Photosynthesis - Mars Home



Artificial Photosynthesis: a low power recycling life support system

Robert B. Dyck

Ardeco Consulting Ltd., 170 Dearborn Ave, Winnipeg, Manitoba R2L 0M3, Canada

Purpose

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 is actually quite similar to photosynthesis. The light reaction produces twice as much hydrogen as required for glucose; the excess is combined with the oxygen released from CO2 to form water.

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?

From the beginning

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. Leaf cells have tiny organs (organelles) inside them where photosynthesis happens. It is called a chloroplast.

Photosynthesis has two parts:

1. The Light Reaction, known as Photophosphorylation, 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.

2. The Dark Reaction, known as the Calvin-Benson cycle, 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.

Chloroplast

A chloroplast is a highly structured biochemical machine. The light reaction occurs in the surface membrane of a folded bag called the thylakoid. As water is cleaved into oxygen and positive hydrogen ions, those ions are released inside the thylakoid. Electron transport pumps additional H+ ions into the thylakoid. That ion imbalance is used to power one step in the chemical process. The space outside the thylakoid is called the stroma. Part of the dark reaction occurs there. The rest of the dark reaction occurs in the intermembrane space.

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.

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This is highly optimized. Photosystem II has 2 pheophytin molecules close together because directly creating a single O2 molecule requires less energy than mono-atomic oxygen. To create a single glucose molecule, the Calvin-Benson cycle requires 12 H+ ions, 12 NADPH molecules, and 18 ATP molecules. Each pair of water molecules release 4 H+ ions and 4 electrons. Those 4 electrons and 2 H+ ions create 2 NADPH molecules, the other 2 H+ ions remain. The cytochrome complex pumps 4 additional H+ ions into the thylakoid, for a total of 8 H+ ions for each pair of water molecules cleaved. The outflow of those ions powers ATP synthase to create 3 ATP molecules. To ensure the H+ ions flow out with sufficient force, the folds of the thylakoid are shaped into disks which are stacked like the anodes of a capacitor. Each stack is called a granum.

ATP Synthesis

The ratio of H+ ions to ATP synthesised of 8:3 is surprising; this is not a single integer value. However, there is a reason. ATP synthase is a rotary molecule. The F1 portion has 3 binding sites where it synthesises ATP. This results in 3 molecules of ATP for each rotation. The F0 portion is the motor which drives it, and operates as a stepper-motor. The rotor of the F0 portion consists of several c-subunits, each of which requires one H+ ion to rotate one step. The number of c-subunits is not fixed, but a different value for each species. In fact, the number of c-subunits of ATP synthase in chloroplasts is different than the number of c-subunits of ATP synthase in mitochondria of the same leaf cell.

The F1 portion of ATP synthase consists of 5 subunits: 3α:3β:1γ:1δ:1ε. The 3 α-subunits and 3 β-subunits alternate in a ring around the γ-subunit, which forms a rod up the middle. The δ-subunit binds the α-β ring to the b support rod from the F0 portion. This support rod provides the physical connection between the F0 and F1 portions of ATP synthase as well as preventing the ring from rotating. The F0 portion is the molecular motor, the a-subunit is the stator and the c-subunit is the rotor. The ε-subunit is on one side of the γ-subunit and rotates with it, but changes its attachment to the α-, β-, γ-, or c-subunits as the rotor turns. The ε-subunit acts as a cam. The a-subunit acts as a port of entry for an H+ ion on the positive side to bind to the acidic residue of a c-subunit, neutralizing its negative charge. Only then can it turn away from the positive charge of the a-subunit. The a-subunit also has a port of exit for an adjacent c-subunit to release its H+ ion to the negative side, returning the c-subunit to its negative charge which pulls it toward the positive pole of the a-subunit. Static charge across the membrane drives binding and releasing H+ ions to the c-subunit. Electrical engineers will recognise the alternating charge of the rotor poles as classical stepper-motor design.

This is complicated by the fact that the c-, γ-, ε-subunit complex is fixed, and α-β ring rotates. In fact, the a-subunit rotates around the c-subunit while embedded in the cell wall. This rotation also has a reason: it increases the rate of chemical binding of ADP molecules.

The number of c-subunits in the rotor is matched to the charge potential across the membrane. A fixed amount of work is required per rotation. A greater force per step permits a greater angle of rotation with each step, which translates to fewer steps per rotation. Life forms have tuned the number of c-subunits to match the electrical potential across the membrane in which that molecule of ATP synthase will operate. The rotor steps leverage the motive force available. Another way to say this is that a reduced voltage across the membrane requires a greater current to provide the necessary power.

Chloroplasts have the fewest number of c-subunits discovered so far; chloroplasts have 8, E. coli have 12, Ilyobacter tartaricus has 11, and yeast mitrochondria have 10. For chemical balance, in photosynthesis only 8 H+ ions are available for 3 ATP molecules. This reduced current has demanded a voltage increase, which is why thylakoids are shaped into disk stacks called grana which act like capacitors.

So the 8:3 ratio of H+ ions to ATP is correct, and the chemical formula is balanced.

Dark Reaction details

CO2 is added to Ribulose bisphosphate (RuBP). The result is broken into two molecules of 3-phosphoglycerate (3PG). ATP and NADPH are used to attach phosphates and hydrogen to 3PG to create 3-phosphate glyceraldehyde (G3P). From 12 molecules of G3P, two are removed to make glucose. The other ten are converted by ATP to reform 6 RuBP molecules.

Glucose is a monosaccharide, which is a single carbon-ring sugar.

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The exact process of the Calvin-Benson cycle is complicated, but can be summarized as:

6 CO2 + 18 ATP + 12 H2O + 12 NADPH + 12 H+ → C6H12O6 + 18 Pi + 18 ADP + 12 NADP+

In the light reaction, ATP synthase combines ADP and Pi to form ATP and releases H2O.

Since the Calvin-Benson cycle consumes 12 molecules of water, and ATP synthase releases 18 molecules of water, the net release is 6 molecules of water. That means photosystem II must cleave 12 molecules of water to produce enough NADPH and H+ to produce 1 molecule of glucose, but net consumption of water is only 6 molecules.

Calvin-Benson Cycle

Key:

3PG = 3-phosphogycerate

G3P = glyceraldehydes 3-phosphate

DHAP = dihydroxyacetone phosphate

FDP = fructose 1,6-diphosphate

F6P = fructose 6-phosphate

G6P = glucose 6-phosphate

E4P = erythrose 4-phosphate

X5P = xylulose 5-phosphate

SDP = sedoheptulose 1,7-diphosphate

S7P = sedoheptulose 7-phosphate

R5P = ribose 5-phosphate

Ru5P = ribulose 5-phosphate

RuBP = ribulose 1,5-biphosphate

Replicating photosynthesis

The obvious initial response is to design a chemical system to replicate photosynthesis without use of plants. The dark reaction creates sugar, so how could we simply harvest the enzymes for dark reaction and use them in a tank? However, the light reaction provides ATP, NADPH, and H+ ions. The enzyme ATP synthase can create ATP, but it requires a flow of H+ ions across a membrane to power it. NADP reductase could be powered by electric charge delivered by ferredoxin, by how do you provide the H+ flow? A synthetic thylakoid could replace the light reaction, and we could simply use ATP synthase and NADP reductase, but we have to recreate the required charge potential across the membrane. Cleaving water to create O2 and H+ ions would require a catalyst to ensure we don't destroy the membrane; pheophytin would be the best candidate to accomplish that. To ensure electrical efficiency we would require 2 molecules of pheophytin in close proximity to generate O2 directly. To keep the stoichiometry balanced, we would also have to pump 2 H+ ions into the synthetic thylakoid for each water molecule cleaved. We would have to reproduce every part of the light reaction, with the sole exception of replacing the chlorophyll photocollectors with wires to deliver electricity directly. Connecting that wireing would be a very difficult assembly problem. A spacecraft would use photovoltaic arrays to collect sunlight and convert it to electricity, so why replace the photocollectors of the two photosystems? Why not just illuminate the photosynthesis tank with sunlight through a window? This leaves us with whole, unmodified chloroplasts.

Another way to look at this is that the light reaction is a very efficient, highly tuned nanomachine; the dark reaction is an inefficient mess. For one thing the dark reaction does not directly release oxygen from CO2; it combines oxygen with hydrogen to form water. This doubles the amount of water the light reaction must cleave for each molecule of monosaccharide. That doubles the energy required. From an engineering perspective, you don't want to replace efficient systems, you want to replace inefficient ones and leave the efficient systems alone. But chemically replacing the dark reaction would be a very complicated chemical engineering problem.

Food safety

Using whole chloroplasts from a food crop has a food safety issue associated with it. Every life support system must be assessed for safety in the case of breakdown. Leakage of the chemical intermediaries into the food produced would mean the mashed potato substitute is contaminated with peas. There may be individuals who do not like to mix their peas with their mashed potatoes on their plate, but it will not kill them.

Evolution trivia

As a point of trivia, since the light reaction is so very efficient and highly tuned, while the dark reaction is a mess and inefficient; that implies the light reaction evolved first. The fact that in presence of oxygen the enzyme RuBP carboxylase can undo formation of monosaccharide means that the dark reaction evolved in a CO2 atmosphere. This is consistent with the theory that chloroplasts evolved from cyanobacteria, which were the first oxygenic organism on this planet and did evolve in a CO2 atmosphere.

Light spectrum and filtration

Since the chloroplasts used in this device are stored outside of a eukaryotic cell, the chloroplasts cannot repair themselves. Ultraviolet light can cause sunburn just as it does for humans. The intense broad-spectrum light of space has a great deal of UV light. Direct sunlight must be filtered to prevent damaging the chloroplasts. Spectrally selective coatings have been developed for commercial windows. Spectrally selective low-e glazing will transmit 80% of blue light and 85% of green, while blocking 98% of UV-B, and UV-A blocking tapers from 20% to 98%. Longer wavelength response tapers from 60% transmittance for orange light to 30% for deep red. The charts below for Anacharis indicate primary yellow absorption is by the accessory pigments phycoerythrin and phycocyanin, with a small absorption spike for chlorophyll at reddish-orange. To avoid over heating, infrared light is also filtered; it transmits 10%-45% of IR depending on frequency.

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Solar Transmission Spectra of Commercial Glazing

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Harvesting Chloroplasts

Isolating chloroplasts has been a laboratory technique since 1939. Today it is an exercise for undergraduate students; the process need only be automated to scale up to larger production. The easiest plant from which to harvest chloroplasts is a pea. The protocol is:

Grow pea seedlings in compost for 7-10 days at 18-22ºC

o Light intensity should be relatively low (40-50 μE/m2/s)

o Only young tissue (2-3 days after leaf emergence) should be used

• Grid medium (0.35 M sucrose, 25 mM Hepes-NaOH, pH 7.6, 2mM EDTA)

• Sorbitol medium (50 mM Hepes-KOH, pH 8.4, 0.33 M sorbitol)

• 40% Percoll in sorbitol buffer

• 80% aqueous acetone

1. Harvest leaves from pea seedlings and mix with semi-frozen grinding medium at a ratio of 20 g leaves per 100 ml medium.

2. Homogenize the leaves with two 3 sec bursts of the polytron at 75% full speed.

3. Strain the homogenate gently through eight layers of muslin to remove debris.

4. Pour the suspension into 50 ml or 100 ml centrifuge tubes and centrifuge at 4000 g for 1 min. Discard the supernat in one motion (the pellets are quite firm at this stage) and wipe the inside of tubes.

5. Resuspend the pellet gently in a small volume (4-8 ml) of sorbitol medium using a cotton swab or small paint brush, and layer the suspension on to an equal volume of 40% Percoll (Pharmacia) in sorbitol buffer. Centrifuge at 2500 g for 7 min (with the brake off). Intact chloroplasts are pelleted whereas lysed organelles fail to penetrate through the Percoll pad.

6. Wash the pellet in 5 ml sorbitol medium and resuspend the pellet in 1 ml sorbitol medium. Check the intactness of the organelles under phase-contrast microscopy; intact organelles appear bright green, often with a surrounding halo, whereas broken chloroplasts appear darker and more opaque. The majority of the organelles (up to 95%) should be intact.

Capacity

Two light quanta must be absorbed, one by each photosystem, to cause flow of one electron from H2O to NADP+. To generate one molecule of O2, four electrons must be transferred, which requires eight quanta. To evolve six molecules of O2, 48 light quanta must be absorbed. The energy of a mole of light quanta with 400nm wavelength is 72 kcal, but for 700nm it is only 41 kcal. Assuming an average of 52 kcal per mole of light quanta, that requires 2496 kcal of light to generate 6 moles of oxygen and 1 mole of glucose. To convert that to metric:

1 cal = 4.180 J, and 1 kWh = 3.60 x 106 J, therefore, 2496 kcal = 2.8981 kWh.

Humans require 0.84kg oxygen per day, and molecular oxygen masses 31.9988 grams per mole, therefore a chloroplast life support system requires 12.68 kWh of light per person per day. This doesn't include electricity to run the pumps and fans, or electricity for the water recycling system; this is just light to generate oxygen and glucose. One mole of glucose masses 180.1572 grams; however one mole of a subunit of polysaccharide masses 162.1420 grams, therefore the chloroplast life support system will produce 4.256 kg of carbohydrate per person per day.

Equipment

The in-space equipment to operate a chloroplast based lift support system is remarkably simple. Chloroplasts require carbon dioxide and water to produce oxygen and sugar. Isolated chloroplasts would be contained within a sterile medium in a plastic bag. 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 if that is not possible without water loss then a sorption bed can extract carbon dioxide from cabin air. The CO2 can be introduced by bubbling it under pressure to carbonate water; this is the same process to carbonate soda pop.

Water will be circulated inside the "leaf" bags to ensure oxygen and carbon dioxide circulation to the chloroplasts. The circulation system will also extract carbohydrate. This system is intended for a two year mission to Mars, so filters must not be consumed. Carbohydrate is intended as a food, so it must be extracted without contamination by the filtration system. Since this is an extraction, not all carbohydrate need be removed on a single pass; it can be extracted the next pass through the filter. Carbohydrate/water syrup would require more potable water, but it 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 or incinerated.

Chloroplasts are organic, but are not a complete living organism. To prevent them from decomposing they must be maintained in a sterile medium. Potable water must be filtered to prevent infection by amoebae, bacteria, mould, fungi, etc, that may consume them. In fact, chloroplasts do contain a single plasmid, and are capable of producing some proteins, although they cannot produces everything necessary for reproduction or growth. Chloroplast growth is highly regulated by the nucleus of a eukaryotic cell. But the fact chloroplasts do contain a small piece of DNA and some cellular machinery to produce protein means they are potentially vulnerable to plant viruses and bacteriophages. The water filter must remove all bacteria and phages; that can be accomplished simply with a reverse osmosis filter.

Water recycling can use the system developed by NASA's Johnson Space Center as part of the Advanced Life Support project. That system is reported to have achieved 97% water recycling efficiency. It included an incinerator to extract water from fecal matter; the same incinerator could extract water from excess carbohydrate.

100% oxygen recycling, 97% water recycling

The oxygen recycling system this provides is entirely enclosed and within the spacecraft environment. Any inefficiency extracting carbon dioxide from cabin air would result in a small quantity of air being dissolved into water of the chloroplast bag. The semipermeable membrane of the bag itself would release oxygen back to the cabin. This results in no loss of oxygen from the system. The only matter removed from the system is carbohydrate incinerated in the water recycling system. Using the water recycling figure reported by the Advanced Life Support project, that results in 100% oxygen recycling, 97% water recycling. With water recycling that high, losses can be replenished by residual water in dehydrated food and water produced by metabolizing carbohydrates in stored food.

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Starch and Fermentation

The easiest plant to extract chloroplasts from is peas. Pea chloroplasts convert sugar into starch:

n C6H12O6 → (C6H10O5)nH2O + (n-1) H2O

Where n is somewhere between 50 and several thousand. The sugar/starch solution is a more complex carbohydrate than pure sugar. The starch could be fed to a fermentation tank where yeast would convert some into protein. Raw potatoes contain 78% water, 18% starch, 2.2% protein, 1% ash, and 0.1% fat. Yeast in starch can add protein and fat to the same level as potatoes, with greater vitamin B complex and less ash. The result would have the consistency of pudding and mild flavour. The Hawaiian food poi is made from Taro root. It has the same consistency and no taste at all. You could consider this a synthetic form of poi, but some yeast varieties would add flavour.

The only caution is to select yeast that minimizes production of alcohol; brewers or vintners yeast maximizes alcohol, yeast for food supplements minimizes it. The alcohol can easily be boiled off by cooking, but you have to be careful inside a sealed habitat. You don't want alcohol condensing inside electronics. Alcohol can be condensed by temperature fractionation; and collected for disposal.

Yeast extracts contain many nutrients; autolysate of Saccharomyces cerevisiae contains:

|protein and free amino acids |vitamins per 100 grams |minerals per 100 grams |

|asparaginic acid |thiamine - B1 |calcium |

|6.66% |3.0  mg |120 mg |

| | | |

|threonine * |riboflavin - B2 |magnesium |

|3.20% |11.9  mg |200 mg |

| | | |

|serine |niacin |potassium |

|3.28% |68.0 mg |3.3 g |

| | | |

|glutamic acid |B6 |sodium |

|9.18% |2.3 mg |< 0.5 g |

| | | |

|glycine |folic acid |iron |

|3.17% |3.1 mg |5 mg |

| | | |

|alanine |ca-pantothenate |phosphorus |

|5.53% |30.0 mg |1.8 g |

| | | |

|cystein |biotin | |

|0.45% |0.25 mg | |

| | | |

|valine * | | |

|4.09% | | |

| | | |

|methionine * | | |

|1.12% | | |

| | | |

|isoleucine * | | |

|3.38% | | |

| | | |

|leucine * | | |

|4.83% | | |

| | | |

|tyrosine | | |

|1.92% | | |

| | | |

|phenylalanine * | | |

|2.80% | | |

| | | |

|histidine * | | |

|1.63% | | |

| | | |

|lysine * | | |

|5.51% | | |

| | | |

|arginine * | | |

|1.71% | | |

| | | |

|* essential amino acids | | |

The minerals do raise the question of yeast nutrient. Commercial yeast production for yeast extracts can use glucose syrup as raw material. The usual supplement known as yeast nutrient is di-ammonium phosphate. That provides phosphorus, but it raises the question of a source for the other minerals.

Since a few grams of yeast nutrient can yield kilograms of synthetic poi, a supply of yeast nutrient is a viable consideration for a spacecraft.

Minerals can be supplied to yeast as ash from incinerated human waste.

Regulation of the Dark Reaction

The rate-limiting step in the dark reactions is fixation of CO2 by the ribulose biphosphate carboxylase reaction to form 3-phosphoglycerate (3PG). This enzyme is stimulated by three different changes that result from illumination of chloroplasts:

1. Increase in pH. When chloroplasts are illuminated, H+ ions are transported from the stroma into the thylakoids, resulting in an increase in the stroma pH, which stimulates the carboxylase, located on the outer surface of the thylakoid membrane.

2. Mg+2, which enters the stroma as H+ ions leave when chloroplasts are illuminated.

3. NADPH, which is generated by photosystem I during illumination.

Thus, although CO2 fixation is a dark reaction, it is regulated by the light reaction.

Photorespiration and C3 vs. C4 plants

Most plants in the tropics, as well as temperate-zone crop plants native to the tropics, such as corn, sugar cane, and sorghum, fix CO2 by a route called the Hatch-Slack or C4 pathway. The difference is that for C4 plants, the C3 pathway is preceded by additional steps which fix CO2 into a compound with four carbon atoms before the CO2 is incorporated into phosphoglycerate (3PG).

The C4 pathway requires five high-energy phosphate groups per molecule of CO2 fixed, compared with only three in C3 plants, but C4 plants grow faster and make more biomass per unit leaf area than C3 plants. This indicates inefficiency. Unfortunately for gardeners, crabgrass and many other weeds are among the C4 plants.

The major substrate oxidized by photorespiration in C3 plants is glycolic acid. Glycolate is oxidized in the peroxisomes of leaf cells to glyoxylate, which is converted into glycine and other products. Glycolate is formed by the oxidative breakdown of RuBP by RuBP carboxylase, the same enzyme that that fixes CO2 into 3PG.

RuBP carboxylase can promote the reaction of RuBP with either CO2 or O2. When CO2 concentration is low and O2 is relatively high, O2 not only competes with CO2 but can also replace it. The product of oxygenation undergoes another step to become the substrate oxidized during photorespiration. Photorespiration is the breakdown of that substrate by the mitochondria to generate chemical energy for the cell. C4 plants only respire at night.

In C4 plants the CO2:O2 ratio remains relatively high, and this favours carboxylation. More over, closing of stomata in C4 plants not only avoids water loss but also limits entry of atmospheric oxygen.

By controlling CO2 levels, we can use chloroplasts from the energy efficient C3 plants without losses due to oxidation. Chloroplasts from C3 plants ensure we only need a single organelle.

Conclusion

This system should work, provide oxygen without consuming water, and provide the carbohydrate portion of astronauts' diet. It would be small enough for use in the International Space Station, or a spacecraft to Mars. The simple input requirement prevents the danger of ecological collapse. Long duration stay on the surface of Mars would require greater variety of food, but a surface colony can afford the space and mass for a full greenhouse. A surface greenhouse could be large enough to provide biodiversity to reduce the danger of ecological collapse, but such a system would be too large for a spacecraft. The chloroplast system could be retained on a surface colony as a backup for the greenhouse.

The key is how long chloroplasts will function outside a eukaryotic cell. The undergraduate laboratory exercise is conducted in non-sterile conditions which result in chloroplasts functioning only 20 minutes. For this system to be practical it must function several months. Replacement chloroplast bags could be frozen in liquid nitrogen for storage. At the scale of a chloroplast there is no doubt they would remain functional after they are thawed. However, to be practical a set of bags must function for several months before requiring replacement. This paper describes how chloroplast function in vitro can be extended. That is:

1) retain the chloroplasts in sterile water in the bag

2) introduce water through a reverse osmosis filter to maintain sterility

3) also filter CO2 to retain sterility

4) maintain relatively high CO2 concentration ( CO2/O2 ≈ 0.6)

5) filter sun light to remove UltraViolet

6) maintain stable temperature

Although this theoretical study does demonstrate it should work, it must be tested in the laboratory. Further investigation should also study chloroplast ability for self repair. Since chloroplasts are greatly regulated from the cell nucleus, they should be studied to identify any enzyme which may trigger chloroplast self-destruction or degradation.

References

U.S. Department of Energy, Federal Technology Alerts,

Estrella Mountain Community College,

Photosynthesis: The Role of Light,

Lecture 10, ATP synthase, University of Illinois,

Iowa State University, Botany 513, Lecture 8

Principles of Biochemistry, Albert L. Lehninger, Worth Publishers Inc. ISBN: 0-87901-136-X

Plant Cell Biology, Harris and Oparka, 1994

Isolation of membranes and organelles from plant cells, Hall and Moore, 1983

A Nuclear-encoded RNA Polymerase in Corn Chloroplasts, Rachel Howard

Yeast Extracts: Production, Properties and Components, 9th International Symposium on Yeasts, Sydney, August 1996, Rolf Sommer, Deutsche Hefewerke GmbH & Co. oHG

-----------------------

3 G3P

3 DHAP

12 G3P

2 F6P

2 G3P

3 FDP

3 F6P

12 3PG

2 X5P

2 E4P

2 E4P

2 DHAP

2 SDP

2 S7P

2 G3P

2 R5P

2 X5P

4 X5P

6 Ru5P

6 RuBP

6 CO2

12 ADP

12 Pi

12 NADP+

12 ATP

12 NADPH

12 H+

6 ADP

6 ATP

2 Pi

3 Pi

2 G3P

2 DHAP

2 G3P

2 G3P

G6P

Pi

Glucose

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