The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in plants and algae. Photosynthesis can be described by the simplified chemical reaction
- H2O + CO2 + energy → C6H12O6+ O2
where C6H12O6 represents carbohydrates such as sugars, cellulose, and lignin. The value of the photosynthetic efficiency is dependent on how light energy is defined. On a molecular level, the theoretical limit in efficiency is 25% for photosynthetically active radiation at the Chlorophyll-a absorbance peak (wavelength of 680 nanometers). However, photosynthesis is now known to occur up to 720 nm wavelengths (see Chlorophyll). For actual sunlight, where only 45% of the light is photosynthetically active, 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 an overall photosynthetic efficiency of 3 to 6% of total solar radiation. 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 
Quoted values sunlight-to-biomass efficiency
|Typical crop plants||1–2%|
The following is a breakdown of the energetics of the photosynthesis process from Photosynthesis by Hall and Rao:
Starting with the solar spectrum falling on a leaf,
47% lost due to photons outside the 400–700 nm active range (chlorophyll utilizes photons between 400 and 700 nm, extracting the energy of one 700 nm photon from each one)
30% of the in-band photons are lost due to incomplete absorption or photons hitting components other than chloroplasts
24% of the absorbed photon energy is lost due to degrading short wavelength photons to the 700 nm energy level
68% of the utilized energy is lost in conversion into d-glucose
35–45% of the glucose is consumed by the leaf in the processes of dark and photo respiration
Stated another way:
100% sunlight → non-bioavailable photons waste is 47%, leaving
53% (in the 400–700 nm range) → 30% of photons are lost due to incomplete absorption, leaving
37% (absorbed photon energy) → 24% is lost due to wavelength-mismatch degradation to 700 nm energy, leaving
28.2% (sunlight energy collected by chlorophyl) → 32% efficient conversion of ATP and NADPH to d-glucose, leaving
9% (collected as sugar) → 35–40% of sugar is recycled/consumed by the leaf in dark and photo-respiration, leaving
5.4% net leaf efficiency.
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.). Sugar cane is exceptional in several ways to yield peak storage efficiencies of ~8%.
Photosynthesis increases linearly with light intensity at low intensity, but at higher intensity this is no longer the case. Above about 10,000 lux or ~100 watts/square meter the rate no longer increases. Thus, most plants can only utilize ~10% of full mid-day sunlight intensity. This dramatically reduces average achieved photosynthetic efficiency in fields compared to peak laboratory results. However, real plants (as opposed to laboratory test samples) have lots of 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. In addition, 25 Gigatons of carbon are sequestered annually from cyanobacteria alone; more than the 8.67 gigatons of carbon released from fossil fuels worldwide in 2008.
Worldwide figures 
According to the cyanobacteria study above, this means the total photosynthetic productivity of earth is between ~1500–2250 TW, or 47,300–71,000 exajoules per year. Using this source's figure of 178,000 TW of solar energy hitting the Earth's surface, the total photosynthetic efficiency of the planet is 0.84% to 1.26% (see also Earth's energy budget).
Efficiencies of various biofuel crops 
An analysis[original research?] of a proposed Hawaiian oil palm plantation claimed to yield 600 gallons of biodiesel per acre per year. That comes to 2835 watts per acre or 0.7 W/m2. Typical insolation in Hawaii is around 5.5 kWh/(m2day) or 230 watts. For this particular oil palm plantation, if it delivered the claimed 600 gallons of biodiesel per acre per year, would be converting 0.3% of the incident solar energy to chemical fuel. Total photosynthetic efficiency would include more than just the biodiesel oil, so this 0.3% number is something of a lower bound.
Contrast this with a typical photo-voltaic installation, which would produce an average of roughly 22 W/m2 (roughly 10% of the average insolation), throughout the year. Most crop plants store ~0.25% to 0.5% of the sunlight in the product (corn kernels, potato starch, etc.), sugar cane is exceptional in several ways to yield peak storage efficiencies of ~8%.
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 sugar cane 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.
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 for the synthesis of one molecule of glucose while the C4 pathway requires 30 ATP. C4 is an evolutionary advancement over the simpler C3 cycle which operates in most plants. Corn, 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.
- Renewable biological systems for alternative sustainable energy production. FAO Agricultural Services Bulletins (1997).
- Govindjee, What is photosynthesis?
- The Green Solar Collector; converting sunlight into algal biomass Wageningen University project (2005—2008)
- Light Absorption for Photosynthesis Rod Nave, HyperPhysics project, Georgia State University
- David Oakley Hall; K. K. Rao; Institute of Biology (1999). Photosynthesis. Cambridge University Press. ISBN 978-0-521-64497-6. Retrieved 3 November 2011.
- 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.
- Global carbon budget 2009. env.uea.ac.uk
- Biodiesel Fuel. Ag.ndsu.edu. Retrieved on 2011-11-03.
- PVWATTS: Hawaii. Rredc.nrel.gov. Retrieved on 2011-11-03.
- NREL: In My Backyard (IMBY) Home Page. Nrel.gov (2010-12-23). Retrieved on 2011-11-03.
See also