The living things are arranged in biological levels, organizing the structure of biological organisms and its agroupments. Those levels are|: organelle, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystem and biosphere.
The most basal biological level are the organelles, which are membrane-bound compartments inside the cells responsible for playing different roles in the cell's global function. One example of organelle is the mitochondria. Those organelles are part of the cell, which is another biological level.
The cell is the basic functional and structural unit of life. This means that all living beings are composed of cells, and the cells are the most basal unit that keeps the organisms alive and functioning. A cell is composed of a cytoplasm, in which the organelles are distributed, within a membrane.
The cells can be organized in agroupments of similar cells sharing an extracellular matrix and having a group functioning. This agroupment of cells with the same function is called a tissue. A collective of tissues, on the other hand, composes the next biological level, an organ.
An organ structurally forms a functional unit, all of the tissues working together, performing a particular function. The lungs, the stomach and the heart are examples of organs. Our organs have complementary functions, in order to keep the entire organism functioning.
The organ systems are composed of a group of organs working together to perform different functions. The Digestive System, for example, is composed of the stomach, the liver, the pancreas, amongst other organs, and they all work together so we can digest the food.
The next biological level is the organism, which is an agroupment of molecules arranged in organelles, cells, tissues, organs and systems. The organism can function as a more or les stable whole, exhibiting the properties of life.
An agroupment of organisms from the same species living together at a particular place is called a population. And an interacting group of various species in a common location is a community. But we still have a major agroupment, which includes all living organisms and also the physical environment, and we call it an ecosystem. At last, we have the biosphere, the zone of the Earth where land, water, air and earth interct with each other, supporting life.
Those are the biological leves, from the most basal (organelle) to the most complex (biosphere).