Levels of Life

The organization of life on Earth can be categorized into structural levels of complexity from molecules to the Earth's biosphere in its entirety.

The structure below defines 12 levels: molecule, organelle, cell, tissue, organ, organ system, organism, species, population, community, ecosystem, and biosphere.

Bees on wildflowers, two distinct forms of life.
Bees gathering nectar from wildflowers. Photo by Vicki A Benge. All Rights Reserved.

Follow easycrosswords on Twitter

Molecule
The smallest physical unit of a compound that exhibits all the chemical properties of that compound is known as a molecule. Molecules consist of two or more atoms held together by chemical bonds.

Organelle
Specialized membrane-bound compartments inside cells are known as organelles. An organelle has a particular function to perform. For example, lysosomes break down particles inside the cell. [See The Cell: Building Block of Life.]

Cell
A cell is called the basic unit of living things. It is the smallest independently functioning unit in the structure of an organism, which displays the properties of life. One of the major theories of biology, the cell theory, states that all organisms are made up of cells.

Tissue
A group of similar cells in the body of an organism (both plants and animals) combined to carry out a common function. The four basic types of tissue are nerve, muscle, epidermal, and connective.

Organ
An organ generally consists of two or more kinds of tissue and performs specific functions. For example, the human heart is made up of muscle tissue, nerve tissue, and connective tissue. Other examples of organs that consist of several types of tissue are the lungs and stomach in animals and leaves and roots in plants.

Organ System
Found only in more complex animals, organ systems are a group of related organs that carry out a major activity for the benefit of the body. For example, the human circulatory system consists of the heart, blood, and blood vessels and serves to transport oxygen and nutrients to other organ systems and tissues throughout the body. The circulatory system also transports waste products from the tissues and other organ systems. Other human organ systems include digestive, endocrine, muscular, nervous, respiratory, reproductive, skeletal, and urinary. In an organism with organ systems, all the systems together form the organism's body.

Organism
An organism is an individual living thing. It can be a plant, animal, virus, or bacterium. An organism has, or can develop the ability to act or function independently. Examples of organisms include any living thing from a grizzly bear, to an oak tree, to an octopus, to a virus.

Species
A group of closely related organisms that have a shared gene pool and are capable of interbreeding resulting in fertile offspring are called a species. (Species is the base level of taxonomy, the science of classifying organisms into categories based on shared features.) Scientists estimate that there are tens of millions of different species of life on the Earth. The numbers do not seem quite so dramatic when one stops to realize there are more than 3000 known species of roses and more than 4000 known species of lizards on the Earth.

Population
All the plants or animals of a particular species present in a particular area are known as a population. For example, all the raccoons in a specific region are known, collectively, as the population of raccoons in the area.

Community
All the plants and animals that live in the same region under the influence of the same environment and interact with one another are known as a community.

Ecosystem
An ecosystem is a biological community along with the localized environment the community inhabits. Ecosystems are also called biomes and are generally characterized by the major vegetation in the area, such as grasslands. A major characteristic of an ecosystem is that populations interact by forming food chains. The chain normally begins when plants use photosynthesis to convert solar energy into organic nutrients. This energy is passed from one population to another through food chains. See: Ecosystems of the Earth

Biosphere
The entire area of the Earth's surface, atmosphere, and sea that is inhabited by living things is known as the biosphere of the Earth.

Simple printable crossword: Levels of Life