Early Experiments Questions in English

(C) Robert Mayer,a German physician and physicist,was the first to propose in $1845$ that plants convert light energy into chemical energy. He recognized that the energy stored in the chemical bonds of organic compounds produced by plants originates from sunlight.

(D) The correct answer is $D$. Calvin,Benson,and their colleagues in California,$U.S.A.$,conducted experiments using radioactive $^{14}C$ in carbon dioxide to feed $Chlorella$ and $Scenedesmus$ (which are algae).
Radioactive carbon,$^{14}C$,has a half-life of $5568$ years,allowing the path of $CO_2$ fixation to be traced.
Algal suspension,while performing photosynthesis,was supplied with $^{14}CO_2$.
The algae were killed at specific intervals in near-boiling methanol,which immediately stopped photosynthetic activity due to the denaturation of enzymes.
After evaporating the alcohol and crushing the algae,the resulting paste was analyzed using two-dimensional paper chromatography.
Calvin and his co-workers discovered that after three seconds,radioactivity appeared in phosphoglyceric acid $(PGA)$.
Thus,$PGA$ was identified as the first stable product of photosynthesis in algae.

(C) The correct matches based on historical timelines of scientists are:
$1. 1733-1804$: $r. Joseph Priestley$ (Known for his experiments with bell jars and candles).
$2. 1730-1799$: $p. Jan Ingenhousz$ (Known for his experiments with aquatic plants showing sunlight is essential for photosynthesis).
$3. 1897-1985$: $s. Cornelius Van Niel$ (Known for his studies on purple and green sulfur bacteria).
$4. 1843-1909$: $q. T.W. Engelmann$ (Known for his action spectrum experiment using Cladophora).
Therefore,the correct sequence is $(1-r), (2-p), (3-s), (4-q)$.

(B) Julius von Sachs provided evidence for the production of glucose when plants grow.
In $1854$,he demonstrated that glucose is produced in plants and is usually stored as starch.
He showed that the green parts in plants are where glucose is made,and this glucose is then stored as starch.

(D) Statement $1$ is false because $T$. $W$. Engelmann's experiment demonstrated that photosynthesis occurs primarily in the blue and red regions of the light spectrum,not green and orange. Furthermore,the photolysis of water is a complex process not explicitly described by his action spectrum experiment.
Statement $2$ is true. Engelmann used aerobic bacteria as a tool to detect the sites of $O_2$ evolution. He observed that these bacteria accumulated mainly in the regions of blue and red light,indicating that these are the areas where maximum photosynthesis (and thus $O_2$ release) occurs.

(C) Joseph Priestley,in $1770$,performed a series of experiments that revealed the essential role of air in the growth of green plants.
He placed a burning candle in a closed space (a bell jar) which soon got extinguished.
Similarly,a mouse kept in a closed space would soon suffocate.
He concluded that a burning candle or an animal that breathes the air both somehow damage the air.
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 hypothesized that plants restore to the air whatever breathing animals and burning candles remove.
Thus,he demonstrated that plants purify the air.

(B) Theodore $W$. Engelmann performed a classic experiment using a prism to split light into its spectral components and then illuminated the green alga $Cladophora$ placed in a suspension of aerobic bacteria.
$1$. The bacteria were used to detect the sites of $O_2$ evolution.
$2$. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum.
$3$. This was the first action spectrum of photosynthesis described,which resembles the absorption spectra of chlorophyll $a$ and $b$.

(C) Julius Von Sachs $(1854)$ provided evidence for the production of glucose when plants grow. He demonstrated that the green parts in plants are the sites where glucose is made and that this glucose is usually stored as starch. He also studied the role of chlorophyll in plants.

(C) Cornelius Van Niel performed studies on purple and green sulfur bacteria.
He demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.
In green plants,$H_2O$ acts as the hydrogen donor and is oxidized to $O_2$.
Thus,he concluded that the $O_2$ evolved during photosynthesis comes from water $(H_2O)$ and not from carbon dioxide $(CO_2)$.

(A) Jan Ingenhousz $(1730-1799)$ performed a series of experiments with aquatic plants.
He observed that in bright sunlight,small bubbles were formed around the green parts of the plant,while in the dark,they did not form.
He identified these bubbles as oxygen.
Thus,he concluded that only the green parts of plants can release oxygen in the presence of sunlight.

(B) Melvin Calvin used the radioactive isotope $^{14}C$ in algal photosynthesis studies to trace the path of carbon in the dark reaction of photosynthesis.
This led to the discovery of the Calvin cycle,which describes how carbon dioxide is fixed into organic compounds.

(B) The oxygen released during photosynthesis comes from the photolysis of water $(H_2O)$.
According to the experiments conducted by Ruben,Hassid,and Kamen using the heavy isotope of oxygen $(^{18}O)$,it was proven that the $O_2$ evolved during photosynthesis is derived from water and not from carbon dioxide $(CO_2)$.
Therefore,if a plant releases $O_2$ containing the heavy isotope $^{18}O$,it must have been provided with water labeled with the heavy isotope,i.e.,$H_2^{18}O$.

(D) The discovery that the first stable product of $CO_2$ fixation in photosynthesis is $3$-phosphoglyceric acid $(PGA)$ was made by Melvin Calvin and his colleagues. They used radioactive $^{14}C$ in algal photosynthesis studies,specifically using the green alga $Chlorella$ and $Scenedesmus$. Therefore,the research was conducted using algae.

(B) The oxygen released during photosynthesis originates from water molecules. During the light-dependent reactions of photosynthesis,light energy is captured by chlorophyll in the chloroplasts. This energy facilitates the photolysis of water,which is the splitting of water molecules into protons,electrons,and oxygen gas.
The chemical process is represented as:
$4H_2O \rightleftharpoons 4H^+ + 4OH^-$
$4OH^- \xrightarrow{Mn^{2+}, Cl^-} 2H_2O + O_2 \uparrow + 4e^-$

(D) The experiment described is the famous action spectrum experiment performed by $T$.$W$. Engelmann in $1883$.
He used the green alga $Cladophora$ and placed it in a suspension of aerobic bacteria $(Azotobacter)$.
These bacteria were used to detect the sites of $O_2$ evolution.
He split light using a prism to create a spectrum and illuminated the alga.
He observed that the bacteria accumulated mainly in the regions of blue and red light,where the rate of photosynthesis was highest,leading to maximum $O_2$ release.

(N/A) leaf of a healthy plant is partially covered with black paper.
$(B)$ The other leaf is left fully exposed to sunlight.
$\Rightarrow$ After some time,both leaves are tested for the presence of starch using a dilute iodine solution.
$\Rightarrow$ Observation: Starch is formed in the leaf that was exposed to light (it turns blue-black with iodine).
$\Rightarrow$ The part of the leaf covered by black paper does not turn blue-black,indicating no starch formation.
$\Rightarrow$ Conclusion: This experiment demonstrates that photosynthesis,which leads to starch formation,occurs only in the presence of sunlight.

(N/A) $\Rightarrow$ In this experiment,one half of a leaf is enclosed in a test tube containing cotton soaked in potassium hydroxide $(KOH)$,which absorbs $CO_{2}$,while the other half remains exposed to the atmosphere.
$\Rightarrow$ Test: After some time,the leaf is tested for the presence of starch using an iodine solution.
$\Rightarrow$ Observation: It is observed that the part of the leaf inside the test tube (deprived of $CO_{2}$) does not show the presence of starch,whereas the part exposed to the air tests positive for starch.
$\Rightarrow$ Conclusion: This experiment demonstrates that $CO_{2}$ is essential for the process of photosynthesis.

(N/A) Joseph Priestley $(1733-1804)$ performed a series of experiments in $1770$ that revealed the essential role of air in the growth of green plants.
- Priestley observed that a candle burning in a closed space (a bell jar) soon gets extinguished (Figure $a, b$).
- Similarly,a mouse kept in a closed space would soon suffocate.
- Based on these observations,he concluded that a burning candle or an animal that breathes the air,both somehow damage the air.
- However,when he placed a mint plant in the same bell jar,he found that the mouse stayed alive and the candle continued to burn (Figure $c, d$).
- Priestley hypothesized that plants restore to the air whatever breathing animals and burning candles remove.
- Later,in $1774$,Priestley discovered oxygen.

(N/A) $\Rightarrow$ Jan Ingenhousz $(1730-1799)$ used a setup similar to the one used by Priestley.
$\Rightarrow$ He performed the experiment once in the dark and once in sunlight.
$\Rightarrow$ It was concluded that sunlight is essential to the plant process.
$\Rightarrow$ Sunlight somehow purifies the air fouled by burning candles or breathing animals.
$\Rightarrow$ In an elegant experiment with an aquatic plant,Ingenhousz showed that in bright sunlight,small bubbles were formed around the green parts.
$\Rightarrow$ While in the dark,these bubbles did not occur.
$\Rightarrow$ Later,he identified these bubbles to be of oxygen.
$\Rightarrow$ Hence,he demonstrated that only the green parts of the plants can release oxygen.

(N/A) Julius von Sachs provided evidence for the production of glucose when plants grow in $1854$.
Glucose is usually stored as starch in plants.
His later studies demonstrated that the green substance in plants,now known as chlorophyll,is located within special structures called chloroplasts.
He concluded that the green parts of plants are the sites where glucose is synthesized,and this glucose is subsequently stored as starch.

(N/A) $\Rightarrow$ $T$. $W$. Engelmann used a prism to split light into its spectral components.
$\Rightarrow$ He then illuminated a green alga,$Cladophora$,which was placed in a suspension of aerobic bacteria.
$\Rightarrow$ The bacteria were used to detect the sites of $O_{2}$ evolution.
$\Rightarrow$ He observed that the bacteria accumulated mainly in the regions of blue and red light of the split spectrum.
$\Rightarrow$ Thus,the first action spectrum of photosynthesis was described.
$\Rightarrow$ This spectrum roughly resembles the absorption spectra of chlorophyll $a$ and $b$.

(N/A) $\Rightarrow$ $A$ milestone contribution to the understanding of photosynthesis was made by the microbiologist Cornelius van Niel ($1897$-$1985$),whose studies were based on purple and green bacteria.
$\Rightarrow$ He demonstrated that photosynthesis is essentially a light-dependent reaction,in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.
$\Rightarrow$ General equation: $2H_{2}A + CO_{2} \xrightarrow{\text{light}} 2A + CH_{2}O + H_{2}O$.
$\Rightarrow$ In green plants,$H_{2}O$ is the hydrogen donor and is oxidized to $O_{2}$.
$\Rightarrow$ Some organisms do not release $O_{2}$ during photosynthesis. For purple and green sulfur bacteria,$H_{2}S$ acts as the hydrogen donor.
$\Rightarrow$ The 'oxidation' product in these bacteria is sulfur or sulfate,depending on the organism,and not $O_{2}$.
$\Rightarrow$ Conclusion: He inferred that the $O_{2}$ evolved by green plants comes from $H_{2}O$ and not from carbon dioxide. This was later proven using radioisotopic techniques.
$\Rightarrow$ The correct equation representing the overall process of photosynthesis is: $6CO_{2} + 12H_{2}O \xrightarrow{\text{Light}} C_{6}H_{12}O_{6} + 6H_{2}O + 6O_{2}$.
$\Rightarrow$ Here,$C_{6}H_{12}O_{6}$ represents glucose. The $O_{2}$ released originates from water.

(B) The experiment of two leaves (often referred to as the variegated leaf experiment) is a classic demonstration used to show that chlorophyll is essential for photosynthesis.
In this experiment,a variegated leaf (a leaf with green and non-green patches) is exposed to light.
After testing with iodine,it is observed that starch is formed only in the green parts of the leaf,which contained chlorophyll.
The non-green parts,lacking chlorophyll,do not produce starch.
Therefore,this experiment proves that chlorophyll is necessary for the process of photosynthesis.

(N/A) Joseph Priestley performed a series of experiments in $1770$ that revealed the essential role of air in the growth of green plants.
$1$. He demonstrated that a burning candle or a breathing mouse would soon get extinguished or die in a closed space (bell jar).
$2$. He showed that when a mint plant was placed in the same bell jar,the candle continued to burn and the mouse stayed alive,proving that plants restore to the air whatever breathing animals and burning candles remove.

(B) Joseph Priestley $(1770)$ performed a series of experiments that revealed the essential role of air in the growth of green plants. He demonstrated that plants restore to the air whatever breathing animals and burning candles remove.
Jan Ingenhousz $(1779)$,with a similar setup,showed that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals.
Ingenhousz demonstrated in an elegant experiment with an aquatic plant 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.
Therefore,the key difference is that Ingenhousz specifically identified the role of sunlight and the release of oxygen bubbles in the process.

(D) Julius von Sachs provided evidence for the 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 are where glucose is made,and that the glucose is usually stored as starch.

(B) The relationship between the action spectrum and the absorption spectrum of photosynthesis was first demonstrated by $T.W. Englemann$.
He used the green alga $Cladophora$ and placed it 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.
This experiment showed that the rate of photosynthesis is highest in the blue and red regions of the visible spectrum,which corresponds to the absorption peaks of chlorophyll $a$ and $b$.

(D) The use of radioactive $^{14}C$ by Melvin Calvin in algal (Chlorella) photosynthesis studies led to the discovery that the first carbon dioxide fixation product was a $3$-carbon organic acid.
The first product identified was $3$-phosphoglyceric acid $(PGA)$.

(C) Using a similar setup as used by Priestley,but by placing the experimental plant once in the dark and once in sunlight,Jan Ingenhousz $(1730-1799)$ demonstrated that sunlight is essential to the plant process that purifies the air. He showed that in sunlight,small bubbles were formed around the green parts of the plant,which were identified as oxygen.

(A) Cornelius van Niel conducted pioneering studies on purple and green sulphur bacteria.
He demonstrated that photosynthesis is a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.
In these bacteria,hydrogen sulphide $(H_2S)$ acts as the hydrogen donor instead of water $(H_2O)$,and sulphur or sulphate is produced as an oxidation product instead of oxygen $(O_2)$.
By comparing this with the process in green plants,he inferred that in green plants,$H_2O$ is the hydrogen donor and $O_2$ is evolved as a byproduct,not $CO_2$.

(A) Joseph Priestley,in $1770$,performed a series of experiments that revealed the essential role of air in the growth of green plants.
He demonstrated that plants restore to the air whatever breathing animals and burning candles remove.
He placed a burning candle in a closed bell jar,which soon extinguished,and a mouse in a similar jar,which soon suffocated.
When he placed a mint plant in the same bell jars,he found that the candle continued to burn and the mouse stayed alive,proving that plants purify the air.

(D) In this experiment,half of a leaf was enclosed in a test tube containing $KOH$ (which absorbs $CO_2$),while the other half remained in the air.
When both halves were tested for starch using iodine solution,the part of the leaf exposed to air showed a positive test (blue-black color),whereas the part inside the test tube did not.
This result demonstrates that $CO_2$ is essential for the process of photosynthesis.

(B) The first action spectrum of photosynthesis was described by $T$.$W$. Engelmann. He used a prism to 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. $Chlorella$ was used by Robert Hill in his experiments to study the light and dark phases of photosynthesis,not by Engelmann.

(A) In this experiment,half of a leaf is enclosed in a test tube containing $KOH$ (which absorbs $CO_2$),while the other half remains exposed to the air.
After some time,when both halves are tested for the presence of starch using iodine solution,the part of the leaf exposed to air shows a positive starch test (turns blue-black),whereas the part inside the tube does not.
This demonstrates that $CO_2$ is essential for the process of photosynthesis.

(B) During the process of photosynthesis,the oxygen released as a byproduct comes from the photolysis of water $(H_{2}O)$,not from carbon dioxide $(CO_{2})$.
This was experimentally proven by Ruben and Kamen using the heavy isotope of oxygen,$^{18}O$.
Therefore,if a plant releases oxygen containing $^{18}O$,it indicates that the water supplied to the plant contained the $^{18}O$ isotope.

(D) The 'Half-leaf experiment' is famously known as 'Moll's half-leaf experiment'.
In this experiment,a part of a leaf is enclosed in a test tube containing $KOH$ soaked cotton (which absorbs $CO_2$),while the other half is exposed to air.
After some time,the leaf is tested for starch using iodine solution.
The part exposed to air turns blue-black (indicating starch presence),while the part inside the tube does not.
This proves that $CO_2$ is essential for photosynthesis.
Therefore,both option $A$ and option $C$ refer to the same correct scientific experiment.

(A) In $1770$,Joseph Priestley performed a series of experiments that revealed the essential role of air in the growth of green plants. He observed that a mouse kept in a closed space (bell jar) would soon suffocate and die. Similarly,a burning candle would soon get extinguished in a closed space. However,when he placed a mint plant in the same bell jar,he found that the mouse stayed alive and the candle continued to burn. Based on these observations,he concluded that plants restore to the air whatever breathing animals and burning candles remove.

(B) In 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 Van Niel and later confirmed by Ruben and Kamen using radioactive isotopes,the oxygen released as a byproduct $(O_2)$ comes from the water $(H_2O)$ molecule,not from $CO_2$.
Conversely,the carbon and oxygen atoms from $CO_2$ are incorporated into the glucose $(C_6H_{12}O_6)$ molecule during the Calvin cycle (dark reaction).
Therefore,if the oxygen in $CO_2$ is radiolabeled,the radiolabel will appear in the glucose $(C_6H_{12}O_6)$ produced.

(D) $T$. $W$. Engelmann $(1882)$ performed a classic experiment using the green alga $Cladophora$ placed in a suspension of aerobic bacteria. He used a prism to split light into its spectral components and then illuminated the alga. 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,demonstrating that these wavelengths are most effective for photosynthesis.

(A) The experiment shown in the image is the famous 'Bell Jar Experiment' conducted by Joseph Priestley in $1774$.
In this experiment,Priestley demonstrated that plants restore to the air whatever breathing animals and burning candles remove.
He observed that a mouse kept in a closed space (bell jar) would soon suffocate and die,and a burning candle would soon extinguish.
However,when he placed a mint plant in the same bell jar,the mouse stayed alive and the candle continued to burn.
This led to the discovery of oxygen (which he called 'dephlogisticated air').

(A) In $1770$,Joseph Priestley performed a series of experiments that revealed the essential role of air in the growth of green plants.
He observed that a mouse kept in a closed space (a bell jar) would soon suffocate and die.
Similarly,a burning candle would soon get extinguished in a closed space.
However,when he placed a mint plant in the same bell jar,he found that the mouse stayed alive and the candle continued to burn.
This led him to conclude that plants restore to the air whatever breathing animals and burning candles remove.

(C) Julius von Sachs provided evidence for the production of glucose when plants grow. He demonstrated that glucose is usually stored as starch. His studies showed that the green substance in plants (chlorophyll) 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.

(A) $T$. $W$. Engelmann $(1883)$ performed a classic experiment using a green alga,$Cladophora$,placed in a suspension of aerobic bacteria.
He used a prism to split light into its spectral components and then illuminated the alga.
The aerobic 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.
Thus,he described the first action spectrum of photosynthesis.

(D) 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.
In green plants,$H_2O$ is the hydrogen donor and is oxidized to $O_2$.
This was a significant step in understanding the mechanism of photosynthesis.

(A) The oxygen released during photosynthesis comes from the photolysis of water $(H_2O)$.
This was experimentally proven by $C$.$B$. van Niel,who studied purple and green sulfur bacteria.
He demonstrated that in these bacteria,when $H_2S$ is used instead of $H_2O$,sulfur or sulfate is produced instead of oxygen.
This led to the conclusion that in green plants,the oxygen evolved during photosynthesis originates from water $(H_2O)$ and not from carbon dioxide $(CO_2)$.

(A) In higher plants,photosynthesis involves the photolysis of water $(H_2O)$,which acts as the hydrogen donor to reduce $CO_2$ into carbohydrates,releasing oxygen as a byproduct.
In contrast,purple and green sulfur bacteria do not use water as a hydrogen donor. Instead,they use hydrogen sulfide $(H_2S)$ as the hydrogen donor,which results in the release of sulfur or sulfate instead of oxygen.

(A) Melvin Calvin used radioactive ${}^{14}C$ in algal photosynthesis studies to trace the path of carbon fixation.
He used the technique of radioactive labeling to identify the intermediate compounds formed during the dark reaction of photosynthesis,which is now known as the Calvin cycle.
Therefore,the correct combination is algae and radioactive ${}^{14}C$.

(B) The correct matches are as follows:
$1$. Priestley $(a)$ used a bell jar experiment to show that plants restore air for breathing animals $(ii)$.
$2$. Engelmann $(b)$ used the green alga Cladophora to demonstrate the action spectrum of photosynthesis $(iv)$.
$3$. Van Niel $(c)$ worked on purple and green sulphur bacteria to show that photosynthesis is a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates $(i)$.
$4$. Ingenhousz $(d)$ demonstrated that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals,using an aquatic plant $(iii)$.
Thus,the correct sequence is $a-ii, b-iv, c-i, d-iii$.

(C) Cornelius Van Niel,based on his studies of purple and green sulfur bacteria,demonstrated that photosynthesis is essentially a light-dependent reaction.
He showed that hydrogen from a suitable oxidisable compound reduces $CO_2$ to carbohydrates.
In green plants,$H_2O$ acts as the hydrogen donor and is oxidised to $O_2$.