Many life forms consist of a single cell. As well as simple bacteria, there are more complex organisms, known as protoctists. Unlike bacteria, they have complex internal structures, such as nuclei containing organized strands of genetic material called chromosomes. Most are single-celled, but some form colonies, with each cell usually remaining self-sufficient.
An amoeba is a predatory single cell that does not have a fixed shape. It can project parts of its cell to create jellylike tentacles called pseudopodia. The amoeba uses these to move, touch, and grab prey. Amoebae live in water, where they creep along rotting vegetation. They hunt smaller single cells, such as bacteria.
Slime moulds start out as amoebalike cells hunting for food in damp habitats. Later, the cells join together to build spore-producing structures.
Algae are now classed as protoctists, although scientists used to include them in the plant kingdom. Algae can make food by photosynthesis, as they contain green chloroplasts. Euglena algae live in ponds. They lose their chloroplasts in the dark and then feed like animals. Seaweeds are the best-known algae. They are made up of huge communities of algae cells.
Some protoctists obtain food by invading other organisms and living as parasites. The malaria parasite first enters its human host through the bite of the Anopheles mosquito. Once inside, it multiplies inside the blood and may infect the liver. The parasite causes malaria fever, a disease that may be fatal.
Lens-maker Antoni van Leeuwenhoek made the first practical microscope in 1671. With it, he observed bacteria and protoctists, which he called “animalcules”. Van Leeuwenhoek went on to study yeasts, plant structure, insect mouthparts, and the structure of red blood cells.