These questions ask you to name a level from a description, place levels in order, or classify familiar animal and plant structures. Focus on what the structure contains and whether those parts work together for a shared function.
Count the kind of units working together
Move upward one step at a time. An organelle is a specialized structure inside one cell. Cells with a shared function form a tissue. Several tissue types cooperating within one bounded structure form an organ. Several organs coordinating a major function form an organ system. An organism is one complete living individual. For example, if a structure contains protective, conducting, and storage tissues that cooperate, the presence of several tissue types places the whole structure at the organ level.
| Description signal | Classification test |
|---|---|
| A structure located within one cell | Classify it as an organelle, not as a group of cells. |
| Similar or coordinated cells sharing a function | Classify the group as tissue. Mere proximity without cooperation is not enough. |
| Several tissue types in one structure | Classify the whole structure as an organ. |
| Several organs performing a major function together | Classify the group as an organ system. |
| One complete living individual | Classify it as an organism, whether it has one cell or many. |
Apply the same composition test to plants. A leaf contains several cooperating tissue types and is therefore an organ, while individual roots and leaves belong to the broader root or shoot system.
Avoid these tempting routes
| Tempting route | How to reject it |
|---|---|
| Call an internal cell structure a tissue | A tissue requires cells working together. A specialized part contained within one cell is one level lower. |
| Call any collection of cells a tissue | The cells must cooperate in a shared function; an unrelated collection does not meet the definition. |
| Stop at tissue when several tissue types cooperate | Multiple tissues within one structure signal an organ, in both animals and plants. |
| Treat one organ as the entire system | A system includes several cooperating organs. One digestive structure or one pumping structure is only a member of its wider system. |
| Require every organism to have many organ systems | A single-celled living individual reaches the organism level without tissues, organs, or organ systems. |
| Use one root and one leaf as the names of whole plant systems | Those are individual organs. The broader plant divisions are the root system and shoot system. |
Try it
A separate structure within a multicellular organism contains muscle tissue, nervous tissue, and connective tissue working together to perform a particular function. Which organizational level best fits the structure?
- Organ
- Single cell
- Tissue
- Organism
Count the levels in this usual multicellular hierarchy: organelle, cell, tissue, organ, organ system, organism. Enter the exact number of levels listed.
The heart and blood vessels work together to move blood through the body. At what organizational level is the whole circulatory system?
- Cell
- Tissue
- The organ-system level
- Organelle