Photosynthetic efficiency

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The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in green plants and algae. Photosynthesis can be described by the simplified chemical reaction

Contents

6 H2O + 6 CO2 + energy → C6H12O6 + 6 O2

where C6H12O6 is glucose (which is subsequently transformed into other sugars, starches, cellulose, lignin, and so forth). The value of the photosynthetic efficiency is dependent on how light energy is defined – it depends on whether we count only the light that is absorbed, and on what kind of light is used (see Photosynthetically active radiation). It takes eight (or perhaps ten or more [1] ) photons to use one molecule of CO2. The Gibbs free energy for converting a mole of CO2 to glucose is 114 kcal, whereas eight moles of photons of wavelength 600 nm contains 381 kcal, giving a nominal efficiency of 30%. [2] However, photosynthesis can occur with light up to wavelength 720 nm so long as there is also light at wavelengths below 680 nm to keep Photosystem II operating (see Chlorophyll). Using longer wavelengths means less light energy is needed for the same number of photons and therefore for the same amount of photosynthesis. For actual sunlight, where only 45% of the light is in the photosynthetically active wavelength range, the theoretical maximum efficiency of solar energy conversion is approximately 11%. In actuality, however, plants do not absorb all incoming sunlight (due to reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels) and do not convert all harvested energy into biomass, which results in a maximum overall photosynthetic efficiency of 3 to 6% of total solar radiation. [1] If photosynthesis is inefficient, excess light energy must be dissipated to avoid damaging the photosynthetic apparatus. Energy can be dissipated as heat (non-photochemical quenching), or emitted as chlorophyll fluorescence.

Typical efficiencies

Plants

Quoted values sunlight-to-biomass efficiency

PlantEfficiency
Plants, typical>0.1% [3]

0.2–2% [4]
<1% [5]

Typical crop plants1–2% [3]
C3 plants, peak3.5% [5]
C4 plants, peak4.3% [5]

The following is a breakdown of the energetics of the photosynthesis process from Photosynthesis by Hall and Rao: [6]

Starting with the solar spectrum falling on a leaf,

Stated another way:

Many plants lose much of the remaining energy on growing roots. Most crop plants store ~0.25% to 0.5% of the sunlight in the product (corn kernels, potato starch, etc.).

Photosynthesis increases linearly with light intensity at low intensity, but at higher intensity this is no longer the case (see Photosynthesis-irradiance curve). Above about 10,000 lux or ~100 watts/square meter the rate no longer increases. Thus, most plants can only use ~10% of full mid-day sunlight intensity. [6] This dramatically reduces average achieved photosynthetic efficiency in fields compared to peak laboratory results. However, real plants (as opposed to laboratory test samples) have many redundant, randomly oriented leaves. This helps to keep the average illumination of each leaf well below the mid-day peak enabling the plant to achieve a result closer to the expected laboratory test results using limited illumination.

Only if the light intensity is above a plant specific value, called the compensation point the plant assimilates more carbon and releases more oxygen by photosynthesis than it consumes by cellular respiration for its own current energy demand.
Photosynthesis measurement systems are not designed to directly measure the amount of light absorbed by the leaf. Nevertheless, the light response curves that the class produces do allow comparisons in photosynthetic efficiency between plants.

Algae and other monocellular organisms

From a 2010 study by the University of Maryland, photosynthesizing cyanobacteria have been shown to be a significant species in the global carbon cycle, accounting for 20–30% of Earth's photosynthetic productivity and convert solar energy into biomass-stored chemical energy at the rate of ~450 TW. [7] Some pigments such as B-phycoerythrin that are mostly found in red algae and cyanobacteria has much higher light-harvesting efficiency compared to that of other plants. Such organisms are potentially candidates for biomimicry technology to improve solar panels design. [8]

Efficiencies of various biofuel crops

Popular choices for plant biofuels include: oil palm, soybean, castor oil, sunflower oil, safflower oil, corn ethanol, and sugar cane ethanol.

A 2008 Hawaiian oil palm plantation projection stated: "algae could yield from 5,000-10,000 gallons of oil per acre yearly, compared to 250-350 gallons for jatropha and 600-800 gallons for palm oil". That comes to 26 kW per acre or 7 W/m2. [9] Typical insolation in Hawaii is around 230 W/m2. [10] , so converting 3% of the incident solar energy to chemical fuel. Total photosynthetic efficiency would include more than just the biodiesel oil, so this number is a lower bound.

Contrast this with a typical photovoltaic installation, [11] which would produce an average of roughly 22 W/m2 (roughly 10% of the average insolation), throughout the year. Furthermore, the photovoltaic panels would produce electricity, which is a high-quality form of energy, whereas converting the biodiesel into mechanical energy entails the loss of a large portion of the energy. On the other hand, a liquid fuel is much more convenient for a vehicle than electricity, which has to be stored in heavy, expensive batteries.

Most crop plants store ~0.25% to 0.5% of the sunlight in the product (corn kernels, potato starch, etc.) Ethanol fuel in Brazil has a calculation that results in: "Per hectare per year, the biomass produced corresponds to 0.27 TJ. This is equivalent to 0.86 W/m2. Assuming an average insolation of 225 W/m2, the photosynthetic efficiency of sugarcane is 0.38%." Sucrose accounts for little more than 30% of the chemical energy stored in the mature plant; 35% is in the leaves and stem tips, which are left in the fields during harvest, and 35% are in the fibrous material (bagasse) left over from pressing. [12] [13]

C3 vs. C4 and CAM plants

C3 plants use the Calvin cycle to fix carbon. C4 plants use a modified Calvin cycle in which they separate Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) from atmospheric oxygen, fixing carbon in their mesophyll cells and using oxaloacetate and malate to ferry the fixed carbon to RuBisCO and the rest of the Calvin cycle enzymes isolated in the bundle-sheath cells. The intermediate compounds both contain four carbon atoms, which gives C4. In Crassulacean acid metabolism (CAM), time isolates functioning RuBisCO (and the other Calvin cycle enzymes) from high oxygen concentrations produced by photosynthesis, in that O2 is evolved during the day, and allowed to dissipate then, while at night atmospheric CO2 is taken up and stored as malic or other acids. During the day, CAM plants close stomata and use stored acids as carbon sources for sugar, etc. production.

The C3 pathway requires 18 ATP and 12 NADPH for the synthesis of one molecule of glucose (3 ATP + 2 NADPH per CO2 fixed) while the C4 pathway requires 30 ATP and 12 NADPH (C3 + 12 ATP per CO2 fixed). In addition, we can take into account that each NADPH is equivalent to 3 ATP, that means both pathways require 36 additional (equivalent of) ATP [14] [better citation needed]. Despite this reduced ATP efficiency, C4 is an evolutionary advancement, adapted to areas of high levels of light, where the reduced ATP efficiency is more than offset by the use of increased light. The ability to thrive despite restricted water availability maximizes the ability to use available light. The simpler C3 cycle which operates in most plants is adapted to wetter darker environments, such as many northern latitudes.[ citation needed ] Maize, sugar cane, and sorghum are C4 plants. These plants are economically important in part because of their relatively high photosynthetic efficiencies compared to many other crops. Pineapple is a CAM plant.

Research

Photorespiration

One efficiency-focused research topic is improving the efficiency of photorespiration. Around 25% of the time RuBisCO incorrectly collects oxygen molecules instead of CO
2, creating CO
2 and ammonia that disrupt the photosynthesis process. Plants remove these byproducts via photorespiration, requiring energy and nutrients that would otherwise increase photosynthetic output. In C3 plants photorespiration can consume 20-50% of photosynthetic energy. [15]

Engineered tobacco

The research shortened photosynthetic pathways in tobacco. Engineered crops grew taller and faster, yielding up to 40% more biomass. The study employed synthetic biology to construct new metabolic pathways and assessed their efficiency with and without transporter RNAi. The most efficient pathway increased light-use efficiency by 17%. [15]

Chloroplast biogenesis

Research is being done into RCB and NCP, two non-catalytic thioredoxin-like proteins that activate chloroplast transcription. [16] Knowing the exact mechanism can be useful to allow increasing photosynthesis (i.e. through genetic modification). [17]

Ecosystem research on photosynthetic efficiency

Photosynthesis is the only process that allows the conversion of atmospheric carbon (CO2) to organic (solid) carbon, and this process plays an essential role in climate models. This lead researchers to study the sun-induced chlorophyll fluorescence (i.e., chlorophyll fluorescence that uses the Sun as illumination source; the glow of a plant) as an indicator of photosynthetic efficiency of a region. This is interesting for scientists since its shows them things like the CO2 absorption of a forests, or the productivity of an agricultural region. The FLEX (satellite) is the upcoming satellite program by the European Space Agency designated to this type of measurements.


See also

Related Research Articles

<span class="mw-page-title-main">Chlorophyll</span> Green pigments found in plants, algae and bacteria

Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words χλωρός, khloros and φύλλον, phyllon ("leaf"). Chlorophyll allow plants to absorb energy from light.

<span class="mw-page-title-main">Photosynthesis</span> Biological process to convert light into chemical energy

Photosynthesis is a biological process used by many cellular organisms to convert light energy into chemical energy, which is stored in organic compounds that can later be metabolized through cellular respiration to fuel the organism's activities. The term usually refers to oxygenic photosynthesis, where oxygen is produced as a byproduct and some of the chemical energy produced is stored in carbohydrate molecules such as sugars, starch, glycogen and cellulose, which are synthesized from endergonic reaction of carbon dioxide with water. Most plants, algae and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the biological energy necessary for complex life on Earth.

C<sub>4</sub> carbon fixation Photosynthetic process in some plants

C4 carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960s discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when supplied with 14CO2, incorporate the 14C label into four-carbon molecules first.

<span class="mw-page-title-main">Photorespiration</span> Process in plant metabolism

Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process lowers the efficiency of photosynthesis, potentially lowering photosynthetic output by 25% in C3 plants. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

C<sub>3</sub> carbon fixation Series of interconnected biochemical reactions

C3 carbon fixation is the most common of three metabolic pathways for carbon fixation in photosynthesis, the other two being C4 and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosphoglycerate through the following reaction:

Photobiology is the scientific study of the beneficial and harmful interactions of light in living organisms. The field includes the study of photophysics, photochemistry, photosynthesis, photomorphogenesis, visual processing, circadian rhythms, photomovement, bioluminescence, and ultraviolet radiation effects.

<span class="mw-page-title-main">Calvin cycle</span> Light-independent reactions in photosynthesis

The Calvin cycle,light-independent reactions, bio synthetic phase,dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria. In plants, these reactions occur in the stroma, the fluid-filled region of a chloroplast outside the thylakoid membranes. These reactions take the products of light-dependent reactions and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and reducing power of NADPH from the light dependent reactions to produce sugars for the plant to use. These substrates are used in a series of reduction-oxidation (redox) reactions to produce sugars in a step-wise process; there is no direct reaction that converts several molecules of CO2 to a sugar. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

<span class="mw-page-title-main">Photophosphorylation</span> Biochemical process in photosynthesis

In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of energy available to living organisms, which is sunlight. All organisms produce a phosphate compound, ATP, which is the universal energy currency of life. In photophosphorylation, light energy is used to pump protons across a biological membrane, mediated by flow of electrons through an electron transport chain. This stores energy in a proton gradient. As the protons flow back through an enzyme called ATP synthase, ATP is generated from ADP and inorganic phosphate. ATP is essential in the Calvin cycle to assist in the synthesis of carbohydrates from carbon dioxide and NADPH.

<span class="mw-page-title-main">Photosynthetically active radiation</span> Range of light usable for photosynthesis

Photosynthetically active radiation (PAR) designates the spectral range of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place.

Theoretical production ecology tries to quantitatively study the growth of crops. The plant is treated as a kind of biological factory, which processes light, carbon dioxide, water, and nutrients into harvestable parts. Main parameters kept into consideration are temperature, sunlight, standing crop biomass, plant production distribution, nutrient and water supply.

The light compensation point (Ic) is the light intensity on the light curve where the rate of photosynthesis exactly matches the rate of cellular respiration. At this point, the uptake of CO2 through photosynthetic pathways is equal to the respiratory release of carbon dioxide, and the uptake of O2 by respiration is equal to the photosynthetic release of oxygen. The concept of compensation points in general may be applied to other photosynthetic variables, the most important being that of CO2 concentration – CO2 compensation point (Γ).Interval of time in day time when light intensity is low due to which net gaseous exchange is zero is called as compensation point.

Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule.

<span class="mw-page-title-main">Photosynthetic reaction centre</span> Molecular unit responsible for absorbing light in photosynthesis

A photosynthetic reaction center is a complex of several proteins, pigments, and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along the path of a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and pheophytin, as well as quinones. The energy of the photon is used to excite an electron of a pigment. The free energy created is then used, via a chain of nearby electron acceptors, for a transfer of hydrogen atoms (as protons and electrons) from H2O or hydrogen sulfide towards carbon dioxide, eventually producing glucose. These electron transfer steps ultimately result in the conversion of the energy of photons to chemical energy.

<span class="mw-page-title-main">Phosphoenolpyruvate carboxylase</span> Class of enzymes

Phosphoenolpyruvate carboxylase (also known as PEP carboxylase, PEPCase, or PEPC; EC 4.1.1.31, PDB ID: 3ZGE) is an enzyme in the family of carboxy-lyases found in plants and some bacteria that catalyzes the addition of bicarbonate (HCO3−) to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate and inorganic phosphate:

<span class="mw-page-title-main">Light-dependent reactions</span> Photosynthetic reactions

Light-dependent reactions refers to certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).

<span class="mw-page-title-main">Autotroph</span> Organism type

An autotroph is an organism that produces complex organic compounds using carbon from simple substances such as carbon dioxide, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis). They convert an abiotic source of energy into energy stored in organic compounds, which can be used by other organisms. Autotrophs do not need a living source of carbon or energy and are the producers in a food chain, such as plants on land or algae in water. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and as stored chemical fuel. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide.

<span class="mw-page-title-main">Chlorophyll fluorescence</span> Light re-emitted by chlorophyll molecules during return from excited to non-excited states

Chlorophyll fluorescence is light re-emitted by chlorophyll molecules during return from excited to non-excited states. It is used as an indicator of photosynthetic energy conversion in plants, algae and bacteria. Excited chlorophyll dissipates the absorbed light energy by driving photosynthesis, as heat in non-photochemical quenching or by emission as fluorescence radiation. As these processes are complementary processes, the analysis of chlorophyll fluorescence is an important tool in plant research with a wide spectrum of applications.

<span class="mw-page-title-main">Plant stress measurement</span>

Plant stress measurement is the quantification of environmental effects on plant health. When plants are subjected to less than ideal growing conditions, they are considered to be under stress. Stress factors can affect growth, survival and crop yields. Plant stress research looks at the response of plants to limitations and excesses of the main abiotic factors, and of other stress factors that are important in particular situations. Plant stress measurement usually focuses on taking measurements from living plants. It can involve visual assessments of plant vitality, however, more recently the focus has moved to the use of instruments and protocols that reveal the response of particular processes within the plant

The evolution of photosynthesis refers to the origin and subsequent evolution of photosynthesis, the process by which light energy is used to assemble sugars from carbon dioxide and a hydrogen and electron source such as water. The process of photosynthesis was discovered by Jan Ingenhousz, a Dutch-born British physician and scientist, first publishing about it in 1779.

<span class="mw-page-title-main">Fractionation of carbon isotopes in oxygenic photosynthesis</span>

Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.

References

  1. 1 2 Renewable biological systems for unsustainable energy production. FAO Agricultural Services Bulletins (1997).
  2. ↑ Stryer, Lubert (1981). Biochemistry (2nd ed.). W. H. Freeman. p.  448. ISBN   978-0-7167-1226-8.
  3. 1 2 Govindjee, What is photosynthesis?
  4. ↑ The Green Solar Collector; converting sunlight into algal biomass Wageningen University project (2005—2008)
  5. 1 2 3 Blankenship, Robert E.; Tiede, David M.; Barber, James; Brudvig, Gary W.; Fleming, Graham; Ghirardi, Maria; Gunner, M. R.; Junge, Wolfgang; Kramer, David M. (2011-05-13). "Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement". Science. 332 (6031): 805–809. doi:10.1126/science.1200165. ISSN   0036-8075. PMID   21566184. S2CID   22798697.
  6. 1 2 David Oakley Hall; K. K. Rao; Institute of Biology (1999). Photosynthesis. Cambridge University Press. ISBN   978-0-521-64497-6 . Retrieved 3 November 2011.
  7. ↑ Pisciotta JM, Zou Y, Baskakov IV (2010). "Light-Dependent Electrogenic Activity of Cyanobacteria". PLOS ONE. 5 (5): e10821. doi: 10.1371/journal.pone.0010821 . PMC   2876029 . PMID   20520829.
  8. ↑ "Ingenious 'control panel' in algae provides blueprint for super-efficient future solar cells - News - Utrecht University". www.uu.nl. May 9, 2019.
  9. ↑ Rob Parsons (September 11, 2008). "Slime for change". MauiTime.
  10. ↑ PVWATTS: Hawaii Archived 2015-04-20 at the Wayback Machine . Rredc.nrel.gov. Retrieved on 2011-11-03.
  11. ↑ NREL: In My Backyard (IMBY) Home Page. Nrel.gov (2010-12-23). Retrieved on 2011-11-03.
  12. ↑ Hassan AB, kutigi GI and Tanko OO (2018). "Production of BioEthanol and Bio-Based Materials From Sugar Cane Bagasse". Zaria Journal of Educational Studies. (ZAJES). 19.
  13. ↑ da Rosa, A (2005). Fundamentals of Renewable Energy Processes. Elsevier. pp. 501–502. ISBN   978-0-12-088510-7.
  14. ↑ "Biology –C4 Cycle - askIITians". www.askiitians.com.
  15. 1 2 South, Paul F.; Cavanagh, Amanda P.; Liu, Helen W.; Ort, Donald R. (2019-01-04). "Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field". Science. 363 (6422): eaat9077. doi: 10.1126/science.aat9077 . ISSN   0036-8075. PMC   7745124 . PMID   30606819.
  16. ↑ Yang, Emily J.; Yoo, Chan Yul; Liu, Jiangxin; Wang, He; Cao, Jun; Li, Fay-Wei; Pryer, Kathleen M.; Sun, Tai-Ping; Weigel, Detlef; Zhou, Pei; Chen, Meng (2019). "NCP activates chloroplast transcription by controlling phytochrome-dependent dual nuclear and plastidial switches". Nature Communications. 10 (1): 2630. doi:10.1038/s41467-019-10517-1. PMC   6570768 . PMID   31201314.
  17. ↑ New Scientist, September 2019
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