Download Artificial Photosynthesis - The Mars Homestead Project

Artificial Photosynthesis
a low power recycling life support system
by Robert B. Dyck
Ardeco Consulting Ltd.
Purpose
• A closed system that includes human metabolism
Existing systems
• Apollo and Shuttle carried bottled oxygen and lithium-hydroxide
– fine for a few days supplies too great to last months
• Mir and ISS use electrolysis of water, recycled from dehumidifier and
urine collection, CO2 removed with reusable sorbent
– only half of oxygen breathed is recycled
– other half of oxygen breathed is sequestered in CO2 and dumped
Carbohydrate model
Carbohydrates are relatively simple to model.
Starch, pectin, and dextrose are polysaccharides (C6H10O5)nC6H12O6
Human metabolism starts by hydrating to break into monosaccharide:
(C6H10O5)nC6H12O6 + n H2O → (n+1) C6H12O6
We can use formuli for monosaccharides to analyze all carbohydrates.
Cellular respiration of monosaccharides is:
6 O2 + C6H12O6 → 6 H2O + 6 CO2
Photosynthesis is the reverse of this:
6 H2O + 6 CO2 → 6 O2 + C6H12O6
Replicating photosynthesis
Photosynthesis in plants occurs in an chloroplasts.
This is a two step process:
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Light reaction (Photophosporylation)
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Capture light with chlorophyll
Convert ADP into ATP, and NADP+ into NADPH
Water is broken up, and oxygen released
Dark reaction (Calvin-Benson cycle)
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ATP broken back into ADP, NADP+ into NADPH
Net reaction: CO2 and hydrogen make sugar
Chloroplast structure
• highly structured
biochemical machine
• Light reaction on surface
of thylakoid
• Dark reaction in stroma
and intermembrane space
Light reaction details
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Chlorophyl act as antennae to collect light
pheophytin cleaves 2 H2O into O2, 4 H+, 4 ecytochrome complex pumps 4 more H+ into thylakoid interior
NADP reductase converts NADP+, H+, and 2 e- into NADPH
ATP synthase converts ADP and Pi into ATP, releases H+ from thylakoid
ATP Synthesis
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H+ to ATP ratio 8:3
F0 portion is a stepper-motor
F1 has 3 binding sites:
– produces 3 ATP per rotation
Chloroplast F0 has 8 c-subunits:
– releases 8 H+ ions per rotation
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Granum of stacked thylakoids
increases electrostatic force of H+
Dark reaction details
CO2 is added to RuBP
That is broken into two molecules of 3PG
ATP and NADPH used to attach phosphates and H to 3PG to create G3P
From 12 molecules of G3P, two are removed to make glucose.
The other ten are converted by ATP to reform 6 RuBP molecules.
Calvin-Benson cycle
The Calvin-Benson cycle can be summarized as:
6 CO2 + 18 ATP + 12 H2O + 12 NADPH + 12 H+ → C6H12O6 + 18 Pi + 18 ADP + 12 NADP+
3 Pi
3 FDP
Pi
3 G3P
3 DHAP
G6P
3 F6P
Glucose
12 ADP
12 Pi
12 NADP+
2 F6P
2 G3P
2 G3P
12 G3P
12 ATP
12 NADPH
12 H+
2 X5P
2 E4P
2 DHAP
2 G3P
12 3PG
2 E4P
2 DHAP
2 G3P
6 CO2
2 SDP
6 RuBP
2 Pi
6 ADP
6 ATP
2 S7P
2 G3P
2 R5P
2 X5P
6 Ru5P
4 X5P
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
Light spectrum and filtration
Solar Transmission Spectra of Commercial Glazing
Harvesting Chloroplasts
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Grow pea seedlings in compost for 7-10 days at 18-22ºC
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Light intensity should be relatively low (40-50 μE/m2/s)
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
Harvest leaves from pea seedlings and mix with semi-frozen grinding medium at a ratio of 20 g
leaves per 100 ml medium.
Homogenize the leaves with two 3 sec bursts of the polytron at 75% full speed.
Strain the homogenate gently through eight layers of muslin to remove debris.
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.
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.
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
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2 light quanta to move one electron from H2O to NADP+
For each O2 molecule, 4 electrons, so 8 light quanta
To evolve six molecules of O2, 48 light quanta must be absorbed
1 mole of light quanta = 72kcal @ 400nm, or 41kcal @ 700nm
6 moles of O2 requires 2496 kcal @ 52kcal / mole of light quanta
2496 kcal = 2.8981 kWh
Humans require 0.84kg O2 per day, O2 masses 31.9988g per mole
12.68 kWh of light per person per day
1 mole of a subunit of polysaccharide (C6H10O5) masses 162.142g
4.256kg of carbohydrate produced per person per day
Equipment
Air filter
Blower
Return to cabin
Cabin air
CO2 Sorbent
Compressor
CO2 Storage
Carbonation
Potable
water
O2 release to cabin
through semi-permeable membrane
Reverse osmosis
filter
Pressure
reducer
Sunlight
Chloroplast bag
Water pump
Starch outflow
100% Oxygen recycling, 97% water recycling
• Oxygen recycling enclosed within spacecraft
• Inefficiency extracting CO2 from cabin air means air introduced into
chloroplast bag. The semipermeable membrane would release it
back to the cabin.
• Only matter removed is carbohydrate, incinerated.
• Water losses can be replenished by residual water in dehydrated
food and water produced by metabolizing carbohydrates in stored
food.
Starch and Fermentation
Pea chloroplasts convert sugar into starch and pectin:
n C6H12O6 → (C6H10O5)nH2O + (n-1) H2O
Potatoes: 78% water, 18% starch, 2.2% protein, 1% ash, 0.1% fat
Peas produce roughly 60% starch, 40% pectin
Some carbohydrate fed to yeast to convert it to protein, lipids, vitamin B
Result: pudding consistency, mild flavour, similar to Hawaiian food poi
Yeast nutrient: di-ammonium phosphate
Minerals provided to yeast as ash from incinerated solid human waste
Food nutrition from yeast
Yeast extracts contain many nutrients; autolysate of Saccharomyces cerevisiae:
Protein and free amino acids
vitamins per 100 grams
minerals per 100 grams
Asparaginic acid
Threonine*
Serine
glutamic acid
Glycine
Alanine
Cystein
valine *
methionine *
Isoleucine *
Leucine *
Tyrosine
Phenylalanine *
Histidine *
Lysine *
Arginine *
thiamine - B
13.0 mg
riboflavin - B 211.9 mg
Niacin
68.0 mg
B6
2.3 mg
folic acid
3.1 mg
ca-pantothenate 30.0 mg
Biotin
0.25 mg
Calcium
Magnesium
Potassium
Sodium
Iron
Phosphorus
6.66%
3.20%
3.28%
9.18%
3.17%
5.53%
0.45%
4.09%
1.12%
3.38%
4.83%
1.92%
2.80%
1.63%
5.51%
1.71%
* essential amino acids
120 mg
200 mg
3.3 g
< 0.5 g
5 mg
1.8 g
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.
CO2 fixation is a dark reaction, but it is regulated by the light reaction
Photorespiration and C3 vs. C4 plants
RuBP carboxylase can promote the reaction of RuBP with either CO2 or O2
When CO2 is low relative to O2, oxidation competes with carboxylation
C4 precede the C3 pathway by fixing CO2 into a 4-carbon compound
In C4 plants the CO2:O2 ratio remains high, this favours carboxylation.
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.
References
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U.S. Department of Energy, Federal Technology Alerts,
http://www.pnl.gov/fta/13_glazings/13_glazings.htm
Estrella Mountain Community College,
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html
Photosynthesis: The Role of Light,
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LightReactions.ht
ml
Lecture 10, ATP synthase, University of Illinois,
http://www.life.uiuc.edu/crofts/bioph354/lect10.html
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