Eukaryopolis - The City of Animal Cells: Biology #4. Cell Structure. All living organisms on Earth are divided into cells.
The main concept of cell theory is that cells are the basic structural unit for all organisms. Cells are small compartments that hold the biological equipment necessary to keep an organism alive and successful. Living things may be single-celled or they may be very complex such as a human being. There are smaller pieces that make up cells such as macromolecules and organelles. iCell.