Characteristics and Organization of Life

What separates a living rabbit from a rock? Biologists answer that question with a short checklist of traits shared by everything alive, and with a tidy way of organizing life from the tiniest part up to the whole planet. Both ideas are favorites on science tests, and both are easier than they sound.

This lesson lays out the characteristics all living things share and the levels of organization that build a body.

All living things share a set of characteristics: they are made of cells, use energy, grow and develop, respond to their environment, reproduce, and maintain stable internal conditions. Living things are also organized into levels, from cells to tissues to organs to organ systems to the whole organism.

What are the characteristics of living things?

To count as alive, an organism generally must do all of the following: be made of one or more cells, use energy to power its activities, grow and develop, respond to changes around it, reproduce to make more of its kind, and keep a steady internal state called homeostasis. A car uses energy and moves, but it is not made of cells and cannot reproduce, so it is not alive.

The reason biologists insist on the full list is that no single trait is enough. Fire uses energy and grows, but it is not made of cells and cannot maintain homeostasis. Only living things check every box.

How is life organized in a body?

A complex organism is built in layers, each made from the level below it. Cells are the basic unit of life. Similar cells working together form a tissue. Different tissues combine into an organ like the heart. Organs that cooperate form an organ system like the circulatory system. All the systems together make the organism.

Beyond the organism

Organization does not stop at a single body. A group of the same species living in one area is a population, several populations form a community, and the community plus its physical surroundings make an ecosystem. All the ecosystems on Earth together form the biosphere. This wider ladder connects the study of cells to the study of ecology.

Watch: A Short Video Lesson

CrashCourse walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A routine for organization questions

1. To decide if something is alive, run the full checklist, not just one trait.
2. Remember that everything alive is made of cells.
3. For body organization, go smallest to largest: cell, tissue, organ, organ system, organism.
4. For groupings of organisms, continue: population, community, ecosystem, biosphere.
5. Pick the level that matches the example in the question.

Practice questions

1. Name three characteristics shared by all living things.
2. What is the basic unit of all life?
3. Put these in order from smallest to largest: organ, cell, tissue, organism.
4. What is homeostasis?
5. A group of the same species living in one area is called what?
6. True or false: because fire grows and uses energy, it is alive.

Answers:

1. Any three of: made of cells, uses energy, grows and develops, responds to the environment, reproduces, maintains homeostasis.
2. The cell.
3. Cell, tissue, organ, organism.
4. The maintenance of a stable internal environment.
5. A population.
6. False. Fire is not made of cells and cannot maintain homeostasis or reproduce.

Where this fits

The idea that life is built from cells sets up the whole study of the cell, and the wider levels lead into ecology. To see how organisms are sorted into groups, continue with classification and scientific names. Every topic is gathered on the ASVAB General Science Learning Hub.