The light compensation point (''I''
c) is the light intensity on the
light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...
where the rate of
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 ...
exactly matches the rate of
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 ...
. 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 CO
2 concentration – CO
2 compensation point (Γ).
In assimilation terms, at the compensation point, the net carbon dioxide assimilation is zero. Leaves release CO
2 by
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 i ...
and cellular respiration, but CO
2 is also converted into carbohydrate by photosynthesis. Assimilation is therefore the difference in the rate of these processes. At a given partial pressure of CO
2 (0.343
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in 1980 atmosphere), there is an irradiation at which the net assimilation of CO
2 is zero. For instance, in the early morning and late evenings, the light compensation point ''I''
c may be reached as photosynthetic activity decreases and respiration increases. The concentration of CO
2 also affects the rates of photosynthesis and photorespiration. Higher CO
2 concentrations favour photosynthesis whereas low CO
2 concentrations favor photorespiration, producing a CO
2 compensation point Γ for a given irradiation.
Light compensation point
As defined above, the light compensation point ''I''
c is when no net carbon assimilation occurs. At this point, the organism is neither consuming nor building
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 ...
. The net gaseous exchange is also zero at this point.
''I''
c is a practical value that can be reached during early mornings and late evenings. Respiration is relatively constant with regard to light, whereas photosynthesis depends on the intensity of
sunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
.
Depth
For
aquatic plant
Aquatic plants are plants that have adapted to living in aquatic environments (saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that ...
s where the level of light at any given depth is roughly constant for most of the day, the compensation point is the depth at which light penetrating the water creates the same balanced effect.
CO2 compensation point
The CO
2 compensation point (Γ) is the CO
2 concentration at which the rate of photosynthesis exactly matches the rate of respiration. There is a significant difference in Γ between plants and plants: on land, the typical value for Γ in a plant ranges from 40–100 μmol/mol, while in plants the values are lower at 3–10 μmol/mol. Plants with a weaker
CCM, such as
C2 photosynthesis
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 ...
, may display an intermediate value at 25 μmol/mol.
The μmol/mol unit may alternatively be expressed as the
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
of CO
2 in
pascals
The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defined ...
; for atmospheric conditions, 1μmol/mol = 1
ppm ≈ 0.1 Pa. For modeling of photosynthesis, the more important variable is the CO
2 compensation point in the absence of mitochondrial respiration, also known as the CO
2 photocompensation point (Γ*), the biochemical CO
2 compensation point of
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 ...
. It may be measured by whole-leaf isotopic gas exchange, or be estimated in the Laisk method using an intermediate "appearant" value of C* with correction. C* approximates Γ* in the absence of carbon refixation, i.e. carbon fixation from photorespiration products. In plants, both values are lower than their counterparts. In C2 plants that operate by refixation, only C* is significantly lower.
As it is not yet common to routinely change the concentration of air, the concentration points are largely theoretical derived from modeling and extrapolation, though they do hold up well in these applications. Both Γ and Γ* are linearly related to the partial pressure of oxygen (''p''(O
2)) due to the side reaction of Rubisco. Γ is also related to temperature due to the temperature-dependence of respiration rates. It is also related to irradiation, as light is required to produce
RuBP
Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal acceptor in plants. It is a colourless anion, a double phosphate ester of the ketopentose ( ketone-containing sugar with five c ...
(ribulose-1,5-bisphosphate), the electron acceptor for Rubisco. At normal irradiation, there would almost always be enough RuBP; but at lwo irradiation, lack of RuBP decreases the photosynthetic activity and therefore affects Γ.
[
]
The marine environment
Respiration occurs by both plants and animals throughout the water column, resulting in the destruction, or usage, of organic matter, but photosynthesis can only take place via photosynthetic algae in the presence of light, nutrients and CO2. In well-mixed water columns plankton
Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
are evenly distributed, but a net production only occurs above the compensation depth. Below the compensation depth there is a net loss of organic matter. The total population of photosynthetic organisms cannot increase if the loss exceeds the net production.
The compensation depth between photosynthesis and respiration of phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
in the ocean must be dependent on some factors: the illumination at the surface, the transparency of the water, the biological character of the plankton present, and the temperature. The compensation point was found nearer to the surface as you move closer to the coast. It is also lower in the winter seasons in the Baltic Sea according to a study that examined the compensation point of multiple photosynthetic species. The blue portion of the visible spectrum, between 455 and 495 nanometers, dominates light at the compensation depth.
A concern regarding the concept of the compensation point is it assumes that phytoplankton remain at a fixed depth throughout a 24-hour period (time frame in which compensation depth is measured), but phytoplankton experience displacement due to isopycnal
Isopycnals are layers within the ocean that are stratified based on their densities and can be shown as a line connecting points of a specific density or potential density on a graph. Isopycnals are often displayed graphically to help visualize ...
s moving them tens of meters.
See also
*Photophosphorylation 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 ...
*Critical depth
In biological oceanography, critical depth is defined as a hypothetical surface mixing depth where phytoplankton growth is precisely matched by losses of phytoplankton biomass within the depth interval."Critical depth" is an important term in ...
*CO2 fertilization effect
The CO2 fertilization effect or carbon fertilization effect causes an increased rate of photosynthesis while limiting leaf transpiration in plants. Both processes result from increased levels of atmospheric carbon dioxide (CO2). The carbon fert ...
References
{{DEFAULTSORT:Compensation Point
Photosynthesis