Biochemical Reactions - A First Look
55
effluent concentrations. If substantial amounts of weak alkali or acid (1-2 M concentration) are
added to keep pH constant this extra feed of water may of course have to be considered. The yield
of water by the reaction is in itself of no importance and we shall in general not consider the term
TJWH20 .
The biomass formula (3.17) indicates that a long list of elements appear in the biomass. C, H, O
and N are, however the dominant elements, while the content of S, P, and of all the trace elements
such as Ca, Mg, Na, Fe, Co etc. is small compared to the N-content. These elements must, however,
be present in the feed as e.g. Na2S 0 4 and K2H P 04 to synthesize an active biomass. The composition
of
X
is determined by elemental analysis: A sample of dried biomass is ignited and the combustion
products are analyzed. The residue is termed “ash”, and it consists of inorganic compounds, mostly
oxides. The ash content of biomass is usually in the range 4-8 wt %. It follows from the analytical
procedure that the oxygen content of the sample cannot be measured, but must be calculated. In
most examples of this book calculations are made on the basis of ash-free biomass, but in an
experimental study the ash content of the biomass must of course be accounted for. Otherwise a
significant error is introduced, and the calculation of rates is quite sensitive to the composition of
the biomass. The composition of ash free biomass varies somewhat between different organisms,
but the composition of a given organism varies much more with the growth conditions than
between different organisms.
The yeast
S. cerevisiae
growing at glucose-limited conditions with an ample supply of nitrogen
source has a macromolecular composition shown in Table 3.1. Based on the weight fractions of the
7 main groups of biomass components and their respective formula weight the average composition
and formula weight of the biomass can be calculated
X=CH
1
5960o.396N o.216S 0.OO24P 0.017
(3.19)
which corresponds to a formula weight of A7,=23.57 g (C-mole biomass)*1. Table 3.2 shows the
elemental composition of many organisms studied at different conditions and by different
authors.
Table 3.1
A verage com position o f
S. cerevisiae.
Macromolecule
Elemental comnosition
Percent bv weight
e (C-moleV
1
Protein
CH 1.58^0
3
]Nq
27
S
0
004
57
22.45
RNA
CH |
25
O
0
.
25
N
0
3»P
0
11
16
34.0
DNA
CH i.
15
O
0
.
62
N
0
.
39
P
0.10
3
31.6
Carbohydrates
C H
1 67
O
0 8 3
1 0
27.0
Phospholipids
CH t .
9 1 0 0
.
23
N
0
.
02
P
0.02
10.8
18.5
Neutral fat
C H I84Ooi2
2.5
15.8
Pool o f cellular m etabolites
__CHigOngNn ?Snm____________
0.7
29.7
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