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