206 BIOLOGY
C HAPTER 13
PHOTOSYNTHESIS IN HIGHER PLANTS
13.1 What do we All animals including human beings depend on plants for their food. Have
Know? you ever wondered from where plants get their food? Green plants, in fact,
13.2 Early
have to make or rather synthesise the food they need and all other organisms
Experiments depend on them for their needs. The green plants make or rather synthesise
the food they need through photosynthesis and are therefore called autotrophs.
13.3 Where does You have already learnt that the autotrophic nutrition is found only in plants
Photosynthesis and all other organisms that depend on the green plants for food are
take place? heterotrophs. Green plants carry out ‘photosynthesis’, a physico-chemical
13.4 How many process by which they use light energy to drive the synthesis of organic
Pigments are compounds. Ultimately, all living forms on earth depend on sunlight for
involved in energy. The use of energy from sunlight by plants doing photosynthesis is
Photosynthesis? 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
13.5 What is Light
of oxygen into the atmosphere by green plants. Have you ever thought what
Reaction?
would happen if there were no oxygen to breath? This chapter focusses on
13.6 The Electron the structure of the photosynthetic machinery and the various reactions
Transport that transform light energy into chemical energy.
13.7 Where are the
13.1 W HAT D O W E KNOW?
ATP and NADPH
Used? Let us try to find out what we already know about photosynthesis. Some
13.8 The C4 Pathway simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO2 are required for
13.9 Photorespiration
photosynthesis to occur.
13.10 Factors You may have carried out the experiment to look for starch formation
affecting in two leaves – a variegated leaf or a leaf that was partially covered with
Photosynthesis 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.
2020-21
, PHOTOSYNTHESIS IN HIGHER PLANTS 207
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 CO2), 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 CO2 was required for
photosynthesis. Can you explain how this
conclusion could be drawn? (a) (b)
13.2 E ARLY 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 13.1 a, b, c, d). Similarly, a mouse would
(c) (d)
soon suffocate in a closed space. He concluded that
Figure 13.1 Priestley’s experiment
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.
2020-21
, 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 O2
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 CO2 and water. The empirical equation
representing the total process of photosynthesis for oxygen evolving
organisms was then understood as:
Light
CO2 + H2 O → [CH2 O] + O2
where [CH2O] 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:
Light
2H2 A + CO2 → 2 A + CH2 O + H2 O
In green plants H2O is the hydrogen donor and is oxidised to O2. Some
organisms do not release O2 during photosynthesis. When H2S, instead
is the hydrogen donor for purple and green sulphur bacteria, the
‘oxidation’ product is sulphur or sulphate depending on the organism
and not O2. Hence, he inferred that the O2 evolved by the green plant
comes from H2O, 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:
Light
6CO2 + 12H2 O → C6 H12 O6 + 6H2 O + 6O2
where C6 H12 O6 represents glucose. The O2 released is from water; this
was proved using radio isotope techniques. Note that this is not a single
2020-21
C HAPTER 13
PHOTOSYNTHESIS IN HIGHER PLANTS
13.1 What do we All animals including human beings depend on plants for their food. Have
Know? you ever wondered from where plants get their food? Green plants, in fact,
13.2 Early
have to make or rather synthesise the food they need and all other organisms
Experiments depend on them for their needs. The green plants make or rather synthesise
the food they need through photosynthesis and are therefore called autotrophs.
13.3 Where does You have already learnt that the autotrophic nutrition is found only in plants
Photosynthesis and all other organisms that depend on the green plants for food are
take place? heterotrophs. Green plants carry out ‘photosynthesis’, a physico-chemical
13.4 How many process by which they use light energy to drive the synthesis of organic
Pigments are compounds. Ultimately, all living forms on earth depend on sunlight for
involved in energy. The use of energy from sunlight by plants doing photosynthesis is
Photosynthesis? 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
13.5 What is Light
of oxygen into the atmosphere by green plants. Have you ever thought what
Reaction?
would happen if there were no oxygen to breath? This chapter focusses on
13.6 The Electron the structure of the photosynthetic machinery and the various reactions
Transport that transform light energy into chemical energy.
13.7 Where are the
13.1 W HAT D O W E KNOW?
ATP and NADPH
Used? Let us try to find out what we already know about photosynthesis. Some
13.8 The C4 Pathway simple experiments you may have done in the earlier classes have shown
that chlorophyll (green pigment of the leaf), light and CO2 are required for
13.9 Photorespiration
photosynthesis to occur.
13.10 Factors You may have carried out the experiment to look for starch formation
affecting in two leaves – a variegated leaf or a leaf that was partially covered with
Photosynthesis 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.
2020-21
, PHOTOSYNTHESIS IN HIGHER PLANTS 207
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 CO2), 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 CO2 was required for
photosynthesis. Can you explain how this
conclusion could be drawn? (a) (b)
13.2 E ARLY 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 13.1 a, b, c, d). Similarly, a mouse would
(c) (d)
soon suffocate in a closed space. He concluded that
Figure 13.1 Priestley’s experiment
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.
2020-21
, 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 O2
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 CO2 and water. The empirical equation
representing the total process of photosynthesis for oxygen evolving
organisms was then understood as:
Light
CO2 + H2 O → [CH2 O] + O2
where [CH2O] 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:
Light
2H2 A + CO2 → 2 A + CH2 O + H2 O
In green plants H2O is the hydrogen donor and is oxidised to O2. Some
organisms do not release O2 during photosynthesis. When H2S, instead
is the hydrogen donor for purple and green sulphur bacteria, the
‘oxidation’ product is sulphur or sulphate depending on the organism
and not O2. Hence, he inferred that the O2 evolved by the green plant
comes from H2O, 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:
Light
6CO2 + 12H2 O → C6 H12 O6 + 6H2 O + 6O2
where C6 H12 O6 represents glucose. The O2 released is from water; this
was proved using radio isotope techniques. Note that this is not a single
2020-21
