabolism (respiration) reverses this.
An excellent description (with diagrams) of photosynthesis is given at the Estrella Mountain Community College.
Leaf cells have tiny organs (organelles) inside them where photosynthesis happens. It is called a chloroplast.
Photosynthesis has two parts:
- The Light Reaction captures light with chlorophyll.
It uses that to convert ADP into ATP, and NADP+ into NADPH.
In the process, water is broken up and oxygen released.
- The Dark Reaction does the work of making sugar.
It breaks ATP back down into ADP and NADPH back into NADP+ as its energy source.
The net reaction is to take CO2 and hydrogen from the light reaction to make sugar.
Light Reaction details
50 molecules of chlorophyll a and a half dozen carotenoids (accessory pigments) act as antennas to collect light.
They are embedded in the walls of a folded bag called the thylakoid.
The static electric charge is passed to 2 molecules of pheophytin ("I") which each cleave a water molecule.
The result is 4 positive hydrogen ions, 4 electrons, and one molecule of oxygen (O2).
The hydrogen ions are released inside the bag only, creating an ion imbalance across the membrane.
The electrons are carried by 2 molecules of plastoquinone (PQ) to the cytochrome complex.
"I", PQ and the cytochrome complex are also embedded in the thylakoid membrane.
The cytochrome complex uses the energy of the electrons to pump another 4 positive hydrogen ions into the thylakoid (the bag).
This happens by reducing plastoquinone to plastoquinol, which has two more hydrogen atoms. It then passes electrons to cytochrome b563 and releases the extra hydrogen ions on the opposite side of the membrane, reverting back to plastonquinone. Cytochrome b563 passes its electron to cytochrome f.
The electrons are then passed through plastocyanin (PC) to chlorophyll b.
Two molecules of chlorophyll b boosts the electrons before passing them on. They have an additional 90 molecules of chlorophyll b that act as antennas.
The electrons are passed to an iron-sulphur protein designated P430, then through ferredoxin (FD) to "NADP reductase".
NADP reductase uses a couple electrons to convert two positive hydrogen ions and NADP+ into NADPH and one hydrogen ion.
While all this happens, another protein embedded in the thylakoid wall (called ATP synthase) releases the positive hydrogen ions from the thylakoid, and uses the energy of that flow for its work. It takes Adenosine Di-Phosphate (ADP) plus a Phosphate and combines them into Adenosine Tri-Phosphate (ATP).
There you have your chemical energy sources: NADPH and ATP.
Dark Reaction details
CO2 is added to Ribulose bisphosphate (RuBP). The result is broken into two molecules of phosphoglycerate (PGA).
Energy from ATP and NADPH is used to attach phosphates to PGA to create phosphoglyceraldehyde (PGAL).
From 12 molecules of PGAL, two are removed to make glucose. The other ten are converted by ATP energy to reform 6 RuBP molecules.
Glucose is a monosaccharide, which is a single carbon-ring sugar. It is the simplest sugar, and the only food that the human brain can consume. Actually, if you don't eat enough glucose your liver will make it from the other food you eat. Got to keep that brain thinking!
Carbohydrates are large molecules formed by linking simple sugars (monosaccharides).
Normal table sugar, sucrose, is a disaccharide formed by linking just two monosaccharides.
Thus, human respiration of any carbohydrate can be summarised as the reverse of photosynthesis.
The plastic must be semipermeable to release oxygen into the air, while retaining water and sugar.
Some water loss would be acceptable, but excessive loss would put significant strain on the cabin dehumidifier.
No sugar must get through since that would cake the surface, sealing the plastic from the air and blocking light.
The simplest solution would be to absorb carbon dioxide through the semipermeable plastic, but that is not practicle without water loss since the CO2 molecule is larger than water.
A sorption bed can extract carbon dioxide from cabin air. The CO2 can be introduced by bubbling it under pressure to carbonate the water.
Water will be circulated inside the "leaf" bags to ensure oxygen and carbon dioxide circulation to the chloroplasts.
The circulation system will also extract sugar. This system is intended for a two year mission to Mars, so filters must not be consumed.
Sugar is intended as a food, so the sugar must be extracted without contamination by the filtration system.
Since this is an extraction, not all sugar need be removed on a single pass; it can be extracted the next pass through the filter.
Carbohydrate paste would require more potable water, but would provide that much more water to astronauts' diet.
However, since astronaut's diet will not be solely carbohydrate, much of it will have to be removed from the system. To prevent water loss, the carbohydrate will have to be crystallised.
The easiest plant to extract chloroplasts from is peas. Pea chloroplasts convert some of the sugar into starch:
n C6H12O6 → (C6H10O5)nH2O + (n-1) H2O
Where n is somewhere between 50 and several thousand. Peas convert sugar into 60% starch, 40% pectin.
Pectin has the same chemical formula as starch, but rather than a long chain, it has multiple branches.
The starch/pectin solution is a more complex carbohydrate than pure sugar. It would taste like very bland mashed potatoes or the Hawaiian food poi.
Raw potatoes contain 78% water, 18% starch, 2.2% protein, 1% ash, and 0.1% fat. This synthetic form wouldn't have the protein, ash, or fat.
Living tissue prone to death?
At this level we are talking about the biochemical processes which are life inside part of a leaf cell.
This is too low a level to call it "life" or "death"; it is just chemistry. In fact, a tissue is composed of multiple cells.
This is one of the tiny organelles inside a single leaf cell. This isn't tissue, it is just membranes and protein.
It will not decay unless something breaks it down or eats it. The plastic will keep the inside sterile and protect it from any bacteria that might get into the machine.
We will have to protect it from Ultraviolet light (UV) to ensure that does not break down the bilipid membranes.
A spectrally selective coating will remove 98% of UV from sunlight while transmitting roughly 85% of visible light.
Oxygen causes one of the enzymes to act in reverse: it breaks down the chemical intermediary it is supposed to make as part of the dark reaction.
This is a problem that reduced the metabolic efficiency of C3 plants. C4 plants prevent this by concentrating CO2 in their tissues.
We will duplicate this efficiency step and prevent collateral damage to the enzyme by also concentrating CO2 in the sterile medium.
Chloroplasts are evolved from cyanobacteria, and have a plasmid as well as a protein synthesis capability. This makes them susceptible to bacteriophages.
Water added to the medium will have to be filtered to keep it sterile; the filter must prevent introduction of bacteriophages. A reverse osmosis filter can do this.
The artificial photosynthesis device used to produce sugar during the trip to Mars could be
retained as a backup on the surface.
This system uses one organelle from one type of cell (the mesophyll) of a leaf. The epidermis cells, veins, stoma, and sheath cells are not copied. Nor are the twigs, branches, trunk, or roots. Sugar does not have to feed these other parts of the plant, or the other organelles that make up living cells. Sugar does not have to be used to make cellulose for the support structure of the plant.
This system does require light to illuminate the artificial leaves, so it will be less energy efficient than the Artificial Chloroplast due to energy conversion losses.
Even LED's do not convert 100% of the electricity into light, some energy is lost as heat, and chlorophyll does not absorb all light.
See the absorption spectrum for chlorophyll, and relative rate of photosynthesis.
This machine, however, will be much easier to manufacture than the Artificial Chloroplast, and where sunlight is available that efficiency is moot.
This artificial photosynthesis process is particularly useful on the trip through space, more so than the stay on Mars surface. The spacecraft
has a premium for space and weight. Whole plants are self-assembling and repairing so they are cheap where weight, space, and energy
efficiency are not so critical. A supply of seeds to establish a greenhouse would be more efficient on the surface of Mars.
The machine will be more expensive to make than planting a seed, but its advantages make it a prime candidate for a spacecraft or space station.
The artificial photosynthesis device used to produce sugar during the trip to Mars could be retained as a backup on the surface.
web page photosynthesis - Estrella Mountain Community College
web page Photosynthesis: The Role of Light - John W. Kimball
Principles of Biochemistry, Albert L. Lehninger, Worth Publishers Inc. ISBN: 0-87901-136-X