If matter and energy did not have this facility to transform from one to the other, there would be no life on our planet. Life on our planet depends on two chemical processes: photosynthesis and combustion (metabolism). The two processes depend on each other, that is, for one to take place the other must also take place. Through photosynthesis, plant cells change sunlight into chemical energy; this chemical energy is then burned by the cell itself in the presence of oxygen.
Photosynthesis is a prebiotic function, that is, it is not yet in itself synonymous to life, because life is always associated with combustion (slow combustion or metabolism). Photosynthesis is the manufacturing of food in the form of glucose (sugar) by the cells; life is the combustion of this food using oxygen as the oxidant. Photosynthesis is carried out by plant cells which have the unique ability to fabricate their own food. This means that plants need not move about to search for food like animals.
However, the chemical reaction that keeps them alive is not photosynthesis, but rather combustion. They consume part of the glucose and oxygen they produce to stay alive, and have the energy to perform more photosynthesis. They release the rest of the oxygen into the air, and the stored glucose is used to feed the animals that eat them. Since photosynthesis can only occur during the day, that is, in the presence of sunlight, during the night the plants consume part of the glucose and oxygen they produce during the day. Even so, they still accumulate energy, that is, they grow and release the oxygen that other living beings need to live.
How did Life Appear on Our Planet
Plants generate all the food that exists on Earth; without them, Earth would be a dead planet. Four and a half billion years ago, Earth was formed from the collisions of thousands of meteorites in the emerging solar system. The planet was initially so hot that its surface was an ocean of lava.
Millions of years after the formation of the planet, Earth entered into a process of gradual cooling and this alteration gave rise to the formation of a thin layer of rock that covered the entire planet. With the cooling down of the Earth’s temperature, a large amount of gases and water vapour were expelled from the Earth’s interior. This process resulted in the gases forming the atmosphere, and the water vapour favoring the appearance of first precipitation.
However, as the temperature was still very high, when the rainwater touched the ground it evaporated instantly. It was only when the ground temperature cooled to below the boiling point of water that water accumulated, and oceans were formed. It was in the oceans that life first appeared.
The Origin of Oxygen
It may seem logical to think that just as water came before life appeared, so should oxygen the oxidizing agent of combustion come before the emergence of life. But this is not what happened. Life or a primitive form of life appeared first, and only then did oxygen appear. We could say that oxygen, although inorganic, has an organic origin. There may have been some oxygen in the Earth’s atmosphere 2.4 million years ago, but it was insufficient to create life. What caused the oxygen level to skyrocket and reach 21% in the atmosphere were the microorganisms called cyanobacteria or blue-green algae.
All oxygen in the atmosphere comes from water molecules. The latter is composed of two hydrogen atoms and one oxygen atom. In order for a water molecule to separate into its two simple elements, an electric current must pass through it. The breaking down process is called electrolysis, and it does not occur naturally. If the same result can be secured naturally, we would not need fossil fuel, and we would have an inexhaustible and eternally renewable source of energy.
Many people think that photosynthesis is the process by which plants turn carbon dioxide (CO2) into oxygen (O2), but this is not so. Oxygen does not come from a chemical reaction of carbon dioxide, but from water. It is produced during the first stage of photosynthesis when electrolysis occurs. The first microorganisms known to carry out photosynthesis on Earth are the cyanobacteria.
Cyanobacteria carry out electrolysis in the first stage of photosynthesis, as they are interested in the hydrogen that water contains, but not in the oxygen which they release. In other words, oxygen is a byproduct of this first stage. Cyanobacteria need the hydrogen because when combined with carbon dioxide, during the second stage of photosynthesis, glucose is synthesized. However, cyanobacteria could not extract hydrogen from water (which contains twice as much hydrogen as oxygen) without releasing oxygen as a “waste” product.
The very first microorganisms to emerge were the thermophiles, that is, they were able to survive in very hot environments, and to convert inorganic substances such as sulfur and carbon into energy. When the temperature of the atmosphere fell to 72 ° C, these microorganisms evolved into cyanobacteria, as this is the maximum temperature at which photosynthesis is feasible.
For billions of years, oxygen has been accumulating in the atmosphere, and the one that rose to the highest layers has become ozone. In this way the environment was formed and the conditions for life to diversify were created. All the plants on our planet feed in the same way as this microorganism, that is, through photosynthesis.
The Morphology of Cyanobacteria
Also known as the blue-green algae, already about 1500 species in 150 genera have been catalogued. Most species live in marine waters, lakes, rivers and even in very humid soils. They exist in a wide range of shapes and sizes: rods, spheres and filaments.
They measure only a few micrometers, that is, they can only be seen with the aid of microscopes. They reproduce asexually, are unicellular, but can be found in colonies or filaments. These microorganisms, in the aquatic ecosystem, form the so-called phytoplankton and are at the base of the food chain of this ecosystem. They carry out oxygenic photosynthesis (they use water as an electron source and release oxygen) and are autotrophic, since photosynthesis is their main way of obtaining energy.
Cyanobacteria appeared on Earth about 3 billion years ago. This dating is confirmed from fossils known as stromatolites, which were formed by these microorganisms. Because they have existed for so long, it is believed that they were responsible for producing the oxygen that accumulated in the primitive atmosphere.
The Chronology of Life on Earth
Our planet Earth was formed 4.5 billion years ago. After water settled in the oceans, the first thermophilic microorganisms appeared about one billion years later. Soon after, the cyanobacteria emerged, when the temperature of the seawater began to allow for photosynthesis. Cyanobacteria are simple microorganisms formed of cells without nucleus, that is, they are prokaryotes.
Two billion years ago, the eukaryotic cells appeared, that is, cells with nucleus that are the fundamental cells for life – all living beings, animals and plants, are made up of eukaryotic cells. Approximately 570 million years ago, there was the explosion of Cambrian life, marked by a burst of diversification of life on Earth, and the first non-plant organisms evolved.
About 438 million years ago, plants which until then only existed in the sea, began to populate the Earth. They were the first to leave the sea, which is logical, since all life on Earth depends on them. Years later, around 408 million years, the first amphibians emerged, the ancestors of the reptiles which came 50 million years later.
The extinction of the dinosaurs, the most emblematic reptiles on our planet, occurred 66 million years ago. After they were dethroned, Earth began to be dominated by mammals, and among them, the primates which came about 55 million years ago. Human beings are believed to have appeared 5 million years ago.
The Components of Photosynthesis
For a plant to able to photosynthesize, that is, to use sunlight to transform solar energy into chemical energy, and therefore be able to manufacture its own food, it needs three inorganic elements: water, sunlight, and carbon dioxide. To these elements we add the particularities of the plant cell, which has one component that animal cells do not have -- chloroplasts.
Water
The water in the vacuoles controls the intumescence or swelling of the plant cell. If the plant is left with little water, it withers; a plant with wilted leaves, that is, they have lost much of their intumescence due to a lack of water, decreases in volume and cannot perform photosynthesis well. Due to the thermal control capacity of water, it is possible for plants to absorb a large dose of solar radiation without temperature rising.
Water not only facilitates photosynthesis by keeping the cell vacuoles operational, but it is also an integral part of this chemical process. Plant chloroplast causes the photolysis of water, that is, it breaks down the water molecule into its simple elements: hydrogen and oxygen. It then releases the oxygen into the atmosphere as a byproduct, combining the remaining hydrogen with the carbon dioxide in the air for the subsequent reaction, which is the production of glucose, the energy and food for plant cells.
Sunlight
Light is a wave of electromagnetic radiation made of photons, with wavelength ranging between infrared and ultraviolet radiation. The light that seems white or transparent reveals part of its complexity when it passes through a prism or tiny droplets of moisture in the atmosphere, forming a rainbow. Of this electromagnetic radiation, only one part is visible to the human eye, and this visible light spectrum ranges from 380 to 760 nanometers, from violet to red.
Sunlight is the only source of energy on our planet. All types of energy that we have on Earth, in the final analysis, come exclusively from the sun. Without oxygen there would be no life, without photosynthesis there would be no oxygen, and without the sun there would be no photosynthesis.
It is the sun that sets into motion the cycles upon which life rests: the oxygen cycle, the water cycle, the nitrogen cycle, and many, many other cycles. Without solar energy this would be a dead and dark planet. It is the solar energy that makes it possible for organic elements to combine and produce organic life.
Carbon Dioxide
This gas is nowadays widely talked about as being responsible for the greenhouse effect, which causes the rising of the atmospheric temperature. It is therefore viewed in a negative light as it does not let heat escape from Earth into the space. But plants need carbon dioxide, and life in general also needs it. All organic molecules are a combination of carbon and hydrogen atoms; a molecule which contains both carbon and hydrogen can never be inorganic.
In photosynthesis, this is precisely what happens: the combination of hydrogen, that the plant obtained from the decomposition of water molecule, with carbon dioxide that it gets from the atmosphere produces glucose, that is, a food for life. In fact, the more CO2 humans emit into the atmosphere, the more plants absorb it and the faster they grow.
The ability of plants to remove CO2 from the atmosphere has doubled, it is as if they want to find the solution to our problem of excessive greenhouse effect. That is why despite tropical deforestation, the planet’s vegetative mantle is increasing. But even so, all the terrestrial and marine forests are not enough to purify the atmosphere and prevent its temperature from rising.
Chlorophyll
Chlorophyll is the light absorbing pigment on the thylakoid membrane inside of the chloroplasts. Plant cells are autotrophic, that is, they make their own food. Animal cells are heterotrophic because they feed on life, that is, on other living organisms and do not make their own food. In some ways, we can say that every kind of life depends on plant life, that is, animal life is a parasite of plant life.
It goes without saying that animal life, especially human life, is much more complex than plant life. However paradoxically, at cellular level, the plant cell has practically everything that the animal cell has, and one more organelle that the animal cell does not have: chloroplasts. Whereas animal cells only carry out metabolism, that is, transform matter (food) into energy, plant cells not only carry out metabolism, but also photosynthesis, the opposite of metabolism, where energy is transformed into matter (food).
It is worth remembering that plant cells also have mitochondria where their metabolism takes place, and like in animal cells, is essentially a slow combustion. The difference is that plant cells metabolize or burn the food they themselves have made, carried out with the help of oxygen, the oxidizing agent which they have also made, and still have leftover of both for other living beings that depend on them to survive.
Chlorophyll carries out the main function of photosynthesis: the extraction of energy contained in sunlight to produce carbohydrates. An interesting aspect is that when we look at the atomic structure of chlorophyll, we see a great similarity with the atomic structure of hemoglobin. The chlorophyll molecule consists of a central magnesium atom, which gives it the green colour, surrounded by a nitrogen-containing ring to which a long carbon-hydrogen side chain is attached. The hemoglobin molecule has the same nitrogen-containing ring but with a central iron atom instead, which gives it the red colour. The chlorophyll molecule has a much longer carbon-hydrogen side chain than the hemoglobin molecule.
There are several types of chlorophyll molecules, each one is tuned to or absorbs different wavelengths of light within the visible light spectrum.
- Chlorophyll a: is the pigment that gives the blue/green colour in plants and absorbs violet and orange light the most, at 430 nm and 660 nm, respectively. It doesn’t absorb green, so the green light is reflected to our eyes
- Chlorophyll b: is the yellow/green pigment present in plants and absorbs mostly blue and yellow light, at 450 nm and 640 nm, respectively. It also does not absorb green.
- Carotenoids: these are yellow, orange, and red plant pigments, present in all photosynthetic organisms; and absorb light maximally between 400 to 550 nm.
- Phycobilins: these are red and blue pigments, found in red algae and cyanobacteria, respectively. They absorb light of wavelengths between 500 and 600 nm.
It is the most important biochemical process for life on our planet. The second would be metabolism, but without photosynthesis there would be no metabolism, since it is from photosynthesis that the fuel and the oxidizing agent of combustion that we call metabolism are produced. Photosynthesis generates the base of the food chain of the entire planet.
Every day we feed on the sun, whether we are vegans, vegetarians, carnivores, omnivores or pescetarians. Ultimately, what we are essentially eating every day is sunshine!
The Photochemical Stage (Light Dependent)
In the first stage, water plus sunlight releases oxygen by electrolysis or photolysis. The free electrons from the water molecule use the hydrogen released in the photolysis as a carrier to enter the chloroplast organelle, on the thylakoid membrane inside the chloroplast. Here light energy is converted into chemical energy (ATP) and reducing power or electron donor (NADPH). These two substances trigger the next stage or the biochemical stage of photosynthesis. Therefore, ATP and NADPH are the end products of this first stage, in which sunlight is needed.
The Biochemical Stage (Light Independent)
This stage is much more complex than the first, it is also known as the Calvin Cycle. It consists in short of three steps: fixation, reduction, and regeneration. It starts with the most important enzyme in the world, code name RuBisCo. This substance captures carbon dioxide in the atmosphere, mixes it with a molecule called ribulose bisphosphate (RuBP); in short, fixing the carbon from its inorganic form in CO2 into organic molecules (3-PGA). The next step is a reduction reaction in which electrons are gained by the 3-PGA molecules, supplied by NADPH and ATP, to produce G3P molecules. In the last step, some G3P molecules go to make glucose/sugar, and the rest is sent to regenerate RuBP molecules needed for the fixation step and the cycle repeats. The overall chemical formula for the photosynthesis process is as follows:
6CO2 + 6H2O + sunlight = C6H12O6 (sugar) + 6O2
Conclusion
Today the planet is covered with vast forests and wooded areas; yet 70% of the oxygen in the Earth’s atmosphere does not come from land plants, but from the marine plants in the ocean. It was there that photosynthesis began and it’s still there the photosynthesis that provides us with most of the oxygen that we breathe. We could say then that the photosynthesis of land plants feeds our bodies, and that of the sea allows us to breathe.
The complex chemical reaction that uses sunlight to release from water the oxygen that we breathe and combines the remaining hydrogen with the carbon dioxide in the air to produce the food we eat, is indeed the Big Bang of life on Earth.
Fr. Jorge Amaro, IMC
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