I Spy With My Little Chloroplast

Guess I should mark Darwin Day somehow? Let's talk about the abode of pigments.

Basically in order to make use of light (whether to see or eat it) an organism needs to stick photopigments in its way, generally by embedding those pigments in a layer of cellular membrane exposed to light. But of course, since light easily penetrates thin tissue (much less a single membrane) one layer would be a waste of surface area for how little light it would capture. This problem finds a common solution in both

A) the lettuce you ate for lunch (here's the schematic representation for reference)

and

B) the eyeballs you're using to read this (that TEM's from the University of Utah, but here's Wikipedia's schematic of a retinal rod cell)

While the specific morphology differs a bit (semi-isolated sacs vs. more narrowly anchored shelves) both inside your eye and inside a plant leaf the photopigment-containing membranes within each cell are folded atop each other by the dozens to trap whatever light "leaks" through them sequentially. I'll confess I haven't looked into the exact evolutionary history here but given we've traversed numerous sightless uni/multi-cellular stages of development since we diverged from plants' ancestors, during which we were blind worms etc. this arrangement seems just one more striking example of convergence. Certainly in cyanobacteria thylakoid orientation is far more diverse (scroll down to images in results in this paper) but even there flat layering seems to predominate.

Why?

Because it works.

I mean, if you were trying to deliberately, intelligently design a light-capturing system, you might use denser, sturdier, more specialized materials like you see in solar cells requiring less layering, but evolving from an existing stage of isolated receptors in a membrane, simply stacking more and more redundant membranes maximizes the use of surface area so parsimoniously that your grandkids basically write themselves.