Entropy and Biology Photosynthesis

To live and reproduce, plants and animals need a continuous flow of energy. The energy of the biosphere which originates in the luminous energy of the Sun, is captured by plants and passes from one living form to another along the food chain. The luminous energy captured by chlorophyll, the green pigment in plants, is stored in carbohydrates, molecules rich in energy, by a process called photosynthesis, a term that means 'to make things with light'. This radiant pathway that provides us with great quantities of food, fibers, and energy, all of solar origin, has existed for about 4 billion years, a long time if we think that hominids appeared on the Earth only 3 million years ago and that known history covers only 10 000 years. The ancestors of today's plants were the blue algae, cyanobacteria, that began to practise photosynthesis, assuming a fundamental role in biological evolution.

All vegetation whether natural or cultivated, has been capturing solar energy for millennia, transforming it into food, fibers, materials, and work, and providing the basis for the life of the biosphere.

The vast majority of the energy received by the Earth's surface from the Sun is dispersed: it is reflected, stored in the soil and water, used in the evaporation of water, etc. Only about 1 % of the solar energy that falls on fertile land is fixed by photosynthesis in plants (grass, trees, phyto-plankton) in the form of high-energy organic molecules. By biochemical processes (respiration) the plant transforms this energy into other organic compounds and work.

The food chain considered in terms of energy flows has a logic of its own: the energy degrades progressively in the different phases of the chain (plant producers, animal consumers, microbe decomposers) giving back the elementary substances necessary to build again the molecules of living cells with the help of solar energy.

The organization of living beings in mature ecosystems slows the dispersal of energy fixed by plants to a minimum, using it completely for its complex mechanisms of regulation. This is made possible by large 'reservoirs' of energy (biomasses) and by the diversification of living species. The stability of natural ecosystems, however, means that the final energy yield is zero, except for a relatively small quantity of biomass that is buried underground to form fossils for the future.

Photosynthesis counteracts entropic degradation in so far as it orders disordered matter: the plant takes up disordered material (low-energy molecules of water and carbon dioxide in disorderly agitation) and puts it in order using solar energy. It organizes the material by building it into complex structures. Photosynthesis is therefore the process that by capturing solar energy and decreasing the entropy of the planet, paved the way for evolution. Photosynthesis is the green talisman of life, Maxwell's devil that decreases the entropy of the biosphere.

On the Earth, living systems need a continuous flow of negative entropy (i.e., energy from outside) and this flow consists of the very solar energy captured by photosynthesis. This input of solar energy is what fuels the carbon cycle.

The history of life on the Earth can be viewed as the history of photosynthesis and the history of evolution, as the history of a singular planet that learned to capture solar energy and feed on the negative entropy of the universe for the creation of complex structures (living organisms).

The Sun is an enormous machine that produces energy and offers the Earth the possibility of receiving large quantities of negative entropy (organization, life), allowing a global balance that does not contradict the second law of thermodynamics. Every year the Sun sends the Earth 5.6 x 1024J of energy, more than 10 000 times more energy than mankind consumes in a year. It is as if the Sun sends us 2601 when we consume only 15 kg.

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