5.2 Unicellular and Multicellular Organization

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.

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.