Q.Which pigment acts directly to convert light energy to chemical energy ?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Photosynthetic Pigments
Why do leaves look green?
You already know that plants make their own food using sunlight. But sunlight is just energy — how does a leaf actually catch it? The answer is that leaves contain special molecules called photosynthetic pigments. Think of them as tiny solar panels built into the leaf cells.
A pigment is any substance that absorbs light. Different pigments absorb different colours (wavelengths) of light. The colour you see when you look at a leaf is the colour that the pigment does not absorb — it reflects that colour instead. So a green leaf looks green because its pigments absorb red and blue light strongly, but reflect green light.
A leaf is green not because it uses green light, but because it rejects it. The energy in green light is mostly wasted as far as photosynthesis goes.
The main pigments and what they do
There are four major photosynthetic pigments in higher plants. They work together like a team — each one catches a slightly different slice of the sunlight spectrum.
Chlorophyll a is the star player. Every photosynthetic plant, alga, and cyanobacterium has it. It absorbs light most strongly in the red (~660–680 nm) and blue-violet (~430 nm) regions. This is the only pigment that can directly convert light energy into chemical energy — it sits at the reaction centre of the photosystems.
Chlorophyll b is the backup. It absorbs light in slightly different red and blue bands (around 460 nm and 640 nm). It cannot do the energy conversion itself, but it captures extra light and passes the energy to chlorophyll a. This broadens the range of light the plant can use.
Carotenoids (like beta-carotene) and xanthophylls are the yellow-orange pigments. They absorb blue-green light (around 450–500 nm) — wavelengths that chlorophylls handle poorly. They also have a critical protective role: they safely dissipate excess energy that could otherwise damage the chlorophyll molecules.
Chlorophyll a is the primary pigment — the only one that directly participates in the light reactions. All others are accessory pigments — they capture light and transfer the energy to chlorophyll a.
How they work together
Imagine a leaf as a solar panel made of many small antennae. Each antenna is a cluster of pigments — mostly chlorophyll b and carotenoids — surrounding a single chlorophyll a molecule at the centre. This whole assembly is called a photosystem.
When a photon of light hits any pigment in the antenna, its energy gets passed from molecule to molecule like a hot potato, until it reaches the chlorophyll a at the reaction centre. There, the energy is used to excite an electron, which starts the electron transport chain — the first step of photosynthesis.
| Pigment | Colour absorbed | Colour reflected | Role |
|---------|----------------|------------------|------| …
Photosynthetic pigments are of two kinds: accessory pigments, which merely absorb light and pass the energy on, and the special reaction-centre pigment that actually undergoes a photochemical change and converts the absorbed light energy into chemical energy. …
Chlorophyll a is the only pigment that directly converts light energy into chemical energy; all other pigments (chlorophyll b, xanthophylls, carotenoids) are accessory pigments that merely absorb light and transfer that energy to chlorophyll a.
In the thylakoid membrane, pigments are organised into light-harvesting complexes surrounding a special pair of chlorophyll a molecules at the reaction centre (P680 in Photosystem II, P700 in Photosystem I). Chlorophyll b, xanthophylls, and carotenoids absorb light of wavelengths chlorophyll a does not absorb efficiently, and funnel that captured energy, by resonance transfer, to the chlorophyll a molecules at the reaction centre.
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- CBSE 2025Set ANNUAL1 markMCQQ.Which chlorophyll molecule does not have a phytol tail?(a) Chl-c(b) Chl-a(c) Chl-d(d) Chl-b
›Reveal solutionSolution
Chlorophyll a, b, and d all carry a long hydrophobic phytol chain that anchors them in the thylakoid membrane, but chlorophyll c lacks this phytol tail, making it structurally distinct.
All chlorophyll molecules share a porphyrin "head" (a magnesium-centred tetrapyrrole ring) that absorbs light, but most of them additionally carry a long hydrophobic side chain called the phytol tail, esterified to the porphyrin ring. This phytol tail is what embeds and anchors the chlorophyll molecule within the lipid environment of the thylakoid membrane.
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- CBSE 2024Set ANNUAL1 markMCQQ.A quantasome is present in __________.(a) Golgi bodies(b) Mitochondria(c) Endoplasmic reticulum(d) Chloroplast
›Reveal solutionSolution
Quantasomes, the particulate units that carry out the light reactions of photosynthesis, are located in the thylakoid membranes inside the chloroplast.
The chloroplast is the site of photosynthesis in plant cells. Internally, it consists of a matrix called the stroma, within which lie flattened, membrane-bound sacs called thylakoids, often stacked into structures called grana and interconnected by stroma lamellae.
When the thylakoid membrane was examined under the electron microscope, it appeared studded with small, regularly arranged particles. These particles — named quantasomes by Park and Biggins (1964) — were proposed to be the basic structural and functional units of photosynthesis, each containing an assembly of several hundred chlorophyll and accessory pigment molecules along with the electron-transport components needed to capture light energy (quanta) and drive the light reactions. (The concept has since been refined into today's understanding of discrete photosystems, PS I and PS II, but t …
- CBSE 2024Set botany-sz1 markMCQQ.Which pigment acts directly to convert light energy to chemical energy ?(a) Chlorophyll a(b) Chlorophyll b(c) Xanthophyll(d) Carotenoids
›Reveal solutionSolution
Chlorophyll a is the only pigment that directly converts light energy into chemical energy; all other pigments (chlorophyll b, xanthophylls, carotenoids) are accessory pigments that merely absorb light and transfer that energy to chlorophyll a.
In the thylakoid membrane, pigments are organised into light-harvesting complexes surrounding a special pair of chlorophyll a molecules at the reaction centre (P680 in Photosystem II, P700 in Photosystem I). Chlorophyll b, xanthophylls, and carotenoids absorb light of wavelengths chlorophyll a does not absorb efficiently, and funnel that captured energy, by resonance transfer, to the chlorophyll a molecules at the reaction centre.
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- CBSE 2022Set ANNUAL1 markMCQQ.Pick out the correct pair.(a) Change of Fruit colour in Tomato - Lutein(b) Fruit ripening - Carotenoids(c) Yellowing in Leaf - Chlorophyll(d) Change in Leaf colour - Lycopene
›Reveal solutionSolution
As a fruit ripens, its green chlorophyll degrades, unmasking the underlying carotenoid pigments, which is why ripening fruit changes colour towards yellow, orange, or red.
Let us check each pair:
- Change of Fruit colour in Tomato - Lutein — INCORRECT. The red colour of ripe tomato is due to lycopene (a red carotenoid), not lutein (which is a yellow xanthophyll pigment found mainly in leaves and some flowers, not the main tomato pigment).
- Fruit ripening - Carotenoids — CORRECT. During ripening, chlorophyll breaks down and the carotenoid pigments (carotenes and xanthophylls, including lycopene in tomato) already present in the chromoplasts become visible, giving ripe fruits their yellow/orange/red colours. …
- CBSE 2022Set ANNUAL1 markMCQQ.Match the Column-A item 'Chlorophyll' with the correct term from Column-B.(a) Chromosome(b) Pea-shaped(c) Pectoral girdle(d) Solanaceae(e) Gymnosperm(f) Bones(g) Mg
›Reveal solutionSolution
Chlorophyll's molecular structure contains a magnesium (Mg) atom at the centre of its porphyrin head.
Chlorophyll molecules consist of a porphyrin 'head' — a ring made of four pyrrole units — with a magnesium ion coordinated at its centre, plus a long hydrocarbon 'tail' (phytol) that anchors the molecule in the thylakoid membrane. This Mg-porphyrin structure is what allows chlorophyll to abso …
- CBSE 2019Set ANNUAL1 markMCQQ.Name the spherical structures found in the inner surface of lamellar membrane:(a) Quantasomes(b) Stroma(c) Granum(d) Thylakoid
›Reveal solutionSolution
Quantasomes are the small spherical particles studding the inner surface of the thylakoid (lamellar) membrane, representing the photosynthetic functional units.
Inside the chloroplast, the internal membrane system is organised into flattened sacs called thylakoids, which are stacked into grana and interconnected by stroma lamellae. When the thylakoid membrane is examined closely, tiny spherical particles are seen studding its inner surface — these particles were named quantasomes, and were originally thought to represent the basic structural/functional unit of photosynthesis (containing the pigment molecules and electron-transport components needed to carry out the light reactions).
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- CBSE 2016Set ANNUAL1 markMCQQ.Which of the following is not a photosynthetic pigment?(a) Carotene(b) Xanthophyll(c) Phycobillins(d) Anthocyanin
›Reveal solutionSolution
Correct answer: (d) Anthocyanin
Carotene, xanthophyll and phycobilins are accessory photosynthetic pigments; anthocyanin is a non-photosynthetic vacuolar pigment.
Carotene, xanthophyll (both carotenoids) and phycobilins (found in algae/cyanobacteria) are accessory pigments that absorb light energy and pass it on to chlorophyll during photosynthesis, broadening the range of wavelengths that can be used. Anthocyanin, on the other hand, is a water-soluble flavonoid pigment located in the cell vacuole (not …
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