5.2 Unicellular and Multicellular Organization
Connect the functions of life to single cells, differentiation, tissue cooperation and constraints on cell size.
Estimated time: 49 minutes
IB syllabus: A2.2 · SL and HL
One Cell Must Coordinate Every Life Function
A unicellular organism must obtain nutrients, transform matter and energy, remove waste, maintain internal conditions, sense environmental change, grow and reproduce within one plasma membrane. These functions are not isolated checklist items. Nutrient uptake supports metabolism; metabolism changes solute concentrations; homeostatic transport restores conditions; sensory responses move the organism toward resources or away from harm.
Compartmentalization can occur even within a single eukaryotic cell. A contractile vacuole in a freshwater protist expels water entering by osmosis, food vacuoles isolate digestion, and cilia or flagella generate movement. A bacterium lacks membrane-bound organelles but still localizes reactions using its plasma membrane, protein complexes and cytoskeleton.
Differentiation Creates Division of Labour
Multicellular organisms contain cell types specialized for different tasks. Differentiation occurs when cells with essentially the same genome develop different structures and functions through different patterns of gene expression. A pancreatic secretory cell transcribes genes for digestive enzymes and builds extensive rough ER and Golgi; a neuron expresses proteins for electrical signalling and develops long processes.
Cells with related structure and function form tissues. Tissues interact to produce organism-level properties that no isolated member supplies. Nervous tissue detects and transmits information, muscle tissue generates force, epithelial tissue forms controlled boundaries and connective tissue supports other components. Specialization increases efficiency but also creates interdependence.
Signals from position, neighbouring cells, hormones and external conditions influence gene regulation during development. Differentiation generally changes which genes are active rather than deleting unused genes. This explains why a nucleus from a differentiated cell can, under exceptional experimental conditions, direct development when placed into a suitable egg-cell environment.
Multicellularity Changes Size Constraints
As a cell grows, volume increases faster than surface area. Exchange demand roughly follows volume, whereas membrane area available for exchange follows surface area. A sufficiently large cell cannot rely on its outer surface and diffusion alone to meet every internal demand. Multicellularity permits large bodies made from many small cells, preserving local exchange surfaces while transport systems connect distant regions.
Large organisms can also survive the loss of individual cells, maintain protected internal environments and allocate resources among specialized tissues. These advantages have costs: cells must adhere, communicate, coordinate growth and suppress selfish lineages. Cancer illustrates failure of multicellular control when cells divide and survive in ways that benefit their lineage at the organism's expense.
Multicellularity evolved independently in animals, plants, fungi and several algal lineages. The repeated transition suggests that cooperation can be favoured under many ecological conditions, but the resulting body plans and cell junctions need not have one identical origin.
Specialization atlas
Compare cell models and increase specialization to see why organelle abundance, shape and function vary despite a shared genome.
Boundary · compartment · evidence
Cell origins and structure laboratory
Test Yourself
A liver cell and a motor neuron from one person contain nearly the same DNA sequence but synthesize very different protein sets. Which mechanism explains the difference most directly?