NCERT Textbook - Photosynthesis in Higher Plants NEET Notes | EduRev

NCERT Textbooks (Class 6 to Class 12)

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NEET : NCERT Textbook - Photosynthesis in Higher Plants NEET Notes | EduRev

 Page 1


206 BIOLOGY
All animals including human beings depend on plants for their food. Have
you ever wondered from where plants get their food?  Green plants, in
fact, have to make or rather synthesise the food they need and all other
organisms depend on them for their needs. Green plants carry out
‘photosynthesis’, a physico-chemical process by which they use light
energy to drive the synthesis of organic compounds.  Ultimately, all living
forms on earth depend on sunlight for energy.  The use of energy from
sunlight by plants doing photosynthesis is the basis of life on earth.
Photosynthesis is important due to two reasons: it is the primary source
of all food on earth. It is also responsible for the release of oxygen into the
atmosphere by green plants. Have you ever thought what would happen
if there were no oxygen to breath? This chapter focusses on the structure
of the photosynthetic machinery and the various reactions that transform
light energy into chemical energy.
13.1 WHAT DO WE KNOW?
Let us try to find out what we already know about photosynthesis.  Some
simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO
2
 are required for
photosynthesis to occur.
You may have carried out the experiment to look for starch formation
in two leaves – a variegated leaf or a leaf that was partially covered with
black paper, and one that was exposed to light.  On testing these leaves
for starch it was clear that photosynthesis occurred only in the green
parts of the leaves in the presence of light.
PHOTOSYNTHESIS IN HIGHER PLANTS
CHAPTER  13
13.1 What do we
Know?
13.2 Early
Experiments
13.3 Where does
Photosynthesis
take place?
13.4 How many
Pigments are
involved in
Photosynthesis?
13.5 What is Light
Reaction?
13.6 The Electron
Transport
13.7 Where are the
ATP and NADPH
Used?
13.8 The C
4
 Pathway
13.9 Photorespiration
13.10 Factors
affecting
Photosynthesis
2015-16(19/01/2015)
Page 2


206 BIOLOGY
All animals including human beings depend on plants for their food. Have
you ever wondered from where plants get their food?  Green plants, in
fact, have to make or rather synthesise the food they need and all other
organisms depend on them for their needs. Green plants carry out
‘photosynthesis’, a physico-chemical process by which they use light
energy to drive the synthesis of organic compounds.  Ultimately, all living
forms on earth depend on sunlight for energy.  The use of energy from
sunlight by plants doing photosynthesis is the basis of life on earth.
Photosynthesis is important due to two reasons: it is the primary source
of all food on earth. It is also responsible for the release of oxygen into the
atmosphere by green plants. Have you ever thought what would happen
if there were no oxygen to breath? This chapter focusses on the structure
of the photosynthetic machinery and the various reactions that transform
light energy into chemical energy.
13.1 WHAT DO WE KNOW?
Let us try to find out what we already know about photosynthesis.  Some
simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO
2
 are required for
photosynthesis to occur.
You may have carried out the experiment to look for starch formation
in two leaves – a variegated leaf or a leaf that was partially covered with
black paper, and one that was exposed to light.  On testing these leaves
for starch it was clear that photosynthesis occurred only in the green
parts of the leaves in the presence of light.
PHOTOSYNTHESIS IN HIGHER PLANTS
CHAPTER  13
13.1 What do we
Know?
13.2 Early
Experiments
13.3 Where does
Photosynthesis
take place?
13.4 How many
Pigments are
involved in
Photosynthesis?
13.5 What is Light
Reaction?
13.6 The Electron
Transport
13.7 Where are the
ATP and NADPH
Used?
13.8 The C
4
 Pathway
13.9 Photorespiration
13.10 Factors
affecting
Photosynthesis
2015-16(19/01/2015)
PHOTOSYNTHESIS IN HIGHER P LANTS 207
Another experiment you may have carried out
is the half-leaf experiment, where a part of a leaf is
enclosed in a test tube containing some KOH soaked
cotton (which absorbs CO
2
), while the other half is
exposed to air.  The setup is then placed in light for
some time. On testing for starch later in the two
halves of the leaf, you must have found that the
exposed part of the leaf tested positive for starch
while the portion that was in the tube, tested
negative. This showed that CO
2
 was required for
photosynthesis. Can you explain how this
conclusion could be drawn?
13.2 EARLY E XPERIMENTS
It is interesting to learn about those simple
experiments that led to a gradual development in
our understanding of photosynthesis.
Joseph Priestley (1733-1804) in 1770
performed a series of experiments that revealed the
essential role of air in the growth of green plants.
Priestley, you may recall, discovered oxygen in
1774. Priestley observed that a candle burning in
a closed space – a bell jar, soon gets extinguished
(Figure 13.1 a, b, c, d).  Similarly, a mouse would
soon suffocate in a closed space. He concluded that
a burning candle or an animal that breathe the air,
both somehow, damage the air.  But when he placed a mint plant in the
same bell jar, he found that the mouse stayed alive and the candle
continued to burn.  Priestley hypothesised as follows: Plants restore to
the air whatever breathing animals and burning candles remove.
Can you imagine how Priestley would have conducted the experiment
using a candle and a plant?  Remember, he would need to rekindle the
candle to test whether it burns after a few days. How many different
ways can you think of to light the candle without disturbing the set-up?
Using a similar setup as the one used by Priestley, but by placing it
once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)
showed that sunlight is essential to the plant process that somehow
purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an elegant experiment with an aquatic plant showed that
in bright sunlight, small bubbles were formed around the green parts
while in the dark they did not.  Later he identified these bubbles to be of
oxygen.  Hence he showed that it is only the green part of the plants that
could release oxygen.
(a)
(c)
(b)
(d)
Figure 13.1 Priestley’s experiment
2015-16(19/01/2015)
Page 3


206 BIOLOGY
All animals including human beings depend on plants for their food. Have
you ever wondered from where plants get their food?  Green plants, in
fact, have to make or rather synthesise the food they need and all other
organisms depend on them for their needs. Green plants carry out
‘photosynthesis’, a physico-chemical process by which they use light
energy to drive the synthesis of organic compounds.  Ultimately, all living
forms on earth depend on sunlight for energy.  The use of energy from
sunlight by plants doing photosynthesis is the basis of life on earth.
Photosynthesis is important due to two reasons: it is the primary source
of all food on earth. It is also responsible for the release of oxygen into the
atmosphere by green plants. Have you ever thought what would happen
if there were no oxygen to breath? This chapter focusses on the structure
of the photosynthetic machinery and the various reactions that transform
light energy into chemical energy.
13.1 WHAT DO WE KNOW?
Let us try to find out what we already know about photosynthesis.  Some
simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO
2
 are required for
photosynthesis to occur.
You may have carried out the experiment to look for starch formation
in two leaves – a variegated leaf or a leaf that was partially covered with
black paper, and one that was exposed to light.  On testing these leaves
for starch it was clear that photosynthesis occurred only in the green
parts of the leaves in the presence of light.
PHOTOSYNTHESIS IN HIGHER PLANTS
CHAPTER  13
13.1 What do we
Know?
13.2 Early
Experiments
13.3 Where does
Photosynthesis
take place?
13.4 How many
Pigments are
involved in
Photosynthesis?
13.5 What is Light
Reaction?
13.6 The Electron
Transport
13.7 Where are the
ATP and NADPH
Used?
13.8 The C
4
 Pathway
13.9 Photorespiration
13.10 Factors
affecting
Photosynthesis
2015-16(19/01/2015)
PHOTOSYNTHESIS IN HIGHER P LANTS 207
Another experiment you may have carried out
is the half-leaf experiment, where a part of a leaf is
enclosed in a test tube containing some KOH soaked
cotton (which absorbs CO
2
), while the other half is
exposed to air.  The setup is then placed in light for
some time. On testing for starch later in the two
halves of the leaf, you must have found that the
exposed part of the leaf tested positive for starch
while the portion that was in the tube, tested
negative. This showed that CO
2
 was required for
photosynthesis. Can you explain how this
conclusion could be drawn?
13.2 EARLY E XPERIMENTS
It is interesting to learn about those simple
experiments that led to a gradual development in
our understanding of photosynthesis.
Joseph Priestley (1733-1804) in 1770
performed a series of experiments that revealed the
essential role of air in the growth of green plants.
Priestley, you may recall, discovered oxygen in
1774. Priestley observed that a candle burning in
a closed space – a bell jar, soon gets extinguished
(Figure 13.1 a, b, c, d).  Similarly, a mouse would
soon suffocate in a closed space. He concluded that
a burning candle or an animal that breathe the air,
both somehow, damage the air.  But when he placed a mint plant in the
same bell jar, he found that the mouse stayed alive and the candle
continued to burn.  Priestley hypothesised as follows: Plants restore to
the air whatever breathing animals and burning candles remove.
Can you imagine how Priestley would have conducted the experiment
using a candle and a plant?  Remember, he would need to rekindle the
candle to test whether it burns after a few days. How many different
ways can you think of to light the candle without disturbing the set-up?
Using a similar setup as the one used by Priestley, but by placing it
once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)
showed that sunlight is essential to the plant process that somehow
purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an elegant experiment with an aquatic plant showed that
in bright sunlight, small bubbles were formed around the green parts
while in the dark they did not.  Later he identified these bubbles to be of
oxygen.  Hence he showed that it is only the green part of the plants that
could release oxygen.
(a)
(c)
(b)
(d)
Figure 13.1 Priestley’s experiment
2015-16(19/01/2015)
208 BIOLOGY
It was not until about 1854 that Julius von Sachs provided evidence
for production of glucose when plants grow. Glucose is usually stored as
starch. His later studies showed that the green substance in plants
(chlorophyll as we know it now) is located in special bodies (later called
chloroplasts) within plant cells. He found that the green parts in plants is
where glucose is made, and that the glucose is usually stored as starch.
Now consider the interesting experiments done by T.W Engelmann
(1843 – 1909).  Using a prism he split light into its spectral components
and then illuminated a green alga, Cladophora, placed in a suspension
of aerobic bacteria. The bacteria were used to detect the sites of O
2
evolution. He observed that the bacteria accumulated mainly in the region
of blue and red light of the split spectrum. A first action spectrum of
photosynthesis was thus described. It resembles roughly the absorption
spectra of chlorophyll a and b (discussed  in section 13.4).
By the middle of the nineteenth century the key features of plant
photosynthesis were known, namely, that plants could use light energy
to make carbohydrates from CO
2
 and water.  The empirical equation
representing the total process of photosynthesis for oxygen evolving
organisms was then understood as:
CO H O CH O O
Light
2 2 2 2
+ ? ? ???? + [ ]
where [CH
2
O] represented a carbohydrate (e.g., glucose, a six-carbon
sugar).
A milestone contribution to the understanding of photosynthesis was
that made by a microbiologist, Cornelius van Niel (1897-1985), who,
based on his studies of purple and green bacteria, demonstrated that
photosynthesis is essentially a light-dependent reaction in which
hydrogen from a suitable oxidisable compound reduces carbon dioxide
to carbohydrates. This can be expressed by:
2 2
2 2 2 2
H A CO A CH O H O
Light
+ ? ? ???? + +
In green plants H
2
O is the hydrogen donor and is oxidised to O
2
. Some
organisms do not release O
2 
during photosynthesis.  When H
2
S, instead
is the hydrogen donor for purple and green sulphur bacteria, the
‘oxidation’ product is sulphur or sulphate depending on the organism
and not O
2
. Hence, he inferred that the O
2 
evolved by the green plant
comes from H
2
O, not from carbon dioxide. This was later proved by using
radioisotopic techniques.  The correct equation, that would represent  the
overall process of photosynthesis is therefore:
6 12 6 6
2 2 6 12 6 2 2
CO H O C H O H O O
Light
+ ? ? ???? + +
where C
6
 H
12
 O
6
 represents glucose. The O
2
 released is from water; this
was proved using radio isotope techniques. Note that this is not a single
2015-16(19/01/2015)
Page 4


206 BIOLOGY
All animals including human beings depend on plants for their food. Have
you ever wondered from where plants get their food?  Green plants, in
fact, have to make or rather synthesise the food they need and all other
organisms depend on them for their needs. Green plants carry out
‘photosynthesis’, a physico-chemical process by which they use light
energy to drive the synthesis of organic compounds.  Ultimately, all living
forms on earth depend on sunlight for energy.  The use of energy from
sunlight by plants doing photosynthesis is the basis of life on earth.
Photosynthesis is important due to two reasons: it is the primary source
of all food on earth. It is also responsible for the release of oxygen into the
atmosphere by green plants. Have you ever thought what would happen
if there were no oxygen to breath? This chapter focusses on the structure
of the photosynthetic machinery and the various reactions that transform
light energy into chemical energy.
13.1 WHAT DO WE KNOW?
Let us try to find out what we already know about photosynthesis.  Some
simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO
2
 are required for
photosynthesis to occur.
You may have carried out the experiment to look for starch formation
in two leaves – a variegated leaf or a leaf that was partially covered with
black paper, and one that was exposed to light.  On testing these leaves
for starch it was clear that photosynthesis occurred only in the green
parts of the leaves in the presence of light.
PHOTOSYNTHESIS IN HIGHER PLANTS
CHAPTER  13
13.1 What do we
Know?
13.2 Early
Experiments
13.3 Where does
Photosynthesis
take place?
13.4 How many
Pigments are
involved in
Photosynthesis?
13.5 What is Light
Reaction?
13.6 The Electron
Transport
13.7 Where are the
ATP and NADPH
Used?
13.8 The C
4
 Pathway
13.9 Photorespiration
13.10 Factors
affecting
Photosynthesis
2015-16(19/01/2015)
PHOTOSYNTHESIS IN HIGHER P LANTS 207
Another experiment you may have carried out
is the half-leaf experiment, where a part of a leaf is
enclosed in a test tube containing some KOH soaked
cotton (which absorbs CO
2
), while the other half is
exposed to air.  The setup is then placed in light for
some time. On testing for starch later in the two
halves of the leaf, you must have found that the
exposed part of the leaf tested positive for starch
while the portion that was in the tube, tested
negative. This showed that CO
2
 was required for
photosynthesis. Can you explain how this
conclusion could be drawn?
13.2 EARLY E XPERIMENTS
It is interesting to learn about those simple
experiments that led to a gradual development in
our understanding of photosynthesis.
Joseph Priestley (1733-1804) in 1770
performed a series of experiments that revealed the
essential role of air in the growth of green plants.
Priestley, you may recall, discovered oxygen in
1774. Priestley observed that a candle burning in
a closed space – a bell jar, soon gets extinguished
(Figure 13.1 a, b, c, d).  Similarly, a mouse would
soon suffocate in a closed space. He concluded that
a burning candle or an animal that breathe the air,
both somehow, damage the air.  But when he placed a mint plant in the
same bell jar, he found that the mouse stayed alive and the candle
continued to burn.  Priestley hypothesised as follows: Plants restore to
the air whatever breathing animals and burning candles remove.
Can you imagine how Priestley would have conducted the experiment
using a candle and a plant?  Remember, he would need to rekindle the
candle to test whether it burns after a few days. How many different
ways can you think of to light the candle without disturbing the set-up?
Using a similar setup as the one used by Priestley, but by placing it
once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)
showed that sunlight is essential to the plant process that somehow
purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an elegant experiment with an aquatic plant showed that
in bright sunlight, small bubbles were formed around the green parts
while in the dark they did not.  Later he identified these bubbles to be of
oxygen.  Hence he showed that it is only the green part of the plants that
could release oxygen.
(a)
(c)
(b)
(d)
Figure 13.1 Priestley’s experiment
2015-16(19/01/2015)
208 BIOLOGY
It was not until about 1854 that Julius von Sachs provided evidence
for production of glucose when plants grow. Glucose is usually stored as
starch. His later studies showed that the green substance in plants
(chlorophyll as we know it now) is located in special bodies (later called
chloroplasts) within plant cells. He found that the green parts in plants is
where glucose is made, and that the glucose is usually stored as starch.
Now consider the interesting experiments done by T.W Engelmann
(1843 – 1909).  Using a prism he split light into its spectral components
and then illuminated a green alga, Cladophora, placed in a suspension
of aerobic bacteria. The bacteria were used to detect the sites of O
2
evolution. He observed that the bacteria accumulated mainly in the region
of blue and red light of the split spectrum. A first action spectrum of
photosynthesis was thus described. It resembles roughly the absorption
spectra of chlorophyll a and b (discussed  in section 13.4).
By the middle of the nineteenth century the key features of plant
photosynthesis were known, namely, that plants could use light energy
to make carbohydrates from CO
2
 and water.  The empirical equation
representing the total process of photosynthesis for oxygen evolving
organisms was then understood as:
CO H O CH O O
Light
2 2 2 2
+ ? ? ???? + [ ]
where [CH
2
O] represented a carbohydrate (e.g., glucose, a six-carbon
sugar).
A milestone contribution to the understanding of photosynthesis was
that made by a microbiologist, Cornelius van Niel (1897-1985), who,
based on his studies of purple and green bacteria, demonstrated that
photosynthesis is essentially a light-dependent reaction in which
hydrogen from a suitable oxidisable compound reduces carbon dioxide
to carbohydrates. This can be expressed by:
2 2
2 2 2 2
H A CO A CH O H O
Light
+ ? ? ???? + +
In green plants H
2
O is the hydrogen donor and is oxidised to O
2
. Some
organisms do not release O
2 
during photosynthesis.  When H
2
S, instead
is the hydrogen donor for purple and green sulphur bacteria, the
‘oxidation’ product is sulphur or sulphate depending on the organism
and not O
2
. Hence, he inferred that the O
2 
evolved by the green plant
comes from H
2
O, not from carbon dioxide. This was later proved by using
radioisotopic techniques.  The correct equation, that would represent  the
overall process of photosynthesis is therefore:
6 12 6 6
2 2 6 12 6 2 2
CO H O C H O H O O
Light
+ ? ? ???? + +
where C
6
 H
12
 O
6
 represents glucose. The O
2
 released is from water; this
was proved using radio isotope techniques. Note that this is not a single
2015-16(19/01/2015)
PHOTOSYNTHESIS IN HIGHER P LANTS 209
reaction but description of a multistep process called photosynthesis.
Can you explain why twelve molecules of water as substrate are used
in the equation given above?
13.3 WHERE DOES PHOTOSYNTHESIS TAKE PLACE?
You would of course answer: in ‘the green leaf’ or you may add, ‘in the
chloroplasts’ based on what you earlier read in Chapter 8. You are
definitely right. Photosynthesis does take place in the green leaves of plants
but it does so also in other green parts of the plants. Can you name some
other parts where you think photosynthesis may occur?
You would recollect from previous unit that the mesophyll cells in the
leaves, have a large number of chloroplasts. Usually the chloroplasts align
themselves along the walls of the mesophyll cells, such that they get the
optimum quantity of the incident light. When do you think the
chloroplasts will be aligned with their flat surfaces parallel to the walls?
When would they be perpendicular to the incident light?
You have studied the structure of chloroplast in Chapter 8. Within
the chloroplast there is the membranous system consisting of grana, the
stroma lamellae, and the fluid stroma (Figure 13.2). There is a clear division
of labour within the chloroplast. The membrane system is responsible for
trapping the light energy and also for the synthesis of ATP and NADPH.
In stroma, enzymatic reactions incorporate CO
2
 into the plant leading to
the synthesis of sugar, which in turn forms starch. The former set of
reactions, since they are directly light driven are called light reactions.
The latter are not directly light driven but are dependent on the products
of light reactions (ATP and NADPH). Hence, to distinguish the latter they
are called, by convention, as dark reactions. However, this should not be
construed to mean that they occur in darkness or that they are not light-
dependent.
Figure 13.2 Diagrammatic representation of an electron micrograph of a section of
chloroplast
Outer membrane
Inner membrane
Stromal lamella
Grana
Stroma
Ribosomes
Starch granule
Lipid droplet
2015-16(19/01/2015)
Page 5


206 BIOLOGY
All animals including human beings depend on plants for their food. Have
you ever wondered from where plants get their food?  Green plants, in
fact, have to make or rather synthesise the food they need and all other
organisms depend on them for their needs. Green plants carry out
‘photosynthesis’, a physico-chemical process by which they use light
energy to drive the synthesis of organic compounds.  Ultimately, all living
forms on earth depend on sunlight for energy.  The use of energy from
sunlight by plants doing photosynthesis is the basis of life on earth.
Photosynthesis is important due to two reasons: it is the primary source
of all food on earth. It is also responsible for the release of oxygen into the
atmosphere by green plants. Have you ever thought what would happen
if there were no oxygen to breath? This chapter focusses on the structure
of the photosynthetic machinery and the various reactions that transform
light energy into chemical energy.
13.1 WHAT DO WE KNOW?
Let us try to find out what we already know about photosynthesis.  Some
simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO
2
 are required for
photosynthesis to occur.
You may have carried out the experiment to look for starch formation
in two leaves – a variegated leaf or a leaf that was partially covered with
black paper, and one that was exposed to light.  On testing these leaves
for starch it was clear that photosynthesis occurred only in the green
parts of the leaves in the presence of light.
PHOTOSYNTHESIS IN HIGHER PLANTS
CHAPTER  13
13.1 What do we
Know?
13.2 Early
Experiments
13.3 Where does
Photosynthesis
take place?
13.4 How many
Pigments are
involved in
Photosynthesis?
13.5 What is Light
Reaction?
13.6 The Electron
Transport
13.7 Where are the
ATP and NADPH
Used?
13.8 The C
4
 Pathway
13.9 Photorespiration
13.10 Factors
affecting
Photosynthesis
2015-16(19/01/2015)
PHOTOSYNTHESIS IN HIGHER P LANTS 207
Another experiment you may have carried out
is the half-leaf experiment, where a part of a leaf is
enclosed in a test tube containing some KOH soaked
cotton (which absorbs CO
2
), while the other half is
exposed to air.  The setup is then placed in light for
some time. On testing for starch later in the two
halves of the leaf, you must have found that the
exposed part of the leaf tested positive for starch
while the portion that was in the tube, tested
negative. This showed that CO
2
 was required for
photosynthesis. Can you explain how this
conclusion could be drawn?
13.2 EARLY E XPERIMENTS
It is interesting to learn about those simple
experiments that led to a gradual development in
our understanding of photosynthesis.
Joseph Priestley (1733-1804) in 1770
performed a series of experiments that revealed the
essential role of air in the growth of green plants.
Priestley, you may recall, discovered oxygen in
1774. Priestley observed that a candle burning in
a closed space – a bell jar, soon gets extinguished
(Figure 13.1 a, b, c, d).  Similarly, a mouse would
soon suffocate in a closed space. He concluded that
a burning candle or an animal that breathe the air,
both somehow, damage the air.  But when he placed a mint plant in the
same bell jar, he found that the mouse stayed alive and the candle
continued to burn.  Priestley hypothesised as follows: Plants restore to
the air whatever breathing animals and burning candles remove.
Can you imagine how Priestley would have conducted the experiment
using a candle and a plant?  Remember, he would need to rekindle the
candle to test whether it burns after a few days. How many different
ways can you think of to light the candle without disturbing the set-up?
Using a similar setup as the one used by Priestley, but by placing it
once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)
showed that sunlight is essential to the plant process that somehow
purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an elegant experiment with an aquatic plant showed that
in bright sunlight, small bubbles were formed around the green parts
while in the dark they did not.  Later he identified these bubbles to be of
oxygen.  Hence he showed that it is only the green part of the plants that
could release oxygen.
(a)
(c)
(b)
(d)
Figure 13.1 Priestley’s experiment
2015-16(19/01/2015)
208 BIOLOGY
It was not until about 1854 that Julius von Sachs provided evidence
for production of glucose when plants grow. Glucose is usually stored as
starch. His later studies showed that the green substance in plants
(chlorophyll as we know it now) is located in special bodies (later called
chloroplasts) within plant cells. He found that the green parts in plants is
where glucose is made, and that the glucose is usually stored as starch.
Now consider the interesting experiments done by T.W Engelmann
(1843 – 1909).  Using a prism he split light into its spectral components
and then illuminated a green alga, Cladophora, placed in a suspension
of aerobic bacteria. The bacteria were used to detect the sites of O
2
evolution. He observed that the bacteria accumulated mainly in the region
of blue and red light of the split spectrum. A first action spectrum of
photosynthesis was thus described. It resembles roughly the absorption
spectra of chlorophyll a and b (discussed  in section 13.4).
By the middle of the nineteenth century the key features of plant
photosynthesis were known, namely, that plants could use light energy
to make carbohydrates from CO
2
 and water.  The empirical equation
representing the total process of photosynthesis for oxygen evolving
organisms was then understood as:
CO H O CH O O
Light
2 2 2 2
+ ? ? ???? + [ ]
where [CH
2
O] represented a carbohydrate (e.g., glucose, a six-carbon
sugar).
A milestone contribution to the understanding of photosynthesis was
that made by a microbiologist, Cornelius van Niel (1897-1985), who,
based on his studies of purple and green bacteria, demonstrated that
photosynthesis is essentially a light-dependent reaction in which
hydrogen from a suitable oxidisable compound reduces carbon dioxide
to carbohydrates. This can be expressed by:
2 2
2 2 2 2
H A CO A CH O H O
Light
+ ? ? ???? + +
In green plants H
2
O is the hydrogen donor and is oxidised to O
2
. Some
organisms do not release O
2 
during photosynthesis.  When H
2
S, instead
is the hydrogen donor for purple and green sulphur bacteria, the
‘oxidation’ product is sulphur or sulphate depending on the organism
and not O
2
. Hence, he inferred that the O
2 
evolved by the green plant
comes from H
2
O, not from carbon dioxide. This was later proved by using
radioisotopic techniques.  The correct equation, that would represent  the
overall process of photosynthesis is therefore:
6 12 6 6
2 2 6 12 6 2 2
CO H O C H O H O O
Light
+ ? ? ???? + +
where C
6
 H
12
 O
6
 represents glucose. The O
2
 released is from water; this
was proved using radio isotope techniques. Note that this is not a single
2015-16(19/01/2015)
PHOTOSYNTHESIS IN HIGHER P LANTS 209
reaction but description of a multistep process called photosynthesis.
Can you explain why twelve molecules of water as substrate are used
in the equation given above?
13.3 WHERE DOES PHOTOSYNTHESIS TAKE PLACE?
You would of course answer: in ‘the green leaf’ or you may add, ‘in the
chloroplasts’ based on what you earlier read in Chapter 8. You are
definitely right. Photosynthesis does take place in the green leaves of plants
but it does so also in other green parts of the plants. Can you name some
other parts where you think photosynthesis may occur?
You would recollect from previous unit that the mesophyll cells in the
leaves, have a large number of chloroplasts. Usually the chloroplasts align
themselves along the walls of the mesophyll cells, such that they get the
optimum quantity of the incident light. When do you think the
chloroplasts will be aligned with their flat surfaces parallel to the walls?
When would they be perpendicular to the incident light?
You have studied the structure of chloroplast in Chapter 8. Within
the chloroplast there is the membranous system consisting of grana, the
stroma lamellae, and the fluid stroma (Figure 13.2). There is a clear division
of labour within the chloroplast. The membrane system is responsible for
trapping the light energy and also for the synthesis of ATP and NADPH.
In stroma, enzymatic reactions incorporate CO
2
 into the plant leading to
the synthesis of sugar, which in turn forms starch. The former set of
reactions, since they are directly light driven are called light reactions.
The latter are not directly light driven but are dependent on the products
of light reactions (ATP and NADPH). Hence, to distinguish the latter they
are called, by convention, as dark reactions. However, this should not be
construed to mean that they occur in darkness or that they are not light-
dependent.
Figure 13.2 Diagrammatic representation of an electron micrograph of a section of
chloroplast
Outer membrane
Inner membrane
Stromal lamella
Grana
Stroma
Ribosomes
Starch granule
Lipid droplet
2015-16(19/01/2015)
210 BIOLOGY
13.4 HOW MANY P IGMENTS ARE INVOLVED
IN PHOTOSYNTHESIS?
Looking at plants have you ever wondered why
and how there are so many shades of green in
their leaves – even in the same plant? We can
look for an answer to this question by trying to
separate the leaf pigments of any green  plant
through paper chromatography. A
chromatographic separation of the leaf pigments
shows that the colour that we see in leaves is
not due to a single pigment but due to four
pigments: Chlorophyll a (bright or blue green
in the chromatogram), chlorophyll b (yellow
green), xanthophylls (yellow) and carotenoids
(yellow to yellow-orange).  Let us now see what
roles various pigments play in photosynthesis.
Pigments are substances that have an ability
to absorb light, at specific wavelengths. Can you
guess which is the most abundant plant
pigment in the world? Let us study the graph
showing the ability of chlorophyll a pigment to
absorb lights of different wavelengths (Figure
13.3 a). Of course, you are familiar with the
wavelength of the visible spectrum of light as
well as the VIBGYOR.
From Figure 13.3a can you determine the
wavelength (colour of light) at which chlorophyll
a shows the maximum absorption? Does it
show another absorption peak at any other
wavelengths too? If yes, which one?
Now look at Figure 13.3b showing the
wavelengths at which maximum photosynthesis
occurs in a plant. Can you see that the
wavelengths at which there is maximum
absorption by chlorophyll a, i.e., in the blue and
the red regions, also shows higher rate of
photosynthesis. Hence, we can conclude that
chlorophyll a is the chief pigment associated
with photosynthesis. But by looking at Figure
13.3c  can you say that there is a complete
one-to-one overlap between the absorption
spectrum of chlorophyll a and the action
spectrum of photosynthesis?
Figure 13.3a Graph showing the absorption
spectrum of chlorophyll a, b and
the carotenoids
Figure 13.3bGraph showing action
spectrum of photosynthesis
Figure 13.3cGraph showing action
spectrum of photosynthesis
superimposed on absorption
spectrum of chlorophyll a
2015-16(19/01/2015)
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