ALAIN VIEL: In your everyday life, you are surrounded
by living organisms, non-living things.
What does make the difference between the two?
Are there special atoms, chemicals found only in living organisms?
In this video, you'll discover that inert and living matters are, in fact,
not distinguished by the chemical elements they contain
but by the relative abundance of these elements and the connection
they make with each other.
What are the major atoms found in living versus inert matter?
This periodic table shows a smaller number of elements labeled in green.
These are the major components of living organisms.
They are combined to form a large number of biomolecules that
vary both in structure and complexity.
The elements labeled in yellow are present in trace amounts in living
organisms but still have vital roles that will be discussed in later videos.
As you can see on this table, carbon, oxygen, hydrogen, nitrogen
make up 95% of the weight of a human body.
Carbon by itself represents 50% of dessicated living matter.
In contrast, in Earth's crust, carbon, hydrogen, and nitrogen
are present in low or trace amounts.
In contrast, oxygen, silicon, aluminum, and iron
are the major components of the crust.
Why is so much carbon needed in living organisms?
Aside from water, the majority of biomolecules are carbon-based.
It is efficient for living organisms to use one atom for a lot of purposes.
Its extreme versatility makes carbon a unique atom.
Carbon can be used to build molecules of different shapes
and geometry-- linear chains, branch chains, cyclical structure.
This structural diversity leads to functional diversity.
So let's take a look at a carbon atom.
Carbon has 6 protons, 6 neutrons, and 6 electrons.
The number of electrons and their position in atomic orbitals
are key to carbon's versatility.
Electrons are distributed in space with specific shapes called "orbitals."
For carbon, the first two orbitals-- each containing two electrons--
are spherical and they are called "1s" and "2s."
The 2s orbital is further away from the nucleus than the 1s.
Carbon has also three equivalent barbell-shaped orbitals
called 2px, y, and z.
The remaining two electrons are unpaired,
and each occupies one 2p orbital-- leaving
the third orbital, 2pz, unoccupied.
Remember, covalent bonds between two atoms
are formed when one orbital for each atom carrying an unpaired electron
fuse into a molecular orbital, which will now contain two electrons.
Based on my description on the carbon atom with its two unpaired electrons,
one could predict that a carbon atom can only form two covalent
bonds with other atom.
But you probably know some molecules where carbon makes four bonds.
Methane, a byproduct off the metabolism of some bacteria,
is one of such molecule where one carbon forms four bonds with hydrogen atoms--
as you can see on the three-dimensional structure.
I will take this opportunity to describe how will we represent molecules
in three dimension.
The bonds depicted by straight lines on this figure
are in the plane of the screen.
The bond, represented by a solid wedge, sticks out of the screen-- towards you.
And the bond represented by a hashed wedge
sticks backwards-- behind the screen.
How can carbon make four covalent bonds?
The electron in carbon can shift around, and one electron from the 2s orbital
can be promoted to the empty 2pz orbital.
Now, the 2s the three 2p orbitals contain each an unpaired electron.
So in theory, now four covalent bonds can
be formed, which would be unequal since unpaired electrons are not
on identical orbitals.
Unequal bonds are very unstable.
So what's the solution?
Promoting one electron is not sufficient.
The orbitals must also undergo a process called "hybridization."
The hybridization of the four orbitals equalizes the characteristics
of the four unpaired electrons.
The newly-formed sp3 orbitals have 25% S character and 75% P character.
On this diagram, we can see the four sp3 orbitals of carbon
needed to form a molecule of methane.
These orbitals spread themself into space
as far from each other as possible.
Each fuses with the S orbital of an hydrogen atom
forming four identical bond at 109.5 degree from one another.
The carbon occupy the center of a geometric shape
called a "tetrahedron" with the four hydrogens at the vertices.
Now, how can carbon make bonds with three-- and not four-- other atoms?
In biomolecules such as ethylene-- a plant hormone
controlling the ripening of fruits-- each
carbon bonds three atoms, two hydrogen and the other carbon.
In this case, electrons have to rearrange differently
around the carbon atom.
In other words, a different type of hybridization occurs.
Once again, one electron from the 2s orbital is promoted to a 2p orbital.
However, only three-- and not four-- hybrid orbitals are formed.
These are called the "sp2 orbitals," and they have 33% S character and 67% P
character.
The sp2 orbitals are arranged in a planar, trigonal structure--
120 degree apart from each other.
The remaining unpaired electron still occupies a 2p orbital.
This 2pz orbital is orthogonal to the plane containing the sp2 orbitals.
To make ethylene, two trigonal structures come together.
Two sp2 orbitals fuse head-to-head to form a sigma covalent
bond between the two carbons.
The sideways overlap of the two 2pz orbitals lead to the formation of a new
covalent bonds-- called a "pi bond"-- between the two carbons.
Each of the remaining four sp2 orbitals fuse
with the 1s orbitals of hydrogen atoms to form four carbon hydrogen bonds.
In ethylene, the two carbons are linked by a double bond-- one sigma and one pi
bond.
The electrons involved in the pi bond occupy a ring-shaped space
between the two carbons.
Finally, how can carbon bond with two other atoms?
Once again, one electron from the 2s orbital
is promoted to the empty 2p orbital, and then
two of the orbitals carrying unpaired electrons are hybridized.
The result are two sp orbitals with 50% S character and 50% P character.
The sp orbitals are 180 degrees from each the.
Other two unpaired electrons remain on 2p orbitals-- 2py and 2pz.
When two of these linear structures come together,
two sp orbitals overlap head-to-head to form
a sigma bond between the two carbons.
Sideways overlaps between pairs of 2p orbitals on each carbon
leads to the formation of two pi bonds between carbons.
Therefore, the two carbons are connected by a triple bond.
Finally, the other sp orbitals-- carried by the two carbon-- fuse
with the 1s orbitals of hydrogen atoms to form two carbon hydrogen bonds.
The molecule formed is acetylene.
Acetylene is not produced by living organisms
but can be made synthetically and used to ripen fruits.
Acetylene is extremely combustible due to the instability of the triple bonds.
This instability might explain why almost no biomolecule
contain a triple bond.
So now, what are the consequences of the various types of bonds
found in biomolecules?
Different bonds affect the geometry and the reactivity of biomolecules.
As you can see on this table, we can consider three characteristics.
First, we will consider the differences in bond length.
The shape of a molecule is determined by the type of atom's bond.
For example, a bond between an oxygen and a nitrogen
is shorter than a single bond between two carbons.
The shape of the molecule is also determined by the presence
of a single versus a double bond.
As you can see on the table, the length of a double bond between two carbons
is shorter than the length of a single bond between two carbons.
The second characteristic to take into account is the strength of the bond.
Different bonds have different strengths.
For example, the bond strengths between carbon linked by a double bond
is higher, as you might expect, than that of carbon linked by a single bond
but not twice as high.
Therefore, the pi bond is weaker than the sigma bond.
And the presence of a double bond increases the reactivity of a molecule
because you have now a bond that is weaker and can break easily.
The third characteristic is the electronegativity.
When two different atoms are engaged in a covalent bond,
one atom has a stronger pull on the electrons.
This property is called "electronegativity."
The difference of electronegativity determine the relative position
of the electrons in the molecular orbital between two atoms,
and that will affect the reactivity of the molecule in part--
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