Early Experiments Questions in English

(D) Bacterial photosynthesis is unique because it does not use water $(H_2O)$ as an electron donor. Instead,bacteria use other compounds such as hydrogen sulfide $(H_2S)$,molecular hydrogen $(H_2)$,or organic acids as electron donors. Consequently,oxygen $(O_2)$ is not evolved as a byproduct in bacterial photosynthesis,unlike in green plant photosynthesis where water is split to release oxygen.

(A) Scientific research on photosynthesis began in the $17^{th}$ century. One of the earliest notable experiments was conducted by Stephen Hales in $1727$,but the foundational work is often attributed to the $17^{th}$ century observations by Jan Baptist van Helmont,who demonstrated that plants derive their mass from water rather than soil.

(D) . Pelletier and Caventou $(1818)$ discovered and named the green pigment as chlorophyll.
They successfully extracted this pigment from leaves by boiling them in alcohol.
Chlorophyll is the primary pigment responsible for trapping light energy during the process of photosynthesis.

(D) . Robert Mayer $(1845)$ proposed that light has radiant energy and this radiant energy is converted into chemical energy by plants,which serves to maintain the life of the plants and also animals.

(B) The correct answer is $(b)$.
Jan Ingenhousz $(1779)$ demonstrated that sunlight is essential to the plant process that somehow purifies air fouled by burning candles or breathing animals.
He showed that only the green parts of plants could release oxygen in the presence of sunlight,thereby establishing the significance of light and chlorophyll in photosynthesis.

(D) . Bousingault $(1860)$ reported that the amount of $O_2$ evolved in photosynthesis is equal to the amount of $CO_2$ absorbed, and both these processes occur simultaneously as soon as light is provided.

(C) The correct answer is $C$. Jan Baptist van Helmont conducted a famous experiment with a willow tree. He planted a $5 \ lb$ willow tree in $200 \ lb$ of dried soil and watered it for $5$ years. After $5$ years,the tree weighed $169 \ lb$ and $3 \ oz$,while the soil weight remained almost the same. He concluded that the plant's mass was derived from water and not from the soil.

(A) Early studies on the pathway of $CO_2$ fixation in plants were conducted by Melvin Calvin and his colleagues during the $1940$s.
They used radioactive $C^{14}$ in algal photosynthesis studies,specifically using unicellular green algae like $Chlorella$ and $Scenedesmus$.
This research led to the discovery of the Calvin cycle,which describes the dark reactions of photosynthesis.

(A) The law of limiting factors,proposed by $F.F. Blackman$ in $1905$,states that when a process is conditioned as to its rapidity by a number of separate factors,the rate of the process is limited by the pace of the slowest factor.
This law was criticised by $James$ and $Harder$ because they observed that the rate of photosynthesis does not always increase linearly with the increase in the intensity of a limiting factor; rather,it often shows a gradual transition or a 'curvilinear' response near the saturation point.

(A) In $1930$,$C.B. Van Niel$ proved that purple and green sulphur bacteria use $H_2S$ (in place of water) and $CO_2$ to synthesize carbohydrates as follows:
$6CO_2 + 12H_2S \xrightarrow{Light} C_6H_{12}O_6 + 6H_2O + 12S$
This led $Van Niel$ to the postulation that in green plants,water $(H_2O)$ is utilized in place of $H_2S$ and $O_2$ is evolved as a byproduct in place of sulphur $(S)$. He indicated that water is the electron donor in photosynthesis.
$6CO_2 + 12H_2O \xrightarrow{Light} C_6H_{12}O_6 + 6H_2O + 6O_2$

(C) The correct answer is $C$. Ruben and Kamen used the heavy isotope of oxygen,$O^{18}$,in water $(H_2O^{18})$ instead of normal water $(H_2O^{16})$ during their experiments.
They observed that the oxygen liberated during photosynthesis contains $O^{18}$ derived from water,not from $CO_2$.
The chemical equation for this process is:
$6CO_2 + 12H_2O^{18} \xrightarrow{\text{Light, Chlorophyll}} C_6H_{12}O_6 + 6O_2^{18} + 6H_2O$

(A) In the process of photosynthesis,the oxygen released as a byproduct comes from the photolysis of water $(H_2O)$.
When water labeled with the heavy isotope of oxygen,$^{18}O$,is provided to the photosynthetic system,the oxygen gas $(O_2)$ released during the light-dependent reactions will also contain the $^{18}O$ isotope.
This experiment,famously conducted by Ruben,Hassid,and Kamen,confirmed that the source of oxygen in photosynthesis is water,not carbon dioxide.

(B) The Warburg effect refers to the observation that high concentrations of oxygen inhibit the rate of photosynthesis in plants. Otto Warburg $(1919)$ conducted his experiments on the green alga $Chlorella$ to study the kinetics of photosynthesis and the influence of light and oxygen concentration on the process.

(B) During the process of photosynthesis,the chemical equation is $6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2$.
According to the experiments conducted by $C$.$B$. van Niel using purple and green sulfur bacteria,it was demonstrated that oxygen released during photosynthesis comes from water $(H_2O)$,not from carbon dioxide $(CO_2)$.
Furthermore,isotopic labeling studies using $^{18}O$ have confirmed that the oxygen atoms present in the glucose molecule $(C_6H_{12}O_6)$ are derived from the carbon dioxide $(CO_2)$ molecules,while the oxygen released as a byproduct comes from water $(H_2O)$.

(C) The correct answer is $C$.
$Chlorella$ is a unicellular green alga that has been extensively used in research on photosynthesis.
It was notably used by researchers such as $Ruben$,$Kamen$,and $Warburg$ to study the light-independent reactions and the path of carbon in photosynthesis.
$Chlorella$ is preferred because it is easy to culture,has a simple structure,and performs photosynthesis very efficiently,accounting for a significant portion of global photosynthetic activity.

(A) The fact that oxygen released during photosynthesis originates from water was first demonstrated by Ruben and Kamen using the isotope tracer technique $(O^{18})$.
Photolysis of water occurs in the presence of light and requires $Mn^{++}$ and $Cl^-$ ions as catalysts for water oxidation:
$4H_2O \xrightarrow{Mn^{++}, Cl^-} 4(OH)^- + 4H^+$
Subsequently,the hydroxyl radicals form water and release oxygen as a byproduct:
$4OH \rightarrow 2H_2O + O_2 \uparrow$

(B) The correct answer is $B$.
Robert Emerson and William Arnold conducted experiments on photosynthesis using flashing light in $1932$.
They observed that the rate of photosynthesis could be increased by using short flashes of light followed by dark periods,which provided evidence for the existence of light-dependent and light-independent (dark) reactions.

(C) Moll's half-leaf experiment is a classic demonstration used to prove that $CO_2$ is essential for the process of photosynthesis. In this experiment,half of a leaf is inserted into a bottle containing potassium hydroxide $(KOH)$ solution,which absorbs $CO_2$. The other half remains exposed to the atmosphere. After keeping the plant in sunlight,the starch test (using iodine solution) shows that the part of the leaf exposed to air turns blue-black (indicating starch presence),while the part inside the bottle does not,proving that $CO_2$ is necessary for photosynthesis.

(A) Melvin Calvin and his associates used the unicellular green alga $Chlorella$ for their research on photosynthesis.
They utilized radioactive carbon isotope $^{14}C$ to trace the path of carbon fixation in the Calvin cycle.
This experiment helped in understanding how carbon dioxide is converted into sugars during the light-independent reactions of photosynthesis.

(A) The isotope $C^{14}$ is used to trace the carbon fixation pathway (Calvin cycle) in photosynthesis.
$O^{18}$ is a stable isotope of oxygen used to demonstrate that the oxygen evolved during photosynthesis comes from the splitting of water $(H_2O)$ molecules,not from carbon dioxide $(CO_2)$.

(B) Joseph Priestley performed this experiment in $1774$. He demonstrated that plants restore to the air whatever breathing animals and burning candles remove. He placed a burning candle in a bell jar,which soon got extinguished,and a mouse in a similar bell jar would soon suffocate. However,when he placed a mint plant in the same bell jars,the candle continued to burn and the mouse stayed alive. This experiment proved that plants release oxygen,which is essential for respiration and combustion.

(B) The experiment to demonstrate the evolution of oxygen during photosynthesis is known as the 'Hydrilla experiment'.
In this experiment,a twig of the aquatic plant $Hydrilla$ is placed in a beaker filled with water and covered with a funnel.
$A$ test tube filled with water is inverted over the stem of the funnel.
When the setup is exposed to sunlight,bubbles of gas are seen rising from the cut end of the $Hydrilla$ twig and collecting in the test tube.
These bubbles are identified as oxygen,which is released as a byproduct of photosynthesis.

(A) During photosynthesis,the $O_2$ evolved is derived from the photolysis (hydrolysis) of water.
Since plant $A$ is supplied with $H_2O^{18}$,the oxygen released during the light-dependent reaction will be of the $O^{18}$ isotope type.
Plant $B$ is supplied with $CO_2^{18}$ and normal $H_2O$,so it will release normal $O_2$ because the oxygen in $CO_2$ is incorporated into carbohydrates,not released as gas.
Therefore,plant $A$ releases $O^{18}$ type oxygen.

(C) Variegated leaves contain green areas (with chlorophyll) and yellow areas (lacking chlorophyll). Photosynthesis,which fixes carbon dioxide into organic compounds,occurs only in the green areas where chlorophyll is present. The radioactive $^{14}C$ from the supplied $^{14}CO_2$ is incorporated into the products of photosynthesis (like sugars) in the green parts. The presence of radioactivity in the non-photosynthetic yellow zone of leaf $Y$ (which was kept in the light) is due to the translocation of these radioactive photosynthetic products from the green areas to the yellow areas through the phloem. Therefore,the most likely explanation is that the products of photosynthesis diffuse or are transported into the yellow zone.

(D) $Engelmann$'s experiment used a prism to split light into its spectral components and illuminated $Spirogyra$ placed in a suspension of aerobic bacteria.
He observed that the bacteria accumulated mainly in the regions of blue and red light of the split spectrum.
Since these bacteria require oxygen for respiration,their accumulation indicated that the rate of photosynthesis was highest in these regions.
Therefore,it demonstrated that both blue and red wavelengths are effective for photosynthesis.

(B) In $1937$,Robert Hill demonstrated that isolated chloroplasts,when exposed to light in the presence of an artificial electron acceptor (like ferric salts or viologen dyes),can evolve $O_2$ even in the absence of $CO_2$. This process is known as the Hill reaction. It proves that the oxygen evolved during photosynthesis comes from the photolysis of water.

(D) Robert Hill demonstrated that isolated chloroplasts,when illuminated in the presence of an artificial electron acceptor (like ferric oxalate or $DCPIP$),can evolve oxygen even in the absence of $CO_2$.
This experiment proved that the oxygen evolved during photosynthesis comes from the photolysis of water $(H_2O)$,not from $CO_2$.

(A) Robert Hill $(1939)$ first demonstrated that if chloroplasts extracted from the leaves of $Stellaria \text{ } media$ and $Lamium \text{ } album$ are suspended in a test tube containing a suitable electron acceptor, such as potassium ferrooxalate or potassium ferricyanide, $O_2$ is released due to the photochemical splitting of water. This process is known as the Hill reaction.

(A) Melvin Calvin used radioactive $^{14}C$ in algal photosynthesis studies.
He performed these experiments on the green alga $Chlorella$ and $Scenedesmus$.
Through these experiments,he discovered that the first stable product of carbon fixation in the $C_3$ cycle is $3$-phosphoglyceric acid ($3$-$PGA$).
Therefore,the correct option is $A$.

(A) In bacterial photosynthesis,such as in purple and green sulfur bacteria,$H_2S$ (hydrogen sulfide) acts as the hydrogen donor instead of $H_2O$.
The general reaction is: $CO_2 + 2H_2S \xrightarrow{\text{Light}} (CH_2O) + 2S + H_2O + \text{Energy}$.
Unlike oxygenic photosynthesis in plants where $H_2O$ is the donor and $O_2$ is released,bacterial photosynthesis is anoxygenic and releases sulfur $(S)$ as a byproduct.

(C) In bacterial photosynthesis,such as in purple and green sulfur bacteria,$H_2S$ (hydrogen sulfide) is used as a hydrogen donor instead of $H_2O$ (water).
Because $H_2S$ is used,the by-product released is elemental sulfur $(S)$ or sulfates,rather than oxygen $(O_2)$.
Therefore,the correct by-product of bacterial photosynthesis is $S$.

(B) Theodor Englemann used a prism to split light into its spectral components and illuminated the green alga $Cladophora$ placed in a suspension of aerobic bacteria.
He observed that the bacteria accumulated mainly in the regions of blue and red light.
Since aerobic bacteria require oxygen for survival,their accumulation indicated that these specific wavelengths of light were most effective for photosynthesis,leading to the maximum evolution of $O_2$.
Thus,he concluded that the action spectrum of photosynthesis (which corresponds to $O_2$ evolution) resembles the absorption spectrum of chlorophyll $a$ and $b$,showing peaks in the blue and red regions.

(D) The evidence that $O_2$ released in photosynthesis comes from water is supported by multiple observations:
$1$. $C.B. \text{ van Niel}$ demonstrated that photosynthetic bacteria use $H_2S$ instead of $H_2O$ to produce carbohydrates, releasing sulfur instead of oxygen, which suggested that $O_2$ in plants comes from $H_2O$.
$2$. The use of isotopic oxygen $(^{18}O)$ in $H_2O$ showed that the $O_2$ released during photosynthesis is labeled with the same isotope, confirming its origin from water.
$3$. The Hill reaction showed that isolated chloroplasts can evolve $O_2$ in the presence of an artificial electron acceptor (like potassium ferrocyanide) when illuminated, even without $CO_2$ fixation, proving that water splitting (photolysis) is the source of $O_2$.

(A) Cornelius Van Niel,based on his studies of purple and green sulfur bacteria,demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates. He proposed that in green plants,water $(H_2O)$ acts as the hydrogen donor,releasing oxygen as a byproduct,whereas in sulfur bacteria,hydrogen sulfide $(H_2S)$ acts as the hydrogen donor.

(B) The process of photosynthesis involves the fixation of carbon dioxide and the photolysis of water.
$C^{14}$ (a radioactive isotope of carbon) is used to trace the path of carbon in the Calvin cycle (dark reactions).
$O^{18}$ (a heavy isotope of oxygen) is used to demonstrate that the oxygen evolved during photosynthesis comes from water and not from carbon dioxide.
Therefore,$C^{14}$ and $O^{18}$ are the correct isotopes used in these studies.

(D) The discovery of the first stable product of $CO_2$ fixation was made by Melvin Calvin and his colleagues using radioactive $^{14}C$ in algal photosynthesis experiments.
Specifically,they used the green alga $Chlorella$ and $Scenedesmus$.
They found that $3$-phosphoglyceric acid $(PGA)$,a $3$-carbon compound,was the first stable product formed during the dark reaction of photosynthesis.
Therefore,$PGA$ was discovered as the first product of $CO_2$ fixation in the photosynthesis of algae.

(B) The biochemical phase of photosynthesis,also known as the dark reaction or the Calvin cycle,involves the fixation of $CO_2$ into carbohydrates. This phase was first elucidated by $F.F. Blackman$ in $1905$. He demonstrated that photosynthesis consists of two distinct phases: a light-dependent phase and a light-independent (biochemical) phase. Therefore,the correct answer is $Blackman$.

(B) The radioactive isotope of carbon,$C^{14}$,was used by Melvin Calvin in his experiments to trace the path of carbon in photosynthesis.
This led to the discovery of the Calvin cycle (also known as the $C_3$ cycle).
Therefore,$C^{14}$ is the correct isotope used for studying the carbon fixation pathway in photosynthesis.

(C) In bacterial photosynthesis,such as in purple and green sulfur bacteria,$H_2S$ (hydrogen sulfide) is used as a hydrogen donor instead of $H_2O$ (water).
Because $H_2S$ is used as the electron donor,the byproduct released is sulfur $(S)$ or sulfate,rather than oxygen $(O_2)$.
This is a key distinction between oxygenic photosynthesis (found in plants and cyanobacteria) and anoxygenic photosynthesis (found in many bacteria).

(C) Joseph Priestley,in $1774$,performed a series of experiments that revealed the essential role of air in the growth of green plants. He placed a burning candle and a mouse in a closed bell jar,which eventually extinguished the candle and suffocated the mouse. However,when he placed a mint plant in the same bell jar,the candle continued to burn and the mouse remained alive. This led him to conclude that plants restore to the air whatever breathing animals and burning candles remove. Thus,he suggested that plants purify foul air.

(A) Mohl's half-leaf experiment is a classic demonstration used to prove that $CO_2$ is essential for the process of photosynthesis.
In this experiment,a part of a leaf is enclosed in a bottle containing $KOH$ (potassium hydroxide),which absorbs $CO_2$,while the other part remains exposed to the atmosphere.
After some time,the leaf is tested for starch using iodine solution.
The part of the leaf exposed to air turns blue-black (indicating the presence of starch),while the part inside the bottle remains pale (indicating no starch formation due to the absence of $CO_2$).

(B) Jan Ingenhousz $(1730-1799)$ performed a series of experiments with aquatic plants.
He showed that in bright sunlight,small bubbles were formed around the green parts of the plant,while in the dark,they did not.
He later identified these bubbles to be of oxygen.
Hence,he concluded that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals.
He also demonstrated that only the green parts of the plants could release oxygen.

(C) Photophosphorylation is the process of synthesizing $ATP$ from $ADP$ and inorganic phosphate in the presence of light in chloroplasts.
This phenomenon was first demonstrated by $Daniel \text{ } I. \text{ } Arnon$ and his colleagues in $1954$ using isolated chloroplasts from spinach leaves.
Therefore, the correct option is $C$.

(B) The oxygen released during photosynthesis is derived from the photolysis of water $(H_2O)$.
This was experimentally proven by $C$.$B$. van Niel,who demonstrated that photosynthesis is a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.
In green plants,water acts as the hydrogen donor and is oxidized to release oxygen $(O_2)$ as a byproduct.

(A) The experiment using the $O^{18}$ isotope was conducted by $Ruben$ and $Kamen$ in $1941$.
They used the heavy isotope of oxygen $(O^{18})$ to trace the source of oxygen evolved during photosynthesis.
They provided water containing $O^{18}$ $(H_2O^{18})$ to the green algae $Chlorella$.
It was observed that the oxygen evolved during photosynthesis was $O^{18}$,which confirmed that the oxygen released during photosynthesis comes from water and not from carbon dioxide.

(A) Robert Hill $(1937)$ demonstrated that isolated chloroplasts,when illuminated in the presence of an artificial electron acceptor,can evolve oxygen even in the absence of $CO_2$.
This process is known as the Hill reaction.
He used $2,6$-dichlorophenol indophenol $(DCPIP)$ as an artificial electron acceptor (dye) which changes color from blue to colorless upon reduction,serving as an indicator for the light-dependent reaction of photosynthesis.

(D) $T$.$W$. Engelmann $(1883)$ performed a classic experiment using the green alga Cladophora and aerobic bacteria.
He split light into its spectral components using a prism and then illuminated the alga.
The aerobic bacteria were used to detect the sites of oxygen evolution.
He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum,demonstrating that these wavelengths are most effective for photosynthesis.

(D) In $1941$, $Samuel \text{ } Ruben$, $William \text{ } Hassid$, and $Martin \text{ } Kamen$ used the heavy isotope of oxygen $(^{18}O)$ to trace the source of oxygen evolved during photosynthesis.
They performed experiments using the green alga $Chlorella \text{ } pyrenoidosa$.
By providing water labeled with $^{18}O$ $(H_2^{18}O)$, they observed that the oxygen released as a byproduct was also labeled with $^{18}O$.
This confirmed that the oxygen released during photosynthesis is derived from the splitting of water molecules $(photolysis \text{ } of \text{ } water)$, not from carbon dioxide $(CO_2)$.

(D) In the study of photosynthesis,radioactive isotopes were used to trace the pathways of carbon and the source of oxygen.
$1$. Melvin Calvin used the radioactive isotope $C^{14}$ to trace the path of carbon in the Calvin cycle (dark reaction).
$2$. Scientists used the heavy isotope of oxygen,$O^{18}$,to demonstrate that the oxygen evolved during photosynthesis comes from water $(H_2O)$ and not from carbon dioxide $(CO_2)$.

(B) Jan Ingenhousz ($1730$–$1799$) performed a series of experiments with aquatic plants.
He showed that in bright sunlight,small bubbles were formed around the green parts of the plant,while in the dark,they did not.
He later identified these bubbles to be of oxygen $(O_2)$.
He also demonstrated that green plants release oxygen only in the presence of sunlight and take in carbon dioxide $(CO_2)$ for photosynthesis.