| Chloroplasts are a type of plastid found in algae and plants. These organelles contain chlorophyll and, in higher plants, accessory pigments and their major function is to harness light energy through photosynthesis. However, chloroplasts are also involved in creating amino acids and fatty acids for their cells.
Like mitochondria, chloroplasts contain their own genetic material (DNA) and they are believed to have evolved in a similar fashion. Many researchers think they evolved millions of years ago when the first eukaryotes developed from single-celled organisms that engulfed but did not digest a single-celled photosynthetic organism (the precursor to the chloroplast was probably something like a cyanobacterium). The engulfed organism then became an energy-producing slave for the larger cell and began to reproduce along with it. See the node for endosymbiosis for a full description of this evolutionary process.
Chloroplasts are green and come in various sizes (2 to 10 micrometers in diameter) and shapes. For instance, the alga Spirogyra has very pretty helical ribbon-shaped chloroplasts.
However, the chloroplasts of higher plants tend to look the same. They look like two Frisbees that have been glued together; when they are sliced in half, their shape is reminiscent of an American football.
Algae generally have only one or two chloroplasts per cell, but higher plants can have more than a hundred per cell.
Every cloroplast is bound by a smooth membrane. Inside a chloroplast, one finds a colorless gel called the stroma. The stroma is very similar to the matrix fluid of mitochodria in composition. The stroma contains starch particles, ribosomes, the organelle's DNA strands, and enzymes.
Suspended within the stroma are stacks of coin-shaped structures called thylakoids. The thylakoids contain the chlorophyll, and they are where photosynthesis takes place.
Sometimes, chloroplasts can lose their chlorophyll and they then become chromoplasts.
References:
Introductory Plant Biology by Kingsley R. Stern
Biology by Helena Curtis | 
