The Characteristics of Life and Biological Classification

The Characteristics of Life

Defining Life

Biologists use several key criteria to distinguish living organisms from non-living matter. These characteristics are essential for an entity to be considered alive.

• Unique Molecular Composition: Living things have a different molecular makeup compared to non-living things, often including complex organic molecules such as proteins, nucleic acids, carbohydrates, and lipids.

• Requirement for Energy and Raw Materials: All living organisms need energy (often in the form of ATP) and raw materials to maintain their structure and function.

• Cellular Organization: Living things are composed of one or more cells, which are the basic units of life.

• Homeostasis: The ability to maintain a stable internal environment despite changes in the external environment.

• Responsiveness: Living organisms can sense and respond to stimuli in their external environment.

• Growth and Reproduction: All living things grow and have the capacity to reproduce, passing on genetic information to their offspring.

• Evolution: Populations of living organisms evolve over generations through changes in genetic composition.

Example: Humans maintain a constant body temperature (homeostasis), respond to stimuli (such as moving away from heat), and reproduce sexually.

Classification of Living Things

Taxonomic Hierarchy

Scientists classify living organisms based on shared characteristics, using a hierarchical system that organizes life from the broadest to the most specific categories.

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Mnemonic: "Does King Phillip Come Over For Good Spaghetti?" helps remember the order: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Definition: Species is the most specific level, referring to a group of organisms capable of interbreeding and producing fertile offspring.

Human Classification Example

Humans are classified as follows:

Taxonomic Rank	Human Classification
Domain	Eukarya
Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Primates
Family	Hominidae
Genus	Homo
Species	sapiens

Additional info: The genus name is always capitalized and italicized (Homo), while the species name is lowercase and italicized (sapiens).

Defining Features of Humans

Unique Human Characteristics

Humans possess several features that distinguish them from other living organisms, especially within the animal kingdom.

• Bipedalism: The ability to walk upright on two legs.

• Opposable Thumbs: Thumbs that can touch the tips of the other fingers, allowing for precise grasping and manipulation of objects.

• Larger Brain: Humans have a relatively large brain compared to body size, supporting advanced cognitive functions.

• Capacity for Complex Language: Humans can communicate using complex, symbolic language.

These features have contributed to human survival and adaptation but do not necessarily make humans superior to other species—just different.

Levels of Biological Organization

Hierarchy in Biology

Biological organization refers to the different levels at which life can be studied, from the smallest molecular components to the entire organism and beyond.

• Molecular Level: Atoms and molecules essential for life (e.g., DNA, proteins).

• Cellular Level: The cell is the basic unit of structure and function in living things.

• Tissue Level: Groups of similar cells performing a specific function.

• Organ Level: Structures composed of different tissues working together (e.g., heart, lungs).

• Organ System Level: Groups of organs that perform related functions (e.g., digestive system).

• Organism Level: The complete living being.

Additional info: In Anatomy & Physiology, the focus is often on the molecular, cellular, tissue, and organ system levels.