kloroplastides

Kloroplastides (chloroplasts) are membrane-bound organelles found in the cells of plants and many algae. They carry out photosynthesis, converting light energy into chemical energy stored in sugars. Chloroplasts are plastids that typically contain chlorophyll, the green pigment that enables light capture, along with carotenoids.

Structure and genome: Chloroplasts have a double membrane envelope surrounding a semi-autonomous interior called the stroma. Within the stroma are chloroplast ribosomes and a circular DNA genome. Inner membranes form a network of flattened sacs called thylakoids, which are often stacked into grana. The thylakoid membranes house the photosystems and electron transport chain for the light reactions, while the stroma contains enzymes of the Calvin cycle for carbon fixation. Many chloroplast proteins are encoded in the nucleus and imported via translocons known as TOC (outer membrane) and TIC (inner membrane).

Origin and genetics: Chloroplasts are widely regarded as descendants of ancient cyanobacteria preserved by endosymbiosis. They retain their own genome and ribosomes, but most chloroplast proteins are nucleus-encoded. Some lineages, especially certain algae, possess chloroplasts derived from secondary or tertiary endosymbiosis and may have additional membranes.

Function: In addition to photosynthesis, chloroplasts synthesize fatty acids, amino acids, pigments, and phytohormones, and they can store starch. They can adjust pigment composition and thylakoid organization in response to light conditions.

Distribution and diversity: Chloroplasts are present in plant cells and many algae, including species with primary plastids and those with secondary or tertiary plastids. Animals and fungi do not have chloroplasts.

Size and division: Chloroplasts are typically a few micrometers in size and divide by binary fission, coordinated with cell division, reflecting their bacterial ancestry.