Biology Reference
In-Depth Information
FIGURE 3.1
A Cassini spacecraft image of vaporous, icy jets emerging from fissures on Enceladus.
B. STRUCTURE OF WATER
The chemical composition of water, (2 hydrogens, 1 oxygen), was discovered by Henry
Cavendish around 1781. Water is a polar molecule having both positive and negative poles,
and exists in a tetrahedral arrangement, extending out in all dimensions (
Figure 3.2
). The
partially positively charged hydrogens (each H is
0.41) are at an angle of 104.5 degrees
from each other with respect to the partially negatively charged oxygen (
þ
0.82). The charge
distribution produces a dipole moment of 1.85 D (see
Figure 3.2
). This polar structure is
responsible for water's exceptionally strong self-attraction (cohesion) and numerous unusual
properties that are responsible for its central role in life. Water is both small in size (MW 18)
and highly mobile. The concentration of water in water (1,000 gl
1
/18 gmol
1
) is 55.5 M,
making water both life's solvent, as well as being an essential reagent in life processes.
An important feature of water is its ability to readily form hydrogen bonds (H-bonds) with
itself and other polar entities. H-bonding in water is the major driving force for membrane
stability as first suggested by Latimer and Rodebush in 1920. As discussed in Chapter 2,
the first quarter of the 20
th
century was a critical time period that produced many seminal
membrane studies. Included in these was a first understanding of the importance of water
structure.
- 0.82
O
104.5º
H
H
+ 0.41
FIGURE 3.2
The structure of water.
