Chloroplasts
Chloroplasts (seen through a microscope)
The first cells that could photosynthesize sugars out of sunlight and carbon dioxide and water probably evolved around 3.8 billion years ago. They were prokaryotic cells that looked blue-green, because they needed to absorb all the warmer wavelengths of light and reflect the cooler blue and green ones.
But when other cells ate the blue-green prokaryotes, they found out that it was more useful to let the blue-green prokaryotes live inside them and keep making energy from sunlight, than it was to destroy them. Gradually more and more cells began to have lots of blue-green prokaryotes living inside them, and by around two billion years ago some of the blue-green prokaryotes lost the ability to live on their own and evolved into chloroplasts (KLOR-oh-plasts) that could only live inside other cells (This is very much like what happened to mitochondria).
So a chloroplast is now a piece of a plant cell, but with its own DNA and RNA. One cell of a plant leaf can have a lot of chloroplasts in it - between 20 and 100.
Learn by doing - chloroplasts
Prokaryotes
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