15.2 Water

219

Table 15.4 Morphology and other properties of a typical eukaryotic cellSuperscript normal a^a

Property

Shape

Sphere

Density

1.025 g/cmcubed³

Radius

5muµm

Volume

5 times 10 Superscript negative 165 × 10⁻¹⁶ mcubed³

Surface charge

negative 10−10 fC/muµmsquared²

Coat material

polysaccharide

Coat thickness

10 nm

Coat charge density

negative 5−5MC/mcubed³

Superscript normal a^aA typical prokaryote, such as the organism specified in Table 15.2, would have a diameter about

10 times smaller

The interior of the cell is an exceedingly crowded milieu (compare the quantities of

molecules with the dimensions given in Table 15.4). Although water constitutes about

70% of a typical cell, very little of this water is free, bulk material. The very high con-

centrations of molecules and macromolecules ensure that the cytoplasm is a highly

viscous medium. Moreover, most of the macromolecules (e.g., proteins) are attached

to larger structures such as the internal membranes. Kempner and Miller’s classic

experiments, in which they centrifuged intact cells to separate macromolecules from

the water, demonstrated this very clearly—hardly any macromolecules were found

in the aqueous fraction. This was in sharp contrast to the result of the traditional bio-

chemical procedure of destroying all ultrastructure by mechanical homogenization,

yielding an aqueous cytosol containing many dissolved enzymes (cf. Sect. 14.2).

The effect of the ultrastructure is twofold: to divide the cell up into compartments,

not hermetically separated from one another but allowing access to different zones

to be controlled, and to provide two-dimensional surfaces on which searching for

and finding reaction partners is far more efficient than in an unstructured bulk.¹

The separation of the macromolecules, which of course plays a crucial part in

experimental bioinformatics, is dealt with in Part III.

15.2

Water

As seen from Table 15.2, water is overwhelmingly dominant in the cell. Water (HSubscript 22O)

is a very unusual substance, as can be inferred from its extraordinarily high boiling

point (compared with other molecules of comparable size) and large specific heat. A

salient feature of the molecule is its great polarity—the bond between the oxygen and

the hydrogen has a very strong ionic character. The electrostatic attraction between

the positively charged hydrogen (delta plusδ+) and the negatively charged electron lone pair

on the oxygen (delta minusδ−) constitutes the hydrogen bond (Fig. 15.1). It can be thought of as

1 See Ramsden and Grätzel (1986).