Great question. Chloroplasts (like mitochondria) have their own DNA. WAY back in evolutionary time, some bacteria formed a sort of partnership with other cells, which we call eukaryotic. The chloroplasts can do photosynthesis, using light energy, CO₂, and water to make sugar for the host cell. The host cell provides protection and raw materials for the chloroplast. As you said the host cell even provides proteins needed by the chloroplasts. There’s a nice site about this at:

Apparently there are some chloroplasts that can live outside of cells. There are even some species of sea slug that can hold onto the chloroplasts from the algae that they eat and get sugar from these “orphaned” chloroplasts. The chloroplasts do not reproduce, though.

Chloroplasts seem to be dependent on the host cells for reproduction, but your idea of figuring out which proteins (or other molecules) they need from the host is great. You’re really thinking like a scientist.

If those proteins could be easily synthesized without host cells, it might be possible to use solar energy to make a lot more sugar. Often, though, we find that organisms can synthesize things much more efficiently than we can produce them without using organisms.

Why would you expect the eukaryotic cells of the sea slug NOT to provide the special proteins that chloroplasts need?

You might want to look into a career in cell biology.

An important part of the definition of a cell is that it is the smallest unit of life that can replicate. Though, that doesn’t mean you can’t make individual components of a cell. For instance, you can make DNA in a test tube with just the proteins and other molecules required for DNA reproduction. The interesting thing about chloroplasts is that they were once photosynthetic bacteria that lived by themselves.

The theory is that the ancestor of a plant cell tried to eat a bacterium, but decided instead to let it become a new organelle so that it could get energy from the sun. This idea is motivated by the fact that the chloroplast has its own separate DNA from the plant cell. However, over time, this bacterium has become so dependent on the plant cell and vice-versa that it can’t survive or reproduce on its own. So in that sense the chloroplast couldn’t reproduce outside the cell. Though to answer your question more directly, the processes required to reproduce a chloroplast are not just the sum of their parts. The reproduction of a chloroplast requires a number of interconnected processes arranged in certain compartments in the cell at specific times. The construction of a new chloroplast doesn’t just require the specific component proteins encoded in the nucleus; it requires a bunch of other helper proteins, molecules, and organelles such as the mitochondria to produce the energy for the assembly processes. Ultimately, the reproduction of a cell’s individual parts is a highly complex process that would likely require a living system, such as the cell itself, to complete.

The cell has many complex and related process going on at all times. It wouldn't really be possible to incorporate all of these in a test tube. To create more chloroplasts, a cell needs the proteins that make up the chloroplast, the helper proteins that assemble it, and ATP to power all these proteins. If all of these were present, it is possible that the chloroplast could make itself in a test tube. To my knowledge, however, I don't think this has been done, probably because we don't know every necessary protein yet.

You are right that cholorplasts require proteins encoded by the nucleus to function and divide. Chloroplast division is somewhat of a mystery… Scientists have identified a handful of nucleus-encoded genes that are likely to be involved in chloroplast division. When these genes are mutated, chloroplast number and size is altered. As far as I can find, no one has actually watched a chloroplast divide in real time. Perhaps if we knew all of the proteins required for chloroplast division, we could put them in a solution with chloroplasts and watch the chloroplasts divide. Wouldn't that be cool! I hope you do this interesting in vitro experiment some day.

Here is a link to an article that will give you much more information: