Chloroplasts

Chloroplasts are chlorophyll-containing organelles that can use light energy to fix car- bon dioxide into carbohydrates (photosynthesis). They are bound by double membranes and contain flattened membrane sacs called thylakoids where the light reaction of photosynthesis takes place. In photosynthetic protista and the chlorophyta these organelles are large, almost filling the cell. The pyrenoid is a proteinaceous region within the chloroplast where polysaccharide biosynthesis takes place.