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UNIT 4
The description of structure and variation of living organisms over a
period of time, ended up as two, apparently irreconcilable perspectives
on biology. The two perspectives essentially rested on two levels of
organisation of life forms and phenomena. One described at organismic
and above level of organisation while the second described at cellular
and molecular level of organisation. The first resulted in ecology and
related disciplines. The second resulted in physiology and biochemistry.
Description of physiological processes, in flowering plants as an
example, is what is given in the chapters in this unit. The processes of
photosynthesis, respiration and ultimately plant growth and
development are described in molecular terms but in the context of
cellular activities and even at organism level. Wherever appropriate,
the relation of the physiological processes to environment is also
discussed.
PLANT PHYSIOLOGY
Chapter 11
Photosynthesis in Higher
Plants
Chapter 12
Respiration in Plants
Chapter 13
Plant Growth and
Development
Rationalised 2023-24
Page 2


UNIT 4
The description of structure and variation of living organisms over a
period of time, ended up as two, apparently irreconcilable perspectives
on biology. The two perspectives essentially rested on two levels of
organisation of life forms and phenomena. One described at organismic
and above level of organisation while the second described at cellular
and molecular level of organisation. The first resulted in ecology and
related disciplines. The second resulted in physiology and biochemistry.
Description of physiological processes, in flowering plants as an
example, is what is given in the chapters in this unit. The processes of
photosynthesis, respiration and ultimately plant growth and
development are described in molecular terms but in the context of
cellular activities and even at organism level. Wherever appropriate,
the relation of the physiological processes to environment is also
discussed.
PLANT PHYSIOLOGY
Chapter 11
Photosynthesis in Higher
Plants
Chapter 12
Respiration in Plants
Chapter 13
Plant Growth and
Development
Rationalised 2023-24
MELVIN CALVIN born in Minnesota in April, 1911, received his
Ph.D. in Chemistry from the University of Minnesota. He served
as Professor of Chemistry at the University of California,
Berkeley.
Just after world war II, when the world was under shock
after the Hiroshima-Nagasaki bombings, and seeing the ill-
effects of radio-activity, Calvin and co-workers put radio-
activity to beneficial use. He along with J.A. Bassham studied
reactions in green plants forming sugar and other substances
from raw materials like carbon dioxide, water and minerals
by labelling the carbon dioxide with C
14
. Calvin proposed that
plants change light energy to chemical energy by transferring
an electron in an organised array of pigment molecules and
other substances. The mapping of the pathway of carbon
assimilation in photosynthesis earned him Nobel Prize in 1961.
The principles of photosynthesis as established by Calvin
are, at present, being used in studies on renewable resource
for energy and materials and basic studies in solar energy
research.
Melvin Calvin
Rationalised 2023-24
Page 3


UNIT 4
The description of structure and variation of living organisms over a
period of time, ended up as two, apparently irreconcilable perspectives
on biology. The two perspectives essentially rested on two levels of
organisation of life forms and phenomena. One described at organismic
and above level of organisation while the second described at cellular
and molecular level of organisation. The first resulted in ecology and
related disciplines. The second resulted in physiology and biochemistry.
Description of physiological processes, in flowering plants as an
example, is what is given in the chapters in this unit. The processes of
photosynthesis, respiration and ultimately plant growth and
development are described in molecular terms but in the context of
cellular activities and even at organism level. Wherever appropriate,
the relation of the physiological processes to environment is also
discussed.
PLANT PHYSIOLOGY
Chapter 11
Photosynthesis in Higher
Plants
Chapter 12
Respiration in Plants
Chapter 13
Plant Growth and
Development
Rationalised 2023-24
MELVIN CALVIN born in Minnesota in April, 1911, received his
Ph.D. in Chemistry from the University of Minnesota. He served
as Professor of Chemistry at the University of California,
Berkeley.
Just after world war II, when the world was under shock
after the Hiroshima-Nagasaki bombings, and seeing the ill-
effects of radio-activity, Calvin and co-workers put radio-
activity to beneficial use. He along with J.A. Bassham studied
reactions in green plants forming sugar and other substances
from raw materials like carbon dioxide, water and minerals
by labelling the carbon dioxide with C
14
. Calvin proposed that
plants change light energy to chemical energy by transferring
an electron in an organised array of pigment molecules and
other substances. The mapping of the pathway of carbon
assimilation in photosynthesis earned him Nobel Prize in 1961.
The principles of photosynthesis as established by Calvin
are, at present, being used in studies on renewable resource
for energy and materials and basic studies in solar energy
research.
Melvin Calvin
Rationalised 2023-24
PHOTOSYNTHESIS IN HIGHER PLANTS 133
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. The green plants make or rather synthesise
the food they need through photosynthesis and are therefore called autotrophs.
You have already learnt that the autotrophic nutrition is found only in plants
and all other organisms that depend on the green plants for food are
heterotrophs. 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.
11.1 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 exposed to light.  On testing these leaves for the presence
of 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  11
11.1 What do we
Know?
11.2 Early
Experiments
11.3 Where does
Photosynthesis
take place?
11.4 How many
Pigments are
involved in
Photosynthesis?
11.5 What is Light
Reaction?
11.6 The Electron
Transport
11.7 Where are the
ATP and NADPH
Used?
11.8 The C
4
 Pathway
11.9 hotorespiration
11.10 Factors
affecting
Photosynthesis
Rationalised 2023-24
Page 4


UNIT 4
The description of structure and variation of living organisms over a
period of time, ended up as two, apparently irreconcilable perspectives
on biology. The two perspectives essentially rested on two levels of
organisation of life forms and phenomena. One described at organismic
and above level of organisation while the second described at cellular
and molecular level of organisation. The first resulted in ecology and
related disciplines. The second resulted in physiology and biochemistry.
Description of physiological processes, in flowering plants as an
example, is what is given in the chapters in this unit. The processes of
photosynthesis, respiration and ultimately plant growth and
development are described in molecular terms but in the context of
cellular activities and even at organism level. Wherever appropriate,
the relation of the physiological processes to environment is also
discussed.
PLANT PHYSIOLOGY
Chapter 11
Photosynthesis in Higher
Plants
Chapter 12
Respiration in Plants
Chapter 13
Plant Growth and
Development
Rationalised 2023-24
MELVIN CALVIN born in Minnesota in April, 1911, received his
Ph.D. in Chemistry from the University of Minnesota. He served
as Professor of Chemistry at the University of California,
Berkeley.
Just after world war II, when the world was under shock
after the Hiroshima-Nagasaki bombings, and seeing the ill-
effects of radio-activity, Calvin and co-workers put radio-
activity to beneficial use. He along with J.A. Bassham studied
reactions in green plants forming sugar and other substances
from raw materials like carbon dioxide, water and minerals
by labelling the carbon dioxide with C
14
. Calvin proposed that
plants change light energy to chemical energy by transferring
an electron in an organised array of pigment molecules and
other substances. The mapping of the pathway of carbon
assimilation in photosynthesis earned him Nobel Prize in 1961.
The principles of photosynthesis as established by Calvin
are, at present, being used in studies on renewable resource
for energy and materials and basic studies in solar energy
research.
Melvin Calvin
Rationalised 2023-24
PHOTOSYNTHESIS IN HIGHER PLANTS 133
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. The green plants make or rather synthesise
the food they need through photosynthesis and are therefore called autotrophs.
You have already learnt that the autotrophic nutrition is found only in plants
and all other organisms that depend on the green plants for food are
heterotrophs. 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.
11.1 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 exposed to light.  On testing these leaves for the presence
of 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  11
11.1 What do we
Know?
11.2 Early
Experiments
11.3 Where does
Photosynthesis
take place?
11.4 How many
Pigments are
involved in
Photosynthesis?
11.5 What is Light
Reaction?
11.6 The Electron
Transport
11.7 Where are the
ATP and NADPH
Used?
11.8 The C
4
 Pathway
11.9 hotorespiration
11.10 Factors
affecting
Photosynthesis
Rationalised 2023-24
134 BIOLOGY
Another experiment you may have carried out
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 the presence of starch later in the two
parts 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?
11.2 EARLY EXPERIMENTS
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 11.1 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 11.1 Priestley’s experiment
Rationalised 2023-24
Page 5


UNIT 4
The description of structure and variation of living organisms over a
period of time, ended up as two, apparently irreconcilable perspectives
on biology. The two perspectives essentially rested on two levels of
organisation of life forms and phenomena. One described at organismic
and above level of organisation while the second described at cellular
and molecular level of organisation. The first resulted in ecology and
related disciplines. The second resulted in physiology and biochemistry.
Description of physiological processes, in flowering plants as an
example, is what is given in the chapters in this unit. The processes of
photosynthesis, respiration and ultimately plant growth and
development are described in molecular terms but in the context of
cellular activities and even at organism level. Wherever appropriate,
the relation of the physiological processes to environment is also
discussed.
PLANT PHYSIOLOGY
Chapter 11
Photosynthesis in Higher
Plants
Chapter 12
Respiration in Plants
Chapter 13
Plant Growth and
Development
Rationalised 2023-24
MELVIN CALVIN born in Minnesota in April, 1911, received his
Ph.D. in Chemistry from the University of Minnesota. He served
as Professor of Chemistry at the University of California,
Berkeley.
Just after world war II, when the world was under shock
after the Hiroshima-Nagasaki bombings, and seeing the ill-
effects of radio-activity, Calvin and co-workers put radio-
activity to beneficial use. He along with J.A. Bassham studied
reactions in green plants forming sugar and other substances
from raw materials like carbon dioxide, water and minerals
by labelling the carbon dioxide with C
14
. Calvin proposed that
plants change light energy to chemical energy by transferring
an electron in an organised array of pigment molecules and
other substances. The mapping of the pathway of carbon
assimilation in photosynthesis earned him Nobel Prize in 1961.
The principles of photosynthesis as established by Calvin
are, at present, being used in studies on renewable resource
for energy and materials and basic studies in solar energy
research.
Melvin Calvin
Rationalised 2023-24
PHOTOSYNTHESIS IN HIGHER PLANTS 133
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. The green plants make or rather synthesise
the food they need through photosynthesis and are therefore called autotrophs.
You have already learnt that the autotrophic nutrition is found only in plants
and all other organisms that depend on the green plants for food are
heterotrophs. 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.
11.1 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 exposed to light.  On testing these leaves for the presence
of 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  11
11.1 What do we
Know?
11.2 Early
Experiments
11.3 Where does
Photosynthesis
take place?
11.4 How many
Pigments are
involved in
Photosynthesis?
11.5 What is Light
Reaction?
11.6 The Electron
Transport
11.7 Where are the
ATP and NADPH
Used?
11.8 The C
4
 Pathway
11.9 hotorespiration
11.10 Factors
affecting
Photosynthesis
Rationalised 2023-24
134 BIOLOGY
Another experiment you may have carried out
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 the presence of starch later in the two
parts 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?
11.2 EARLY EXPERIMENTS
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 11.1 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 11.1 Priestley’s experiment
Rationalised 2023-24
PHOTOSYNTHESIS IN HIGHER PLANTS 135
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 11.4).
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
Rationalised 2023-24
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FAQs on NCERT Textbook: Photosynthesis in Higher Plants - NCERT Textbooks (Class 6 to Class 12) - CTET & State TET

1. What is photosynthesis?
Ans. Photosynthesis is a process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of glucose or food. It is the process by which green plants produce their food and release oxygen into the atmosphere.
2. What are the pigments involved in photosynthesis?
Ans. The pigments involved in photosynthesis are chlorophyll a, chlorophyll b, carotenoids, and phycobilins. These pigments absorb light energy and transfer it to the reaction center of the photosystem, where it is converted into chemical energy.
3. How is ATP produced during photosynthesis?
Ans. During photosynthesis, ATP is produced by a process called photophosphorylation. In this process, light energy is used to produce a proton gradient across the thylakoid membrane, which is then used by ATP synthase to synthesize ATP from ADP and inorganic phosphate.
4. What is the role of carbon dioxide in photosynthesis?
Ans. Carbon dioxide is one of the reactants required for photosynthesis. It enters the plant through stomata and is fixed into organic molecules during the light-independent reactions of photosynthesis. The carbon from carbon dioxide is used to build sugars and other organic molecules that the plant uses for energy and growth.
5. What factors affect the rate of photosynthesis?
Ans. The rate of photosynthesis is affected by several factors, including light intensity, temperature, carbon dioxide concentration, and water availability. Higher light intensity, optimal temperature, and higher carbon dioxide concentration increase the rate of photosynthesis, while water scarcity can limit photosynthesis.
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