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:

1. Light reaction (Photophosporylation)– Capture light with chlorophyll– Convert ADP into ATP, and NADP+ into NADPH– Water is broken up, and oxygen released

2. Dark reaction (Calvin-Benson cycle)– 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

• Chlorophyl act as antennae to collect light• pheophytin cleaves 2 H2O into O2, 4 H+, 4 e-

• cytochrome 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

• 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

• 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+

12 G3P2 F6P2 G3P

3 F6P

3 G3P3 DHAP

3 FDP

12 3PG

2 X5P2 E4P

2 SDP

2 E4P2 DHAP

2 S7P2 G3P

2 R5P2 X5P

4 X5P

6 Ru5P

6 RuBP

6 CO2

12 ADP12 Pi

12 NADP+

12 ATP12 NADPH

12 H+

6 ADP

6 ATP

2 Pi

3 Pi

2 G3P

2 DHAP

2 G3P

2 G3P

G6P

Pi

Glucose Key:3PG = 3-phosphogycerateG3P = glyceraldehydes 3-phosphateDHAP = dihydroxyacetone phosphateFDP = fructose 1,6-diphosphateF6P = fructose 6-phosphateG6P = glucose 6-phosphateE4P = erythrose 4-phosphateX5P = xylulose 5-phosphateSDP = sedoheptulose 1,7-diphosphateS7P = sedoheptulose 7-phosphateR5P = ribose 5-phosphateRu5P = ribulose 5-phosphateRuBP = ribulose 1,5-biphosphate

Light spectrum and filtrationSolar Transmission Spectra of Commercial Glazing

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

– 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

• 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

EquipmentBlower

CO2 Sorbent

Air filter

Compressor

Chloroplast bag

Pressurereducer

CO2 Storage

Carbonation

Water pump

Reverse osmosisfilter

Return to cabin

Potablewater

Cabin air

Starch outflow

O2 release to cabinthrough semi-permeable membrane

Sunlight

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

Protein and free amino acids

Asparaginic acid 6.66%Threonine* 3.20%Serine 3.28%glutamic acid 9.18%Glycine 3.17%Alanine 5.53%Cystein 0.45%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

vitamins per 100 grams

thiamine - B 13.0 mgriboflavin - B 211.9 mgNiacin 68.0 mgB6 2.3 mgfolic acid 3.1 mgca‑pantothenate 30.0 mgBiotin 0.25 mg

minerals per 100 grams

Calcium 120 mgMagnesium 200 mgPotassium 3.3 gSodium < 0.5 gIron 5 mgPhosphorus 1.8 g

Yeast extracts contain many nutrients; autolysate of Saccharomyces cerevisiae:

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• 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.html

• 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