Ciencias
Chlorophyll: The Green Pigment That Traps Light
Chlorophyll absorbs red and blue light and reflects green, which is the colour a leaf throws away. It sits above the arrow in photosynthesis, not on the left.
Chlorophyll
Chlorophyll is the green pigment inside chloroplasts that absorbs light energy, mostly red and blue, and makes that energy available for photosynthesis.
- Dónde lo encuentra el alumnado
- Grade 5 science, when leaves and photosynthesis are first taught, and again in Grade 9 biology where the amount of chlorophyll appears as one of the things that can limit the rate.
La respuesta corta
Chlorophyll is the green pigment held inside the chloroplasts of plant cells. It absorbs light from the red and blue ends of the spectrum and reflects green, which is why leaves look green. Its job is to capture the light energy that drives photosynthesis; it is not used up by the reaction.
Un ejemplo
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy, chlorophyll written above the arrow)
Look at where chlorophyll goes. It is written above the arrow, with light, and not on the left with the carbon dioxide and water. Everything on the left of a chemical equation is consumed and ends up inside the product; chlorophyll does not. A leaf that photosynthesised all summer has the same chlorophyll in it at the end as at the start, which is exactly why it can keep going.
Why a leaf looks green
The colour you see is the light the plant rejected. Chlorophyll absorbs strongly in the blue-violet region, around 430 nanometres, and again in the red, around 660 nanometres. Green light in the middle is absorbed poorly, so it bounces off the leaf and reaches your eye. Green is the one part of the visible spectrum chlorophyll is worst at using.
There is more than one pigment in a leaf. Chlorophyll a and chlorophyll b absorb slightly different wavelengths, and orange and yellow carotenoids sit alongside them. Those other pigments are present all year; they are simply drowned out. In autumn the chlorophyll breaks down first, and the carotenoids that were there the whole time become visible.
A trap, not an ingredient
Students often list chlorophyll alongside carbon dioxide and water as something photosynthesis uses up. It is not. Carbon dioxide and water are taken apart and rebuilt into glucose; chlorophyll absorbs a photon, passes the energy on and is ready for the next one. Calling it a trap rather than an ingredient keeps the equation straight.
The variegated leaf experiment shows this cleanly. Take a leaf with green patches and white patches, remove the chlorophyll with hot ethanol, and test with iodine. The parts that were green go blue-black because starch was made there. The white parts, which never had chlorophyll, stay orange-brown. Same leaf, same air, same light — the only variable was the pigment.
When there is not enough of it
Because chlorophyll is what captures the energy, a shortage of it caps the rate of photosynthesis in the same way a shortage of light or carbon dioxide does. This is why it is treated as a limiting factor: a plant with pale, damaged leaves cannot be made to photosynthesise faster by moving it into brighter light.
Every chlorophyll molecule is built around a single magnesium atom at its centre. A plant grown in soil short of magnesium cannot make enough of the pigment, and the symptom is specific and recognisable: yellowing between the leaf veins while the veins themselves stay green.
Preguntas frecuentes
Is chlorophyll used up during photosynthesis?
No. It absorbs light energy and passes it on, then does the same again. That is why it is written above the arrow rather than on the reactant side of the equation. Chlorophyll is eventually replaced as leaf cells age, but the photosynthesis reaction itself does not consume it.
Why is chlorophyll green and not black?
Because it absorbs blue and red light well and green light badly, so green is reflected back to your eye. A black pigment would absorb everything and, in energy terms, would be more efficient. Chlorophyll is what evolved, not what a designer would have chosen.
Where exactly is chlorophyll found in a plant?
Inside chloroplasts, which are small structures found mainly in the palisade and spongy mesophyll cells of the leaf. Root cells have no chloroplasts, which is why roots are not green. Guard cells around each stoma do have them, so they can open and close the pore.
Can a plant survive without chlorophyll?
Not on its own. A seedling can live for a short time on the food stored in its seed, and the white patches of a variegated leaf survive on sugars made by the green patches nearby. But a plant with no chlorophyll anywhere cannot make its own food at all.
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