OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Which of the following best defines an organ in biological terms?

1. A single type of cell

2. A group of similar cells

3. Groups of tissues working together

4. Individual organelles within cells

The correct answer is: Groups of tissues working together

An organ is best defined as groups of tissues working together to perform specific functions within an organism. In biological systems, organs are composed of multiple types of tissues that collaborate to execute complex tasks vital for the organism's survival. For example, the heart is made up of muscle tissue, connective tissue, and nerve tissue, all working in unison to pump blood throughout the body. The other options lack the necessary complexity to define an organ. A single type of cell does not constitute an organ, as organs require a variety of cell types and structures. Similarly, a group of similar cells refers more to tissue rather than an organ, since tissue is defined by the similarities of its cellular makeup. Lastly, organelles are specialized structures within cells (such as mitochondria or the nucleus) and are not classified as organs themselves. This hierarchical structure, where cells form tissues, tissues form organs, and organs form systems, highlights why the correct definition focuses on the collaboration of multiple tissue types.