By John Palka — Posted December 27, 2015
In 1939, the year I was born, Richard Llewellyn wrote a magical novel that is still widely read today, How Green Was My Valley. The story is set in the mining area of Wales, and it offers lyrical descriptions of the countryside, including these wonderfully crafted sentences: “How green was my Valley that day, too, green and bright in the sun.” And to close the book: “How green was my Valley then, and the Valley of them that have gone.”
Valleys, with their forests, fields, and meadows, are still green. The needles and leaves of the trees and the shrubs and the delicate plants of the understory, the fronds of the ferns, the tiny leaflets of the mosses—a living valley is a world of green.
Where does all this magical green come from? It comes from the presence of the pigment chlorophyll in the cells of the leaves and whatever other parts of a plant are green. And chlorophyll is of central importance to life on Earth. It is the starting point for photosynthesis, the creation of sugars out of the carbon dioxide of the air and the water of the soil. A byproduct of this process is the release into the atmosphere of oxygen, O2, molecules that we animals need to survive. Without the green of the forests and the fields, and also of the plankton floating in the upper layers of the oceans, the lives of the Earth’s animals, including our lives as human beings, would not be possible.
Photosynthesis begins with the capture of the energy of sunlight, and ends with using that energy to make sugar. What does it actually mean to capture the energy of sunlight? Here is one way of looking at it. In Nature’s Depths posts “Our Brightly Colored World” and “Seeing Colors” we explore the idea that pigments look colored to us because they absorb some wavelengths of light while failing to absorb others. Our main focus there is on the wavelengths that are not absorbed, and therefore remain available to our eyes to be seen as colors. In examining photosynthesis, our focus is instead on the wavelengths that are absorbed.
Absorbing light of a particular wavelength is the same as capturing that light’s energy. Think about what happens when you sit under an infrared lamp or beside a fire—you can feel the heat energy, reaching you as infrared radiation, warm up your skin. The same with getting a sunburn from too much ultraviolet—the energy of the UV radiation (invisible to our eyes but readily seen by insects) is so great that absorbing it results in actual damage to our molecules, and therefore to our cells and tissues. Chlorophyll is excellent at absorbing not infrared and ultraviolet, but red and blue wavelengths of light. Absorbing this light is the same as capturing the energy it brings to Earth from the Sun at these wavelengths. Isn’t this a startling concept? Chlorophyll is the molecule that links the incomprehensively gigantic nuclear explosions occurring in the Sun directly to all of life on Earth, with the exception of certain groups of bacteria.
Virtually all of life on Earth depends on chlorophyll. Why is this true? Let’s take the explanation in steps.
First, in photosynthesis green leaves use the energy of sunlight captured by chlorophyll to power the combining of carbon dioxide and water to make sugar. This biosynthetic process would not take place without the availability of solar energy to drive the necessary chemical reactions.
Second, whenever energy is used to make a chemical compound, it is effectively stored in this compound and can be retrieved later by taking the compound apart. In biology, we call the disassembly of molecules to provide the energy for life processes cellular respiration. Here we are not speaking of the breath (breathing, confusingly, is often also called respiration)—or of digestion in the gut, or of absorption across the gut’s wall. Cellular respiration is the extraction of energy from small molecules that we have taken into our cells for the express purpose of retrieving the energy those molecules contain. Through the disassembly of such molecules (principally sugars, fatty acids, and amino acids) we are able to recover the energy they contained while they were intact and use it to power our own processes, from building cells and tissues, to muscle contraction, to thinking.
Third, deriving energy for life from cellular respiration is an example of the operation of a fundamental physical principle called the Conservation of Energy. This principle states that energy can be neither made nor destroyed, only converted from one form into another. The principle has sweeping application. It is, for example, at the heart of current discussions about global warming: We have been using fossil fuels (chemical energy derived originally from light energy) via several steps to generate the mechanical energy needed to move our cars, as well as the electrical energy needed to provide light for our houses. More recently we have started to use solar panels to convert light energy from the sun directly into electrical energy, which gives us back light energy emitted by light bulbs and also mechanical energy to move our cars.
Fourth, and finally, energy transformations work in living systems fundamentally in the same way. In cells containing chlorophyll, light energy is converted into the chemical energy contained in sugars. These sugars are used elsewhere as a source of energy to do many things—to make more complex molecules such as fats (that in turn store this chemical energy) or to generate the electrical signals of nerve cells (electrical energy) or to make our muscles contract as we go for our walk in the forest (mechanical energy). Instead of using energy, as photosynthesis does, respiration releases energy that our bodies use.
In this photograph, taken at the Greenbank Farm on Whidbey Island, we see the principle of the Conservation of Energy in action in multiple ways all at the same time: Solar to electrical in man-made solar panels, solar to chemical by all the greenery, chemical to chemical within the plants and the grazing horses, chemical to mechanical as the horses move, and more. It is surely one of the beauties of science that such diverse phenomena as the operation of solar cells and the contraction of muscles can be understood, at least in part, as manifestations of a single underlying principle.
These are large-scale, holistic, and sometimes abstract concepts. To make them concrete, practice seeing the conservation and transformation of energy in your everyday life. For starters, enjoy a piece of vegetarian food, perhaps an apple or a slice of bread. Remember that the carbohydrates in that food were made in the leaves of plants by photosynthetic reactions driven by sunlight. Let yourself realize that it is the energy of sunlight, delivered to you through the medium of sugars made by plants, that is sustaining your life.