Green algae may be colonial, unicellular, multicellular or filamentous. The cell walls consist of two layers of cellulose fortified by pectin. Green algae also has green chloroplast that carries out photosynthesis.
Green algae is split into the classes Chlorophyceae, Charophyceae and Ulvophyceae. Chlorophyceae contains the most species, and its members have two or more flagella. They also produce a zygospore after sexual reproduction, but they can reproduce asexually. Charophyceae contains asymmetrical cells that may or may not be motile. Its nuclear envelopes dissolve during mitosis, which is different from the other two classes. Ulcophyceae contains mostly marine organisms that reproduce by the alternation of generation.