What Defines a Living Organism? – Key Characteristics in Biology

Introduction

This lecture introduces the foundational question of biology: What is life? Rather than a philosophical query, the focus is on the scientific criteria that determine whether something is alive. The instructor emphasizes the importance of note‑taking, pausing, and reviewing the material to master the concepts.

Unifying Themes in Biology

Throughout the course, several recurring ideas will appear across seemingly unrelated topics: - All organisms share certain characteristics. - Structure and function are tightly linked. - Homeostasis is essential for survival. - Evolution explains both unity and diversity of life. These themes provide a framework for understanding every unit, from cells to ecosystems.

Defining an Organism

An organism is any individual living entity, also called a biotic factor. Organisms range from single‑celled bacteria, amoebas, algae, and fungi to complex multicellular plants and animals.

The Seven Characteristics of Life

The lecture breaks down the criteria that must be met for something to be considered alive.

1. Cellular Organization
2. All living things are composed of one or more cells.
3. Unicellular organisms (e.g., bacteria, amoebas) perform all life functions within a single cell.

4. Multicellular organisms (e.g., plants, animals) have many specialized cells. Every cell contains the same DNA, but different genes are expressed to create diverse cell types.

5. Organization (Hierarchy of Structure)

6. Life is organized from atoms → molecules → organelles → cells → tissues → organs → organ systems → whole organism.

7. Each level has a specific function, illustrating the principle that structure determines function.

8. Energy and Metabolism

9. Organisms require energy to drive metabolism— the sum of all chemical reactions that build up or break down substances.
10. Energy ultimately originates from the Sun.
11. Autotrophs (photoautotrophs like plants, chemoautotrophs in deep‑sea vents) produce their own food.

12. Heterotrophs (herbivores, carnivores, omnivores, decomposers) obtain energy by consuming other organisms.

13. Response to Stimuli

14. Living things must detect and react to environmental changes (light, temperature, touch, etc.).

15. Example: A plant growing toward a light source demonstrates stimulus (light) and response (directional growth).

16. Reproduction

17. Ensures the continuation of a species.
18. Asexual reproduction: one parent, offspring genetically identical.
19. Sexual reproduction: two parents, offspring genetically diverse.

20. Both strategies have advantages and disadvantages, often illustrated with Venn diagrams.

21. Growth and Development

22. Growth: increase in size or mass.
23. Development: change in abilities or complexity (e.g., a frog’s metamorphosis).

24. Both processes are directed by genetic information (DNA/RNA).

25. Homeostasis

26. Maintenance of a stable internal environment (temperature, pH, blood sugar, etc.).
27. Achieved through feedback mechanisms and behavioral responses (e.g., pulling a hand away from a hot pan).

28. Failure to maintain homeostasis can be life‑threatening.

29. Evolution (Population Level)

30. Evolution occurs in populations, not individual organisms.
31. Adaptations are inherited traits that improve survival and reproduction.
32. Genetic variation within a population enables natural selection to act, fostering resilience to environmental changes.

Recap

The lecture concludes by reminding students to review the material, answer all embedded questions, and ends with a light‑hearted joke about bacteria crossing a microscope slide.

Use this guide to fill out your notes organizer, pause the video as needed, and ensure you can explain each characteristic of life without referring back to the video.

Life is defined by a set of observable, scientific criteria—cellular makeup, organized structure, energy use, responsiveness, reproduction, growth/development, homeostasis, and evolutionary change. Mastering these characteristics provides a clear, testable answer to the question “What is a living organism?”