Hydrophobicity

As anyone who has looked at a petunia or a pansy can testify, plants generate flowers with characteristic numbers and patterns of organs. The floral organs are derived from a pool of stem cells called the floral meristem. In wild-type Arabidopsis thaliana flowers (shown at lower left) the meristem gives rise to four sepals that enclose the bud, four white petals, six pollen-bearing stamens and a central gynoecium comprised of two fused, ovule-containing carpels. The other flowers shown above have mutations in genes that control floral organ number. The wiggum mutant flower at the top has extra organs of all types, mainly sepals and petals. These mutants have wider floral meristems than the wild type, which allows formation of more organs in the outer regions of the flower. The clavata3 mutant flower (lower centre) has extra organs of all types, mainly stamens and carpels. These mutants have taller floral meristems than the wild type, allowing formation of more organs in the inner region of the flower. The perianthia mutant flower (lower right) has five each of sepals, petals and stamens. In these mutants the size of the floral meristem is not affected; instead, the distance between floral organ initiation events is altered. What is it? Hydrophobic means 'water-hating' (although no-one has yet defined an SI unit of 'hate'). An impressive example of the effect of hydrophobicity in nature is the stabilization of the interior of phospholipid membranes — the fluid-like phase that separates the 'inside' from the 'outside' of every animal cell. Other oft-quoted examples include binding of substrates to enzymes, conformational changes to biopolymers, and association of subunits to form enzymes. To date, it is not clear whether all these effects are due to hydrophobicity, or to some other weird property of water, such as hydrogen bonding. How do I know if it's hydrophobicity? Hard to say. It's certainly a candidate for the most over-used and least well-understood word in biology. According to some, if it behaves unusually and it's in water (… or 'remembers' water…), it must be the hydrophobic effect. So, how do I know if it's really hydrophobicity? The characteristic feature of hydrophobicity is the heat capacity of transfer of a solute molecule from the gas phase to water, which is large and positive over a wide range of temperature. This implies that although the transfer is opposed by entropy at room temperature, it is opposed …