English: Membrane structre of an algal chromista cell (cryptophyceae). These originated by secondary intracellular enslavement of a red algal plant cell. Both target nuclear-coded proteins to plastids by transit peptides (TPs) recognised by outer membrane (OM, blue) Toc receptors and to mitochondria (enslaved α-proteobacteria) by topogenic sequences recognised by OM Tom receptors. For clarity, Golgi, peroxisomes and lysosomes omitted.
Cryptophytes retain the enslaved red algal nucleus (simplified to a tiny nucleomorph), starch, and cytosolic ribosomes within the periplastid space (PS), and phycobilins (shown in red but can be blue instead) in the thylakoid lumen; all other euchromists (haptophytes, Ochrophytina, not shown) lost these four components and stack their thylakoids in threes not pairs, but like cryptophytes retained the red algal plasma membrane as the periplastid membrane (PPM) and a periplastid reticulum (PR) here argued to be the relict trans-Golgi network (TGN) of the enslaved red alga and topologically distinct from the PPM.