HE­LEN CZERSKI … THE HID­DEN LIFE OF A LEAF

BBC Earth (Asia) - - Comment & Analysis - Dr He­len Czerski is a physi­cist and BBC sci­ence pre­sen­ter. Her book, The Storm In A Teacup, is out now

“THE STRUC­TURE IS FANTASTICALLY CLEVER AND DEL­I­CATE, DE­SIGNED TO MAX­IMISE PHOTOSYNTHESIS”

Leaves are green. It’s one of those state­ments that is so self-ev­i­dent you could eas­ily re­verse it: we can all agree on what colour green re­ally is be­cause it’s the colour of a leaf and it’s pretty much the same for all leaves. But while mak­ing a lasagne the other evening, I started to won­der whether we re­ally know what that colour is. I was mi­crowav­ing spinach in batches be­fore chop­ping it up, and the dif­fer­ence in colour be­tween the fresh spinach go­ing into the mi­crowave and the grow­ing pile of cooked spinach was re­ally strik­ing. What went in was a cheer­ful light green, but what came out was much darker. Why the change? And which colour is the real green?

The leaves are the pow­er­house of a plant: they’re the flex­i­ble or­ganic fac­tory in which light fu­els a re­ac­tion be­tween car­bon diox­ide and water to make sugar. The ac­tual work is done in­side chloro­plasts, self-con­tained pouches of chem­i­cal ac­tiv­ity that sit in­side the cells in the mid­dle of the leaf. And the most im­por­tant chem­i­cal of all is the chloro­phyll mol­e­cule, the green pig­ment re­spon­si­ble for har­vest­ing light. This lies within the chloro­plasts and is what makes the spinach look green – chloro­phyll ab­sorbs red and blue light very strongly, mostly leav­ing the green alone.

But it turns out that the struc­ture of a spinach leaf isn’t just pack­ag­ing for chloro­phyll. The leaf struc­ture is fantastically clever and del­i­cate, specif­i­cally de­signed to max­imise photosynthesis. Each chloro­plast needs a sup­ply of car­bon diox­ide, so per­haps a quar­ter to a third of the leaf is taken up by air spa­ces that act as a sup­ply route. The con­se­quence is that the in­side of the leaf is full of sur­faces that de­flect the light again and again, so it bounces around in­stead of go­ing straight through. There­fore, it makes sense for the cell to have chloro­plasts dis­trib­uted through­out the en­tire leaf, be­cause it’s pretty likely that some light will reach them even­tu­ally. But that’s not all.

Per­haps the ob­vi­ous strat­egy for a spinach plant would be to pack all the chloro­plasts into the thin layer on the top sur­face of each leaf. How­ever, it turns out that there’s a max­i­mum amount of light that each in­di­vid­ual chloro­plast can use. Bounc­ing the light around the in­side shares out the avail­able light so that more chloro­plasts get a us­able amount. And the fi­nal beau­ti­ful trick is when I said that chloro­phyll ‘mostly’ leaves the green light alone, be­cause when light has many opportunities for ab­sorp­tion as it’s be­ing bounced around so much, the green doesn’t go to waste af­ter all.

Each hand­ful of fresh leaves is pale green in colour be­cause of this in­ter­nal ar­chi­tec­ture. Hav­ing all that struc­ture means that more light (of all colours) bounces off the leaf, so I see some­thing closer to white: light green.

Spend­ing a while ap­pre­ci­at­ing the na­ture of a leaf wasn’t enough to stop me de­stroy­ing it. Cooking spinach breaks down the cell walls so that the con­tents spills out, and the beeps when the mi­crowave has fin­ished in­di­cate that the beau­ti­ful airy struc­ture is ru­ined. Most light goes straight in and is ab­sorbed by the chloro­phyll, but doesn’t come back out. This dark brood­ing green is the true colour of chloro­phyll. And now I ap­pre­ci­ate the bright colour of a grow­ing leaf much more – that pale green shade is a de­light­ful by-prod­uct of the el­e­gance of a leaf’s in­ner life.

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