Catching light

Section 2 of 4

Chlorophyll is the molecule that catches the light. You have met it as the green of leaves: a big, flat ring with a magnesium atom at the centre.

Chlorophyll a, the main kind, is C₅₅H₇₂MgN₄O₅. Its big ring is built from four smaller five-membered rings, each holding one nitrogen atom, joined by single carbon atoms. The four nitrogens face inward and hold the magnesium between them, and a run of alternating double bonds goes all the way round the ring.

Chlorophyll a as a skeletal formula A big flat ring made of four smaller five-membered rings, each holding one nitrogen atom, joined by single carbon atoms. A magnesium atom sits at the centre, held by the four nitrogen atoms. Double and single bonds take turns all the way round the big ring, and these double bonds are highlighted. A fifth ring with a C=O group sits on one side, and short groups stick out round the edge. From the bottom, an ester link joins a long zigzag tail of 20 carbon atoms with four methyl branches. Mg O O O O O N N N N tail of 20 carbon atoms
Chlorophyll a, C₅₅H₇₂MgN₄O₅. The highlighted double bonds take turns with single bonds all the way round the big ring. Two nitrogen atoms hold the magnesium with bonds and two with their lone pairs (dashed). The ring is flat, and the tail can twist and bend.

The long tail is a hydrocarbon chain of 20 carbon atoms, joined to the ring by an ester link. Like the tail of a soap molecule, it mixes with oily things: it holds chlorophyll in the stacked membranes inside the chloroplast, which are made of fat-like molecules.

Red and blue

The longer a run of alternating double bonds, the further toward red the light it absorbs. Chlorophyll's run is long and closes into a ring, and its electrons have two main jumps they can make. One matches blue light and the other red light. Green light, in between, is barely absorbed, so leaves send it back to your eyes.

The colours of light that chlorophyll a absorbs A line chart of how much light chlorophyll a absorbs at each wavelength, above a bar of the visible colours from violet at 400 nanometres to red at 700 nanometres. The line has a tall peak in the blue at about 430 nanometres and a second peak in the red at about 660 nanometres. Between them, across the green, the line stays low. 400 nm 500 nm 600 nm 700 nm light absorbed blue, 430 nm red, 660 nm green: little absorbed
How strongly chlorophyll a, dissolved in ether, absorbs each colour of light, drawn as a simplified curve. It peaks in the blue at about 430 nm and in the red at about 660 nm.

Chlorophyll b, a close relative, carries a -⁠CHO group where chlorophyll a has a -⁠CH₃. Its peaks sit at about 453 nm and 642 nm, so it catches some of the light chlorophyll a misses. Carotenoids such as beta-carotene catch blue-green light and pass its energy on as well.

Passing the energy on

In a chloroplast, chlorophyll molecules sit in proteins, a few hundred working together. When one absorbs light, the energy hops from molecule to molecule until it reaches a reaction centre, a special pair of chlorophyll molecules. There the energy drives an electron right out of the chlorophyll and on along a chain of other molecules. At that moment, the energy of light becomes chemical energy.

Quick check

Which part of chlorophyll absorbs the red and blue light?