According to the cell theory, living organisms are composed of cells. Organisms consisting of only one cell carry out all functions of life in that cell. Surface area to volume ratio is important in the limitation of cell size. Multicellular organisms have properties that emerge from the interaction of their cellular components. Specialized tissues can develop by cell differentiation in multicellular organisms. Differentiation involves the expression of some genes and not others in a cell’s genome. The capacity of stem cells to divide and differentiate along different pathways is necessary in embryonic development and also makes stem cells suitable for therapeutic uses. Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae. Application: Investigation of functions of life in Paramecium and one named photosynthetic unicellular organism. Application: Use of stem cells to treat Stargardt’s disease and one other named condition. Application: Ethics of the therapeutic use of stem cells from specially created embryos, from the umbilical cord blood of a new-born baby and from an adult’s own tissues. Skill: Use of a light microscope to investigate the structure of cells and tissues, with drawing of cells. Calculation of the magnification of drawings and the actual size of structures and ultrastructures shown in drawings or micrographs. (Practical 1) Guidance: Students are expected to be able to name and briefly explain these functions of life: nutrition, metabolism, growth, response, excretion, homeostasis and reproduction. Chlorella or Scenedesmus are suitable photosynthetic unicells, but Euglena should be avoided as it can feed heterotrophically. Scale bars are useful as a way of indicating actual sizes in drawings and micrographs.
Essential idea: The evolution of multicellular organisms allowed cell specialization and cell replacement.
Things to study Cell theory and atypical examples Specialization, differentiation Stem cells