HELEN CZERSKI … THE HIDDEN LIFE OF A LEAF
“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.
