. . . . . Light and Life
. . . . . Light and Life
Without light, life as we know it would not exist. Plants harvest solar energy by photosynthesis, and provide energy to other organisms through the food chain. Humans and animals depend on light through vision and other photoresponses. Biological effects of artificial light are the basis of a variety of medical treatments and diagnostic techniques. Light can also have deleterious effects. For example, ozone in the atmosphere protects living things from damage by ultraviolet radiation. Increases in the amount of ultraviolet light reaching the earth's surface as a result of changes in the ozone layer may result in increased rates of skin cancer and skin aging as well as undesirable effects on agriculture, oceanic plankton and the aquatic food chain. Recognition of the importance of light in biology has led to the development of the science of photobiology: the study of the myriad effects of light on life.
In recognition of the growing awareness of the influence of light (both beneficial and harmful) on living organisms and the need for scientific information concerning these effects, the American Society for Photobiology was founded in 1972. The Society, with nearly 2000 members from all over the world, holds annual scientific meetings, publishes the international journal Photochemistry and Photobiology, and in general serves as a resource for information about photobiology.
Members of the American Society for Photobiology study a variety of scientific subjects concerned with the interaction of light and living things. The sections below introduce some active areas of photobiological research and outline their importance to society.
To avoid the sun would be to exist without one of the great pleasures of life. But as with most enjoyable things, indiscrim- inate exposure and lack of understanding of the possible unpleasant consequences can result in unhappiness and even serious aftereffects. There are over 25 diseases that are caused or aggravated by sunlight. The field of photomedicine includes the study of such diseases and their treatments.
Harmful effects of light. Sunlight is implicated in several skin diseases, including premature aging of the skin and skin cancer. Skin sensitivity to sunlight is controlled by the genetic ability of an individual to produce melanin, the pigment that helps protect the skin from light-induced injury. Genetic variations in the capacity to form melanin, proximity to the equator, and personal habits of sun exposure determine the susceptibility of an individual to skin aging (actinic elastosis) and cancer of the skin (basal cell carcinomas, squamous cell carcinomas and probably malignant melanomas). Deficiencies in cellular capacity to repair sun-induced damage of DNA, as in the inherited disorder called xeroderma pigmentosum, are responsible for the early onset of sun sensitivity and freckling, which can lead to sunlight-induced skin cancer. Certain drugs or chemicals also augment skin reactivity to solar radiation, and lead to transient phototoxic effects or chronic photoallergic reactions. In the synthesis of hemoglobin, the substance that gives the red color to human blood and carries oxygen, genetic deficiencies of certain enzymes lead to metabolic overproduction of hemoglobin precursors called porphyrins. These porphyrins absorb light and cause severe, disabling photosensitivity.
Beneficial effects of light. Photomedicine is also concerned with the beneficial effects of light. For example, phototherapy is useful for treating jaundice in premature babies, and light-based therapies can be effective in treating psoriasis. The use of light-based techniques to treat tumors and to inactivate the human immunodeficiency virus that causes AIDS and other viruses present in blood is being explored. Phototherapies for several other diseases exist or are under development by the photomedical community.
Photoprotection. Both topical and systemic sunscreen agents prevent the acute and chronic effects of sunlight. They enable people to work outdoors and enjoy outdoor activities with reduced risk of sun-induced injury. The damage that absorbed light creates in the skin, such as the changes recognized as ~aging~ of the skin, is preventable by using new types of water- and sweat- resistant sunscreens.
Photoimmunology. Light exposure can affect the immune system. For example, irradiation of mice with ultraviolet light not only produces skin tumors at the site of exposure, but also alters the entire immune system, allowing transplantation of the tumors to areas not exposed to light. Studies of these effects may ultimately help explain the molecular basis for skin cancer in humans. This and other effects of light on the immune system are currently under active investigation in many photobiology laboratories.
Photosynthesis is one of the most important biological processes on earth. By consuming carbon dioxide and liberating oxygen, it has transformed the world into the hospitable environment we know today. Directly or indirectly, photosynthesis fills all of our food requirements and many of our needs for fiber and building materials. The energy stored in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the energy in firewood, which is a major fuel in many parts of the world. This being the case, scientific research into photosynthesis is extremely important. If we can understand and control the intricacies of the photosynthetic process, we can learn how to increase crop yields of food, fiber, wood, and fuel, and how to use our lands more benignly. The energy-harvesting secrets of plants can be adapted to synthetic systems which will provide new, efficient ways to collect and use solar energy. Because photosynthesis helps control the makeup of our atmosphere, understanding photosynthesis is crucial to understanding how carbon dioxide and other ~greenhouse gases~ affect the global climate.
In photosynthesis, the energy of the absorbed light is converted into chemical and electrochemical energy which can be used to support cell growth. The process occurs in pigment- protein complexes called reaction centers, which are embedded in biological membranes. Light is gathered by antenna complexes and efficiently transferred to the chlorophyll of a reaction center where the primary photochemical events occur. As a result of the primary photochemical change, a charge imbalance and a proton gradient develop across the membrane. The resulting high-energy state is ultimately used to make chemicals that satisfy the energy needs of the organism. Green plants carry out additional energy-conserving photochemical reactions and produce oxygen, which is released to the atmosphere.
Research in photosynthesis is at a particularly exciting state since the molecular structures of many of the relevant proteins and pigment-protein complexes are now being characterized in detail. Amino acid sequences of important polypeptides are becoming available, as is 3-dimensional structural information from x-ray diffraction and electron microscopy. Model systems are being constructed to help elucidate the chemistry and physics of the in vivo system, and to discover principles for possible application in the construction of efficient solar cells. Also of great interest are questions relating to the development and control of photosynthetic organisms. Such studies may ultimately lead to better crop yields.
Environmental Photobiology
Environmental photobiology is a new, multidisciplinary research area. It is concerned with the effects of artificial light on the human environment, the effects of sunlight on ecosystems, and human influences on the quality of sunlight reaching the earth~s surface.
By the release of chlorofluorocarbons, society has the capacity to change the spectral quality of sunlight by destroying the ozone layer in our stratosphere, which filters out much of the damaging short-wavelength ultraviolet radiation. What would be the ecological consequences of such a change? Could this have a deleterious effect on agriculture? Would it result in increased skin cancer risk? These are the kinds of questions studied by environmental photobiologists.
The role of artificial light on the human environment has only begun to receive serious attention. Mammalian cells grown in the laboratory are mutated by fluorescent room light. Light absorbed through the eye is known to affect the functioning of the pineal gland. Visible light, especially red light, penetrates deeply into tissues. Are there beneficial effects of light on humans other than those mediated by vision? The possibility of extraocular photoreception in humans is an exciting challenge for the future.
Photosensitization. This phenomenon occurs not only in humans, as described in the section on photomedicine, but also in other organisms. For example, some plants contain potent photosensitizing chemicals. When cattle, sheep or other animals eat these plants they become light sensitive and may even die if they remain in the sunlight. Grazing animals with liver dysfunctions also become light sensitive due to the accumulation of chlorophyll metabolites that are photosensitizers. Even foodstuffs can suffer from photodamage. Some snack foods such as potato and corn chips develop an ~off flavor~ when exposed to light. This apparently results from the photooxidation of unsaturated oils that remain in the chips after cooking.
Ultraviolet radiation effects. Photobiologists working with ultraviolet radiation are concerned with identifying the photochemical changes that are produced in living tissue by the absorption of ultraviolet light and determining the biochemical and physiological responses of cells to this damage. The most important discovery in this research area in recent years is that all cells have a remarkable capacity to repair damage that is produced in their deoxyribonucleic acid (DNA) by ultraviolet radiation. It is now known that cells can also repair their DNA when it has been damaged by other types of radiation, such as x- rays, and by chemical carcinogens. Furthermore, normal cellular metabolism produces agents that damage DNA, and DNA repair systems play an important role in protecting the genetic material from the deleterious effects of this damage.
Light-induced damage of DNA may lead to aging. Many of the earlier theories of aging have been unified into the genetic alteration theory. The basic tenet of this theory is that the proper functioning of a cell or organism depends on the proper functioning of its DNA. If DNA damage is not repaired properly, the organism will undergo aging. Cells from patients with hereditary diseases that predispose them to early aging have been found to be deficient in DNA repair.
Photochemical changes are responsible for biological responses to light, and can have important effects on the environment of all organisms, including humans. Photochemistry is the study of the basic chemistry and physics of such transformations. Understanding and control of any photobiological process requires a knowledge of the underlying photochemistry. Photochemists investigate photochemical reactions using the tools of modern chemical analysis, including spectroscopic methods of many kinds. Once the detailed mechanism of a photochemical reaction is known, it is usually possible to learn how to modify the photochemistry, and thus to improve the efficiency of beneficial reactions or inhibit detrimental processes.
Photochemistry is becoming increasingly important as a tool in biological research. For example, the understanding of many complex photobiological processes can be enhanced through the preparation and study of synthetic photochemical models. Photochemistry can also be used to study the spatial relationships of molecules in complex biological structures. In this approach, light is used to induce chemical bonds between adjacent molecules. Subsequent identification of these attachments indicates the spatial relationship of the molecules in the native biological structure.
Many important industrial and manufacturing processes are based on photochemistry. Photocopying, photography and photolithography are just a few examples. Also, natural and synthetic chemicals (e.g., medications, industrial chemicals, herbicides and pesticides) can sometimes be altered by sunlight to produce compounds toxic to humans and other organisms or harmful to the environment. For example, the action of sunlight on automobile exhaust contributes to smog. It is therefore important to study the photochemistry of all common chemicals that may be exposed to sunlight.
Bioluminescence. Although most areas of photobiology deal with the consequences of the absorption of light, bioluminescence deals with the biological emission of light. In nature, bioluminescence reactions are used for sexual signaling (firefly), to attract food (Australasian glowworm) and for protection by frightening predators. In deep regions of the oceans, bioluminescence provides the only source of light. The photochemical basis of bioluminescence is the reaction of two molecules to form a molecule in a higher-energy state, which can emit light. Studies of this process in the laboratory have led to commercial light sources and methods for the measurement of important biological molecules such as adenosine triphosphate (ATP).
Spectroscopy. The first law of photochemistry states that only light that is absorbed can produce chemical change. Spectroscopists study the absorption and emission of light by molecules. Spectroscopy can provide information about the chemical structure of a molecule and the energy states it can assume. It can also be used to determine the amounts of specific chemicals present in mixtures of materials. These analytical techniques are so sensitive that they are often the methods of choice for industrial quality control, medical analyses, monitoring of environmental pollutants and contaminants, analysis of ores, etc. There are many spectroscopic techniques available to photochemists, including those based on lasers of various kinds. Photoacoustic spectroscopy is a recent innovation that allows the determination of the spectra of solid materials as well as liquid solutions.
Phototechnology. Continued progress in the science of photobiology depends on the timely development of new light sources to solve specific problems, and equipment to measure their intensity and spectral quality. For example, a commonly used phototherapy, for psoriasis, was fully developed only after suitable ultraviolet light sources were invented. Some of the more sophisticated developments in phototechnology have been the laser, and laser-based equipment such as biological cell sorters, cytofluorographs and photoacoustic spectrometers.
Photosensory Biology
Chronobiology. The ability to distinguish time of day without reference to external light or darkness is found in both plants and animals. Light has important effects on this time sense, or circadian clock, as it is sometimes called. Light keeps the timing cycle synchronous with day and night, adjusts it to long or short days, and even stops or starts it under certain conditions. Animals respond to changes in day length with accompanying changes in reproduction, migration, overwintering behavior, etc.
In humans, mental acuity varies with the time of day, as do body temperature, hormone levels and many other physiological functions. Even the sensitivity to drugs varies according to a circadian rhythm: a dose that is toxic at one time of day may have little deleterious effect at another. Most travelers have experienced ~jet lag,~ which is the result of our circadian clock getting out of adjustment. Understanding and control of circadian rhythms promises to lead to significant improvements in our quality of life.
Photomorphogenesis. Nature has produced a number of light- absorbing molecules that enable organisms to respond to changes in the natural light environment. Light signals can regulate changes in structure and form, such as seed germination, leaf expansion, stem elongation, flower initiation and pigment synthesis. These photomorphogenic responses confer an enormous survival advantage on organisms. For example, timing must be very precise in order for seed to be produced before the first killing frosts and yet allow the photosynthetic process to accumulate enough stored food to support seedling growth in the spring.
Currently, commercial greenhouse growers regulate the production of floral crops such as Easter lilies and poinsettias by artificially regulating the length of night and day. With more knowledge about light control of the photomorphogenic triggering responses, other commercial applications should be forthcoming. These may improve crop resistance to external stresses.
Photomovement. Photomovement involves any light-mediated behavior that results in movement of an organism. A common example is the bending of plants toward a light source. Some flowers, such as the sunflower, move to face the sun throughout the day. Organisms that can move about can respond to light by moving either toward or away from the source. This ability can be ecologically important, as when it enables photosynthetic organisms to move into a favorably lighted environment. Such responses depend upon the organism being able to determine the intensity and direction of light. Some organisms use the sun as a directional compass for migration (European starling) or food gathering (honeybee).
Since light is an easily manipulated stimulus, the study of photomovement is particularly attractive when compared to investigations of other stimulus/response systems. Photomovement studies will increase our understanding of the molecular basis of behavior. Much exciting research on how different organisms perceive the direction and intensity of light remains to be done.
Photoreception. The perception of light by receptors other than true eyes is well documented for both invertebrates and vertebrates. A classical example is the house sparrow. It uses the cyclic annual change in day length to synchronize its reproductive cycle with the appropriate season. The receptor for this light signal is not in the eye, but in the brain. It receives light that passes through the feathers, skin and skull at the top of the head.
In mammals, most responses to light seem to be mediated by the eyes. A well-documented exception is that photoreception outside the eye affects the level of the neurotransmitter pineal serotonin in newborn (but not adult) rats. If such extraretinal photoreception also occurs in newborn humans, then it is appropriate to be concerned with the occasionally extreme lighting conditions used in hospital nurseries. If it occurs in adult mammals, then currently used artificial lighting schemes might require adjustment.
Vision. Only a relatively narrow band of the spectrum is visible to the human eye. Various animals have different wavelength sensitivities. For example, some insects ~see~ best in the ultraviolet region. Because humans are so visually oriented, it is not surprising that a lot of effort has gone into understanding the molecular and physiological bases of vision. Vision research deals not only with the initial photochemistry that occurs in the eye upon being exposed to light, but also with the conversion of these changes to nerve impulses that lead ultimately to perception by the brain. Nutritional factors, variations in light/dark cycles and environmental light intensity levels have been shown to have profound effects on visual photoreceptors. Further research in these areas may provide ways to maintain healthy visual function throughout life.
This brief introduction describes a few of the many ways that animals and plants are affected by light. Some of these effects are beneficial (and indeed, make life possible), and others are highly detrimental. Yet, in truth, we know relatively little about the influence of light on biological systems, including human beings. The goals of the science of photobiology can be roughly divided into four categories:
The future holds many exciting challenges and rewards for this young science. More information about photobiology and photobiologists may be obtained by contacting:
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