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Food is needed by all living organisms mainly for growth and repair.
Several organisms need food to maintain body temperature as well. A large
variety of substances are taken as food from single celluar organisms like
amoeba to the complex multicellular organisms like the human body.
Even with in the human body the cells require a wide variety of substances
as food. The mode of acquiring food also varies from cell to cell and
organism to organism.
You have studied in your previous classes about how different
organisms get their food. Let us recall some of them.
• How do heterotrophs get their food?
• How do autotrophs get their food?
Let us study about autotrophic and heterotrophic modes of nutrition
and find out why most plants are called as autotrophs.
Autotrophic Nutrition
We know that autotrophs are the organisms capable of using light
energy to synthesize chemical compounds. They acquire nutrients like
some minerals and water from the soil as well as some gases from the air.
They are capable of producing complex compounds like carbohydrates,
proteins, lipids etc. from these very simple substances. These compounds
produced by them are utilized for providing energy to most of the living
organisms and all animals including human beings.
Most of the things that we eat are obtained from plants. Even if we
depend on animal products, we would find that those animals depend on
1
Chapter
2 X Class Nutrition - Food supplying system
fig-1: Photosynthesis
oxygen
sunlight
water
carbon dioxide
plants for their food. But what plants do use to carry out their life processes?
Scientists have been working for centuries to find out how plants carry
out these life processes. We know that among all life processes, the process
of photosynthesis makes plants “the universal food provider” for all living
organisms.
You have studied something about photosynthesis in your earlier
classes. Von Helmont and other scientists believed that plants get their
food material not only from soil but also from other sources.
• Can you think of some raw materials needed for photosynthesis?
• What could be the end products of the process of photosynthesis?
Let us study the process of photosynthesis in detail to know more
about this.
Photosynthesis
Photosynthesis is the process by which
plants containing the green pigment
‘chlorophyll’ build up complex organic
molecules from relatively simple inorganic
ones, using sun light as an energy source.
The process of photosynthesis is very
complex .There are several steps in it and
several intermediary compounds are
formed. Scientists had tried to formulate a
simple equation for photosynthesis over past
200 years. An equation that was readily
accepted and is still widely used is the one
formulated and proposed by C.B. Van Neil
in the year 1931 which is as follows. His opinion was, “for each molecule
of carbohydrate formed, one molecule of water and one molecule of
oxygen is produced”. This is a very simplified equation and does not reflect
the complexity of the process of photosynthesis, yet we shall use it for
now.
CO2
+ 2H2
0 CH2
O + H2
0 + O2
What would be the reaction to show that glucose (C6
H12O6
) is being
synthesized? Write down a balanced equation to show this.
(Refer chemical equations and reactions, carbon and its compounds
chapters).
Do you know?
Van Neil first worked on purple sulphur bacteria and found light plays a specific
role in photosynthesis. Instead of H2
O they used H2
S as a starting material. Here no
oxygen is liberated during photosynthesis instead, elemental sulphur is evolved. Later
he envisioned a similar process for photosynthesis in plants and proposed the above
mentioned equation. Later Robert Hill showed O2
is released from water. Then the
equation was modified as follows (Modifications are still being made).
6CO2
+ 12H2
0 C6
H12O6
+ 6H2
0 + 6O2
Light
Chlorophyll
It is known that plants synthesize carbohydrates, the smaller simpler
ones first and from them, the more complex ones like starch and cellulose.
Plants are also capable of synthesizing all other compounds like proteins,
fats etc.
Animals are not capable of synthesizing carbohydrates and they have
to depend on plants for the same.
Can we state that photosynthesis is the basic energy source for most
of the living world? Why, why not?
Let us examine the presence of carbohydrates in plant parts .
Activity-1
Presence of starch (a type of carbohydrate) in leaves
Let us take a leaf from a plant (we can select such plants that have soft
thin leaves) well exposed to sun light.
Boil the leaf in methylated spirit over a water bath. It becomes palewhite
due to the removal of chlorophyll. Observe the leaf.
Take the leaf carefully from test tube by using a brush.
Spread the leaf in a petridish and add a few drops of tincture iodine/
betadine solution on it. Again observe the leaf.
• What do you see?
The presence of starch will be indicated by a blue-black colour. Do
you think solar energy transforms into chemical energy by the process of
photosynthesis? Try doing the same test on the leaf of a potted plant kept
in the dark for around 10 days to test the effect of sun light.
Materials essential for the process of Photosynthesis
What are the materials that you think would be essential for the
synthesis of carbohydrates in the process of photosynthesis? (Hint:
Equation proposed by Van Neil)
• Do you think the equation tells us about all the materials involved?
It took scientists over 300 years to find out about them. We still do
not know about several materials involved in the process.
Let us study how scientists experimented to find out about some of
the materials required for the process of photosynthesis.
Water and Photosynthesis
In class VII we already studied how Von Helmont found that water was
essential for the increase of plant mass.
He did not know about photosynthesis then. Later, it was found that
increase in plant body mass or material occurred due to the process of
photosynthesis. We shall study more about it in the following sections.
Read the chapter on ‘Nutrition in Plants’ in class VII discuss with
your friends and write a note on Von Helmont’s experiment focusing on
how he concluded that water was important for plant growth and increase
in body mass.
Air and Photosynthesis
Let us discuss an experiment on photosynthesis. We have studied some
experiments in our earlier classes. This one helps us to find out about the
role of air in the process of photosynthesis. It is interesting to learn about
the experiment which was one of the several milestones in the gradual
development of our understanding of Photosynthesis.
Joseph Priestly (1733-1804) in 1770 performed a series of experiments that revealed the essential role of air on the growth
of green plants (photosynthesis was still not known
to scientists at that time). You may recall, Oxygen
was discovered by Priestly in 1774 the name oxygen
was coined later by Lavoisier in the year 1775.
Priestly observed that a candle burning in a closed
bell jar, soon gets extinguished. Similarly, a mouse
would soon suffocate in a closed space of the bell
jar. He concluded that a burning candle or an animal,
both somehow, damage air. When he placed a mint
plant in the same bell jars, he found that the mouse
stayed alive and the candle when lighted from
outside continued burning in the presence of the
mint plant. Priestly hypothesized as follows: ‘Plants
restore air what breathing animals and burning
candles remove’.
What had Priestly done to introduce the mint plant without disturbing
the experimental set up?
How did he light the candle from outside?
Priestey’s experiment confirmed that gaseous exchange was going on
and plants were giving out a gas that supported burning and was essential
for the survival of animals.
But how do plants take in air and utilize carbon dioxide for
photosynthesis and oxygen for respiration?
How do they make the choice?
Massive amounts of gaseous exchange occur through the stomata
(usually present in leaves) as long as they are open .While plants also
carry on gaseous exchange through loose tissues on stems, roots etc.
It is actually at the level of the organelles involved in the process of
photosynthesis and respiration that the choice of the gas required is made.
Activity-2
Carbon dioxide is necessary for Photosynthesis
We need a destarched plant to start with. For destarching we need to
keep the plant in the dark for nearly a week to remove the starch
(destarching) from the leaves.
Arrange the apparatus as shown in the figure.
• Take a wide mouthed transparent bottle.
Put potassium hydroxide pellets or potassium hydroxide solution in
the bottle. Potassium hydroxide absorbs carbon dioxide.
• Insert splitted cork in the mouth of the bottle.
• Insert one of the leaf of destarched plant (through a split cork) into
transparent bottle containing potassium hydroxide dioxide pellets/
potassium hydroxide solution.
• Leave the plant in sunlight.
• After a few hours, test this leaf and any other leaf of this plant for
starch. As mentioned in activity-1.
• The leaf which was exposed to the atmospheric air becomes bluishblack,
and the one inside the flask containing potassium hydroxide
which absorbs carbon dioxide in the bottle becomes brown instead of
blue-black, showing that carbon dioxide is necessary for photosynthesis.
• Why was the plant kept in dark and then in sun light ?
• Why did we study two leaves in this experiment?
We have so far discussed the role of water and gasses in the process
of photosynthesis. Scientist who had been working on these lines had
observed some other factors that affect the process of photosynthesis.
Light and Photosynthesis
In Priestley’s time, scientists didn’t understand about energy, but later
on much was discovered about it. If energy is released when carbon dioxide
and water is formed by combining oxygen with carbon and hydrogen, then
what about the reverse?. What about forming oxygen again and putting it
back in the air. Eventually, scientists learned that the energy situation would
also reverse. Oxygen formation would use up energy. That means if plants
fig-4: Mohl’s half leaf experiment
KOH
wide mouthed
boule
split cork
potted plant
blue black
(starch formed)
no starch
Free distribution by A.P. Government 7
funnel
test tube
beaker
water
oxygen
hydrilla plant
fig-5: Hydrilla experiment
form oxygen they have to get energy to make it possible. Where did the
energy come from?
A Dutch scientist, Jan Ingenhousz (1730-1799), found the answer. He
studyed the way in which plants formed oxygen. In 1779, he noticed that it
happened only in the presence of light. In an experiment with the aquatic
plant, Hydrilla, he observed that in bright sunlight, small bubbles formed
around the green parts while in the dark they did not form. He also found
that the gas present in the bubbles was oxygen.
It was further confirmed when Engelman in the early 20th century
ingeniously detected the point of maximum rate of photosynthesis. He
used a strand of algae and exposed it to different colours of light (the
colours that we see in a rainbow) He then used oxygen sensitive bacteria
and found they crowd around areas illuminated with red and blue rays of
light. This led to more studies on effect of light on photosynthesis, the
role of different coloured compounds called pigments in plants and the
utilization of light energy.
Oxygen is produced during photosynthesis in the presence of light
• Arrange the apparatus as shown in the figure. Make
two identical sets.
• Place some water plant like Elodea or Hydrilla in
a short stemmed funnel and keep it in a beaker
containing water.
• Invert a test-tube full of water over the stem of the
funnel. Ensure that the level of water in the beaker
is above the level of stem of the inverted funnel.
Place one apparatus in the sun and the other in the
dark for at least 2-3 hours. You would see that in place
of water there is air that fills in the set up kept in sun. It
is actually a gas that will collect in the test-tube.
Observe the other set up kept in dark. Is there any
difference in the amount of gas collected?
Test the gas in the test-tube by inserting a glowing match stick or
incense stick which would burst into flames. This shows the presence of
oxygen.
• What precautions do you need while removing test tube from the
beaker. Discuss with your teacher.
Lab Activity
bubbles
8 X Class Nutrition - Food supplying system
Activity-3
Sunlight is necessary to form starch in green plants
fig-6: Black paper experiment
• Take a potted plant with destarched leaves. Remember the process of
destraching leaves mentioned in activity-1.
• Cover one of its leaves with black paper on which a design is cut. Fix the
paper on the leaf in such a manner that light does not enter the dark part.
• Place this potted plant in sun light.
• After few hours of exposure to bright sunlight, test the leaf which is
covered by black paper for the presence of starch.
• Which part of leaf turms blue black? What about the remaining
part?
• Observe the colour of leaf stained with iodine. Can you tell why it
is stained differently?
• It will be observed that only the parts of the leaf, which could get light
through the cut out design, turns blue-black showing the presence of
starch.
Chlorophyll and Photosynthesis
Ingenhousz wanted to find out more about photosynthesis and carried
out several other experiments. He proposed that only green plant parts
could carry out the process of photosynthesis.
What about plants having coloured leaves? How is it that new leaves
which look dark red in colour in several plants turn green? Do plants having
reddish or yellowish leaves also carry out photosynthesis? What made
plants carry out photosynthesis while even green coloured animals (like
some birds )could not? Questions like these remained challenges until
scientists could isolate the green coloured substance from plant parts and
study its nature.
Establishment of Ingenhousz’s proposition came after several
experiments till the mid 20th century when scientists could also locate the
site of photosynthesis and even isolate it. Around four decades after
Ingenhouszs’ proposition scientists could only isolate the green substance
to observe its nature and find out whether photosynthesis could be carried
out with it. This had become possible in the year 1817 due to the work of
two scientists Pelletier and Caventou who obtained an extract of the green
colored substance and named it ‘chlorophyll’ meaning green leaf.
It was also found that pigments other than green could also aid in the
process of photosynthesis by passing on the energy of sunlight trapped by
them to chlorophyll.
• But where is chlorophyll and other pigments present in the plant?
Where does Photosynthesis take place?
Try to name some parts where you think photosynthesis occurs.
• Do you think the new reddish leaves of plants also carry out
photosynthesis? What could be the role of their colour?
The exact location of the photosynthetic part or a part containing
chlorophyll was not known till another 6 decades after Pelletier and
Caventou discovered chlorophyll. It was believed to be spread in the cells
of green plant parts. In 1883, Julius Von Sachs, observed that chlorophyll
in plant cells is not spread through out the entire cell. It is rather found in
organelles within the cell. Such organelles were named as ‘chloroplasts’.
These are present in large numbers in the cells (around 40 – 100) of parts
like the stomatal guard cells and ground tissues of plants .
Do you know?
If a cell is broken up, the chloroplasts also break into pieces, so it becomes a very
difficult task to isolate them to study the different steps of photosynthesis.It was not
until 1954, that Daniel I. Arnon was able to break up plant cells so gently that whole
chloroplasts could be obtained that could carry through photosynthesis.
It has been found that the chloroplast is
a membranous structure, consisting of 3
membranes. The third layer forms stacked
sack like structures called as granum. It is
believed to be a site for trapping of solar
energy. The intermediary fluid filled portion
is called as stroma. It is believed to be
responsible for enzymatic reactions leading
to the synthesis of glucose, which in turn
join together to form starch.
Substances found in chloroplast which capture sunlight are called
photosynthetic pigments. There are several types of photosynthetic
pigments involved in the process to produce organic molecules like
glucose in plants.
Chlorophyll is such a pigment which contain one atom of megnisium.
It is similar in structure to the heam of haemoglobin. (The iron containing
red pigment that transports oxygen in blood.) Two major kinds of
chlorophylls are associated with thylakoid membranes. Chlorophyll ‘a’ is
blue-green in colour and chlorophyll ‘b’ is yellow-green colour. Around
250 to 400 pigment molecules are grouped as light harvesting complex or
photosynthetic unit in each granum. Such innumerable units function
together in chloroplasts of green plants in the process of photosynthesis.
During photosynthesis several events occur in the chloroplast some
of them are:
1. Conversion of light energy to chemical energy
2. Splitting of water molecule
3. Reduction of carbondioxide to carbohydrates
Light is required to initiate several events while several may continue
even in absence of it. That would mean, once light energy has been captured
it can help reactions to continue even in the dark. Light dependent events
Free distribution by A.P. Government 11
or reactions are called light reactions and it has been found to take place
in grana, while the rest are called light independent or dark reactions and
they occur in the stroma.
Mechanism of Photosynthesis
1. Light dependent reaction (Photochemical phase)
In this reaction light plays a key role. A series of chemical reactions
occur in a very quick succession initiated by light and therefore the phase
is technically called the photochemical phase or light dependent reaction.
The light reaction takes place in chlorophyll containing thylakoids called
grana of chloroplasts. Several steps occur in the light dependent reaction.
Step-I : The chlorophyll on exposure to light energy becomes activated
by absorbing photons. (Photon is the smallest unit of light energy)
Step-II: The energy is used in splitting the water molecule into two
component ions named hydrogen (H+
), hydroxyl ion (OH-
).
H2
O H+ + OHThe
reaction is known as photolysis, which means splitting by
light (photo means light, lysis means breaking). This was
discovered by Hill. Hence it is also called Hill’s reaction.
Step-III: The highly reactive ions of water undergo quick change as
described below.
OH-
ions through a series of steps produce water (H2
0) and Oxygen
(O2
). Water may be used by the plant inside, but O2
is useually released
into the atmosphere. H+ ions undergo series of changes in dark reaction.
compounds that can trap energy like ATP (Adenosine Tri phosphate) and
NADPH (Nicotinamide Adenosine Dinucleotide Hydrogen Phosphate) are
formed at the end of the light reaction.
2. Light independent reaction (Biosynthetic phase)
This reaction does not require the presence of light and extension of
the phases after day time may occur in some plants (time gap between the
two being less than even one thousandth of a second) and some times even
in the dark.
This is also called dark reaction. But the term dark reaction or light
independent reaction does not mean that they occur when it is dark at night.
It only means that these reactions are not depend on light. H+
Ions produced
in photolysis are immediately picked up by special compound NADP to
form NADPH. In the dark phase the hydrogen of the NADPH is used to
combine it with CO2
by utilizing ATP energy and to produce glucose
12 X Class Nutrition - Food supplying system
(C6
H12O6
). This synthesis occurs in a number of steps using certain special
intermediate compounds (mainly RUBP- Ribulose bis Phosphate) and
enzymes. Finally the glucose is converted to starch.
Plants are capable of surviving under a range of situations, from very
hot, dry and brightly lighted conditions to wet, humid and dimly lighted
ones. The requirement of light and other factors varies from one plant to
another.