Foundations of General Biology: Characteristics, Organization, Taxonomy, Scientific Method, and Evolution

Characteristics of Living Organisms

Defining Life

Biology is the study of life. Determining what qualifies as "living" involves identifying several key characteristics shared by all living things.

• Sensing and Responding to Stimuli: Organisms detect and respond to changes in their environment.

• Regulation and Homeostasis: Organisms regulate their internal environment to maintain homeostasis, a stable internal balance.

• Reproduction: Organisms reproduce independently, either sexually (offspring with genetic variation) or asexually (clones of the parent).

• Energy Utilization: Organisms require a source of energy and convert it into usable forms to perform work. For example, plants use photosynthesis to convert light energy into chemical energy, while animals consume organic molecules and use cellular respiration.

• Growth and Development: Organisms grow by adding cells and may change physically over their lifetime.

• Adaptation and Evolution: Organisms possess traits that allow them to thrive in specific environments, a result of natural selection.

Levels of Biological Organization

Hierarchy from Atoms to Biosphere

Biological organization is structured from the smallest to the largest units:

• Atoms: The basic building blocks of matter.

• Molecules: Formed by chemically bonding atoms together.

• Organelles: Specialized structures within cells (e.g., mitochondria, nucleus).

• Cells: The smallest unit of life; can be unicellular or multicellular.

• Tissues: Groups of similar cells performing a specific function.

• Organs: Structures composed of multiple tissues working together.

• Organ Systems: Groups of organs that perform complex functions.

• Organism: A complete living entity.

• Population: A group of organisms of the same species living in a defined area.

• Community: All populations of different species in an area.

• Ecosystem: Includes all organisms and physical factors (light, water, temperature) in an area.

• Biosphere: All places on Earth where life exists.

Taxonomy and Classification

Organizing Life

Taxonomy is the science of classifying organisms into groups based on shared characteristics. The traditional system uses physical traits, but modern taxonomy relies heavily on genetic similarity.

• Taxon: A classification group; plural is taxa.

• Hierarchy of Taxa (from largest to smallest):

  1. Domain

  2. Kingdom

  3. Phylum

  4. Class

  5. Order

  6. Family

  7. Genus

  8. Species

Binomial Nomenclature: Scientific naming uses genus and species, e.g., Peromyscus polionotus (the oldfield mouse).

Domains and Kingdoms

There are three recognized domains, each containing distinct types of organisms:

Domain	Cell Type	Key Features
Bacteria	Prokaryotic	Small, lack nucleus, single compartment
Archaea	Prokaryotic	Unique DNA, cell wall chemistry, many extremophiles
Eukarya	Eukaryotic	Nucleus, organelles, unicellular and multicellular forms

Within Domain Eukarya, several kingdoms are recognized:

• Kingdom Plantae: Multicellular autotrophs that perform photosynthesis.

• Kingdom Animalia: Multicellular heterotrophs that consume other organisms.

• Kingdom Fungi: Unicellular (yeasts) and multicellular (molds, mushrooms) heterotrophs, often saprotrophs (decomposers).

• Kingdom Protista: Diverse group, includes unicellular and multicellular organisms; classification is under revision based on genetic data.

Note: Modern taxonomy is increasingly based on DNA similarity rather than physical traits.

The Scientific Method

Process of Scientific Inquiry

The scientific method is a systematic approach to investigating natural phenomena.

1. Observation: Gather information and previous knowledge.

2. Hypothesis: Formulate a rational, testable explanation.

3. Experimentation: Design experiments with at least two groups:

   • Experimental Group: Receives the variable being tested.

   • Control Group: Used as a baseline; known and understood result.

4. Variables:

   • Independent Variable: Manipulated by the experimenter.

   • Dependent Variable: Measured response.

   • Standardized Variables: Factors kept constant across all groups.

5. Analysis: Interpret results to accept or reject the hypothesis.

6. Peer Review and Publication: Findings are reviewed and published in scientific journals.

7. Consensus: Strongly supported ideas become scientific theories.

Example: Testing the effect of mouse coloration on predation rates. Independent variable: color of mouse model; dependent variable: frequency of predation.

Evolution and Natural Selection

Unity and Diversity of Life

Life on Earth is approximately 4.6 billion years old. Over time, genetic changes have led to the diversity of organisms observed today. Evolution is the process by which populations change genetically over generations.

• Natural Selection: Mechanism of evolution where organisms with advantageous traits survive and reproduce more successfully.

• Key Concepts (Darwin):

  • Variation exists within populations.

  • Traits are passed from parents to offspring.

  • More offspring are produced than can survive due to limited resources.

  • Survival and reproduction lead to propagation of advantageous traits.

Common Ancestry: All life shares a common genetic heritage, as evidenced by DNA.

Molecular Basis of Life: DNA and Gene Expression

Structure and Function of DNA

Deoxyribonucleic acid (DNA) is the molecule that stores genetic information in all cells. DNA is composed of four nitrogenous bases:

• Adenine (A)

• Thymine (T)

• Cytosine (C)

• Guanine (G)

A human cell contains about 6.4 billion bases. The sequence of these bases encodes instructions for making proteins.

Central Dogma of Molecular Biology

The flow of genetic information in cells follows this pathway:

• Transcription: DNA is copied into RNA.

• Translation: RNA is used to synthesize proteins.

Equation:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Proteins are the functional molecules of the cell, responsible for structure and function.

Genetic variation arises through sexual reproduction (mixing parental DNA) and mutation (changes in DNA sequence), driving evolution and adaptation.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Size	1-10 μm	10-100 μm
Organelles	Absent	Present
Domains	Bacteria, Archaea	Eukarya

Key Terms and Concepts

• Homeostasis: Maintenance of stable internal conditions.

• Autotroph: Organism that produces its own food (e.g., plants).

• Heterotroph: Organism that consumes other organisms for food (e.g., animals, fungi).

• Saprotroph: Organism that feeds on dead or decaying organic matter (e.g., fungi).

• Mutation: Change in DNA sequence.

• Adaptation: Trait that increases an organism's fitness in its environment.

Additional info: Modern taxonomy increasingly uses genetic data to classify organisms, and the Kingdom Protista is being reorganized into supergroups based on genetic similarity.