Michael Gross:
Light and Life
Oxford University Press April 2003
Hardback: ISBN 0-1985-6480-5
£ 16.99, pp 161

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Light, like no other physical phenomenon, is linked in a wide variety of ways with the biological phenomenon of life. You are able to read this page, for instance, because light is reflected from it and carries the information to your retina, where it can be converted to nerve signals. While you're doing this, you breathe oxygen which was produced by green plants using the energy of sunlight. Your inner organs are breaking down food the chemical energy of which can also be traced back to photosynthesis. You're sufficiently awake to understand these thoughts (unless you happen to be jet-lagged), because your inbuilt biological clock has set itself using the cues of light and dark and tells you that you're still within the active period of your day. Thus, at this very moment, you are making direct or indirect use of three fundamentally important interfaces between light and life: vision, photosynthesis, and the biological clock.

This close relationship with light is something we share with most species alive. There may be other kinds of life in the Universe, but the only kind that we know about owes its existence to the energy of the Sun, and its family tree was shaped by the invention of photosynthesis more than by any other event. As a result, today's inhabitants of our planet tend to rely on light for energy, information, and guidance in space/time. No wonder ancient cultures acknowledged its importance by worshipping the Sun as a life-giving, god-like being. Had this small yellow star acquired less fuel and failed to light up, all life-forms that we know of would be non-existent. This would probably also be the result if you took the planet a bit further away from the Sun. Planet Earth orbiting between Mars and Jupiter would be as lifeless as the moon.

The second most important factor in shaping life on our planet involves the technological revolution which introduced a better way of making use of solar energy, but incidentally changed everything else, too. If evolution hadn't come up with photosynthesis some 3.5 billion years ago, there would be no oxygen in the atmosphere, no protective ozone layer above it, and probably no life on dry land. Planet Earth minus the invention of photosynthesis would be inhospitable for all higher organisms that are around today, peopled only by primitive bacteria in the oceans or under the surface of the Earth.

Unraveling photosynthesis was one of the major challenges of modern biology. From the fundamental studies tracing the paths of carbon atoms using radioactive isotopes throught to today's structure-function analysis helped by crystal structures of essential parts and even of one entire photosynthetic reaction centre, the quest to understand the natural solar powerstations has a rich history of difficulties. Similarly, mimicking their performance in technological systems is a goal which we only approach at a painstakingly slow pace.

In contrast to the fundamental importance of sunlight and photosynthesis, the second connection between light and life, bioluminescence, is almost an eccentricity and adds an element of comic relief. Although many organisms have obvious evolutionary reasons for producing light of their own, some cases are less easily explained, and there may even be organisms for which light is the unwanted byproduct of a biochemical reaction run for entirely unrelated reasons. Ironically, however, technological applications of bioluminescence systems are much more advanced than any technology derived from photosynthesis.

That organisms rely on light in their orientation in space and time is easily explained as a direct consequence of both the importance of light for the energy balance and the regularity with which diurnal and annual patterns of light change are repeated. More light often means more food even for organisms not directly involved in photosynthesis. Movement toward light (phototaxis) is therefore observed even in the most primitive groups of bacteria. And plants, of course, move towards the light for obvious reasons.

Plants are also interesting to study for the influence of light on biological time-keeping. As their "behaviour" is more restricted than that of animals of similar complexity, elementary reactions triggered by changes in luminosity, day length etc. can best be studied in plants. Only recently, breakthrough discoveries have been made which put the molecular cogwheels of plant circadian clocks in evolutionary relationships both with their counterparts in animals including insects and mammals, and with the mechanisms of plant phototropic behaviour.

Animals like ourselves tend to have an inbuilt clock which can only crudely keep a 24 hour day and needs recalibration with the help of the light/dark changes every day. The way we are influenced by light changes are very much linked to ancient, sub-conscious processing pathways, including hormonal response. Misadjustment of biological rhythm or light intake can therefore make us ill and depressive without us even realising that light is the underlying problem. Recent discoveries suggest that this response to light and dark may be independent of the eyes. In one controversial experiment, it was triggered by illuminating the backside of a person's legs.

More conscious but still not entirely under our control is the process of seeing and perceiving. Although we think we know wat we're looking at, psychological research has shown that perception is far from being a video recording of what we choose to look at. The cooperation between our rather ancient eyes (which come from the same line of development as those of all other vertebrates) and the much more recent conscious brain with its higher order processing and its mechanisms designed to make sense of the information flooding in can come up with the most intriguing phenomena, many of which can be demonstrated by simple optical illusions.

In summary, life wouldn't be here without light. It wouldn't be as highly evolved as it is, if it had not made the best possible use of light's energy and information content for using photosynthesis, biological clocks, and vision. It has also developed ingenious ways of producing its own light for a variety of purposes. All these aspects of the fascinating interplay between light and life will be explored with a host of intriguing examples from up-to-date research in these areas.




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