Organisms

1. Seven Characteristics of All Organisms

To be considered living, an organism must meet basic criteria:

  • Cells: The basic unit of life. Organisms consist of one or more cells.

    • Prokaryotic: Simple cells lacking a membrane-bound nucleus or organelles.

    • Eukaryotic: Complex cells containing a membrane-bound nucleus and specialized organelles.

    • Unicellular vs. Multicellular: Single cell (e.g., bacteria) or multiple eukaryotic cells (e.g., humans).

  • Complexity in Organization: Living things possess an intricate structure that cannot simply be dismantled and reassembled like non-living objects.

  • Growth and Development:

    • Growth: Increase in overall size via cell division, cell enlargement, or noncellular material accumulation.

    • Development: Lifelong structural and physiological changes, including differentiation (unspecialized cells becoming specialized).

  • Energy and Metabolism: Taking in and consuming molecules (food) to generate energy required for growth, repair, and cellular function.

    • Metabolism: The sum total of all chemical processes occurring within a living organism.

    • Anabolism: Building complex biological molecules from simpler ones (requires energy).

    • Catabolism: Breaking down complex molecules into simpler ones to release energy.

  • Stable Internal Environment (Homeostasis): The ability to maintain stable internal conditions despite external changes (e.g., sweating or shivering to maintain body temperature).

    • Feedback Mechanisms: Systems (negative and positive feedback) used to maintain physiological equilibrium.

  • Responsiveness (Irritability): Adjusting to internal or external environmental changes via a stimulus and a corresponding response (e.g., pupil constriction in bright light).

  • Reproduction and Heredity: Passing genetic material (DNA) from parent(s) to offspring to continue the species.

    • Asexual Reproduction: Production of genetically identical offspring from a single parent.

    • Sexual Reproduction: Combination of genetic material from two parents to produce genetically diverse offspring.

  • Adaptation and Evolution: Trait changes over generations that enhance survival and reproduction in specific environments.

2. Levels of Structural Organization

Living organisms are structured from the smallest chemical components to complex organisms:

  1. Chemical Level

    • Atoms: Basic unit of matter and elements.

    • Molecules: Two or more atoms bonded together (e.g., water, proteins, sugars).

    • Macromolecules: Large complex molecules essential for life (e.g., nucleic acids, carbohydrates, lipids, proteins).

  2. Cellular Level

    • The smallest independently functioning unit of life; composed of organelles and structures.

    • Organelles: Specialized sub-cellular structures that perform specific tasks (e.g., mitochondria, nucleus).

  3. Tissue Level

    • A group of similar specialized cells working together to perform a common function.

    • Four main tissue types: Epithelial, Connective, Muscle, and Nervous.

  4. Organ Level

    • Distinct structure composed of two or more tissue types performing specific functions (e.g., skin, heart, stomach).

  5. Organ System Level

    • A group of organs working together for major physiological functions (e.g., skeletal system; humans have 11 organ systems).

  6. Organism Level

    • An individual living entity made up of integrated organ systems capable of performing all processes of life.