Chloroplasts and Leaves: Controlled Parasitism

What if the photosynthetic mechanism of the leaf, the chloroplast, is actually a sort of parasite? You might wonder how this is possible. After all, chloroplasts "feed" plant cells by converting solar energy into chemical energy.

But if you think about it another way there's a much different interpretation possible. Think about spring (which I seem to do all the time). Inside each expanding leaf cell new mechanisms are being constructed. In photosynthetic cells (not all of them are), chloroplasts are reproducing, and in each chloroplast a maze of membranes are being built. Embedded in the membranes are myriads of proteins that run the photosynthetic process. And some of the proteins hold the crown jewel of photosynthesis, chlorophyll. Where do the resources come from that provide the raw materials for all this building?

The plant, through its relationship with the soil and its nutrients, shuttles needed materials up to the leaves. Nitrogen, carbon, magnesium, just to name a few, are all provided to the chloroplast via its "host" plant. The resources sent by the plant are materials a single-cell organism (like the chloroplast) might not be able to obtain that easily. Especially a photosynthetic organism stuck in a terrestrial environment. Yet safely ensconced in a leaf cell, the chloroplast gets everything it needs spoon-fed by the plant. What's more, the plant cell, an aquaeous (watery) environment par excellence, bathes the chloroplast in an aquatic environment where dissolved nutrients are readily available in a nano-habitat easily penetrated by sunlight.

There's more. Consider this time of year when every plant puts on lots of new growth. Every shrub in my garden, every tree, has put on a good 6-8 inches of new growth. In effect, the plant, by expanding, is developing new infrastructure for the chloroplasts that live inside it. Every new leaf, every inch the plant grows closer to the sun, provides new opportunities for expansion by the chloroplast community harbored inside the plant. Certainly sunlight, warmth, and ample moisture aid these processes, but they are programmed by the plant's DNA, perhaps in conjunction with the endosymbiont chloroplasts.

I wonder what this all means in the context of Richard Dawkins' "The Selfish Gene."

More Spring Thoughts

We spent the weekend in northern Vermont and New Hampshire and so had a second chance at observing processes of spring. Up there in the forests you see the greening process in a whole new light. Each individual tree is putting out its leaves. But the whole forest is transforming. What is at the heart of this transformation?

The photosynthetic "machinery" inside each leaf is becoming established for the season. That machinery resides in the chloroplasts, bodies that are descended from once free-living bacteria that live in every photosynthetic plant cell. The cells of plants were invaded by these bacteria eons ago, but in a sense they re-infect the plant (and the forest) with the beginning of each photosynthetic season.

So in a way when we observe a mountainside with its steadily greening trees we are seeing a wave of bacteria (beneficial ones) spreading across the population. Photosynthesis is a partnership between the leaf and the chloroplasts, between the host plant and its endosymbiotic bacteria. It's the spread of these bacteria, the chloroplasts, as they invade in a wave over the weeks of greening, that transform a northern mountainside into a shimmering green solar collector.

The Greening

Absolutely amazing to see the greening of the trees over these weeks of spring. While the flowers come quickly and the leaves seem to pop right out overnight, a closer look shows us that the greening is a long process, one that seems to take more than a month to complete. The beauty of this process lies as much in its complexity as in the gorgeous outcome. Bright green leaves are a joy to behold but they contain much more than we perceive at first glance.

What's going on there? As the leaves assemble their photosynthetic apparatus there are a number of variables to consider. First and foremost, chlorophyll, the green pigment that initiates photosynthesis, is an expensive molecule to produce. Chlorophyll is a large molecule, but more important, its component parts are relatively rare and difficult for the plant to procure. The active part of chlorophyll, an assemblage of atoms called the porphyrin ring, is the focus. It contains four nitrogen atoms and at their center, a single atom of magnesium. Magnesium is a trace element and nitrogen, while abundant in its inert form in the atmosphere, is not abundant in the soil. In addition, nitrogen is needed for many other molecular roles in the plant, for example DNA, RNA, and proteins.

If you think about it, there are millions upon millions of chlorophyll molecules in each leaf. Consider the number of leaves on the tree and the amount of expensive molecular components needed for photosynthesis is mind boggling.

Consider also the many proteins that are involved in the photosynthetic process, proteins that bind to the chlorophyll, proteins that shuttle electrons back and forth during photosynthesis, and proteins that produce ATP, the energy currency of the cell, with the help of energy from sunlight. Proteins require massive amounts of nitrogen, and the molecular machinery that forms the proteins themselves is also nitrogen-rich. Again, more competition for available nitrogen.

Think about it. If the tree were to experience a late frost, the leaves and all their components would die, wasting the extensive resources that are found in each leaf. So the slow rate of "greening" each spring can be attributed to two things: 1) the slow rate of production of photosynthetic molecules (primarily chlorophyll and proteins), partly based on a scarcity of ingredients, and 2) an innate conservatism that favors the full development of the leaf's photosynthetic apparatus during a relatively risk-free time, once the possibility of frost has passed.