................................... The Earth is an inhabited planet

Introduction. Our planet has living beings and so far we do not know of any other star that does. That is, for the moment, we only have this house to live in and, therefore, it is needed to take care of it.

We know that living beings nourish, interact and reproduce. We also know what they are made of and what chemical reactions are taking place in its interiors, but we still do not know how they are able to control all these reactions instead of starting a process of disorganization as happens when they die. Nor do we know how did the first living creature emerged and whether it appeared on our planet or came from the outside.

Our ideas about the origin of the life on Earth are constantly changing. For example, for many years it was believed that because of the atmosphere being rich in oxygen, our planet could have had live beings. Today we know that it is not exactly true, but just the opposite, that now, thanks to having live organisms, our atmosphere is rich in oxygen. Today we know that the early Earth's atmosphere had no oxygen and that thanks to the microscopic single celled-organism, called cyanobacteria, which through executing photosynthesis originated oxygen contained in present atmosphere, plants and animals existence is allowed. All of this is explained In this interesting chapter.

Activities to do. Read the explanations about the characteristics of living things and the conditions that stars must be fulfilled for the existence of living beings, and perform Multiple Choice Test 9.1. Then read the text on the chemical composition of living beings and levels of organization of matter and perform Multiple Choice Test 9.2, Crossword 9 and Complete Sentences 9.

1. Concept of living being. A living being is that being who is able to nurture, socialize and reproduce, that is, to perform three vital functions.

  • Nutrition. It is the ability to capture matter and energy from the outside in order to grow, develop and perform all other vital functions.
  • Relationship. It is the ability to capture the variations of the external environment, called stimulation, and emit adequate responses.
  • Reproduction. It is the ability to generate new individuals. As the length of life of an organism is limited, without the reproduction life would be extinguished at the death of the first living.

2. Characteristics of living beings.

  • They are very complex beings.They consist of many different chemicals that react in a controlled manner.
  • They are made of organic matter. For the most part they are constituted by organic matter, which in nature is the matter that is only presented by living beings or their natural derivatives, such as carbon and petroleum. The organic material consists essentially of carbon and hydrogen atoms.
  • They present high water content. This is necessary since all the biological reactions that occur in them are given in this liquid.
  • They work for themselves. They work independently and searching for their own benefit.

3. Environmental characteristics that have to be fulfilled by a star in order to accommodate living beings.

The characteristics of living beings imply that for a star (planet, satellite or asteroid) to contain living beings, two environmental properties must be submitted:

  • A source of energy for living beings to feed themselves. For example, plants need light to nourish through the process of photosynthesis, and animals need to consume plants or other animals that eat plants. In conclusion, without a source of energy such as light, a planet cannot permanently sustain any organisms
  • A temperature that allows the existence of water. The organisms require liquid water in its interior to perform its vital functions. Without a water, cells could not perform any biological reactions inside them, and therefore, the plants and animals would die.

4 . Astronomical features that should be presented by an astral in order to have environmental characteristics that allow life.

For an astral to present a source of energy and a temperature that allow the existence of life, astronomical characteristics should be as follows:

1 . An appropriate distance to a star. An excessive proximity cause a temperature too high and water would evaporate, and an excessive distance imply that all the water would be in the form of ice.

2 . A properly sized planet. It should be large enough for its gravity to hold an atmosphere. This is essential because the atmosphere does the following:

  • Contains gases that are required by living beings. For example animals require oxygen and plants require oxygen and carbon dioxide.
  • Prevents the surface from dangerous radiation. These are ultraviolet and X radiation.
  • Thanks to the greenhouse effect it prevents sudden temperature changes between day and night, that occur on planets and satellites that, because of their small size, have no atmosphere.


5 . Life in the Universe. Our planet, the Earth, is located at such a distance from the Sun, that makes its water to be liquid and in the gas state. Moreover, its size makes that gravity is able to maintain a layer of gas above it, called atmosphere. Thanks to all of it, on our planet the existence of live beings is possible. At present, there is no evidence of the existence of any living being elsewhere in the universe, but given its immensity and that we only know a small part of it, most scientists believe that it is likely that there is life in Universe. Regarding to our solar system, the general opinion is very differential, that is not believed to be life on other planets in the solar system, there is only a certain possibility on Mars, if at certain depth was liquid water, given that on its surface it is impossible, because it is covered with ice. Let us remember that water is the essence to the existence of active life.

Multiple Choice Test (Spanish activity. Activity in English is under construction)


6. The chemical composition of living organisms.

Living things are composed of the same chemical elements that are on the surface of our planet, but in a very different proportions, because some elements are much more suited to constitute living beings than others. We remember that the chemical elements are the different types of atoms that link together, two or more atoms, give molecules. Well, there are many types of atoms that do not serve to constitute the molecules which form living organisms, because the links between them are too weak and break. It has a similar reason to a childish play, construing buildings from parts, where cubes or prisms are very good to do it, but if the pieces were bent or had no flat surfaces, construction would be impossible or very difficult.

The elements which form living things are called biochemical elements. There are about 70 of them and the most important is carbon, since it constitutes the basis of the majority of the molecules that form the living organisms. The molecules consisting essentially only carbon and hydrogen are called organic molecules and these molecules are composing the organic matter. The main types of organic molecules that are presented to be living beings are carbohydrates, lipids and proteins.

The molecules that are not made only of the carbon and hydrogen are called inorganic molecules and are forming the inorganic matter or mineral matter. The main types of inorganic molecules which are presented in live beings are water, dissolved salts that are in the blood, and un-dissolved salts which form a skeleton.

There are two groups of bioelements:

  • Primary bioelements. Are necessary to form the various types of organic matter, ie to form glucides, lipids, proteins and nucleic acids, There are six: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P) and sulfur (S).
  • Secondary bioelements. The remaining bioelements. The most important are: sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), chlorine (Cl) and silicon (Si). In some organisms, some may be very abundant. For example, calcium in bivalve shells is calcium carbonate.

7 . Carbon. It is the most abundant element in organic matter, along with hydrogen, it is needed to form it. As living beings are basically an organic matter, carbon is essential for life. This is not due to its abundance in nature, which is little, but to its properties, which are:

  • It can form stable unions. The bonds between carbons and between these and other bioelements are stable (which allows to form structures of organisms) but liable to break and release energy, which frees energy to grow, socialize and reproduce.
  • It can form three-dimensional unions. Unions or chemical bonds are arranged in three directions of space. This allows to build structures of the exact shape that is needed to perform a particular function. They can also form large molecular chains and this allow them to store information and work as an organism.

8 . The levels of complexity of living matter. To facilitate the study of living matter seven levels of organization are differenced, which are:

  • Subatomic level. Includes subatomic particles. For example protons and electrons.
  • Atomic level. Includes atoms. For example carbon atoms, hydrogen atoms, etc.
  • Molecular level. Includes molecules that are the union of two or more atoms. For example water molecules, glucose molecules, etc.
  • Cellular level. Includes cells. For example nerve cells, muscle cells, etc.
  • Multicellular level. Includes tissues, organs, systems and organs. For example connective tissue, kidney, nervous system, respiratory system, etc.
  • Population level. Includes populations, which means individuals of the same species that occupy the same area at a given time. For example the population of sparrows that are currently in a particular area
  • Ecosystem level. Includes ecosystems, which means the set of stock that are in a particular area and the relations established between them and between them and the environment.
Multiple Choice Test (Spanish activity. Activity in English is under construction)
Crossword. (Spanish activity. Activity in English is under construction)
Complete sentences (Spanish activity. Activity in English is under construction)
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