Photosynthetic efficiency
Encyclopedia
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
where CH2O represents carbohydrates such as sugar
s, 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
.
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 by D.O.Hall & K.K.Rao says that photosynthesis increases linearly up to about 10,000 lux or ~100 watts/square meter before beginning to exhibit saturation effects. 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. 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.
, 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 were released from fossil fuels worldwide in 2008.
, the total photosynthetic efficiency of the planet is 0.84% to 1.26%.
, soybean
, castor oil
, sunflower
oil, safflower
oil, corn ethanol
, and sugar cane ethanol.
An analysis of a proposed Hawaiian oil palm plantation claimed to yield 600 gallons of biodiesel per acre per year. That adds to 2835 watts per acre or 0.7 W/m2. Typical insolation
in Hawaii is closer to 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 leave
s and stem
tips, which are left in the fields during harvest, and 35% are in the fibrous material (bagasse
) left over from pressing.
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 OO2 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.
Chemical energy
Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction or, to transform other chemical substances...
during photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
in plants and algae. Photosynthesis can be described by the simplified chemical reaction
- H2O + CO2 + energy → CH2O + O2,
where CH2O represents carbohydrates such as sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s, cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, and lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
.
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
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
). 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
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
, 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
Non-photochemical quenching
Non-photochemical quenching is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls via enhanced internal conversion to the ground state , thus harmlessly dissipating...
), or emitted as chlorophyll fluorescence
Chlorophyll fluorescence
Chlorophyll fluorescence is light that has been re-emitted after being absorbed by chlorophyll molecules of plant leaves. By measuring the intensity and nature of this fluorescence, plant ecophysiology can be investigated....
.
Plants
Quoted values sunlight-to-biomass efficiency| Plant | Efficiency |
|---|---|
| Plants, typical | 0.1% 0.2–2% |
| Typical crop Crop Crop may refer to:* Crop, a plant grown and harvested for agricultural use* Crop , part of the alimentary tract of some animals* Crop , a modified whip used in horseback riding or disciplining humans... plants |
1–2% |
| Sugarcane | 7–8% peak |
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
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
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
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
, etc.). Sugar cane is exceptional in several ways to yield peak storage efficiencies of ~8%.
Photosynthesis by D.O.Hall & K.K.Rao says that photosynthesis increases linearly up to about 10,000 lux or ~100 watts/square meter before beginning to exhibit saturation effects. 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. 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
Compensation point
The compensation point is the amount of light intensity on the light curve where the rate of photosynthesis exactly matches the rate of respiration...
the plant assimilates more carbon and releases more oxygen by photosynthesis than it consumes by cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
for its own current energy demand.
Algae and other monocellular organisms
From a 2010 study by the University of MarylandUniversity of Maryland
When the term "University of Maryland" is used without any qualification, it generally refers to the University of Maryland, College Park.University of Maryland may refer to the following:...
, photosynthesizing Cyanobacteria have been shown to be a significant species in the global carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
, 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 were 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 TJ of solar energy hitting the Earth's surface, see also Earth's energy budgetEarth's energy budget
The Earth can be considered as a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. The planet is approximately in equilibrium, so the sum of the gains is approximately equal to the sum of the losses.Note on accompanying images:...
, the total photosynthetic efficiency of the planet is 0.84% to 1.26%.
Efficiencies of various biofuel crops
Popular choices for plant biofuels include: oil palmOil palm
The oil palms comprise two species of the Arecaceae, or palm family. They are used in commercial agriculture in the production of palm oil. The African Oil Palm Elaeis guineensis is native to West Africa, occurring between Angola and Gambia, while the American Oil Palm Elaeis oleifera is native to...
, soybean
Soybean
The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...
, castor oil
Castor oil
Castor oil is a vegetable oil obtained from the castor bean . Castor oil is a colorless to very pale yellow liquid with mild or no odor or taste. Its boiling point is and its density is 961 kg/m3...
, sunflower
Sunflower
Sunflower is an annual plant native to the Americas. It possesses a large inflorescence . The sunflower got its name from its huge, fiery blooms, whose shape and image is often used to depict the sun. The sunflower has a rough, hairy stem, broad, coarsely toothed, rough leaves and circular heads...
oil, safflower
Safflower
Safflower is a highly branched, herbaceous, thistle-like annual. It is commercially cultivated for vegetable oil extracted from the seeds. Plants are 30 to 150 cm tall with globular flower heads having yellow, orange or red flowers. Each branch will usually have from one to five flower heads...
oil, corn ethanol
Corn ethanol
Corn ethanol is ethanol produced from corn as a biomass through industrial fermentation, chemical processing and distillation. Corn is the main feedstock used for producing ethanol fuel in the United States and it is mainly used as an oxygenate to gasoline in the form of low-level blends, and to a...
, and sugar cane ethanol.
An analysis of a proposed Hawaiian oil palm plantation claimed to yield 600 gallons of biodiesel per acre per year. That adds to 2835 watts per acre or 0.7 W/m2. Typical insolation
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
in Hawaii is closer to 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
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
, etc.), sugar cane is exceptional in several ways to yield peak storage efficiencies of ~8%.
Ethanol fuel in Brazil
Ethanol fuel in Brazil
Brazil is the world's second largest producer of ethanol fuel and the world's largest exporter. Together, Brazil and the United States lead the industrial production of ethanol fuel, accounting together for 87.8% of the world's production in 2010. In 2010 Brazil produced 26.2 billion litres 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
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
accounts for little more than 30% of the chemical energy stored in the mature plant; 35% is in the leave
Leave
Leave may refer to:* Leave , a 2002 Mandopop album* "Leave ", a 2004 R&B song* "Leave" , a 2000 pop rock song...
s and stem
Stem
-Science:* Plant stem, the above ground structures that have vascular tissue and that support leaves and flowers Also see similar:**Stipe **Stipe * Stem cell...
tips, which are left in the fields during harvest, and 35% are in the fibrous material (bagasse
Bagasse
Bagasse is the fibrous matter that remains after sugarcane or sorghum stalks are crushed to extract their juice. It is currently used as a biofuel and as a renewable resource in the manufacture of pulp and paper products and building materials....
) left over from pressing.
C3 vs. C4 and CAM plants
C3 plantsC3 carbon fixation
carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate through the following reaction:...
use the Calvin cycle
Calvin cycle
The Calvin cycle or Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle or C3 cycle or CBB cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms...
to fix carbon. C4 plants
C4 carbon fixation
C4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...
use a modified Calvin cycle in which they separate Ribulose-1,5-bisphosphate carboxylase oxygenase
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
(RuBisCO) from atmospheric oxygen, fixing carbon in their mesophyll
Mesophyll
Mesophyll can refer to:* Mesophyll tissue, in plant anatomy, photosynthetic parenchyma cells that lie between the upper and lower epidermis layers of a leaf...
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
Crassulacean acid metabolism
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. The stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide...
(CAM), time isolates functioning RuBisCo
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
(and the other Calvin cycle enzymes) from high oxygen concentrations produced by photosynthesis, in that OO2 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
Pineapple
Pineapple is the common name for a tropical plant and its edible fruit, which is actually a multiple fruit consisting of coalesced berries. It was given the name pineapple due to its resemblance to a pine cone. The pineapple is by far the most economically important plant in the Bromeliaceae...
is a CAM plant.
See also
- Photosynthetically active radiationPhotosynthetically active radiationPhotosynthetically active radiation, often abbreviated 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...