Living organisms are material bodies characterized
by specific features: Birth: this is the beginning of an organized
existence originated from an intimate connection with other similar bodies. Growth: living organisms increase their body mass and develop multiple capabilities
through energy and material harvesting. Reproduction: this is the generation of
other entities belonging to the same species. Death: the step of dissolution of any
mass aggregation. The loss of any living capability. In order to assess the relevance of energy
in the natural world, the next step is to consider the link with biological life. First,
we need to look at the network of relationships between the environment and living organisms.
L.U.C.A – the Last Universal Common Ancestor –formed around 3,8 billion years ago and
differentiated in a multitude of species, humans included. Each of these species had a major role in its interaction with the environment and other organisms. A complex web developed to guarantee the availability of nutrients and energy.
The figure describes the multiple links between different living organisms and the environment in harvesting and discharging of materials and energy flows.
Life is possible once energy is available to produce organic matter and to drive vital
functions. Almost all living organisms harness energy from the Sun.
Most of autotrophs use solar energy directly by means of photosynthesis.
On the other side, heterotrophs use energy from the Sun indirectly by feeding on organisms
capable to grow through solar energy. In figure, it is clear how the removal of
autotroph producers would damn other species to not access energy anymore.
Although the Earth receives from the Sun a flow of radiant energy of 174 million of billions
watts, the existence of life is guaranteed by the 0,03% used by photosynthesis.
The great cycles of the hydrosphere and atmosphere involve much higher energy amounts!
Human beings are heterotrophs and therefore not able to use directly solar energy for
body mass synthesis. At the same time, the metabolism of fossil-fueled civilization requires
an amount of energy equal to about 25% of the photosynthetic production.
These two values are too close to exclude catastrophic consequences for humankind. All complex life on this planet, all its incredible diversity, all our hopes and worries, are but trasmutations of the Sun’s light, and photosynthesis is the agent of this miracle. Living organisms have the particular ability
to synthesize organic biomass of their bodies withdrawing nutrients and energy from the
external environment. A widely accepted classification divides living
organisms into two groups, basing on how they provide nutrients and energy.
Autotrophs are organisms that can transform inorganic carbon, hydrogen and small amounts of nitrogen and minerals into complex organic compounds. Most of them use solar energy to drive these transformations and therefore they are called phototrophs. Some other autotrophic species harvest energy from particular chemical reactions. They are known as chemotrophs.
On the other side, heterotrophs synthesize their biomass by feeding on other living organisms.
Herbivores eat plants, carnivores other animals, and saprophytes the organic material obtained
from the decomposition of died organisms. Finally, omnivores, like most humans, eat
both autotrophic and heterotrophic species. The existence of heterotrophs is constrained
by autotroph organisms because the latter are the door to access energy from food.
More generally, when looking at trophic chains, we discover they have rarely more than 5 links.
In order to understand this natural pattern, we need to look at the efficiency of each
step of the chain. The efficiency of photosynthetic conversion
is actually lower than 5%. Biomass-to-biomass conversion efficiency in primary consumers
rarely exceeds 15%, while it is much lower for higher order consumers. A trophic chain with a high number of links would therefore rely on an extremely low efficiency utilization of solar energy. This inefficient scheme is hardly pervasive in nature.
The engine of life is photosynthesis. We will know better the major processes and carriers
of life energetics in the next talk.