Which of the following exists at the organ level of organization?

The levels of organization in living things, from simplest to most complex, are: cell, tissue, organ, organ system, and organism. An organ is a structure made up of two or more different types of tissues working together to perform a specific function.

An onion bulb is an organ. It is a modified underground stem composed of several tissue types: the fleshy scale leaves (which store food), the outer dry protective scales, the basal plate (a compressed stem), and the growing point. These different tissues work together to store nutrients and enable vegetative reproduction, which is the hallmark of organ-level organization.

Euglena and Paramecium are unicellular (single-celled) organisms. They exist at the cell level of organization because all their life processes are carried out within a single cell. They do not have tissues or organs.

Hydra is a simple multicellular organism that exists at the tissue level of organization. It has two cell layers (ectoderm and endoderm) that form simple tissues, but these tissues are not organized into distinct organs.

Exam tip: To identify the level of organization, ask whether the structure contains multiple tissue types working together. If yes, it is at least at the organ level.