Characteristics and Organization of Life

A rabbit moves and responds to its surroundings. A dormant seed may do neither visibly, yet both are alive. To judge life, examine cellular organization and biological processes across an appropriate time scale, rather than choosing one conspicuous behavior.

The same approach also helps you distinguish levels of organization: a cell, an organ, and a population are different kinds of biological systems.

Living systems share cellular organization, metabolism, regulation, inherited information, and a history of reproduction and evolution. These properties describe life collectively. A sterile animal is still alive, and a dormant seed may show little visible activity. Multicellular bodies can also organize cells into tissues, organs, and organ systems.

What are the characteristics of living things?

Look for several connected properties: cells, energy use, regulation, response, inherited information, and participation in a lineage capable of reproduction and evolution. Individual organisms need not display every property at every moment. A mule cannot normally reproduce, but its cells, metabolism, and biological ancestry leave no doubt that it is alive.

No single visible behavior proves that something is alive. Fire can spread and use fuel, yet lacks cellular organization and a genetic system. Assess the whole biological system rather than counting isolated behaviors.

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. Cells and extracellular material organized for related functions 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.

Does every organism need organs?

No. A bacterium performs its essential activities within one cell; it does not have tissues, organs, or organ systems. The cell-to-organ-system sequence describes complex multicellular bodies. It should not be applied as a requirement for all life. Even multicellular organisms vary in the kinds of tissues and organs they possess.

Viruses show why the boundary deserves care. They contain genetic material and evolve, but they rely on host cells for reproduction and lack independent cellular metabolism. Introductory biology therefore usually treats them as acellular biological entities at the boundary of life, rather than as ordinary cells. Explain the criteria being used instead of relying on movement or size.

Watch: A Short Video Lesson

This Crash Course Biology lesson follows biological organization. As you watch, distinguish the structures inside one organism from groups of organisms in an ecosystem.

A routine for organization questions

1. To evaluate life, consider its connected properties and biological history; do not demand every visible behavior from each individual.
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 properties used together to recognize living systems.
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. Examples include cellular organization, metabolism, regulation, inherited information, and membership in a reproducing and evolving lineage. An individual need not reproduce to be alive.
2. The cell.
3. Cell, tissue, organ, organism.
4. Regulation of internal variables within workable ranges, allowing normal variation.
5. A population.
6. False. Fire lacks cellular organization and a genetic system; its ability to spread does not establish biological reproduction.

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.