The notion of Energy

In order to breathe, move, or talk, we need energy. A car, an airplane or a ship needs energy to move. In general, energy consumption is the basis for every change of a physical object in our world.

Energy is a very common concept in the field of natural sciences. It is not so easy to define it, though. We could say that a certain body encloses energy if, under the appropriate circumstances, is able to cause a transformation to itself or to its environment.

Forms of Energy

There are many different forms of energy: Kinetic energy, potential energy, chemicalenergy, nuclearenergy, electricenergy, radiant energy (including light), thermalenergy, etc.

The two basic forms of energy are kinetic and potential energy.

Kinetic energy is energy contained in a physical body because of its motion. Potential energy is energy contained in a physical body because of its position (e.g. because of the height at which it lies) or because of its shape (e.g. a squashed spring). Using kinetic and potential energy as a basis, we are able to describe all other forms of energy. Chemical energy stored in food, for instance, is the sum of the potential energy stored in the molecules that constitute food itself. In the same way, electric energy is the energy which is carried by the electric current as a result of the motion of electrons (kinetic energy).

The Laws of Energy

The total energy of a system can not change (!). Energy cannot be created out of nothing, nor can it be lost. It can only be stored or transformed. During the transformation of energy from one form to another, however, there is always an amount of energy that is released to the environment as thermal energy, which is very difficult to collect and, consequently, to use. Therefore, we assume this energy to be non-useable and, because of that, we say that it is, in a sense, “lost”.

Because of the fact that all machines and all organisms need energy in order to perform their functions, we have to be extremely mindful of how to use it –for no source of energy on planet Earth is inexhaustible, while, as we have already said, during the transformations some of it is released to the environment in the form of thermal energy which we cannot reuse.
Imagine life without food, electricity, refrigerators, computers or the medical machines able to keep a man to life after a serious accident.
In order to understand how valuable energy is, take the following test: Try not to consume electric energy for a whole day.

The Sun is the Basic Energy Source

The Earth, according to what scientists know so far, is the only planet of our solar system that hosts different forms of life.These forms of life must absorb or acquire energy from the environment in order to survive.

The SOURCE of energy for all living organisms is the SUN

Energy reaches Earth in the form of sunlight (solar energy) and then escapes Earth in the form of heat (thermal energy). Even though thermal energy may stay trapped in Earth’s atmosphere for quite a long time, eventually it will again be released in space.

The bottom line is that -in all the different ecosystems- energy flows towards one direction only, and that it, finally, escapes EARTH.

Hence, the maintenance of life on earth (terrestrial and aquatic ecosystems) requires the constant resupply of energy through the sun’s light.

Energy Flow in the Natural Ecosystems

Most of solar energy reaching Earth is absorbed by the atmosphere, by the oceans and land. This leads to their heating.

Only a small proportion of the total solar energy reaching Earth’s surface is used by the PLANTS (0.2%).

Plants absorb solar energy, transforming a part of it into stored chemical energy. How do they manage that?

This is achieved through a process called Photosynthesis:: Apart from solar energy, plants absorb CO2 found in the atmosphere, as well as water and other nutrients from the soil, to compose their own nutrition: Glucose.
Plants are thus called producers or autotrophs (from the Greek eaftos “self” and trefo “nourish”.)

In other words, we could say that plants are chemical reactors which transform a small amount of the solar energy reaching Earth into chemical energy storing it in their tissues.

Plants use most of the total chemical energy which is produced in their cells in order to:
1. Build their main structural components (their bodies) –stems, roots, leaves, fruits
2. Grow (to increase in size)
3. Perform all their functions
Thermal energy is released inside the cells of a plant, through the above processes.

In conclusion, just a small part of the total chemical energy produced by plants can actually be found as stored chemical energy in their bodies. This small amount of stored chemical energy is the only available source of energy for the herbivore animals.

Plants are the nutritional basis of all animals because they are the nutrition for all other ETEROTROFH organisms (from the Greek etero“other” and trefo“nourish”). Animals can be herbivores, carnivores or omnivores, depending on the kind of nutrition they consume. If not the omnivores, then the herbivores are the intermediates in the energy flow between plants and carnivore animals.
We could thus say that the energy pool for herbivores and carnivores alike is the stored chemical energy which they acquire through the consumption of plants or other animals respectively!

This energy, as we have already discussed before, deriving from the solar energy collected by plants, is actually DECREASING as we move along from one trophic level to the next.

Only 10% of a trophic level’s total energy passes on to the next trophic level.

Plants and animals use the energy they acquire to grow and perform their functions. Most of this energy is eventually lost in the environment as heat (cellular respiration).

This is the reason why more herbivores than carnivores are found: Carnivores cannot use but the remaining energy of the herbivores. If there were more carnivores than herbivores, then both would go extinct (e.g. if, in a wood, wolves were greater in number than deer, then all the deer would be eaten, something that would quickly result in the starvation of all the wolves).
Consequently, an ecosystem needs to be balanced in such a way that it contains all the energy required for every single organism in order to survive, including humans.


Where does all this energy go after organisms die?
There comes a time for all the leaves to fall, for grass to shrivel and for all living organisms to die. Yet, there is a group of organisms which feeds from dead organisms or from the dead parts of the living organisms.

hese organisms are called DECOMPOSERSand have a very important part to play inside the ecosystem because they decompose matter (organic matter) into simpler molecules (inorganic matter) which is then expelled to the ground.

Decomposers can be: Scavengers (e.g. vultures) decomposing large pieces of dead matter, like bones Detritivores(or, else, saprophages) like earthworms which feed on organic matter partly decomposed Chemical decomposers (many bacteria and fungi species, like mushrooms) that decompose organic matter at the molecular level.
Thus, decomposers use organic matter as a source for nutrition and ENERGY. They need energy in order to grow and to decompose.
Nevertheless, part of the energy acquired by these organisms from their nutrients is again released to the environment as heat. And these organisms too, are then consumed by other animals, etc.

The Formation of Fossil Fuels

During the Paleozoic and Mesozoic geologic era, 100-500 million years ago, the surface of the Earth was widely covered by swamps and shallow seas sustaining many flora species and phytoplankton. Decomposition of the dead tissues by decomposers was blocked by the anaerobic conditions prevailing at the deeper layers of these water collections. As a result, massive quantities of dead organic matter were accumulated. During the passing of millions of years, that organic matter got buried under sediments, and consequently, because of the pressure and the high temperature, it was transformed. Sediments that were buried 2,000-2,500 meters underground were transformed into petroleum. Sediments buried more than 5,000 meters underground were transformed into natural gas. Thus, natural gas can be discovered together with petroleum or during the process of drilling for petroleum. Finally, there is coal, which can be found buried both deep or close to earth’s surface.

Petroleum, natural gas and coal are the fossil fuels. Fossil fuels are the storages of Earth’s energy because, as it was stressed before, they derive from from the remnants of plants and therefore they contain part of the solar energy initially collected by the plants of the Earth millions of years ago.

Sources and Usage of Energy


Fossil Fuels (Coal, Petroleum, Natural Gas)
Fossil fuels are not considered renewable resources -even though they are still produced through the natural processes until this day- because the rate by which they are consumed by man is much more rapid than the rate of their formation.
Our civilization is heavily based on coal, natural gas and petroleum to produce energy. To be more concise, 81% of global energy consumption comes from fossil fuels. These basic resources are linked in various ways with common energy consumption in our everyday life.
Coal: it is mined and then transferred to electric power generator facilities (electric energy).
Petroleum: Transportations (cars, airplanes, tractors, etc) depend almost solely on petroleum. Moreover, petroleum is used for home heating as well as for the production of cosmetics and pharmaceuticals.
Natural Gas: Most of natural gas consumption is linked with industrial and domestic usage (heating and cooking, for example). However, it is increasingly used for electricity generation (electric energy). In addition to that, it is also used as a fuel for vehicles such as cars and buses (transportation).
Nuclear Energy: The steam released by the fission of uranium atoms sets a motor in motion. This is how nuclear power plants produce electric energy (electricity). Today, more than 100 nuclear power plants are fully functional in all over the world, covering the 17% of global energy production.


Solar Energy: In increasing rates, we are transforming solar energy that reaches Earth into electric power using photovoltaics (electricity). We are also able to utilize solar energy for the heating of the water we use (heater) or for the heating of buildings, by making use of the orientation and the insulation of a house.
Hydro-electric power:: Modern usage of the power of the water falling from aloft into great dams, results in the production of electric energy (electricity).
Επίσης Wind Power It is the energy produced by the exploitation of the movement of the wind. Wind parks consist of thousands of wind generators which in their turn transform wind power into electric power (electricity). Moreover, wind power is used for the motion of sailing-ships (transportation).
Geothermal Energy: Is the energy that is coming from inside the Earth to the its crust. In various places of the world there are sources of hot water or steam which are used for heating buildings (thermal energy) or for electric energy production (electricity).
Biomass: Biomass (wood, agricultural waste, organic human waste, etc) contains solar energy collected by plants through photosynthesis. We use biomass by burning wood or waste in order to produce thermal energy (heat) or electric energy (electricity). Moreover, biogas is produced from the rotting of waste. Biogas can be used as a biofuel (transportation).

Earth as a Greenhouse

The rays of the sun penetrate the glass walls of a greenhouse, and then the heat of the sun stays trapped inside. This is approximately how the atmosphere of the Earth works as well. The gases in the upper layers of our atmosphere (carbon dioxide, most importantly) formulate something which is known as the “greenhouse’s glass wall” by preventing part of the heat to escape in space. This is the reason why Earth is a warm and hospitable planet. These gases are called “greenhouse gases” and the whole phenomenon is called the “greenhouse effect”. The “greenhouse effect” plays a very important role in maintaining life on Earth as we know it, because without it the average temperature on Earth’s surface would be approximately -19°C. Evidently, the greenhouse gases are acting as a blanket which keeps our planet warm.

The Enhanced Greenhouse Effect

High carbon dioxide -and the rest of the greenhouse gases- concentrations lead to the intensification of the natural greenhouse effect. This intensification leads to global warming and climate change.

Climate Change researchers agree on the fact that human activity (such as the burning of fossil fuels) is at least partly responsible for the global warming. They also predict that in the following years drastic climate changes will occur. If global warming is not confronted, then all the countries of the world will suffer the consequences, which will differ depending on their geographic position. Some of the expected environmental effects would be: Long periods of droughts, sea level rise, intensified hurricanes and typhoons, fierce heat waves, fires, decrease in water supplies, destructive effects on agricultural production. Many other disturbances will also follow that will cause gradual biodiversity loss, difficulties concerning viability (access to food and water) as well as the emergence of millions of climate refugees.

Actions for reducing the greenhouse gasses emission and preventing the occurrence of severe weather phenomena

In terms of politics, it is very important to enact a specific global threshold for greenhouse gases emission. Moreover, new investments are needed in order to minimize the dependence of energy production from the burning of fossil fuels and also to secure the growth of the industry based on alternative energy recourses.

Nuclear power is an alternative energy resource which doesn’t contribute significantly to the increase of Earth’s temperature. However, the usage of nuclear power can be dangerous. Nuclear plants not only produce radioactive waste but can also cause a nuclear accident inducing uncontrolled consequences for the living organisms of our planet.

Thus, the plan for economic growth must focus on the idea of managing our energy resources reasonably (less energy consumption) and using more renewable energy resources (e.g. solar energy, wind power, etc.) All the above should be insured by the state. Moreover, all he states of the world must encourage the usage of public transports by promoting them in the urban areas.

On the other hand, and on the social level, it is necessary for the citizens to adopt an environmentally responsible behavior. This is why it is should be a priority for all states to ensure that all schoolchildren are familiar with the idea of sustainable growth through their formal institutional education.

What can you do to stop global warming?

Reducing energy consumption begins in your everyday life, in your own home. The triptych “Reduce, Reuse, Recycle” can be the basis of the biggest changes.

Finally, do not forget: it is very important to share your knowledge with other people. If you can change the way you think about your old habits, then you can become an example for other people and help them change their own way of thinking.

Download and Print the Green Agents Posters to be reminded of what you've learned in class here!

Download and Print the Green Agents Posters to be reminded of what you've learned in class!