- Photosynthesis is a physico-chemical process by which - Ultimately, all living forms depend on sunlight for energy.
green plants use light energy (solar energy) to synthesise Importance of Photosynthesis
organic compounds. So they are autotrophs. • It is the primary source of all food on earth.
- It is the basis of life on earth. • It releases oxygen into the atmosphere.
EXPERIMENTS RELATED WITH PHOTOSYNTHESIS
1. Variegated leaf experiment Experiments by Julius von Sachs (1854)
- Take a variegated leaf (or leaf partially covered with black - He proved that
paper) that was exposed to light. o Glucose is produced when plants grow and it is usually
- Test the leaves for starch. It shows that photosynthesis stored as starch.
occurs only in green parts of the leaves in presence of light. o Chlorophyll is located in special bodies (chloroplasts).
2. Half-leaf experiment o Glucose is made in the green parts of plants.
- A part of a leaf is enclosed in a test tube containing KOH Experiments by T.W Engelmann (1843 – 1909)
soaked cotton (which absorbs CO2). - He split the light using a prism into its spectral components
- The other half of leaf is exposed to air. and illuminated a green alga (Cladophora) placed in a
- Place this setup in light for some time. suspension of aerobic bacteria.
- Test the leaf for presence of starch. Exposed part shows - The bacteria were used to detect the sites of O2 evolution.
positive for starch and portion in the tube shows negative. - He observed that the bacteria accumulated mainly in the
This proves that CO2 is required for photosynthesis. region of blue and red light of the split spectrum.
EARLY EXPERIMENTS - It was a first described action spectrum of photosynthesis.
It resembles the absorption spectra of chlorophyll a & b.
Experiments by Joseph Priestley (1770)
- By the middle of 19th century, it is discovered that plants
- Priestley performed experiments to prove the role of air in use light energy to make carbohydrates from CO2 & H2O.
the growth of green plants. - Empirical equation of the process of photosynthesis is
- He discovered oxygen in 1774.
- He observed that a candle burning in a closed bell jar gets
extinguished. Similarly, a mouse suffocated in closed jar. Where, [CH2O] represents a carbohydrate (e.g. glucose).
He concluded that a burning candle or a breathing animal Experiments by Cornelius van Niel (1897-1985)
damage the air. - Van Niel (microbiologist) conducted some studies in
- He placed a mint plant in the same bell jar. He found that purple and green bacteria.
the mouse stayed alive and the candle continued to burn. - He demonstrated that photosynthesis is a light-dependent
- He hypothesised that plants restore to the air whatever reaction in which hydrogen from an oxidisable compound
breathing animals and burning candles remove. reduces CO2 to carbohydrates.
Experiments by Jan Ingenhousz (1730-1799)
- He conducted the same experiment by placing in darkness
- In plants, H2O is the hydrogen donor and is oxidised to O2.
and sunlight.
- Purple & green sulphur bacteria use H2S as H-donor. So
- He showed that sunlight is essential to the plant for
the ‘oxidation’ product is sulphur or sulphate and no O2 is
purifying the air fouled by burning candles or animals.
produced.
- He repeated this experiment with an aquatic plant. It
- Thus, he inferred that the O2 evolved by the green plant
showed that in bright sunlight, small bubbles were formed
comes from H2O, not from CO2. This was later proved by
around green parts while in the dark they did not.
using radio isotopic techniques.
- Later he identified these bubbles to be of oxygen. Thus he
- Therefore overall correct equation for photosynthesis is:
showed that only the green part of plants release O2.
PHOTOSYNTHESIS: SITE AND PIGMENTS
- Photosynthesis occurs in green leaves & other green parts. - The membrane system traps light energy and synthesise
- Chloroplasts present in the walls of mesophyll cells of ATP and NADPH. It is called light reactions.
leaves. It helps to get optimum quantity of incident light. - In stroma, enzymatic reactions synthesize sugar, which in
- Chloroplast contains a membranous system. It consists of turn forms starch. It is called dark reactions (carbon
grana, stroma lamellae and matrix stroma. reactions). It does not mean that they occur in darkness or
- Each granum is a group of membrane-bound sacs called that they are not light dependent.
thylakoids (lamellae). They contain leaf pigments.
1
, Graph showing
action
spectrum of
photosynthesis
PIGMENTS INVOLVED IN PHOTOSYNTHESIS
Graph showing
- Pigments are substances that have ability to absorb light at
action spectrum of
specific wavelengths. photosynthesis
- Chromatography shows the following leaf pigments: superimposed on
o Chlorophyll a (bright or blue green in chromatogram) absorption
spectrum of
o Chlorophyll b (yellow green) chlorophyll a
Accessory
o Xanthophylls (yellow) pigments
o Carotenoids (yellow to yellow-orange)
- Functions of accessory pigments: Photosystems
o They absorb light at different wavelength and transfer - Pigments are organised into two Photosystems called
the energy to chlorophyll a. Photosystem I (PSI) & Photosystem II (PSII). These are
o They protect chlorophyll a from photo-oxidation. named in the sequence of their discovery.
- The absorption spectrum & action spectrum coincide - Each photosystem has a chlorophyll a and accessory
closely showing that photosynthesis is maximum at the pigments bound by proteins.
blue & red regions of the spectrum. - All pigments (except one molecule of chlorophyll a) form
- The graphs also show that chlorophyll a is the chief a light harvesting
pigment associated with photosynthesis. complex (LHC or
antennae).
- Single chlorophyll a acts
Graph showing as reaction centre.
absorption
spectrum of
- In PS I, the reaction
chlorophyll a, b centre absorbs light at
& carotenoids 700 nm, and so called
P700.
- In PS II, the reaction
centre absorbs light at 680 nm, and so called P680.
LIGHT REACTION (PHOTOCHEMICAL PHASE)
- Light reactions include light absorption, water splitting,
oxygen release and formation of ATP & NADPH (high-
energy chemical intermediates).
The Electron Transport
- When PS II absorbs red light of 680 nm wavelength,
electrons are excited and transferred to an electron acceptor.
- The electron acceptor passes them to a chain of electrons
transport system consisting of cytochromes.
- This movement of electrons is downhill, in terms of redox
potential scale.
- The electrons are transferred to the pigments of PS I.
- Simultaneously, electrons in PS I are also excited when
they receive red light of 700 nm and are transferred to Z scheme of light reaction
another accepter molecule having a greater redox potential. Splitting of Water (Photolysis)
- These electrons are moved downhill to a molecule of
- The water splitting complex in PS II is located on the inner
NADP+. As a result, NADP+ is reduced to NADPH + H+.
side of the thylakoid membrane.
- Transfer of electrons from PS II to PS I and finally downhill - Water is split into H+, [O] and electrons.
to NADP+ is called the Z scheme, due to its zigzag shape.
2H2O → 4H+ + O2 + 4e−
This shape is formed when all the carriers are placed in a
- So PS II can supply electrons continuously by replacing
sequence on a redox potential scale.
electrons from water splitting.
2
green plants use light energy (solar energy) to synthesise Importance of Photosynthesis
organic compounds. So they are autotrophs. • It is the primary source of all food on earth.
- It is the basis of life on earth. • It releases oxygen into the atmosphere.
EXPERIMENTS RELATED WITH PHOTOSYNTHESIS
1. Variegated leaf experiment Experiments by Julius von Sachs (1854)
- Take a variegated leaf (or leaf partially covered with black - He proved that
paper) that was exposed to light. o Glucose is produced when plants grow and it is usually
- Test the leaves for starch. It shows that photosynthesis stored as starch.
occurs only in green parts of the leaves in presence of light. o Chlorophyll is located in special bodies (chloroplasts).
2. Half-leaf experiment o Glucose is made in the green parts of plants.
- A part of a leaf is enclosed in a test tube containing KOH Experiments by T.W Engelmann (1843 – 1909)
soaked cotton (which absorbs CO2). - He split the light using a prism into its spectral components
- The other half of leaf is exposed to air. and illuminated a green alga (Cladophora) placed in a
- Place this setup in light for some time. suspension of aerobic bacteria.
- Test the leaf for presence of starch. Exposed part shows - The bacteria were used to detect the sites of O2 evolution.
positive for starch and portion in the tube shows negative. - He observed that the bacteria accumulated mainly in the
This proves that CO2 is required for photosynthesis. region of blue and red light of the split spectrum.
EARLY EXPERIMENTS - It was a first described action spectrum of photosynthesis.
It resembles the absorption spectra of chlorophyll a & b.
Experiments by Joseph Priestley (1770)
- By the middle of 19th century, it is discovered that plants
- Priestley performed experiments to prove the role of air in use light energy to make carbohydrates from CO2 & H2O.
the growth of green plants. - Empirical equation of the process of photosynthesis is
- He discovered oxygen in 1774.
- He observed that a candle burning in a closed bell jar gets
extinguished. Similarly, a mouse suffocated in closed jar. Where, [CH2O] represents a carbohydrate (e.g. glucose).
He concluded that a burning candle or a breathing animal Experiments by Cornelius van Niel (1897-1985)
damage the air. - Van Niel (microbiologist) conducted some studies in
- He placed a mint plant in the same bell jar. He found that purple and green bacteria.
the mouse stayed alive and the candle continued to burn. - He demonstrated that photosynthesis is a light-dependent
- He hypothesised that plants restore to the air whatever reaction in which hydrogen from an oxidisable compound
breathing animals and burning candles remove. reduces CO2 to carbohydrates.
Experiments by Jan Ingenhousz (1730-1799)
- He conducted the same experiment by placing in darkness
- In plants, H2O is the hydrogen donor and is oxidised to O2.
and sunlight.
- Purple & green sulphur bacteria use H2S as H-donor. So
- He showed that sunlight is essential to the plant for
the ‘oxidation’ product is sulphur or sulphate and no O2 is
purifying the air fouled by burning candles or animals.
produced.
- He repeated this experiment with an aquatic plant. It
- Thus, he inferred that the O2 evolved by the green plant
showed that in bright sunlight, small bubbles were formed
comes from H2O, not from CO2. This was later proved by
around green parts while in the dark they did not.
using radio isotopic techniques.
- Later he identified these bubbles to be of oxygen. Thus he
- Therefore overall correct equation for photosynthesis is:
showed that only the green part of plants release O2.
PHOTOSYNTHESIS: SITE AND PIGMENTS
- Photosynthesis occurs in green leaves & other green parts. - The membrane system traps light energy and synthesise
- Chloroplasts present in the walls of mesophyll cells of ATP and NADPH. It is called light reactions.
leaves. It helps to get optimum quantity of incident light. - In stroma, enzymatic reactions synthesize sugar, which in
- Chloroplast contains a membranous system. It consists of turn forms starch. It is called dark reactions (carbon
grana, stroma lamellae and matrix stroma. reactions). It does not mean that they occur in darkness or
- Each granum is a group of membrane-bound sacs called that they are not light dependent.
thylakoids (lamellae). They contain leaf pigments.
1
, Graph showing
action
spectrum of
photosynthesis
PIGMENTS INVOLVED IN PHOTOSYNTHESIS
Graph showing
- Pigments are substances that have ability to absorb light at
action spectrum of
specific wavelengths. photosynthesis
- Chromatography shows the following leaf pigments: superimposed on
o Chlorophyll a (bright or blue green in chromatogram) absorption
spectrum of
o Chlorophyll b (yellow green) chlorophyll a
Accessory
o Xanthophylls (yellow) pigments
o Carotenoids (yellow to yellow-orange)
- Functions of accessory pigments: Photosystems
o They absorb light at different wavelength and transfer - Pigments are organised into two Photosystems called
the energy to chlorophyll a. Photosystem I (PSI) & Photosystem II (PSII). These are
o They protect chlorophyll a from photo-oxidation. named in the sequence of their discovery.
- The absorption spectrum & action spectrum coincide - Each photosystem has a chlorophyll a and accessory
closely showing that photosynthesis is maximum at the pigments bound by proteins.
blue & red regions of the spectrum. - All pigments (except one molecule of chlorophyll a) form
- The graphs also show that chlorophyll a is the chief a light harvesting
pigment associated with photosynthesis. complex (LHC or
antennae).
- Single chlorophyll a acts
Graph showing as reaction centre.
absorption
spectrum of
- In PS I, the reaction
chlorophyll a, b centre absorbs light at
& carotenoids 700 nm, and so called
P700.
- In PS II, the reaction
centre absorbs light at 680 nm, and so called P680.
LIGHT REACTION (PHOTOCHEMICAL PHASE)
- Light reactions include light absorption, water splitting,
oxygen release and formation of ATP & NADPH (high-
energy chemical intermediates).
The Electron Transport
- When PS II absorbs red light of 680 nm wavelength,
electrons are excited and transferred to an electron acceptor.
- The electron acceptor passes them to a chain of electrons
transport system consisting of cytochromes.
- This movement of electrons is downhill, in terms of redox
potential scale.
- The electrons are transferred to the pigments of PS I.
- Simultaneously, electrons in PS I are also excited when
they receive red light of 700 nm and are transferred to Z scheme of light reaction
another accepter molecule having a greater redox potential. Splitting of Water (Photolysis)
- These electrons are moved downhill to a molecule of
- The water splitting complex in PS II is located on the inner
NADP+. As a result, NADP+ is reduced to NADPH + H+.
side of the thylakoid membrane.
- Transfer of electrons from PS II to PS I and finally downhill - Water is split into H+, [O] and electrons.
to NADP+ is called the Z scheme, due to its zigzag shape.
2H2O → 4H+ + O2 + 4e−
This shape is formed when all the carriers are placed in a
- So PS II can supply electrons continuously by replacing
sequence on a redox potential scale.
electrons from water splitting.
2
