Biology is the study of life. Since life is such a broad topic, scientists break it down into several different levels of organization to make it easier to study. These levels start from the smallest unit of life and work up to the largest and most broad category.
TL;DR (Too Long; Didn't Read)
The levels, from smallest to largest, are: molecule, cell, tissue, organ, organ system, organism, population, community, ecosystem, biosphere.
Molecules are made of atoms, the smallest unit of chemical elements. They can be found in all matter, living and non-living. Molecules make up the most basic structures of living beings. Two biological disciplines that focus on this level are biochemistry and molecular biology.
A cell is the basic unit of life. There are two kinds of cells: plant cells, which have a rigid cell wall made of cellulose molecules, and animal cells, which have flexible cell membranes. Cell biologists consider questions such as metabolism and other questions about structure and function within and between cells.
Tissue is made of cells that work together to perform a certain task. Muscle tissue, connective tissue, and neural tissue are some types of tissue. Histologists are an example of biologists who work at this level.
An organ is a system of tissues that work together on a larger scale to do certain jobs within an animal's body. Examples of organs are the brain, heart and lungs. Anatomy is an example of a biology specialty concerned with this level.
An organ system is a group of organs that work together to perform specific bodily functions. The respiratory system, for example, uses the lungs, airways and respiratory muscles to inhale oxygen and release carbon dioxide in animals. Physiologists study the function of parts of the body as they work together. Though physiologists can work at any level of biological organization, they often answer questions related to organ systems.
An organism is a recognizable, self-contained individual. Organisms can be unicellular organisms such as bacteria or amoebae, or multi-cellular organisms comprised of organs and organ systems. A human being is an example of a multi-cellular organism.
A population is a group of multiple organisms of the same species within a specific area. For example, a pride of lions in Kenya, Africa, is a population.
A community consists of all the different species within a certain area. The population of lions in Kenya, plus the populations of gazelles, giraffes, elephants, dung beetles, and all other species in that area, add up to a community.
An ecosystem is made up of all the communities in a certain area, as well as all the non-living, physical components of the environment. Rocks, water and dirt are a part of an ecosystem. Ecologists may study populations, communities, or whole ecosystems.
The biosphere is all of the ecosystems on Earth added together. Every animal, plant, bacteria, rock, and molecule is a part of the Earth's biosphere. Non-biologists, such as meteorologists and geologists, may join biologists to answer questions at this level of biology organization.