The major difference between plant and animal cells is the photosynthetic¹（光合作用的） process, which converts light energy into chemical energy. When light isn't available, energy is generated by breaking down carbohydrates³（碳水化物） and sugars, just as it is in animal and some bacterial⁴ cells. Two cellular⁵ organelles are responsible for these two processes: the chloroplasts for photosynthesis⁶ and the mitochondria（线粒体） for sugar breakdown⁷. New research from Carnegie's Eva Nowack and Arthur Grossman has opened a window into the early stages of chloroplast（叶绿体） evolution. Their work is published online by the Proceedings⁸ of the National Academy of Sciences in the week of February 27-March 2.

It is widely accepted that chloroplasts originated from photosynthetic, single-celled bacteria called cyanobacteria（蓝藻细菌） , which were engulfed⁹ by a more complex, non-photosynthetic cell more than 1.5 billion years ago. While the relationship between the two organisms was originally symbiotic¹⁰, over evolutionary¹¹ time the cyanobacterium transferred most of its genetic¹³ information to the nucleus¹⁴ of the host organism, transforming the original cyanobacterium into a chloroplast that is no longer able to survive without its host. A similar process resulted in the creation of mitochondria.

To sustain the function of the organelle, proteins encoded by the transferred genes¹⁵ are synthesized in the cytoplasm, or cell's interior, and then imported back into the organelle. In most systems that have been studied, the transport of proteins into the chloroplast occurs through a multi-protein import complex that enables the proteins to pass through the envelope membranes¹⁶ that surround the chloroplast.

Clearly the events that gave rise to chloroplasts and mitochondria changed the world forever. But it is difficult to research the process by which this happened because it took place so long ago. One strategy used to elucidate¹⁷（阐明） the way in which this process evolved has relied on identifying organisms for which the events that resulted in the conversion¹⁸ of a bacterium¹² into a host-dependent organelle（细胞器） occurred more recently.

Nowack and Grossman focused their research on a type of amoeba called Paulinella chromatophora, which contains two photosynthetic compartments¹⁹ that also originated from an endosymbiotic cyanobacterium, but that represent an earlier stage in the formation of a fully²⁰ evolved organelle.

These compartments, called chromatophores, transferred more than 30 of the original cyanobacterial genes to the nucleus of the host organism. While gene² transfer has been observed for other bacterial endosymbionts, the function of the transferred genes has been unclear, since it does not appear that the endosymbionts (in contrast to organelles) are equipped to recapture those proteins, because they do not have appropriate protein import machineries.