Currently there is much discussion (and some action) concerning “green” technologies of energy production, including the development of solar cells and panels for conversion of light energy from the sun to electricity.
Photoelectric cells are a recent innovation for mankind, but conversion of light into electricity is something that photosynthetic organisms have been doing over aeons of time. There is fossil evidence for photosynthetic microorganisms dating back thousands of millions of years, long before any plants existed. Walk beneath a leafy canopy, and uncountable numbers of little green solar panels are waving above your head. We think of photosynthesis as the synthesis of organic compounds from the raw materials carbon dioxide and water, at the expense of energy from the sun. This is indeed the end result. But before any chemical synthesis takes place, the plant converts the light energy to electricity.
Let us home in on a green leaf. The naked eye sees a green surface, with a pattern of veins, perhaps some hairs (fig.1).
Fig.1. The green “solar panels” that supply energy for life on earth, as illustrated by the grapevine (Vitis vinifera). Photo Helgi Öpik.
With a microscope, we can see leaf cells and magnifying more, we see in the cells the chloroplasts, little green lens-shaped organelles, the sites of photosynthesis. A plant cell can contain anything from less than 10 to several hundred – cell sizes vary greatly. The diameter of a typical chloroplast is about 5 μm (micrometres; for units, see end). Yet within that minute dot there is a complex substructure of membranes which contain the light-absorbing pigment molecules. These membranes are folded and convoluted to form a light-absorbing surface much greater than the overall surface of the chloroplast. Light absorbed by the green pigment chlorophyll activates electrons to move as an electric current in the membranes. This current energizes a complex series of biochemical reactions producing sugar. The raw materials are carbon dioxide and water, while oxygen is released as a byproduct. For long-term storage, the sugar can be converted to starch or oil.
In one single chloroplast, some 50 million electrons may be moving per second; a small leaf, area 10 square cm, can contain 550 million chloroplasts, which gives us:
27 thousand million electrons moving in the leaf each second!
This number is enormous. However, we are dealing with subatomic particles and the amount of electricity each single electron carries is infinitesimally small. Nobody got electrocuted by a sunlit leaf yet!
Fig. 2. Chloroplasts in living cells of a moss (Plagiomnium affine) seen under a microscope. The thinness and transparency of a moss leaf makes its chloroplasts easy to observe. The cells measure approx. 80 x 40 μm (or 0.08 x 0.04 millimetres).From Wikipedia, under the terms of the GNU Free Documentation Licence © Kristian Peters - Fabelfroh
BUT: add together all the electric currents in all the chloroplasts of all the leaves in the world, and in all the chloroplasts of all the algae floating in the oceans, and in all the photosynthetic bacteria which are almost like free-living chloroplasts – and we have the driving force for nearly all life on earth. (Only chemosynthetic bacteria subsist at the expense of chemical reactions, in habitats containing suitable chemicals). The photosynthetically produced organic compounds serve both as the raw material for building the bodies of living organisms, and as fuel for respiration, which again is an energy-conversion process. Respiration provides living cells with energy in a form that is usable for growth, movement and maintenance, and produces the body heat of warm-blooded organisms. It uses oxygen and releases carbon dioxide and water, completing the natural carbon cycle, i.e. cycling of carbon between carbon dioxide and organic carbon compounds:
Since the first humans appeared some 2.6 million years ago, the human species has depended on plant photosynthesis for the fuel to yield the energy that runs our bodies and for fuel to yield energy for our industries, transport, communications, heating, cooking and lighting. The human population of the earth has already reached a size and a stage of technological development demanding a large supply of energy and this demand is still growing. All the electricity obtained from fossil fuels is solar energy, converted by photosynthesis to chemical energy of organic materials. This process occurred 100s of millions of years ago, and it took 100s of millions of years to fossilize the plant material to coal, oil or natural gas. These resources are finite. It is becoming imperative to find alternative means of generating electric energy. One of these alternatives is to do what the plants do: convert sunlight energy to electricity. Solar cells and panels that do this are already manufactured commercially and their use is increasing.
For humans, however, there are fundamental problems in this field that do not affect plants: the problems of scale and of storage and distribution of electric energy.
The power stations of the plant are built on a microscale, of molecular subunits. That, combined with the special properties of the molecules involved (e.g. chlorophyll) enables the plants to run their photoelectric conversion silently, imperceptibly, efficiently, at low temperatures, and from the viewpoint of present-day life on earth, cleanly without waste. (But see endnote). The process is spread out over an enormous surface area. The total leaf area on earth is currently estimated at108 (hundred million) square km. That does not include seaweeds, nor microscopic photosynthetic plankton. The plants’ electricity production occurs in the same place, in the chloroplasts, where the electric power is used to synthesise organic compounds.
The plants do not need to conduct electricity nor store it in batteries. They store the energy chemically in sugars, starches and oils, which suits their mode of life (fig. 3).
Fig. 3. Storage cells in a bean seed (Phaseolus vulgaris), imaged by scanning electron microscopy (SEM), which shows structure in three dimensions. The large rounded bodies are starch grains, with diameters reaching over 30 μm; the small granules are of storage protein. The cells have been cut open, causing some cracks and falling out of starch grains. Photo Helgi Öpik.
Mankind, however, needs to generate on a large scale and transport electricity over long distances. To build a solar station of the same capacity as a current fossil fuel, hydroelectric or nuclear power station would require an enormous surface area for collecting light, especially when allowing for low light in temperate regions during winter or under heavy cloud, The Sahara desert has been considered as a good place for solar generation, but that implies power lines over very long distances to inhabited areas. Would inhabitants in Britain, say, be happy to depend on electricity generation in the Sahara, at any rate while the world is riddled with wars and terrorism? Moreover, since electricity does not store well, there would always and everywhere be the difficulty of night-time storage, unless one works on such a global scale that electricity is transported right round the globe, with electricity stations coming “on line”sequentially as the earth revolves. Such an organization is still in the science fiction realm.
At present, it seems more sensible to follow the plant example and generate solarelectricity as near as possible to where it is to be utilized, with solar panels on individual buildings and modest scale power stations for small communities. Regarding improvements in efficiency, plant systems have yielded some ideas and might well yield us some more. After all, humanity has been making solar cells only for decades; photosynthesis has been going for some 2500 million years.
Endnote: Something to think about
We regard solar energy as clean energy. However, photosynthesis once resulted in what could be regarded as the most significant pollution event during the history of life on earth: it enriched the atmosphere with oxygen, its by-product. Thereby photosynthesis changed the entire chemistry of the earth’s surface, and directed the evolution of living organisms into new channels. We would not be here without oxygen – but it is highly likely that many primitive organisms were killed by the oxygen and never fulfilled their evolutionary potential.
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Units of length: the μm, micrometre, is generally used for measuring cells and organelles. If we start with the mm, millimetre, the eye can see objects down to about 1/10 mm; 1 μm =1/1000 of a mm.
Acknowledgement: this article is based on one previously published in the Newsletter of the Friends of the City of Swansea Botanical Complex , No. 14, (2002).