The Characteristics and Organization of Life

Chapter 1: Human Biology, Science, and Society

Characteristics of Life

Biologists use a set of criteria to distinguish living organisms from non-living things. All seven characteristics must be present for an entity to be considered alive.

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

• Require Energy and Raw Materials: Organisms must obtain and use energy and materials from their environment to maintain life processes.

• Composed of Cells: The cell is the basic unit of life; all living things are made up of one or more cells.

• Maintain Homeostasis: Living organisms regulate their internal environment to maintain stable, life-supporting conditions.

• Respond to External Environment: Organisms can sense and react to changes in their surroundings.

• Grow and Reproduce: Living things increase in size and produce offspring, passing on genetic information.

• Populations Evolve: Over generations, populations of living organisms undergo genetic changes, leading to evolution.

Example: Humans maintain a constant body temperature (homeostasis), respond to stimuli (like moving away from heat), and reproduce to ensure the survival of the species.

Classification of Living Things

Living organisms are classified into hierarchical categories based on shared characteristics. This system helps scientists organize and understand the diversity of life.

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Mnemonic: "Does King Phillip Come Over For Good Spaghetti?" helps remember the order of taxonomic ranks.

Human Classification

Humans are classified as follows:

• Domain: Eukarya

• Kingdom: Animalia

• Phylum: Chordata (organisms with a neural cord)

• Class: Mammalia (mammals with mammary glands)

• Order: Primates (includes lemurs, monkeys, apes, and humans)

• Family: Hominidae (great apes and humans)

• Genus: Homo

• Species: sapiens

Note: The genus name (Homo) is capitalized and italicized; the species name (sapiens) is lowercase and italicized.

Defining Features of Humans

Humans possess several unique characteristics that distinguish them from other species:

• Bipedalism: Ability to walk upright on two legs.

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

• Larger Brain: Relative to body size, humans have a larger and more complex brain.

• Complex Language: Capacity for sophisticated verbal and written communication.

These features have contributed to human survival and adaptation but do not imply superiority over other species.

Biological Organization of Humans

Levels of Organization

Biological organization refers to the hierarchy of complex biological structures and systems that define life, from the smallest chemical building blocks to the entire biosphere. In human biology, these levels are especially important for understanding structure and function.

Level	Description
Atom	Smallest unit of an element; consists of protons, neutrons, and electrons.
Molecule	Two or more atoms held together by chemical bonds.
Cell	Smallest structural and functional unit of life.
Tissue	Group of similar cells that perform a specific function.
Organ	Structure composed of two or more tissue types working together to perform specific functions.
Organ System	Group of organs that work together to perform major functions or meet physiological needs of the body.
Organism	An individual living being composed of multiple organ systems.
Population	Group of individuals of the same species living in a specific area.
Community	All living organisms in a particular area.
Ecosystem	All living organisms plus the non-living components of their environment in a particular area.
Biosphere	The sum total of all ecosystems on Earth.

Example: The human digestive system is an organ system composed of organs such as the stomach and intestines, which are made of tissues, which in turn are made of cells.

Summary

• Seven characteristics define life; all must be present for an organism to be considered living.

• Taxonomic classification organizes life from domain to species; humans are Homo sapiens.

• Humans are set apart by bipedalism, opposable thumbs, large brains, and complex language.

• Biological organization ranges from atoms to the biosphere, with each level building on the previous one.

Additional info: The study of human biology often focuses on the molecular, cellular, tissue, and organ system levels, as these are critical for understanding health and disease.