This MCQ module is based on: Early Experiments Pigments
TOPIC 46 OF 90
Early Experiments Pigments
🎓 Class 11
Biology
CBSE
Theory
Ch 11 – Photosynthesis in Higher Plants
⏱ ~14 min
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Photosynthesis — Early Experiments and Pigments
All animals, including human beings, depend on plants for their food. Green plants have to synthesise the food they need, and all other organisms depend on them. Green plants carry out photosynthesis, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds, and are therefore called autotrophs; organisms that depend on them are heterotrophs.
Why photosynthesis matters, in two lines. It is the primary source of all food on earth, and it is responsible for the release of oxygen into the atmosphere by green plants. Ultimately all living forms on earth depend on sunlight for energy.
11.1 What Do We Know?
Simple experiments from earlier classes have already shown that chlorophyll (the green pigment of the leaf), light and CO₂ are required for photosynthesis to occur.
The variegated leaf / half-covered leaf experiment
A variegated leaf, or a leaf partly covered with black paper, is exposed to light and then tested for starch. Starch appears only in the green parts of the leaf that received light. This establishes two requirements at once — chlorophyll and light.
The KOH experiment
Part of a leaf is enclosed in a test tube containing KOH-soaked cotton, which absorbs CO₂, while the other half is exposed to air. The setup is placed in light. On testing for starch, the exposed part tests positive and the enclosed part tests negative.
In-text question — how can that conclusion be drawn? Both halves of the same leaf had the same chlorophyll and the same light; the only variable was CO₂, which the KOH removed from the enclosed half. Since starch formed only where CO₂ was available, CO₂ must be required for photosynthesis. This is a controlled experiment in which the leaf serves as its own control.
11.2 Early Experiments
Joseph Priestley (1733–1804) — plants restore the air
In 1770 Priestley performed a series of experiments revealing the essential role of air in the growth of green plants. He had discovered oxygen in 1774. He observed that a candle burning in a closed bell jar soon goes out, and a mouse in a closed space would soon suffocate. He concluded that a burning candle or a breathing animal somehow damages the air. But when he placed a mint plant in the same bell jar, the mouse stayed alive and the candle continued to burn.
Priestley's hypothesis. Plants restore to the air whatever breathing animals and burning candles remove.
In-text question — how would Priestley rekindle the candle? The candle must be relit without opening the jar, or the experiment is ruined. Possible ways: focus sunlight onto the wick with a converging lens from outside the glass; use a burning lens or mirror to concentrate the sun's rays; or fit the jar in advance with a candle that can be ignited by an external electrical spark through sealed wires. The point of the question is to make you see that a good experiment must keep the closed system closed.
Jan Ingenhousz (1730–1799) — sunlight is essential
Using a setup similar to Priestley's, but placing it once in the dark and once in the sunlight, Ingenhousz showed that sunlight is essential to the plant process that purifies air fouled by burning candles or breathing animals. In an elegant experiment with an aquatic plant he showed that in bright sunlight small bubbles formed around the green parts, while in the dark they did not. He later identified these bubbles as oxygen, showing that it is only the green part of the plant that releases oxygen.
Julius von Sachs (1854) — glucose, and where it is made
Von Sachs provided evidence for the production of glucose when plants grow, and that glucose is usually stored as starch. His later studies showed that the green substance in plants — chlorophyll, as we now know it — is located in special bodies later called chloroplasts within plant cells, and that the green parts are where glucose is made.
T. W. Engelmann (1843–1909) — the first action spectrum
Using a prism he split light into its spectral components and illuminated the 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 blue and red regions of the split spectrum. A first action spectrum of photosynthesis was thus described, and it resembles roughly the absorption spectra of chlorophyll a and b.
The empirical equation
By the middle of the nineteenth century the key features were known — that plants could use light energy to make carbohydrates from CO₂ and water:
CO₂ + H₂O —[light]→ [CH₂O] + O₂
where [CH₂O] represented a carbohydrate such as glucose, a six-carbon sugar.
Cornelius van Niel (1897–1985) — where the oxygen comes from
A milestone contribution came from the microbiologist van Niel, who, from 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:
2H₂A + CO₂ —[light]→ 2A + CH₂O + H₂O
In green plants H₂O is the hydrogen donor and is oxidised to O₂. Some organisms do not release O₂ during photosynthesis: when H₂S is the hydrogen donor, as in purple and green sulphur bacteria, the oxidation product is sulphur or sulphate, not O₂. Van Niel therefore inferred that the O₂ evolved by green plants comes from H₂O, not from carbon dioxide — later proved using radioisotopic techniques.
The correct overall equation.
6CO₂ + 12H₂O —[light]→ C₆H₁₂O₆ + 6H₂O + 6O₂
Note that this is not a single reaction but a description of a multistep process.In-text question — why twelve molecules of water? Water appears on both sides. Twelve molecules are consumed as the hydrogen donor, and six are regenerated, so the net consumption is six. Twelve are written on the left because the oxygen released must be accounted for: splitting 12 H₂O supplies the 24 electrons and 24 protons needed, and liberates 6 O₂. Writing only 6H₂O would balance the atoms but would wrongly suggest that some of the released O₂ came from CO₂ — which van Niel's work disproved.
11.3 Where Does Photosynthesis Take Place?
Photosynthesis takes place in the green leaves of plants, but it does so also in other green parts.
In-text question — which other parts? Any green, chloroplast-bearing tissue: green stems (as in Opuntia and other cacti, where flattened stems take over entirely), green sepals, young green fruits such as unripe mango or pea pods, green petioles and photosynthetic roots of some epiphytic orchids.
The mesophyll cells in the leaves have a large number of chloroplasts. Usually the chloroplasts align themselves along the walls of the mesophyll cells, so that they receive the optimum quantity of incident light.
In-text question — when do chloroplasts lie flat, and when edge-on? In moderate or low light the chloroplasts turn their flat faces parallel to the cell wall and perpendicular to the incoming light, presenting the greatest possible area for absorption. In very intense light they turn edge-on, their flat surfaces parallel to the light, so that the exposed area is minimised and the pigments are protected from photo-oxidation.
Division of labour inside the chloroplast
Within the chloroplast there is a membranous system consisting of grana, the stroma lamellae, and the matrix stroma. There is a clear division of labour.
| Compartment | Function | Name of the reactions |
|---|---|---|
| Membrane system (grana + stroma lamellae) | trapping light energy; synthesis of ATP and NADPH | Light reactions (photochemical reactions) — directly light driven |
| Stroma | enzymatic reactions that synthesise sugar, which in turn forms starch | Dark reactions (carbon reactions) — dependent on ATP and NADPH |
A warning about the term "dark reaction". The name is a convention only. It must not be taken to mean that these reactions occur in darkness, or that they are not light-dependent. They are not directly light driven, but they depend completely on the ATP and NADPH made by the light reactions.
11.4 How Many Types of Pigments Are Involved in Photosynthesis?
A chromatographic separation of leaf pigments by paper chromatography shows that the colour we see in leaves is not due to a single pigment but to four.
| Pigment | Colour in the chromatogram | Role |
|---|---|---|
| Chlorophyll a | bright or blue green | the chief pigment associated with photosynthesis; forms the reaction centre |
| Chlorophyll b | yellow green | accessory pigment |
| Xanthophylls | yellow | accessory pigment |
| Carotenoids | yellow to yellow-orange | accessory pigment |
Pigments are substances that have the ability to absorb light at specific wavelengths.
In-text question — the most abundant plant pigment in the world? Chlorophyll a. It occurs in every oxygen-evolving photosynthetic organism on earth, from cyanobacteria to forest trees, and is the pigment at the reaction centre of both photosystems.
Absorption spectrum and action spectrum
In-text questions on Figure 11.3.
(a) Where does chlorophyll a absorb maximally? In the blue region (around 430 nm), with a second peak in the red region (around 662 nm). Absorption is lowest in the green, which is why leaves look green — green light is largely reflected rather than absorbed.
(b) Is there a complete one-to-one overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis? No. The two curves peak in the same blue and red regions, which is why chlorophyll a is judged the chief pigment — but the action spectrum stays appreciably above the chlorophyll a absorption curve at intermediate wavelengths. Photosynthesis continues at wavelengths where chlorophyll a absorbs poorly, because the accessory pigments absorb there and hand the energy on.
(a) Where does chlorophyll a absorb maximally? In the blue region (around 430 nm), with a second peak in the red region (around 662 nm). Absorption is lowest in the green, which is why leaves look green — green light is largely reflected rather than absorbed.
(b) Is there a complete one-to-one overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis? No. The two curves peak in the same blue and red regions, which is why chlorophyll a is judged the chief pigment — but the action spectrum stays appreciably above the chlorophyll a absorption curve at intermediate wavelengths. Photosynthesis continues at wavelengths where chlorophyll a absorbs poorly, because the accessory pigments absorb there and hand the energy on.
What the accessory pigments do. Chlorophyll b, xanthophylls and carotenoids absorb light and transfer the energy to chlorophyll a. They therefore do two things: they enable a wider range of wavelengths to be utilised for photosynthesis, and they protect chlorophyll a from photo-oxidation.
🎯 Interactive: Which scientist proved what?
Select a finding and see who established it, and how.
Scientist: —
Choose a finding to see the experiment behind it.
🧪 Activity 11.1 — Separating the four leaf pigmentsL3 Apply
The claim that a green leaf contains four pigments is not something to take on trust — paper chromatography shows it directly in about twenty minutes.
Predict: A spinach leaf looks uniformly green. How many coloured bands will appear on the chromatography paper, and in what order from the bottom?
- Grind a few fresh spinach or Amaranthus leaves in a mortar with a little acetone to extract the pigments.
- Apply a concentrated spot of the extract about 2 cm from one end of a strip of filter paper. Let it dry, then reapply two or three times on the same spot.
- Suspend the strip in a jar so the lower edge — but not the spot — dips into a shallow layer of petroleum ether or a similar solvent. Cover the jar.
- Let the solvent climb until it is about 2 cm from the top. Remove the strip, mark the solvent front and let it dry.
- Record the number, colour and position of the bands.
Four bands appear. From the bottom of the strip upwards: chlorophyll b (yellow green), chlorophyll a (bright or blue green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange) travelling furthest.
Why they separate. The pigments differ in solubility in the moving solvent and in how strongly they cling to the wet paper. The least polar pigment — carotene — dissolves best in the non-polar solvent and is carried highest; the more polar chlorophylls are retained lower down.
What this explains. The single green we perceive is a mixture. It explains why leaves show many shades of green even on one plant, since the proportions of the four pigments differ with age and light exposure. It also explains the autumn colours of deciduous leaves: when chlorophyll breaks down, the more stable yellow xanthophylls and carotenoids that were always present become visible.
🎯 Competency-Based Questions
Scenario: A student sets up three bell jars in bright sunlight. Jar A contains a lit candle only. Jar B contains a lit candle and a healthy mint plant. Jar C contains a lit candle and a mint plant whose leaves have all been painted over with thick black paint. All three jars are sealed at the same moment.
Q1. Predict what happens to the candle in each jar, and name the scientist whose work this reproduces. L3 Apply
This reproduces Priestley's experiment of 1770. In jar A the candle soon goes out, because burning uses up the oxygen and nothing replaces it. In jar B the candle continues to burn for much longer, because the mint plant photosynthesises and restores to the air what the candle removes. In jar C the candle goes out much like jar A: the black paint prevents light from reaching the leaves, so no photosynthesis occurs and no oxygen is returned.
Q2. Jar C is the crucial control. What single conclusion can be drawn from comparing B with C that cannot be drawn from comparing B with A? L4 Analyse
Comparing B with A shows only that the presence of a plant helps. Comparing B with C isolates the variable of light reaching the leaf, since both jars contain a living plant of the same kind. The conclusion is that the plant's restoring effect depends on light, not merely on the plant being alive — which is exactly the advance that Ingenhousz made over Priestley by placing identical setups in the dark and in the sunlight.
Q3. Fill in the blanks: The oxygen released in photosynthesis comes from ______, as demonstrated by ______ from his studies on purple and green sulphur bacteria, in which the hydrogen donor is ______ and the oxidation product is sulphur or sulphate. L1 Remember
water (H₂O); Cornelius van Niel; H₂S. Van Niel's general equation is 2H₂A + CO₂ → 2A + CH₂O + H₂O, and the conclusion was later confirmed by radioisotopic techniques.
Q4. A leaf is exposed to light of wavelength 540 nm only. Chlorophyll a absorbs very poorly at this wavelength, yet some photosynthesis is measured. Explain. L4 Analyse
The accessory pigments — chlorophyll b, xanthophylls and carotenoids — absorb in regions where chlorophyll a does not, and they transfer the absorbed energy to chlorophyll a at the reaction centre. Photosynthesis therefore proceeds even at wavelengths chlorophyll a cannot use directly. This is precisely why the action spectrum does not overlap the chlorophyll a absorption spectrum one-to-one but sits above it in the intermediate regions.
Q5. A classmate argues that the dark reaction should be renamed, since it is not really independent of light. Evaluate this claim. L5 Evaluate
The classmate is right in substance, and NCERT itself cautions that the term must not be taken to mean these reactions occur in darkness or are not light-dependent. The carbon reactions are not directly driven by photons, which is the only sense in which "dark" is accurate — but they consume the ATP and NADPH made by the light reactions, so they stop shortly after light is withdrawn and resume when it returns. The better names, used in this chapter, are biosynthetic phase or carbon reactions. The claim is therefore sound, provided one recognises that "dark reaction" is a historical convention rather than a factual description; the experimental proof is that the biosynthetic process continues for a short time after light becomes unavailable and then stops.
Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Leaves appear green to our eyes.
Reason (R): Chlorophyll absorbs strongly in the blue and red regions and poorly in the green region, so green light is largely reflected.
Answer: A. The absorption spectrum has peaks in the blue and red and a trough in the green; the unabsorbed green light reaches our eyes.
Assertion (A): The oxygen released during photosynthesis comes from carbon dioxide.
Reason (R): In the overall equation, six molecules of CO₂ yield six molecules of O₂.
Answer: D. The assertion is false — van Niel showed that the O₂ comes from water, later confirmed by radioisotope techniques. The reason states a true numerical coincidence in the balanced equation, and it is exactly that coincidence which misled earlier workers; it is not evidence for the source of the oxygen.
Assertion (A): Accessory pigments make photosynthesis more efficient.
Reason (R): They absorb wavelengths that chlorophyll a absorbs poorly and transfer the energy to chlorophyll a, besides protecting it from photo-oxidation.
Answer: A. Both roles are stated in Section 11.4, and widening the usable spectrum while shielding the reaction-centre pigment is precisely how efficiency is raised.
Coming next. Part 2 takes up Sections 11.5 and 11.6 — the light harvesting complex, photosystems I and II with their P700 and P680 reaction centres, the Z scheme of electron transport, the splitting of water, and cyclic versus non-cyclic photophosphorylation.
Frequently Asked Questions - Early Experiments and Photosynthetic Pigments
Why are twelve molecules of water written in the photosynthesis equation?
The balanced equation 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ has water on both sides, so the net consumption is six molecules. Twelve are written on the left because splitting twelve water molecules supplies the electrons and protons needed and liberates the six O₂ molecules. Writing only six would still balance the atoms but would wrongly imply that some released oxygen came from CO₂, which van Niel disproved.
What did Priestley, Ingenhousz, von Sachs, Engelmann and van Niel each establish?
Priestley (1770) showed that plants restore to the air what burning candles and breathing animals remove. Ingenhousz showed sunlight is essential and that only the green parts release oxygen. Von Sachs (1854) showed plants make glucose, usually stored as starch, in green parts, and located chlorophyll in bodies later called chloroplasts. Engelmann produced the first action spectrum, finding photosynthesis fastest in blue and red light. Van Niel showed the oxygen released comes from water, not carbon dioxide.
Which four pigments are found in a green leaf?
Paper chromatography of leaf pigments separates four: chlorophyll a, which appears bright or blue green and is the chief pigment associated with photosynthesis; chlorophyll b, yellow green; xanthophylls, yellow; and carotenoids, yellow to yellow-orange. The last three are accessory pigments that absorb light and transfer the energy to chlorophyll a.
What is the difference between the absorption spectrum and the action spectrum?
The absorption spectrum shows how strongly a pigment absorbs each wavelength; the action spectrum shows the rate of photosynthesis at each wavelength. Both peak in the blue and red regions, which identifies chlorophyll a as the chief pigment, but the overlap is not one-to-one — the action spectrum stays higher at intermediate wavelengths because accessory pigments absorb there and pass the energy on to chlorophyll a.
Why should the dark reaction not be called a dark reaction?
Because the name misleads. These carbon reactions are not directly driven by photons, but they depend entirely on the ATP and NADPH produced by the light reactions, so they are fully light-dependent in practice. The proof is that the biosynthetic process continues only briefly after light is withdrawn and then stops, restarting when light returns. Biosynthetic phase or carbon reactions are the better names.
What is the division of labour inside a chloroplast?
The membrane system — the grana and the stroma lamellae — traps light energy and synthesises ATP and NADPH; these are the light reactions or photochemical reactions. The stroma carries the enzymatic reactions that synthesise sugar, which in turn forms starch; these are the carbon reactions, conventionally called dark reactions.
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