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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 --> CH2O + O2,

where CH2O 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 percent[1] for photosynthetically active radiation (wavelengths from 400 to 700 nanometer). For actual sunlight, where only 45 percent of the light is photosynthetically active, the theoretical maximum efficiency of solar energy conversion is approximately 11 percent. 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 percent of total solar radiation.[1]

Contents

Typical efficiencies

Quoted values sunlight-to-biomass efficiency

Plant Efficiency
Plants, typical 0.1%[2]

0.2–2%[3]

Typical crop plants 1–2%[2]
Sugarcane 7-8% peak[2][4]

The following is a breakdown of the energetics of the photosynthesis process from Photosynthesis by Hall and Rao:[5]

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) [Photosynthetically_active_radiation]
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 700nm 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-bio-available-photons-waste-47% leaving-->
53% (in 400--700nm range) --30%-of-photons-lost due to incomplete absorption leaving-->
37% (absorbed photon energy) --24%-lost-due-to-wavelength-missmatch-degradation-to-700nm-energy-level 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

net efficiency of a leaf at 25°C is about 5%
many plants lose most of the rest of this doing things like 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.

Efficiencies of Various Energy Crops

Popular energy crops include Oil Palm, Soybean Oil, 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's 2835 watts per acre or 0.7 watts per square meter [1]. Typical insolation in Hawaii is more like 5.5 kW-hrs/square meter/day or 230 watts [2]. So this particular Hawaiian 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 [3], which would produce an average of roughly 22 watts per square meter (roughly 10% of the average insolation), throughout the year.

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 per square meter. Assuming an average insolation of 225 W per square meter, 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 Plants

C3 plants use the Calvin cycle. The C4 plants separate Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) from atmospheric oxygen fixing carbon in the 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, hence the name C4. 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 and Sugar Cane are examples of C4 plants. These plants are economically important in part because of their relatively high photosynthetic efficiencies compared to many other crops.

References

Photosynthetically_active_radiation
John Kimball's online textbook is a really good overview.

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