The photosynthetic efficiency is the fraction of light energy converted into
chemical energy
Chemical energy is the energy of chemical substances that is released when they undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (2018). "How ...
during
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
in green plants and algae. Photosynthesis can be described by the simplified chemical reaction
:6 H
2O + 6 CO
2 + energy → C
6H
12O
6 + 6 O
2
where C
6H
12O
6 is
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
(which is subsequently transformed into other
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
s,
starches,
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
,
lignin
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...
, 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 Photosynthetically active radiation (PAR) designates the spectral range (wave band) 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 o ...
). It takes eight (or perhaps ten or more
[Renewable biological systems for unsustainable energy production](_blank)
FAO Agricultural Services Bulletins (1997).) photons to use one molecule of CO
2. The
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...
for converting a mole of CO
2 to glucose is 114
kcal
The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of o ...
, whereas eight moles of photons of wavelength 600
nm contains 381 kcal, giving a nominal efficiency of 30%. 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
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
operating (see
Chlorophyll
Chlorophyll (also chlorophyl) 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 , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
). 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
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
, which results in a maximum 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 (NPQ) 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 (Chl) via enhanced internal con ...
), or emitted as
chlorophyll fluorescence
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 ...
.
Typical efficiencies
Plants
Quoted values sunlight-to-biomass efficiency
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 uses 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 used energy is lost in conversion into d-
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
* 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 chlorophyll) → 68% is lost in 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 or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
, etc.).
Photosynthesis increases linearly with light intensity at low intensity, but at higher intensity this is no longer the case (see
Photosynthesis-irradiance curve
The PI (or photosynthesis-irradiance) curve is a graphical representation of the empirical relationship between solar irradiance and photosynthesis. A derivation of the Michaelis–Menten curve, it shows the generally positive correlation betwee ...
). 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.
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 light compensation point (''I''c) 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 re ...
the plant assimilates more carbon and releases more oxygen by photosynthesis than it consumes by
cellular respiration
Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
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
The University of Maryland, College Park (University of Maryland, UMD, or simply Maryland) is a public land-grant research university in College Park, Maryland. Founded in 1856, UMD is the flagship institution of the University System of M ...
, photosynthesizing
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
have been shown to be a significant species in the global
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
, accounting for 20–30% of Earth's photosynthetic productivity and convert solar energy into biomass-stored chemical energy at the rate of ~450 TW.
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
Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from grc, βίος (''bios''), life, and μίμησ ...
technology to improve
solar panels
A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a phot ...
design.
Efficiencies of various biofuel crops
Popular choices for plant
biofuel
Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (E ...
s include:
oil palm
''Elaeis'' () is a genus of palms containing two species, called oil palms. They are used in commercial agriculture in the production of palm oil. The African oil palm ''Elaeis guineensis'' (the species name ''guineensis'' referring to its co ...
,
soybean
The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses.
Traditional unfermented food uses of soybeans include soy milk, from which tofu an ...
,
castor oil
Castor oil is a vegetable oil pressed from castor beans.
It is a colourless or pale yellow liquid with a distinct taste and odor. Its boiling point is and its density is 0.961 g/cm3. It includes a mixture of triglycerides in which about ...
,
sunflower
The common sunflower (''Helianthus annuus'') is a large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), as ...
oil,
safflower
Safflower (''Carthamus tinctorius'') is a highly branched, herbaceous, thistle-like annual plant in the family Asteraceae. It is commercially cultivated for vegetable oil extracted from the seeds and was used by the early Spanish colonies along ...
oil,
corn ethanol, and
sugar cane
Sugarcane or sugar cane is a species of (often hybrid) tall, perennial grass (in the genus ''Saccharum'', tribe Andropogoneae) that is used for sugar production. The plants are 2–6 m (6–20 ft) tall with stout, jointed, fibrous stalks t ...
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/m
2. Typical
insolation in Hawaii is around 230 W/m
2., so converting 3% of the incident solar energy to chemical fuel. Total photosynthetic efficiency would include more than just the
biodiesel
Biodiesel is a form of diesel fuel derived from plants or animals and consisting of long-chain fatty acid esters. It is typically made by chemically reacting lipids such as animal fat (tallow), soybean oil, or some other vegetable oil with ...
oil, so this number is a lower bound.
Contrast this with a typical
photovoltaic
Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially us ...
installation, which would produce an average of roughly 22 W/m
2 (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
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
, etc.)
Ethanol fuel in Brazil
Brazil is the world's second largest producer of ethanol fuel. Brazil and the United States have led the industrial production of ethanol fuel for several years, together accounting for 85 percent of the world's production in 2017. Brazil produc ...
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/m
2. Assuming an average insolation of 225 W/m
2, the photosynthetic efficiency of
sugarcane
Sugarcane or sugar cane is a species of (often hybrid) tall, Perennial plant, perennial grass (in the genus ''Saccharum'', tribe Andropogoneae) that is used for sugar Sugar industry, production. The plants are 2–6 m (6–20 ft) tall with ...
is 0.38%."
Sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
accounts for little more than 30% of the chemical energy stored in the mature plant; 35% is in the
leaves
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
and
stem
Stem or STEM may refer to:
Plant structures
* Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang
* Stipe (botany), a stalk to support some other structure
* Stipe (mycology), the stem of a mushro ...
tips, which are left in the fields during harvest, and 35% are in the fibrous material (
bagasse
Bagasse ( ) is the dry pulpy fibrous material that remains after crushing sugarcane or sorghum stalks to extract their juice. It is used as a biofuel for the production of heat, energy, and electricity, and in the manufacture of pulp and building ...
) left over from pressing.
C3 vs. C4 and CAM plants
''C3'' plants use the
Calvin cycle
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 ...
to fix carbon.
C4 plants
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 1960's discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when suppl ...
use a modified Calvin cycle in which they separate
Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) from atmospheric oxygen, fixing carbon in their
mesophyll
A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, s ...
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, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night. ...
(CAM), time isolates functioning
RuBisCO
Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...
(and the other Calvin cycle enzymes) from high oxygen concentrations produced by photosynthesis, in that O
2 is evolved during the day, and allowed to dissipate then, while at night atmospheric CO
2 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 fixed) while the C4 pathway requires 30 ATP and 12 NADPH (C3 + 12 ATP per 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
etter 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.
Maize
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
,
sugar cane
Sugarcane or sugar cane is a species of (often hybrid) tall, perennial grass (in the genus ''Saccharum'', tribe Andropogoneae) that is used for sugar production. The plants are 2–6 m (6–20 ft) tall with stout, jointed, fibrous stalks t ...
, and
sorghum
''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many othe ...
are C4 plants. These plants are economically important in part because of their relatively high photosynthetic efficiencies compared to many other crops.
Pineapple
The pineapple (''Ananas comosus'') is a tropical plant with an edible fruit; it is the most economically significant plant in the family Bromeliaceae. The pineapple is indigenous to South America, where it has been cultivated for many centuri ...
is a CAM plant.
Research
Photorespiration
One efficiency-focused research topic is improving the efficiency of
photorespiration
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 ...
. Around 25% of the time RuBisCO incorrectly collects oxygen molecules instead of , creating 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.
[
]
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
Synthetic biology (SynBio) is a multidisciplinary area of research that seeks to create new biological parts, devices, and systems, or to redesign systems that are already found in nature.
It is a branch of science that encompasses a broad ran ...
to construct new metabolic pathways and assessed their efficiency with and without transporter RNAi
RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by ...
. The most efficient pathway increased light-use efficiency by 17%.
Chloroplast biogenesis
Research is being done into RCB and NCP, two non-catalytic thioredoxin-like proteins that activate chloroplast transcription. Knowing the exact mechanism can be useful to allow increasing photosynthesis (i.e. through genetic modification).[''New Scientist'', September 2019]
See also
* Energy conversion efficiency
Energy conversion efficiency (''η'') is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The input, as well as the useful output may be chemical, electric power, mechanical work, light (radia ...
* FLEX (satellite)
The FLuorescence EXplorer (FLEX) is a planned mission by the European Space Agency to launch a satellite to monitor the global steady-state chlorophyll fluorescence in terrestrial vegetation. FLEX was selected for funding on 19 November 2015 and ...
* Phototroph
Phototrophs () are organisms that carry out photon capture to produce complex organic compounds (e.g. carbohydrates) and acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a common misconcep ...
* Photosynthetically active radiation Photosynthetically active radiation (PAR) designates the spectral range (wave band) 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 o ...
References
{{modelling ecosystems
Ecological metrics
Photosynthesis