Life is Complex
Nope, you are not in Philosophy class. Even though "What is life?" is the kind of mind-draining question we could spend days talking about over espresso and chocolate-drizzled croissants, a biologist would tell you that all living things—organisms, creatures, beasties, take your pick—have five characteristics in common. That's right; you are not officially alive if you cannot do the five things on this and the following four pages. Hand-eye coordination is not among them. You can breathe again now.
Living things are complex and organized.
Living creatures are well-oiled machines. Despite the nicely packaged outward appearance, living creatures have various different levels of organization that are extremely complex.
Starting at the lowest level of organization, subatomic particles, such as protons, neutrons, and electrons, make up atoms, which can combine with other atoms to form a dizzying variety of molecules. Atoms turn into molecules that get together and form cellular structures, including organelles, or as we like to call them, "mini organs."
Many different organelles work together inside a cell. The cell is the most basic functional unit of life.
In a multicellular organism, a group of cells that all share the same tasks is called a tissue, and one or more tissues form an organ. Organs are organized into organ systems, and a whole organism has many organ systems working together to get things done.
However, a single-celled individual lacks tissues, organs, and organ systems, but it still has the same levels of organization below the cellular level: protons, neutrons, electrons, atoms, and molecules.
Many individuals living in the same place form a population, and all the interbreeding populations of a certain kind of living organism make up a species. When populations of different species interact, they form a community…and have lots of parties.
An ecosystem includes all the living organisms in a certain area in addition to the nonliving parts of their environment, which usually includes many different communities. Finally, the highest level of organization is the biosphere–encompassing all the living things on Earth. Phew.