Explain emergent properties, differentiation, apoptosis, stem-cell potency and the opportunities and constraints of multicellularity.
Estimated time: 95 minutes
IB syllabus: B2.3 · C3.1 · SL and HL
Multicellularity means more than many cells occupying the same place. Cells must remain attached, communicate and divide their labor. Membrane proteins form junctions between neighboring animal cells and connect to the cytoskeleton. Tight junctions restrict leakage between epithelial cells, desmosomes resist mechanical stress, and communicating junctions allow selected substances or electrical signals to pass. The resulting tissue has properties that an isolated cell does not possess.
An emergent property arises from interactions among components. Cardiac muscle cells can contract individually, but a chamber that contracts in a coordinated sequence can create pressure and pump blood. Epithelial cells can each transport ions, but an aligned sheet with apical and basal surfaces can move solutes directionally across an organ. Emergence does not imply that lower levels are irrelevant; it means that component identity alone is insufficient without organization and interaction.
Organization is hierarchical. Similar specialized cells and their extracellular material form tissues; multiple tissue types arranged for a common purpose form an organ; and cooperating organs form an organ system. The stomach, for example, contains epithelial, connective, muscle and nervous tissues. Its digestive function depends on secretion, mechanical mixing, blood supply and neural or hormonal regulation acting together.
Most cells of an organism inherit essentially the same genome, yet a pancreatic beta cell synthesizes insulin while a skeletal muscle fiber accumulates contractile proteins. The difference lies mainly in gene expression. Transcription factors, chromatin state, chemical signals and a cell's position activate some genes and repress others. The resulting protein set changes cell shape, metabolism, receptors and behavior, stabilizing a specialized identity.
Development is sculpted by both cell production and controlled cell removal. During apoptosis, an internal program dismantles a cell, packages its contents and permits removal with little inflammatory damage. Apoptosis separates developing digits, removes transient structures and balances cell number. It differs from uncontrolled tissue injury: death occurs at a regulated time and contributes constructively to form.
In early animal development, cell position and signals guide cells into ectoderm, mesoderm and endoderm. Ectoderm contributes epidermis and nervous tissue; mesoderm contributes muscle, skeleton, blood and much of the circulatory and urogenital systems; endoderm contributes epithelial linings and organs associated with the digestive and respiratory systems. These are developmental origins, not simple layers retained unchanged in an adult.
A stem cell can self-renew and can generate differentiated descendants. Totipotent cells can produce every embryonic and extra-embryonic cell type needed for a complete organism; the zygote and its earliest descendants have this capacity. Pluripotent cells can form cell types from all three embryonic germ layers but cannot independently form all supporting extra-embryonic tissues. Multipotent adult stem cells generate a narrower family of cells associated with a tissue.
Adult stem cells remain in specialized microenvironments called niches. Hematopoietic stem cells in bone marrow replenish red blood cells, platelets and immune-cell lineages. Stem cells near hair follicles contribute to recurring hair growth and skin repair. Signals from surrounding cells and extracellular material balance dormancy, self-renewal and differentiation. Removing a cell from its niche can therefore change its behavior.
Stem-cell therapy must solve more than the problem of making the desired cell. Cells must survive, integrate into the correct tissue, respond appropriately and avoid immune rejection or uncontrolled division. Cord-blood stem cells can rebuild blood-forming tissue after destructive treatment, but a limited cell number and tissue matching constrain use. Embryonic sources raise ethical questions because obtaining the cells may destroy an embryo; induced pluripotent cells reduce some ethical and matching problems but retain technical risks.
Division of labor permits efficient digestion, movement, sensing and defense, and a large body can maintain a stable internal environment. Size can reduce predation risk and enable access to new resources. The costs are equally important: surface-area-to-volume ratio falls, diffusion distances increase, cells become mutually dependent, reproduction and development require coordination, and failures of growth control can produce cancer. Transport and communication systems are therefore consequences of multicellular scale, not optional additions.
Adjust expression specificity and adhesion, then test how apoptosis refines a developing tissue into an integrated organ.
Structure · gradient · exchange · feedback
A researcher isolates cells from three embryonic layers. A cell forms neurons, cardiac muscle and intestinal epithelium in culture but cannot generate placental tissue. What is the strongest classification?
Practice focused questions or see how IB combines this topic with ideas from elsewhere in the course.
Matching part: 7(d)
Matching part: 1