Introduction to Biology: Characteristics and Classification of Living Organisms

Introduction to Biology

Course Overview

This section introduces the foundational concepts of General Biology, focusing on the characteristics that define living organisms and the principles of biological classification.

Organisms

Etymology and Definition

The term organism is derived from the word organ, meaning "a tool or something that works," and the suffix -ism, which denotes a state, condition, or action. The word is closely related to "organization," reflecting the structured nature of living things.

• Organism: Any living entity that exhibits the properties of life.

• Organization: The ordered structure of biological systems, from molecules to cells to tissues and organs.

The Characteristics of Living Organisms

Defining Features

Living organisms share several key characteristics that distinguish them from non-living matter. These features are essential for the maintenance and perpetuation of life.

• Organization: Living things are highly organized, with cells as the basic unit of life.

• Acquire Materials and Energy: Organisms require external sources of materials and energy to maintain their organization and carry out life’s activities. Energy is defined as the capacity to do work.

• Reproduction: Organisms produce new individuals, passing on hereditary information encoded in DNA. Genes are units of information within DNA that direct cellular functions.

• Respond to Stimuli: Organisms detect and respond to environmental changes, often by moving toward or away from stimuli. Behavior is directed toward avoiding injury, acquiring food, or mating.

• Homeostasis: The maintenance of a relatively constant internal environment, such as stable body temperature in humans.

• Growth and Development: Growth refers to an increase in size or number of cells, while development encompasses all changes from conception to death.

• Capacity to Adapt: Organisms can adapt to changing environments. Adaptations are features that make individuals better suited to their environment. Natural Selection is the differential reproductive success of adapted individuals, leading to evolution, or changes in the frequency of traits in populations over time.

NASA's Definition of Life

NASA defines life as "a self-sustaining chemical system capable of Darwinian evolution." This emphasizes the ability of living systems to maintain themselves and evolve over generations.

Organization in Living Organisms

Cellular Organization

All living organisms are composed of cells, which may be prokaryotic (lacking a true nucleus) or eukaryotic (containing a true nucleus). The organization extends from molecules to cells, tissues, organs, and organ systems.

• Prokaryotic Cells: Simple cells without a nucleus (e.g., bacteria).

• Eukaryotic Cells: Complex cells with a nucleus and organelles (e.g., plants, animals, fungi, protists).

Classification of Living Organisms

Systematics and Taxonomy

Living organisms are classified based on similarities and evolutionary relationships. Systematics is the discipline of identifying and classifying organisms, while taxonomy assigns a binomial (two-part) scientific name to each species.

• Binomial Nomenclature: Each species is given a two-part name: Genus (capitalized) and species (lowercase), both italicized. Examples: Homo sapiens, Pisum sativum, Felis domesticus

Hierarchical Classification

Organisms are classified into a hierarchy of categories, from most inclusive to least inclusive:

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Domains of Life

There are three domains, based on biochemical and genetic evidence:

• Domain Archaea: Unicellular prokaryotes that live in extreme environments (e.g., high temperature, salinity, acidity).

• Domain Bacteria: Unicellular prokaryotes found in diverse environments; some are beneficial, others cause disease.

• Domain Eukarya: Eukaryotic organisms, further divided into four kingdoms: Protista, Fungi, Plantae, Animalia.

Kingdoms of Eukarya

• Protista: Mostly unicellular, diverse group (e.g., algae, protozoa).

• Fungi: Multicellular or unicellular, absorb nutrients (e.g., mushrooms, yeast).

• Plantae: Multicellular, photosynthetic (e.g., plants).

• Animalia: Multicellular, ingest food (e.g., animals).

Example Table: Classification of Humans

The following table summarizes the classification of humans:

Category	Name	Key Characteristics
Domain	Eukarya	Cells with nucleus
Kingdom	Animalia	Multicellular, ingest food
Phylum	Chordata	Spinal cord
Class	Mammalia	Hair, mammary glands
Order	Primates	Opposable thumbs
Family	Hominidae	Large brain, upright posture
Genus	Homo	Human-like
Species	Homo sapiens	Modern humans

The Process of Science

Scientific Method

Biology is a scientific discipline that relies on the scientific method to investigate natural phenomena.

1. Observation: Using senses to gather information about the natural world.

2. Inductive Reasoning: Combining isolated facts into generalizations.

3. Hypothesis: A tentative, testable, and falsifiable explanation for an observation.

4. Experiment/Further Observation: Testing hypotheses using deductive reasoning and controlled experiments. Experimental Design: The plan for conducting an experiment, including controls and models.

5. Data: Observable and objective results, often presented in tables or graphs. Statistical Analysis: Used to determine if results are due to chance.

6. Conclusion: Analysis of data to determine if the hypothesis is supported or refuted. May lead to new hypotheses.

7. Scientific Theory: A well-supported concept that unifies related hypotheses and is backed by extensive evidence. Examples:

   • Cell Theory: All organisms are composed of cells; new cells arise from preexisting cells.

   • Homeostasis: Organisms maintain a stable internal environment.

   • Evolution: All organisms share a common ancestor and adapt to their environment.

Example of Scientific Reasoning

• If organisms are composed of cells, then examination of an organism should reveal cells.

Thought Question

Are Viruses Alive?

Viruses challenge the definition of life because they do not exhibit all characteristics of living organisms independently. They require host cells to reproduce and do not maintain homeostasis.

Summary

Understanding the characteristics and classification of living organisms is fundamental to biology. The scientific method provides a systematic approach to studying life, leading to the development of scientific theories that explain biological phenomena.