Practice question

Plastids comprise diverse double-membrane organelles in plant and algal lineages originating from primary endosymbiosis of cyanobacterium engulfed by eukaryote over billion years ago, retaining circular plastid genome encoding photosystem components and ribosomal RNAs and ability to divide by binary fission via FtsZ ring. Differentiation depends on tissue and environmental cues: chloroplasts in mesophyll contain thylakoid stacks grana where photosystem II, cytochrome b6f, photosystem I perform light reactions generating proton gradient for ATP synthase and NADPH for Calvin cycle fixing CO2 via Rubisco, chlorophyll a and b absorbing 680 and 700 nm light; chromoplasts develop from chloroplasts during fruit ripening accumulating carotenoids lycopene in tomato and beta-carotene in pepper conferring orange-red coloration and antioxidant nutritional value attracting frugivores; amyloplasts in roots and seeds store amylose and amylopectin starch granules and function as statoliths sedimenting for gravity perception; etioplasts in dark-grown seedlings hold prolamellar body. Unlike mitochondria focused on oxidative phosphorylation, plastids specialize in pigment storage and photosynthesis.