Final answer:
The described organism is multicellular with cells having a nucleus, endoplasmic reticulum, and mitochondria, but no chloroplasts, suggesting it is likely an animal or a fungus, as these are eukaryotic and do not perform photosynthesis.
Step-by-step explanation:
Characteristics of Eukaryotic Cells
The organism described is multicellular and its cells contain a nucleus, endoplasmic reticulum, and mitochondria. These characteristics are consistent with eukaryotic organisms such as animals, protists, and fungi. Given that the organism lacks chloroplasts, which are organelles required for photosynthesis in plants, it cannot be a plant. Animal cells do not have chloroplasts and are eukaryotic; hence, animals are heterotrophic consumers. Fungi are also multicellular eukaryotes that lack chloroplasts and are mostly decomposers in ecosystems. Protists can be tricky, as some are photosynthetic while others are not; however, the majority of protists are unicellular, and the organism in question is multicellular.
Considering these characteristics, the organism is more likely to be an animal or a fungus. These two groups of organisms are comprised of multicellular eukaryotic cells that do not contain chloroplasts. While some protists are multicellular and could fit this description, animals and fungi are the groups most consistently matching the characteristics described.