1.2 Levels of Organization

Transcript

The biological world is organized as a hierarchy, the largest of which is the biosphere, containing all of life on Earth. Within the biosphere, are many different ecosystems, like this forest. Ecosystems consist of many communities made up of populations of different species that share resources and interact with one another.

On the organismic level, organ systems within the body work together to ensure survival and reproduction. Each organ, such as the heart, is made up of a variety of tissues, like cardiac muscle and connective tissue, and each tissue is made up of millions of cells, the basic structural and functional units of life.

Inside cells are organelles, functional compartments like mitochondria and endoplasmic reticula, which build, break down and transport many different types of molecules, from sugars to proteins.

Overview

Biological organization is the classification of biological structures, ranging from atoms at the bottom of the hierarchy to the Earth’s biosphere. Each level of the hierarchy represents an increase in complexity that builds upon the previous level.

Molecules Are Composed of Atoms and Biomolecules Are Assembled from Molecules

The most basic levels include atoms, molecules, and biomolecules. Atoms, the smallest unit of ordinary matter, are composed of a nucleus and electrons. Molecules comprise two or more atoms held together by chemical bonds, most commonly covalent, ionic, or metallic bonds.

Biomolecules are molecules found in living organisms, including proteins, nucleic acids, lipids, and carbohydrates. Biomolecules are often polymers—large molecules that are created from smaller, repeating units. For instance, proteins are composed of amino acids, and nucleic acids are composed of nucleotides.

Biomolecules can be endogenous or exogenous. Endogenous means that the biomolecule is produced inside a living organism. Biomolecules can also be consumed; for example, a cow gets carbohydrates from digesting grass (exogenous), but the grass must produce the carbohydrates through photosynthesis (endogenous).

Organelles Are Cellular Compartments Built from Biomolecules

The next hierarchical level comprises subcellular structures called organelles. Organelles are made up of biomolecules and compartmentalize eukaryotic cells. Organelle means “little organ” as they have specialized functions within a cell. For example, the lysosome allows degradation of molecules without detrimental effects to other structures within the cell, and chloroplasts enable plants to perform photosynthesis.

Organisms Can Have One, Multiple or Several Trillions of Cells

The next level of biological organization is cells. Organisms can be unicellular (i.e., consist of a single cell) or multicellular. Scientists estimated that the human body consists of 37 trillion cells.

Tissues, Organs and Organ Systems Are Formed by Specialized Cells

In complex organisms such as plants and animals, cells form tissues, organs, and organ systems. Tissues are groups of cells that carry out a similar function. For example, connective tissue, one of the four types of animal tissue, is found between organs.

Organs are groups of tissues that carry out a specific function or set of functions and may be comprised of either similar or different types of tissue. The heart, for example, is found in most animals and pumps blood throughout the body to provide other organs with oxygen and nutrients and remove waste products.

Finally, groups of organs make up organ systems that work together to carry out vital processes. For example, the animal cardiovascular system is comprised of the heart, vessels, and blood. In plants, a diverse set of tissues create the root and shoot system that provide different functions.

Individual Organisms Create Populations and Communities

Several organisms of the same species that exist at the same place and time, form populations. Communities are composed of individuals of different species at the same location and time. Communities can include two or dozens of species.

Ecosystems comprise not just the living (biotic) community, but also the non-living (abiotic) environmental factors that influence the community. For example, an oasis is an ecosystem that may contain populations of date palms, fig trees and small lizards found at the same location and time. This community lives and interacts with an environment of fertile soil and fresh water in an otherwise arid environment.

The biosphere comprises all areas (air, soil, and water) of planet Earth that harbors living organisms. As such, the biosphere is composed of many ecosystems.