The red algae form a distinct group. They have eukaryotic cells without flagella and centrioles. Their chloroplasts lack external endoplasmic reticulum. These chloroplasts have unstacked (stroma) thylakoids. Phycobiliproteins are accessory pigments, which give them their red color. What these pigments do is the same as what chlorophyll does: absorb sunlight as energy, which is then used to fuel the building of organic compounds.