Levels of Organization of Life

All life on planet Earth is defined by a rigid hierarchy that organizes all living organisms and natural systems within the Earth’s biosphere. These are the Levels of Organization of Life.

The various scalar resolution of physical reality that leads to, composes, and organizes living things. These levels range from the smallest parts of an organism, to its most fundamental mechanisms, and the complex social dynamics between the members of a species, members of other species, or aspects of an environment, within an ecosystem.

The Levels of Organization of Life

Atoms and Molecules

At the most fundamental level every living creature in existence is composed of atoms. Of course we cannot know for certain the conditions of life in other galaxies, superclusters of galaxies, or universes. But based on the sum totality of everything that we know about reality at this present time, at least, we know that atoms are intrinsic.

For practical purposes the smallest building block of matter is the atom. Every thing – both animate and inanimate – is composed of atoms. They are composed of an atomic nucleus which contains small objects called protons and neutrons orbited by small objects called electrons.

Furthermore, there are a number of different types of atoms which differ predominantly by the number of protons and neutrons in the nucleus called elements. These elements are different types of atoms, such as gold, silver, iron, hydrogen, and all the rest of the periodic table.

These are the fundamental building blocks of physical reality. Though if we are being precise, technically speaking both protons and neutrons are actually composed of even smaller structures, while even the atom itself is 99.9999% space. What these objects are is still a matter of investigation.

The next scalar resolution of reality occurs with the combination of more than one atom (or type of atomic element) into what we call molecules. Molecules are composed of at least two atoms held together by at least one chemical bond, which we then refer to as a chemical structure. An individual “piece” of water is called a water molecule expressed chemically as $H_2O$ because it is composed on two hydrogen atoms each chemically bonded to one oxygen atom.

The next of the Levels of Organization of Life occurs when molecules come together to form even larger molecules. These are called macromolecules, which literally means “large-molecules.” Macromolecules are enormous, relative to a single molecule. A water molecule is composed of only three atoms, however a macromolecule can typically be composed of thousands of atoms.

Each of these atoms are connected by covalent bonds, bonds that form between atoms when they share electrons, these shared electrons then being called bonding pairs. Carbohydrates (sugars), lipids (fats), proteins, nucleic acid, and deoxyribonucleic acid (DNA) are all examples of macromolecules.

Macromolecules are typically formed through the polymerization (when monomers combine to form polymers and/or macromolecules).

Cells & Organisms

We have so far discussed the Levels of Organization of Life below what we commonly classify as Living Creatures based on the 8 Properties of Life. With the construction of macromolecules due to natural forces, the organic dynamics and processes of the Universe, this next incredible transition is where we see the emergence of structures of actual living organisms, with the development of what we call the cell.

All living creatures are composed of cells. At their most simplistic level cells are semi-permeable membranes that will generally protect specific types of macromolecules within this boundary. Such as macromolecules like the mitochondria which stores chemical energy within the cell, to be used by the cell, or chloroplasts, which are structures that enable plants to convert sunlight into chemical energy, sugars, to be used as food.

These types of macromolecules are called organelles. Organelles are to a cell what an organ is to a human; they perform necessary tasks for the perpetuation of life. The stomach breaks down food, the gastrointestinal tract absorbs nutrients, the heart pumps blood, and the blood distributes nutrients and oxygen throughout the body.

Single-celled organisms were the first to evolve. However, (relatively) shortly afterwards multicellular organisms came into existence. We can classify cells under two different categories: Prokaryotes are single-celled organisms that do not have a membrane-bound nucleus (which is the home of DNA), whereas Eukaryotes do possess a membrane which houses both organelles and a cellular nucleus. These are often called microorganisms and are the smallest units of life.

Tissues and Organs

Each individual creature is called an organism. This distinction applies to any living creature, representing the next of the Levels of Organization of Life once single-celled prokaryotes and eukaryotes emerged through evolution. A human being is an organism, like a tree, lion or ant, while smaller single-celled or multi-celled organisms are generally referred to as microorganisms.

Larger organisms similar types of cells that perform similar functions group together to form tissues, which then group together in structures that we call organs (such as the adrenal gland), which perform specific functions determined by the specific functions of the tissues, which are the majority of times necessary for life.

In larger more complex mammals organs then work together to form organ systems. The Hypothalamus-Pituitary-Adrenal (HPA) axis is an example of an organ system connecting the glands named above which regulates the production of hormones and chemical signals to control things like blood pressure or the fight-or-flight response.

Population, Ecosystem, and Biosphere

Part of biology involves considering Levels of Organization of Life beyond the single cell, organ, or even organism. In the natural world every individual of a species amounts to a greater organization of said species which we can call a population, such as the total human population, or the entire tree population in a forest, or all the fish in the sea.

A community is then the distinction of a grouping of living organisms within a specific region, which includes a number of interpenetrating populations collectively forming an ecosystem. This includes all abiotic (non-living) aspect of the ecosystem such as viruses, rocks of the landscape, nitrogen or mineral deposits in the soil, etc.

The mice, trees, insects, flowers, birds, bacteria, and mammals of a forest form a community, and collectively comprise that specific forest ecosystem. Just like all of the creatures of the ocean form a complex ecosystem distinct from those on land.

Yet in the end all ecosystems are one. Even on our finite planet, we are united by the limitations imposed by the atmosphere of trapped gas, defining the biosphere where all life has miraculously evolved, and thrived for millions of years.

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