Eukaryotes are a very diverse group, and their cell structures are equally diverse. Many have cell walls; many do not. Many have chloroplasts, derived from primary, secondary, or even tertiary endosymbiosis; and many do not. Some groups have unique structures, such as the cyanelles of the glaucophytes , the haptonema of the haptophytes , or the ejectisomes of the cryptomonads . Other structures, such as pseudopods , are found in various eukaryote groups in different forms, such as the lobose amoebozoans or the reticulose foraminiferans .
Today, mitochondria are found in fungi, plants, and animals, and they use oxygen to produce energy in the form of ATP molecules, which cells then employ to drive many processes. Scientists believe that mitochondria evolved from aerobic , or oxygen-consuming, prokaryotes. In comparison, chloroplasts are found in plant cells and some algae, and they convert solar energy into energy-storing sugars such as glucose. Chloroplasts also produce oxygen, which makes them necessary for all life as we know it. Scientists think chloroplasts evolved from photosynthetic prokaryotes similar to modern-day cyanobacteria (Figure 4). Today, we classify prokaryotes and eukaryotes based on differences in their cellular contents (Figure 5).