photosynthesis: NATURAL AND ARTIFICIAL
what --- why --- how --- development
natural Photosynthesis
In a picture:
In words:
In plants and algae, photosynthesis takes place in organelles called chloroplasts. The multi-layered membrane around each chloroplast contains a fluid that surrounds stacks of disc-shaped thylakoids, which are the site of photosynthesis. The membranes that surround the thylakoids contain chlorophyll and other pigments used in photosynthesis.
Cell --> Chloroplast membrane --> Thylakoid membrane (chlorophyll, carotenes, xanthophylls, etc.)
First half: Light Reaction
2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2
The photolysis of water results in adenosine triphosphate (ATP, used as the energy source in cellular respiration), oxygen (as waste), and hydrogen ions and nicotinamade adenine dinucleotide triphosphate+H (NADPH), which are both used in the second half of the photosynthetic reaction.
Light Reaction
Second Half: Calvin Cycle ("Dark Reaction"):
3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
Carbon dioxide, NADPH, ATP, and hydrogen are synthesized into sugar (most importantly), and water (as waste).
Dark Reaction
Photosynthesis as a whole
CATALYSTS
Both the light and dark reactions require catalysts: in plants, magnesium, manganese, and calcium are used .
Without a catalyst, the part of the light reaction that oxidizes water is very endothermic, requiring high temperatures (at least 2500 K).
In the dark reaction, enzymes called hydrogenases act as proton-to-hydrogen converting catalysts.
Currently this is where a large amount of research energy is going - developing artificial hydrogenases for the production of hydrogen fuel.
ON THAT NOTE....
ARTIFICIAL PHOTOSYNTHESIS
Why? To produce clean (solar) energy that:
- avoids the land-use issues of biofuels and solar PV...
- addresses the food equity issues brought on by increased food prices
- avoids the inefficiencies of photovoltaic processes resulting from a) the indirectness of the energy generation process and b) the inconsistency of sunshine over time
(and as we've discussed, we currently lack an efficient method of storing the energy produced this way).
Artificial photosynthesis focuses on:
1) Inorganic systems that undergo photocatalytic water splitting, a process that converts water into protons (eventually yielding hydrogen fuel) and oxygen.
2) Organic systems that perform light-driven carbon dioxide reduction (carbon fixation, or the processes plants use to produce sugars). Performed by living microalgae to produce combustible carbohydrates.
challenges:
1) Separating the parts of photolysis that produce hydrogen and oxygen, respectively. It is less complex and cheaper to allow them to share generation space, but it creates a potentially explosive mixture of gases.
2) To develop a biomimetic enzyme to catalyze the carbon fixation reaction (photocatalyst). The enzymes used by plants for this process are not fully understood.
Nocera's group developed one that uses cobalt+phosphate, that he says is cheap to manufact- ure, self-heals, and can be used with fresh, salt, or waste water.
- Current research is mostly happening in innovation hubs: a federal clean-tech investment strategy
- Less controversial than the model that funded efforts like the failed Solyndra deal (grants/loan guarantees)
Current Research:
- There is a new artificial photosynthesis innovation hub - the Joint Center for Artificial Photo- synthesis (JCAP)
- at Caltech, with researchers at Lawrence Berkeley National Lab and more than 20 other research centers
- brings together a large number of experts in different areas, including catalysis, optics, and membrane technology
- should receive $122 million over five years.
JCAP's theoretical goal:
Catalysts attached to dense wires in the upper membrane would use sunlight to split water into hydrogen and oxygen. More sunlight-triggered catalysts in the middle membrane would use carbon dioxide to turn the hydrogen into liquid fuel ideal for transportation utility. The bottom layer would wick the fuel away into storage.
Accomplishments:
- JCAP has developed an ink-jet printing process that can churn out millions of slightly different variations of promising catalysts. Each sample is as small as a pixel on a screen.
- And have built two prototype systems that can produce fuel from sunlight—though not yet economically. The plan is to have at least four or five different versions of the devices, each with different strengths and weaknesses. They want multiple versions because they can’t predict where the next materials advance will be.
artificial photosynthesis
By erikahan
