B2.3 Cell specializationIB Biology SL: Revision notes

After fertilization, the zygote divides by mitosis to produce many unspecialized cells. These develop into specialized cells by differentiation: particular genes are switched on and others off, so each cell type makes different proteins. All cells keep the same genome.

In an early embryo, gradients of signalling molecules (morphogens) determine which genes each cell expresses. For example, Bicoid protein is concentrated at the anterior end of a fruit fly embryo, and cells with high concentrations develop into head structures.

A stem cell can divide endlessly and can differentiate along different pathways into more than one cell type.

• Totipotent: can form any cell, including extra-embryonic cells such as the placenta, so a complete organism. Cells of the very early embryo.
• Pluripotent: can form any body cell, but not a whole organism. Cells of the embryo soon after (e.g. inner cell mass).
• Multipotent: can form a limited range of related cells, e.g. adult bone marrow stem cells form blood cells.

A stem cell niche is the location and microenvironment in which stem cells are found. The niche can maintain the stem cells in an undifferentiated state or promote their proliferation and differentiation when new cells are needed.

• Bone marrow: stem cells produce red blood cells, platelets and white blood cells.
• Hair follicles: stem cells in the follicle produce cells for hair growth and help repair skin after wounding.

Human cells vary greatly in size according to function:

• Egg (about 110 μm) is large, storing nutrients and cytoplasm for the embryo; sperm is small, with little mass to propel.
• Red blood cells (about 7.5 μm) are small enough to pass through capillaries; white blood cells are larger (about 12 μm for a neutrophil).
• Neurons can have axons up to 1 m long; striated muscle fibres can be several centimetres long.

Exchange of materials across a cell surface depends on its surface area, but the need for exchange depends on the cell's volume. For a cube of side $l$: surface area $6l^2$, volume $l^3$, so SA:V $= 6/l$. When $l$ doubles, area ×4 but volume ×8, so the ratio halves.

Larger cells therefore have a lower SA:V and cannot exchange materials fast enough, which limits cell size. Cells divide before they get too large.

Models are simplified versions of complex systems. Agar cubes of different side lengths show how SA:V falls as size increases: indicator colour change shows how far a substance diffuses in a set time, and the proportion of each cube reached falls as the cube gets bigger.

Cells are not cubes, but scale factors operate in the same way, so the model is valid for the principle. Limitations: agar is not metabolically active and cannot transport actively.