Core: Organisms
IB Biology HLΒ· Theme A: Unity and DiversityΒ· 20 min read
1. Cell Theory and the Definition of Organismsβ β ββββ± 7 min
Organism
A contiguous living system that is capable of independent metabolism, growth, reproduction and response to stimuli, aligned with cell theory principles.
Example:
A single E. coli bacterium and a giant redwood tree are both organisms; an isolated human cell in culture is not considered a full organism in this context.
All recognised living organisms follow the core tenets of cell theory, which unites all life on Earth by its basic structural unit.
Cell Theory
The fundamental biological theory with three core claims: (1) All living organisms are composed of one or more cells, (2) The cell is the basic unit of structure and organisation in organisms, (3) All cells arise from pre-existing living cells.
A researcher discovers a new self-replicating particle made of protein and RNA that lacks a cell membrane. Is it classified as an organism under cell theory?
- 1
Step 1: Recall the core requirement for classification as an organism under cell theory.
- 2
Cell theory requires all organisms to be composed of one or more cells, which are defined by a cell membrane boundary.
- 3
Step 2: Compare the particle to the requirement.
- 4
The particle lacks a cell structure and cell membrane, so it does not meet the core requirement.
- 5
Conclusion: The particle is not classified as an organism.
Exam tip:
Always link answers about organism classification back to cell theory tenets to earn full marks.
2. Unicellular vs Multicellular Body Plansβ β ββββ± 6 min
Organisms are broadly classified by body plan into two groups: unicellular, where the entire organism is one single cell, and multicellular, where the organism is made of many specialised, interdependent cells.
Unicellular Organism
An organism that consists of a single cell that carries out all essential life functions required for survival and reproduction.
Example:
Escherichia coli (prokaryote), Chlamydomonas (eukaryotic alga), yeast (eukaryotic fungus)
Multicellular Organism
An organism that consists of multiple differentiated cells that work together, with different cells specialised for different life functions.
Example:
Humans, oak trees, button mushrooms
Contrast how the function of nutrition is carried out in unicellular yeast vs multicellular humans.
- 1
Step 1: Describe nutrition in unicellular yeast.
- 2
The single yeast cell performs all steps of nutrition independently: it takes in nutrients across its own cell membrane, breaks them down via metabolism, and uses the products for growth and reproduction.
- 3
Step 2: Describe nutrition in multicellular humans.
- 4
Nutrition is split between many specialised cells and tissues: intestinal cells absorb nutrients from food, liver cells process and store nutrients, muscle cells use glucose for energy, and fat cells store excess energy. No single cell carries out all steps of nutrition.
Test your classification understanding
Which of the following is a unicellular eukaryote?
Escherichia coli bacteria
A human red blood cell
Saccharomyces cerevisiae (brewer's yeast)
A maple leaf cell
Reveal answer
2 βCorrect! Yeast are single-celled eukaryotic fungi. E. coli is prokaryotic, and human red blood cells/maple leaf cells are only parts of larger multicellular organisms, not full organisms.
3. Emergent Properties in Multicellular Organismsβ β β βββ± 7 min
A defining feature of multicellular life is the development of emergent properties that cannot exist in individual isolated cells, even when that same cell type is present.
Emergent Property
A new property that arises from the interaction of individual components of a system. The property is not present in any individual component alone, hence the phrase "the whole is greater than the sum of its parts".
Emergent properties develop as cells differentiate and organise into tissues, organs, and organ systems. This allows for far greater complexity and functional specialisation than is possible in unicellular life.
Explain why long-distance water transport in a large oak tree is an emergent property.
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Step 1: Confirm that the property does not exist in individual cells.
- 2
A single oak tree cell cannot transport water from roots 2 meters underground to leaves 30 meters above ground on its own.
- 3
Step 2: Describe how the property arises from cell interaction and organisation.
- 4
Many dead, hollow xylem cells arrange into long, interconnected tubes to form xylem tissue. The interaction of this tube structure, cohesion of water molecules, and transpiration pull from leaves creates a continuous flow of water from roots to leaves.
- 5
Conclusion: Long-distance water transport is only possible when multiple cells are organised into the correct structure, so it is an emergent property.
Exam tip:
Always mention that emergent properties arise from interaction of components, not just the presence of multiple cells, for full marks.
4. Common Pitfalls
Wrong move:
Claiming all unicellular organisms are prokaryotes.
Why:
Many eukaryotes are entirely unicellular, including yeast, algae and protozoa.
Correct move:
State that unicellular organisms can be either prokaryotic or eukaryotic, while all multicellular organisms are eukaryotic.
Wrong move:
Classifying viruses as living organisms.
Why:
IB Biology defines organisms as cellular life, so non-cellular viruses are not classified as organisms.
Correct move:
State that viruses are non-cellular and not classified as living organisms per IB syllabus definitions.
Wrong move:
Describing emergent properties as just "new functions in big organisms".
Why:
This misses the core point that emergent properties arise from interaction between components, not just larger size.
Correct move:
Define emergent properties as properties that arise from the interaction of individual components, and do not exist in any single component alone.
Wrong move:
Claiming multicellular organisms are just a collection of independent cells.
Why:
This ignores the specialisation and interdependence of cells in multicellular life.
Correct move:
Describe cells as specialised, interdependent components that work together to produce functions no single cell can perform alone.
5. Quick Reference Cheatsheet
Feature | Unicellular Organism | Multicellular Organism |
|---|---|---|
Total number of cells | 1 | Many (thousands to trillions) |
Cell specialisation | None (one cell does all functions) | Extensive differentiated cells for specific roles |
Cell type allowed | Prokaryotic or Eukaryotic | Eukaryotic only |
Emergent properties | No complex emergent properties | Many emergent properties from cell interaction |
Example | Chlamydomonas, E. coli | Human, Oak tree, Mushroom |
6. Frequently Asked
Are viruses classified as organisms by the IB syllabus?
No. The IB syllabus follows the consensus that organisms are cellular systems, and viruses are non-cellular, so they are classified as non-living/non-organisms, as they cannot carry out independent metabolism or reproduction.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 Β· 1
Compare unicellular vs multicellular life
- 2021 Β· 2
Explain emergent properties in plants
Going deeper
What's Next
Understanding the core definition and classification of organisms is the foundation for all further study in IB Biology. From here, you will explore how evolutionary processes generate the diversity of life we see today, how cells differentiate to build complex multicellular bodies, and how different groups of organisms interact with each other and their environments. This core concept links across all IB Biology themes: from the molecular processes that keep cells alive in Theme B, to ecosystem interactions in Theme C, and the evolution and adaptation of organisms in Theme D. Mastery of this topic will make all subsequent topics easier to connect to the core principles of life.