Chlorophyll: Why Plants Are Green

Look out of a window and the dominant colour of the living world is green. That colour comes from one family of molecules: chlorophyll. It’s the pigment that captures sunlight for photosynthesis, and its structure explains not only why plants are green, but also why leaves turn yellow and red in autumn, and why chlorophyll is such an effective light catcher.

The structure of chlorophyll

A chlorophyll molecule has two main parts.

1. The light-absorbing head: a ring with magnesium at its centre

The head is a large, flat ring system called a chlorin, closely related to the porphyrin ring in the haem of your blood. It’s built from four smaller five-membered rings (pyrrole rings), each containing a nitrogen atom, linked into one big ring.

At the centre, the four nitrogen atoms hold a single magnesium ion (Mg²⁺). (In haem, the same position is occupied by iron, which is why blood is red and plants are green: similar rings, different metals and side groups.)

The ring has an extended system of alternating single and double bonds, a conjugated system. Electrons in this system are delocalised over the whole ring. That delocalisation is what makes chlorophyll absorb visible light strongly.

2. The anchoring tail: phytol

Attached to the head is a long hydrocarbon chain of 20 carbons called phytol. Being non-polar, the tail anchors chlorophyll into the lipid membranes of the thylakoids (see phospholipids and cell membranes), where it’s held in precise positions by proteins.

The molecular formula of chlorophyll a is C₅₅H₇₂O₅N₄Mg, with a molar mass of about 893 g mol⁻¹. You can check the magnesium’s properties on the magnesium element page.

Why chlorophyll absorbs light: conjugation

Molecules absorb light when a photon’s energy exactly matches the gap between two electron energy levels (see photons and energy levels).

• In small molecules, those gaps are large, corresponding to ultraviolet light, which is why most simple organic compounds are colourless.
• In molecules with long conjugated systems, electrons spread over many atoms, and the energy gaps become smaller. The absorbed light moves into the visible region.

Chlorophyll’s big conjugated ring has energy gaps that match blue and red light. This is the same principle that gives colour to carrots (β-carotene), tomatoes (lycopene) and many dyes. The technique used to measure it is UV–vis spectroscopy.

Why plants are green

Chlorophyll absorbs strongly in two regions:

• blue-violet light, around 430–450 nm;
• red light, around 640–680 nm.

It absorbs green light (about 500–570 nm) only weakly. Green light is mostly reflected or transmitted, so that’s the colour our eyes see.

It’s a slightly surprising result: sunlight is most intense in the middle of the visible spectrum, near green, yet plants’ main pigment largely ignores it. Scientists still debate why. One suggestion is that absorbing the most intense light could overload and damage the photosynthetic machinery, and that absorbing at the edges gives a steadier energy supply.

Chlorophyll a and chlorophyll b

Plants contain two main forms:

	Chlorophyll a	Chlorophyll b
Structural difference	–CH₃ group on one ring	–CHO (aldehyde) group in the same position
Colour	Blue-green	Yellow-green
Absorption peaks (in solvent)	About 430 and 662 nm	About 453 and 642 nm
Role	Main pigment in reaction centres	Accessory pigment that passes energy to chlorophyll a

One small change, a methyl group replaced by an aldehyde group, shifts the absorption peaks by several nanometres, so together the two forms capture a wider range of wavelengths.

Accessory pigments

Plants also contain other pigments that absorb light chlorophyll misses and pass the energy on:

• Carotenoids (orange and yellow), such as β-carotene and xanthophylls, absorb blue-green light around 450–500 nm. They also protect the photosynthetic machinery by absorbing excess energy and neutralising reactive oxygen species.
• Phycobilins in cyanobacteria and red algae absorb green and orange light, which is why some seaweeds are red.

You can separate these pigments yourself by chromatography of a leaf extract: the pigments travel different distances up the paper, showing bands of yellow-orange carotene, yellow xanthophylls, blue-green chlorophyll a and yellow-green chlorophyll b (see paper chromatography).

What happens when chlorophyll absorbs light

When a chlorophyll molecule absorbs a photon, an electron jumps to a higher energy level: the molecule is in an excited state (see excited state vs ground state). In a leaf, what happens next is carefully controlled:

1. Energy transfer. Most chlorophyll molecules act as antennae. Within about a trillionth of a second, they pass the excitation energy to neighbouring pigment molecules, funnelling it towards a special pair of chlorophyll a molecules in a reaction centre.
2. Charge separation. At the reaction centre, the excited chlorophyll actually gives away an electron to a nearby acceptor molecule. This is the moment light energy becomes chemical energy: a separated positive and negative charge. It drives the light-dependent reactions.

If you extract chlorophyll into a solvent, there’s no reaction centre to pass the energy to. The excited molecules release the energy as red fluorescence instead: shine a strong blue or UV light through a chlorophyll solution and it glows deep red. This is an easy and striking demonstration.

Why leaves change colour in autumn

Chlorophyll is constantly broken down and rebuilt. In autumn, as days shorten and temperatures fall, deciduous trees stop making it and break down what’s there, recovering valuable nitrogen and magnesium before the leaves drop.

As the green fades:

• the carotenoids, present all along but masked by chlorophyll, become visible as yellows and oranges;
• in some species, new anthocyanin pigments are made, giving reds and purples. Anthocyanins are the same family of pigments that make red cabbage indicator change colour with pH (see red cabbage indicator).

Chlorophyll and magnesium in the diet and soil

• Plants short of magnesium can’t make enough chlorophyll and show yellowing between the leaf veins (chlorosis). Magnesium-containing fertilisers correct this.
• Green vegetables are a good dietary source of magnesium.
• When green vegetables are overcooked, especially in acidic water, the Mg²⁺ at the centre of chlorophyll is replaced by hydrogen ions, forming an olive-brown compound called pheophytin. That’s why overcooked broccoli turns dull brownish-green.

Key takeaways

• Chlorophyll has a flat chlorin ring with a central Mg²⁺ ion and a long phytol tail that anchors it in membranes.
• Its large conjugated system lets it absorb blue and red light strongly; it reflects green, which is why plants look green.
• Chlorophyll a is the main reaction-centre pigment; chlorophyll b and carotenoids are accessory pigments that widen the range of light absorbed.
• Absorbed energy is funnelled to reaction centres, where an electron is transferred, starting photosynthesis.
• In autumn, chlorophyll breaks down, revealing carotenoids and new anthocyanins. For the bigger picture, see photosynthesis: the chemistry.