Why do chloroplasts have a double membrane

Individual chloroplasts are about 5-6 ┬Ám (micrometers) long. Like the mitochondria and the cell nucleus, chloroplasts are also enclosed by two semi-permeable cell membranes, which enables the release and absorption of substances (e.g. water). This double membrane can be explained by the so-called endosymbiont hypothesis, which represents the current state of research on the origin of eukaryotes. According to this, chloroplasts were created by the immigration of cyanobacteria into the precursors of plant cells. Means that chloroplasts were once autonomous, photosynthesis-inducing bacteria and that they were taken up by the progenitor cells of plants during phylogenesis, e.g. through phagocytosis.

The intermembrane space is located between the two membranes. The inner membrane of the chloroplasts is turned inside out and forms the thylakoids. The thylakoids form stacks called grana; this is where the light reaction, which is decisive for photosynthesis, takes place. Inside the chloroplasts is the stroma, the cytosol of the chloroplasts. The stroma also contains the DNA and ribosomes of the chloroplasts. Just like the mitochondria, the chloroplasts also have their own (circular) DNA. Chloroplasts replicate independently of the cell cycle of the rest of the cell and thus represent an autonomous organelle. All these facts are further arguments for the endosymbiotic theory.