The principal function of the leaves is the production of
nutritional substances through some reactions called
photosynthesis. The photosynthesis lets produce glucose
(C6H12O6) and oxigen (O2) starting from water (H2O) and
carbon dioxide (CO2).
The oxigen is released in the
atmosphere.
6 CO2 + 6H2O ==> C6H12O6 + 6O2
The photosynthesis is carried out by the chlorophyll, thanks to
sunlight.
A second physiological activity that is done by leaves is
represented by the transpiration (a loss of water in the form of
steam).
From the first years of the XX century the English botanist
F.F.Blackman showed that there are two phases in the
photosynthesic process.
He noticed that the speed of photosynthesis increased with the light
intensity, but this increase was influenced by the temperature. This
observation was used by Blackman in order to conclude that, in the
photosynthesic process, there are reactions which are determined
by the presence of light and other reactions, which are determined
by enzymes that lose their efficacy beyond a certain temperature.
Nowadays we know that the
photosynthesis reactions develop in two
phases.
In the first one, which is influenced by
light, the daylight hits some molecules of
chlorophylle A.
The molecules of chlorophyll A lose one
electron which is used to make energy.
In this phase also the molecules of water
become ion H+, gaseous oxigen and
electrons which replace the chlorophyll’s
ones. In the second phase of
photosynthesis without light the energy
that has been produced in the previous
phase is used to produce sugar.
Besides the carbon dioxide is used to
create a structure of other organic
molecules.
This process is called “Cycle of Calvin”.
A system is a restricted group of
matter, distinguishable from the
surrounding environment. In
every system there is one or more
phases, which are portions of
matter with the same features
delimited by sharp and definite
surfaces.
A system with one phase is called
homogeneous. If there is only one
substance this is called pure, but if
there are two or more phases,
which are not identified with the
traditional methods of observation
it is called solution. When the
system is composed by two or
more phases it is called
heterogeneous.
MATTER
HOMOGENEOUS
SYSTEMS
SOLUTIONS
HETEROGENEOUS
SYSTEMS
PURE
SUBSTANCES
The most common techniques of separation used to divide the
different components of the systems are:
Decantation;
Filtration;
Centrifugation;
Distillation;
Crystallization;
Extraction with solvents;
Chromatography.
The extraction with solvent is usually used
to divide the solutions. This technique is
based on the affinity of a solute with
another solvent different from the solvent
in which it is dissolved.
In order to extract the substance dissolved
into the solution we can use a solvent
which has a more affinity with this
substance.
The only aspect that we have to take into
consideration is that the two solvents
aren’t soluble among them because in this
way the solution becomes more complex.
The chromatography is the most versatile among the separation
techniques. It was created in 1906 by the Russian botanist
Michael Tswett, this technique doesn’t differ from the simple
extraction, but multiplies its efficacy. The solvent, that in this
case is called mobile phase conveys the components of the system
through a fixed phase. In the chromatography on layer, the fixed
phase, composed by a thin coat of inert matter such as silica gel or
alumina, is fixed on a sheet of aluminium. A thin line is drawn
and some drops of solution are put on it. This zone is called
sowing zone. Then a small part of the sheet is immersed in a
container in which there is another solvent (for example water,
acetone, hexane…)so that the initial solution moves through the
fixed phase due to the capillary action; as the various substances
of the solution move at different speeds they separate.
Take some leaves of spinach, break into bits and put them
into the mortar with some sand to facilitate the crumbling.
Crush and pour some alcohol.
Add some calcium bicarbonate with the spatula
Pick up a small quantity of solution with
capillary.
Take a sheet of aluminium with silica gel and
draw a line with a pencil
Sow some drops of the solution on the line.
Place the sheet upright into the pot and close
it immediately so that the solvent doesn’t
evaporate.
Wait until the solvent goes up the whole sheet
carrying the pigments.
When the solvent reaches the top of the sheet
take it out and wait so that the solvent
evaporates.
YELLOW – ORANGE band:
carotenoids
GREEN- BLUE band: chlorophyll b
GREEN- YELLOW band:
chlorophyll a
LEMON YELLOW band: xantophyll
SOWING ZONE
During the experiment we can note a solid and a liquid phase, in
fact when we pour the alcohol in the mortar we obtain a solution of
pigments and alcohol . After the evaporation of the solvent it is
possible to see the separation of the pigments of spinach. The
pigments are arranged in this way:
PIGMENT
DISTANCE FROM SOW LINE
Xantophyll
1,8 cm
chlorophyll a
2,2 cm
Chlorophyll b
2,4 cm
Β carotene
5,6 cm
The leaves can catch the sunlight thanks to the pigments.
The chlorophyll is the most important chloroplast and it is contained in
the membrane of thylakoids.
There are two kinds of chlorophyll: chlorophyll A which changes bright
energy into chemical energy.
A lot of cells also contain chlorophyll B that has a different colour.
Usually there is a second group of pigments that are orange or red: the
carotenoids.
Normally they are masked by the green of chlorophyll because in the
leaves there is more chlorophyll.
In autumn the chlorophyll is the first pigment that disappears and the
carotenoids become visible.
The chlorophyll is fundamental
for the photosynthesis. The
chlorophyll molecule is
composed of two parts. The
first one, called “head”,
intervenes in the light phase
and takes part into the
photochemical
transformations; it is
composed of a ring in which
there is an atom of
magnesium (Mg). The second
part is an hydrocarbon tail
linked to the first part and it
has the function of introducing
the chlorophyll into the
thylakoids.
This molecule is composed of a long hydrocarbon chain that
ends with two rings of carbon atoms.
The classical red of carotenoids is the direct consequence of
the molecular structure of these mixtures.
The elements composing the chain present some bonds
among them which consent the structure to change its
position and so let that the light spectrum changes and the
colour of the pigments changes too.
The bright energy can be used by the living
beings only if it is first absorbed. The
substances that can absorb the light are called
pigments. Some pigments take up all the
wavelengths of light therefore they appear
black. Other pigments only absorb some
wavelengths so they reflect those that they
don’t absorb.
Through this experience we have learnt that although the
colour of leaves is determined by the union of several pigments,
the chlorophyll prevails. It is for this reason that the only colour
we can almost always see is green.
.
Through chromatography it is also possible to determine which
pigments are contained into the leaves cells.