Food vacuole - a membrane enclosed vesicle where engulfed food is digested. A few protists are surrounded by a plasma membrane. Protozoans have internal shells that they produce. Many algae and other protists have cell walls. Although most protists reproduce asexually and sexually, some groups are asexual only.
Sexual reproduction is syngamy - the union of two gametes.
For convenience, we will continue to use the term protist in an informal sense to refer to unicellular eukaryotes and their close multicellular relatives. Archaezoa lack mitochondria and may represent early eukaryotic lineages. They may lack mitochondria because the lineages of these organisms diverged before the endosymbiotic event that gave rise to mitochondria. One subgroup of the archaezoa called the diplomonads contains a parasite called Giardia lamblia . Giardia infects the human intestine and causes abdominal cramps and severe diarrhea.