Photobiology

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Photobiology is the scientific study of the beneficial and harmful interactions of light (technically, non-ionizing radiation) in living organisms. [1] The field includes the study of photophysics, photochemistry, photosynthesis, photomorphogenesis, visual processing, circadian rhythms, photomovement, bioluminescence, and ultraviolet radiation effects. [2]

Contents

The division between ionizing radiation and non-ionizing radiation is typically considered to be a photon energy greater than 10 eV, [3] which approximately corresponds to both the first ionization energy of oxygen, and the ionization energy of hydrogen at about 14 eV. [4]

When photons come into contact with molecules, these molecules can absorb the energy in photons and become excited. Then they can react with molecules around them and stimulate "photochemical" and "photophysical" changes of molecular structures. [1]

Photophysics [5]

This area of Photobiology focuses on the physical interactions of light and matter. When molecules absorb photons that matches their energy requirements they promote a valence electron from a ground state to an excited state and they become a lot more reactive. This is an extremely fast process, but very important for different processes. [5]

Photochemistry [6]

This area of Photobiology studies the reactivity of a molecule when it absorbs energy that comes from light. It also studies what happens with this energy, it could be given off as heat or fluorescence so the molecule goes back to ground state.

There are 3 basic laws of photochemistry:

1) First Law of Photochemistry: This law explains that in order for photochemistry to happen, light has to be absorbed.

2) Second Law of Photochemistry: This law explains that only one molecule will be activated by each photon that is absorbed.

3) Bunsen-Roscoe Law of Reciprosity: This law explains that the energy in the final products of a photochemical reaction will be directly proportional to the total energy that was initially absorbed by the system.

Plant Photobiology

Plant growth and development is highly dependent on light. Photosynthesis is one of the most important biochemical processes for life on earth and its possible only due to the ability of plants to use energy from photons and convert it into molecules such as NADPH and ATP, to then fix carbon dioxide and make it into sugars that plants can use for their growth and development. [7] But photosynthesis is not the only plant process driven by light, other processes such as photomorphology and plant photoperiod are extremely important for regulation of vegetative and reproductive growth as well as production of plant secondary metabolites. [8]

Photosynthesis

Photosynthesis is defined as a series of biochemical reactions that phototrophic cells perform to transform light energy to chemical energy and store it in carbon-carbon bonds of carbohydrates. [9] As it is widely known, this process happens inside of the chloroplast of photosynthetic plant cells where light absorbing pigments can be found embedded in the membranes of structures called thylakoids. [9] There are 2 main pigments present in the Photosystems of higher plants: chlorophyll (a or b) and carotenes. [7] These pigments are organized to maximize the light reception and transfer, and they absorb specific wavelengths to broaden the amount of light that can be captured and used for photo-redox reactions. [7]

Photosynthetically Active Radiation (PAR)

Due to the limited amount of pigments in plant photosynthetic cells, there is a limited range of wavelengths that plants can use to perform photosynthesis. This range is called "Photosynthetically Active Radiation (PAR)". This range is interestingly almost the same as the human visible spectrum and it extends in wavelengths from approximately 400-700 nm. [10] PAR is measured in μmol s−1m−2 and it measures the rate and intensity of radiant light in terms of micro-moles per unit of surface area and time that plants can use for photosynthesis. [11]

Photobiologically Active Radiation (PBAR)

Photobiologically Active Radiation (PBAR) is a range of light energy beyond and including PAR. Photobiological Photon Flux (PBF) is the metric used to measure PBAR.

Photomorphogenesis

This process refers to the development of the morphology of plants which is light-mediated and controlled by 5 distinct photoreceptors: UVR8, Cryptochrome, Phototropin, Phytochrome r and Phytochrome fr. [12] Light can control morphogenic processes such as leaf size and shoot elongation.

Different wavelengths of light produce different changes in plants. [13] Red to Far Red light for example, regulates stem growth and straightening of the seedling shoots that are coming out of the ground. [14] Some studies also claim that red and far red light increases the rooting mass of tomatoes [15] as well as the rooting percentage of grape plants. [16] On the other hand, blue and UV light regulate the germination and elongation of the plant as well as other physiological processes such as stomatal control [17] and responses to environmental stress. [18] Finally, green light was thought not to be available to plants due to the lack of pigments that would absorb this light. However, in 2004 it was found that green light can influence stomatal activity, stem elongation of young plants and leaf expansion. [19]

Secondary Plant Metabolites

These compounds are chemicals that plants produce as part of their biochemical processes and help them perform certain functions as well as protect themselves from different environmental factors. In this case, some metabolites such as anthocyanins, flavonoids, and carotenes, can accumulate in plant tissues to protect them from UV radiation and very high light intensity [20]

Photobiologists

See also

Related Research Articles

<span class="mw-page-title-main">Fluorescence</span> Emission of light by a substance that has absorbed light

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation. A perceptible example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the electromagnetic spectrum, while the emitted light is in the visible region; this gives the fluorescent substance a distinct color that can only be seen when the substance has been exposed to UV light. Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.

<span class="mw-page-title-main">Photosynthesis</span> Biological process to convert light into chemical energy

Photosynthesis is a biological process used by many cellular organisms to convert light energy into chemical energy, which is stored in organic compounds that can later be metabolized through cellular respiration to fuel the organism's activities. The term usually refers to oxygenic photosynthesis, where oxygen is produced as a byproduct, and some of the chemical energy produced is stored in carbohydrate molecules such as sugars, starch and cellulose, which are synthesized from endergonic reaction of carbon dioxide with water. Most plants, algae and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the biological energy necessary for complex life on Earth.

<span class="mw-page-title-main">Photochemistry</span> Sub-discipline of chemistry

Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet, visible light (400–750 nm) or infrared radiation (750–2500 nm).

<span class="mw-page-title-main">Photosystem</span> Structural units of protein involved in photosynthesis

Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae, and cyanobacteria. These membranes are located inside the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: PSI and PSII.

Actinism is the property of solar radiation that leads to the production of photochemical and photobiological effects. Actinism is derived from the Ancient Greek ἀκτίς, ἀκτῖνος. The word actinism is found, for example, in the terminology of imaging technology, medicine, and chemistry, and the concept of actinism is applied, for example, in chemical photography and X-ray imaging.

In developmental biology, photomorphogenesis is light-mediated development, where plant growth patterns respond to the light spectrum. This is a completely separate process from photosynthesis where light is used as a source of energy. Phytochromes, cryptochromes, and phototropins are photochromic sensory receptors that restrict the photomorphogenic effect of light to the UV-A, UV-B, blue, and red portions of the electromagnetic spectrum.

<span class="mw-page-title-main">Photosynthetically active radiation</span> Range of light usable for photosynthesis

Photosynthetically active radiation (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 the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place.

Far-red light is a range of light at the extreme red end of the visible spectrum, just before infra-red light. Usually regarded as the region between 700 and 750 nm wavelength, it is dimly visible to human eyes. It is largely reflected or transmitted by plants because of the absorbance spectrum of chlorophyll, and it is perceived by the plant photoreceptor phytochrome. However, some organisms can use it as a source of energy in photosynthesis. Far-red light also is used for vision by certain organisms such as some species of deep-sea fishes and mantis shrimp.

In particle physics, the quantum yield of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.

<span class="mw-page-title-main">Photosensitizer</span> Type of molecule reacting to light

Photosensitizers are light absorbers that alters the course of a photochemical reaction. They usually are catalysts. They can function by many mechanisms, sometimes they donate an electron to the substrate, sometimes they abstract a hydrogen atom from the substrate. At the end of this process, the photosensitizer returns to its ground state, where it remains chemically intact, poised to absorb more light. One branch of chemistry which frequently utilizes photosensitizers is polymer chemistry, using photosensitizers in reactions such as photopolymerization, photocrosslinking, and photodegradation. Photosensitizers are also used to generate prolonged excited electronic states in organic molecules with uses in photocatalysis, photon upconversion and photodynamic therapy. Generally, photosensitizers absorb electromagnetic radiation consisting of infrared radiation, visible light radiation, and ultraviolet radiation and transfer absorbed energy into neighboring molecules. This absorption of light is made possible by photosensitizers' large de-localized π-systems, which lowers the energy of HOMO and LUMO orbitals to promote photoexcitation. While many photosensitizers are organic or organometallic compounds, there are also examples of using semiconductor quantum dots as photosensitizers.

Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule.

<span class="mw-page-title-main">Extreme ultraviolet</span> Ultraviolet light with a wavelength of 10–121nm

Extreme ultraviolet radiation or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths shorter that the hydrogen Lyman-alpha line from 121 nm down to the X-ray band of 10 nm, and therefore having photons with energies from 10.26 eV up to 124.24 eV. EUV is naturally generated by the solar corona and artificially by plasma, high harmonic generation sources and synchrotron light sources. Since UVC extends to 100 nm, there is some overlap in the terms.

A light-harvesting complex consists of a number of chromophores which are complex subunit proteins that may be part of a larger super complex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. The light which is captured by the chromophores is capable of exciting molecules from their ground state to a higher energy state, known as the excited state. This excited state does not last very long and is known to be short-lived.

The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in green plants and algae. Photosynthesis can be described by the simplified chemical reaction

Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative stress caused by excess or fluctuating light conditions. Humans and other animals have also developed photoprotective mechanisms to avoid UV photodamage to the skin, prevent DNA damage, and minimize the downstream effects of oxidative stress.

<span class="mw-page-title-main">Photoinhibition</span>

Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII. In living organisms, photoinhibited PSII centres are continuously repaired via degradation and synthesis of the D1 protein of the photosynthetic reaction center of PSII. Photoinhibition is also used in a wider sense, as dynamic photoinhibition, to describe all reactions that decrease the efficiency of photosynthesis when plants are exposed to light.

<span class="mw-page-title-main">Light-dependent reactions</span> Photosynthetic reactions

Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI),

Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.

<span class="mw-page-title-main">Non-ionizing radiation</span> Harmless low-frequency radiation

Non-ionizingradiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule. Instead of producing charged ions when passing through matter, non-ionizing electromagnetic radiation has sufficient energy only for excitation. Non-ionizing radiation is not a significant health risk. In contrast, ionizing radiation has a higher frequency and shorter wavelength than non-ionizing radiation, and can be a serious health hazard: exposure to it can cause burns, radiation sickness, many kinds of cancer, and genetic damage. Using ionizing radiation requires elaborate radiological protection measures, which in general are not required with non-ionizing radiation.

The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically or as a single flash. The photoacoustic effect is quantified by measuring the formed sound with appropriate detectors, such as microphones or piezoelectric sensors. The time variation of the electric output from these detectors is the photoacoustic signal. These measurements are useful to determine certain properties of the studied sample. For example, in photoacoustic spectroscopy, the photoacoustic signal is used to obtain the actual absorption of light in either opaque or transparent objects. It is useful for substances in extremely low concentrations, because very strong pulses of light from a laser can be used to increase sensitivity and very narrow wavelengths can be used for specificity. Furthermore, photoacoustic measurements serve as a valuable research tool in the study of the heat evolved in photochemical reactions, particularly in the study of photosynthesis.

References

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