Earth as a System: Biotic, Abiotic, and the Four Spheres

Earth is conceptualized by scientists as a singular system, rather than a collection of independent parts.
A system is defined as a group of things interacting together in extremely complex ways.
The most fundamental division of these complex interactions is between what is living and what is non-living.

Biotic Factors
- The term "biotic" contains the prefix "bio," which refers to life.
- Verbatim Definition: A biotic factor is a "living factor."
- Biological Example: The human body is a system that contains biotic factors. Gut bacteria residing in the intestines are biotic factors; while they can live independently if needed, they are integral parts of the body's internal system.
- Medical Relationship: Antibiotics are medications designed to combat living factors in the body, typically "bad" bacteria in the blood or other tissues. However, they can unintentionally disrupt "good" biotic factors, such as the gut bacteria essential for digestive health.
Abiotic Factors
- In linguistic terms, placing the letter "a" in front of a word often means "not" or the "opposite of."
- Verbatim Definition: An abiotic factor is a "non-living factor."
- Examples of abiotic factors within a system include:
  - Soil.
  - Air pressure and air molecules.
  - Water pressure.
  - Temperature.
System Interaction: In nearly all systems, biotic and abiotic factors interact with each other continuously to maintain the system's function.

Earth scientists, biologists, and other researchers divide Earth's complex systems into four major subsystems described as "spheres."
Rationale for the term "Spheres": Earth is physically a sphere. If one were to isolate a single component (such as only the water or only the living things) while keeping those elements in their current positions relative to the planet, that component would collectively represent a spherical shape.

The prefix "atmo-" refers to the air surrounding the Earth.
Definition: The atmosphere encompasses all the gases that surround the planet.
Physical Dimensions: The Earth is surrounded by a layer of atmosphere approximately $60\,\text{miles}$ thick.
Relative Perspective: While $60\,\text{miles}$ is a significant distance horizontally on the surface, it constitutes a "very thin layer" when viewed from the perspective of the entire Earth. It appears as a thin, faint layer in satellite imagery.
Conceptual Sphere: If all land, water, and life were removed while the air was kept in place, the gases would still form a sphere.

The root "bio-" signifies life.
Definition: The biosphere consists of all living organisms on Earth.
Population and Scale: This sphere includes trillions and trillions of individual organisms.
Distribution: The vast majority of living organisms in the biosphere are located at or very near the surface of the Earth.

Definition: The hydrosphere refers to all water on Earth, regardless of its location or state.
Geographic Scope: This includes water found both on the surface of the Earth and below the surface.
Forms of Water Included:
- Oceans and seas.
- Lakes and rivers.
- Precipitation and rain.
- Icebergs (water in solid form).

Definition: The geosphere consists of all the non-water, non-gas, and non-living materials that compose the planet.
Composition: This sphere includes the rocks, the continents, and the ocean floor.
Significance: The geosphere makes up the bulk of the mass of the planet Earth. It represents virtually everything that is not water, gas, or a living organism.