Originated as cyanobacteria that lived as symbionts within eukaryotic cells. Still many similarities with cyanobacteria: similar photosynthetic machinery, can divide by binary fission, a giant among organelles. As big as a human red blood cell. Allows large molecules to get in and out: inner envelope membrane. Variety of transporters / pumps / channels present: stroma. Grana: arrangement of stacks within the stroma. The space that the thylakoid sacs enclose is called the lumen: space outside the thylakoid sac is stroma. Flattened membranes that connect thylakoids from different grana. Contains dna and ribosomes and protein translation factors. 90% of the chloroplast proteins are imported into the organelle. Most chloroplast proteins are translated on free ribosomes in the cytosol. Contain a targeting signal that directs them to chloroplasts: chloroplast transit peptide. Stroma proteins have a stroma targeting domain: as part of the transit peptide. Thylakoid proteins have a thylakoid transfer domain: located downstream of the stroma targeting domain.