SL Content Statements

• B2.3.1

  Production of unspecialized cells following fertilization and their development into specialized cells by differentiation

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  Students should understand the impact of gradients on gene expression within an early-stage embryo.
  

• B2.3.2

  Properties of stem cells

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  Limit to the capacity of cells to divide endlessly and differentiate along different pathways.
  

• B2.3.3

  Location and function of stem cell niches in adult humans

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  Limit to two example locations and the understanding that the stem cell niche can maintain the cells or promote their proliferation and differentiation. Bone marrow and hair follicles are suitable examples.
  

• B2.3.4

  Differences between totipotent, pluripotent and multipotent stem cells

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  Students should appreciate that cells in early-stage animal embryos are totipotent but soon become pluripotent, whereas stem cells in adult tissue such as bone marrow are multipotent.
  

• B2.3.5

  Cell size as an aspect of specialization

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  Consider the range of cell size in humans including male and female gametes, red and white blood cells, neurons and striated muscle fibres.
  

• B2.3.6

  Surface area-to-volume ratios and constraints on cell size

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  Students should understand the mathematical ratio between volume and surface area and that exchange of materials across a cell surface depends on its area whereas the need for exchange depends on cell volume.
  NOS: Students should recognize that models are simplified versions of complex systems. In this case, surface-area-to-volume relationship can be modelled using cubes of different side lengths. Although the cubes have a simpler shape than real organisms, scale factors operate in the same way.
  

AHL Content Statements

• B2.3.7

  Adaptations to increase surface area-to-volume ratios of cells
  

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  Include flattening of cells, microvilli and invagination. Use erythrocytes and proximal convoluted tubule cells in the nephron as examples.
  

• B2.3.8

  Adaptations of type I and type II pneumocytes in alveoli

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  Limit to extreme thinness to reduce distances for diffusion in type I pneumocytes and the presence of many secretory vesicles (lamellar bodies) in the cytoplasm that discharge surfactant to the alveolar lumen in type II pneumocytes. Alveolar epithelium is an example of a tissue where more than one cell type is present, because different adaptations are required for the overall function of the tissue.
  

• B2.3.9

  Adaptations of cardiac muscle cells and striated muscle fibres

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  Include the presence of contractile myofibrils in both muscle types and hypotheses for these differences: branching (branched or unbranched), and length and numbers of nuclei. Also include a discussion of whether a striated muscle fibre is a cell.
  

• B2.3.10

  Adaptations of sperm and egg cells

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  Limit to gametes in humans.