Table 1. Computation of atom number densities for ribosomes and calcium phosphate.

	Element	# of atoms	Mole fraction	# atoms/nm3
ribosomal RNA*	C	68671	0.295	31.2
	H	78158	0.336	35.5
	N	27884	0.120	12.7
	O	50462	0.217	22.9
	P	7216	0.031	3.3
	Mg	239	0.001	0.24
Ribosomes†	C	135061	0.186	19.3
	H‡	372416	0.514	53.2
	N	48041	0.0663	6.86
	O‡	161037	0.222	23.0
	S	501	0.000692	0.072
	P	7216	0.00996	1.03
	Mg	239	0.000330	0.034
Ca3(PO4)2	Ca	3	0.231	18.4
as crystalline TCP§	P	2	0.154	12.2
	O	8	0.615	49.1

* The partial specific volume of RNA was taken to be 0.569 cm³/g (Voss and Gerstein, 2005).

^† A volume of 7000 nm³ was estimated to enclose the ribosome (Protein Data Bank ID 4UG0) based on a ~ 2 nm-resolution isosurface calculated using Chimera (Pettersen et al., 2004). Solvent within this isosurface (42%) was treated as bulk vitreous ice for atom number density summations.

^‡ Includes solvent atoms.

^§ A density of 3.14 g/cm³ was taken for crystalline TCP.