The Hierarchy of Life

Investigating the Hierarchical Organization of the Biological World

Jul 23, 2009

There is an ordered regularity to the pandemonium of life on Earth.

At first glance one might not perceive much, if any, order to the chaos of life that inhabits this planet. However, through the work of many biologists in different disciplines over long periods of time, it now appears that the biological world is indeed organized and that this organization is hierarchical. That is, each level builds on the level below it.

Here is a description of the levels of the life hierarchy beginning with the lowest and working up to the largest.

The Atomic and Molecular Level.

Ninety-two naturally-occurring elements (types of atoms) form the physical structure of this planet and its atmosphere. However, the main bulk of any living thing is composed primarily of carbon atoms, some hydrogen, oxygen, and nitrogen atoms, and trace amounts of a few other elements.

Atoms bond (join) together into clusters known as molecules.The molecules that constitute the cells of living things are large and complex assemblages called macromolecules. There are four basic types of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

Carbohydrates provide immediate energy for organisms while lipids allow for long-term storage of energy. Proteins function as structural and movement molecules in animals and form enzymes and hormones, and transport molecules and antibodies in all living things. Nucleic acids (mainly deoxyribonucleic acid or DNA) control and coordinate all cell processes as well as cell replication.

Large biological macromolecules – carbohydrates, lipids, proteins, and nucleic acids –by themselves are not alive but when they are arranged into cells, the next level up in the hierarchy, life appears.

The Cellular Level

Any living thing is composed of cells and cell products (such as bone or shell), be it a one-celled protozoan or the human body, made up of trillions of cells.

“Cells let us walk, talk, think, make love and realize that the bath water is cold.”

If, as systematists (taxonomists) suspect, there truly are tens of millions of different species of living things on this planet, the total number of cells that inhabit it could not even be imagined, let alone accurately determined. However, nature has smiled on biologists for all the untold multitude of cells on earth fall into only two general categories – prokaryote or eukaryote – based on inherent internal properties.

A prokaryotic cell (Gr. pro, before + karyon, kernel or nucleus) has DNA but its DNA is not isolated from the rest of the cell inside a membrane-bound nucleus. Instead, the DNA is a single loop floating free in the cytoplasm. In addition, the prokaryotic cell is less complex and much smaller than it eukaryotic counterpart.

Prokaryotic cells range in size from 1 to 10 µm and lack internal membrane-bound organelles (L, organum = tiny organ), structures devoted primarily to the many metabolic tasks required to maintain the life of the cell. Most prokaryotic life forms are single cells but some types are colonial.

A eukaryotic cell (Gr. eu, true + karyon, kernel or nucleus) contains DNA that is complexed with DNA-binding proteins into linear chromosomes and located within an organelle known as the nucleus. Eukaryotic cells, ranging from 10 to 100 µm, are larger than prokaryotic cells and contain numerous membranous organelles not found in prokaryotic cells.

Any multicellular organism is vested with life at two levels – cellular and organismal. Each individual cell in a creature is alive but somehow the collective lives of each of those cells are transposed to a higher plane, and as a consequence, the entire organism lives. This amazing extrapolation of the life force results in any animal being far more than just the sum of its parts.

The Organismal Level

Cells are grouped into tissues, such as muscle cells, which are grouped into organs, such as a heart and blood vessels, and organs are coordinated into organ systems to form a complex multicellular organism. A total of eleven different organ systems are found in metazoans (complex multicellular organisms): integumentary, muscular, skeletal, respiratory, digestive, circulatory, excretory, nervous, endocrine, immune, and reproductive. Only with the single-celled protozoans, algae, and bacteria does the totality of life and death exist solely at the cellular level.

From the atoms and molecules that build cells to the collection of cells that is an organism, there is an ordered regularity to this pandemonium called life.

The copyright of the article The Hierarchy of Life in Biology is owned by Dennis Holley. Permission to republish The Hierarchy of Life in print or online must be granted by the author in writing.