Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 543

The light phase of photosynthesis consists of two photosystems. Photosystem I (PS I) can
be excited by light of wavelength shorter than 700 nm and generates NADPH. Photosystem II
(PS II) requires light of wavelength shorter than 680 nm and splits H 2 O into 1
2

2H þ . O 2 þ

ATPs are formed as electrons flow from PS II to PS I.

10.12. SUMMARY

In this chapter, you have learned some of the elementary concepts of how cells control their
composition in response to an ever-changing environment. The essence of an organismresides
in the chromosome as a linear sequence of nucleotides that form a language (genetic code)to
describe the production of cellular components. The cell controls the storage and transmission
of such information, using macromolecular templates: DNA or RNA. DNA is responsible for
its own replication and is also a template for transcription of information into RNA species that
serve both as machinery and template to translate genetic information into proteins. Proteins
often must undergo posttranslational processing to perform their intended functions.
The cell controls both the amount and activity of proteins it produces. Many proteins are
made on regulated genes (e.g. repressible or inducible), although other genes are constitutive.
With regulated genes, small effector molecules alter the binding of regulatory proteins to
specific sequences of nucleotides in the operator or promoter regions. Such regulatory proteins
can block transcription or in other cases enhance it. A group of contiguous genes under the
control of a single promoter-operator is called an operon. More global control through regulons
is also evident. Some gene products are not regulated, and their synthesis is constitutive.
Once a protein is formed, its activity may be continuously modulated through feedback
inhibition. A number of alternative strategies are employed by the cell to control the flux of
material through a pathway. Another form of regulation occurs through the interaction of
extracellular compounds with cell surface protein receptors.
Cell senses the availability of substrate(s) due to the transport of substrate(s) from the
medium (abiotic phase) into cytoplasm (or biotic phase) as depicted by Fig. 10.16 . The trans-
port can be passive diffusion through cell membrane,

J S ¼ k p ðC SE C SI Þ (10.27)

or by protein-facilitated transport,

C SE
K SE þ C SE

C SI
K SI þ C SI

J S ¼ J S max (10.33)

where C SI is the concentration of substrate inside the cell (biotic phase) and C SE is the concen-
tration of substrate in the medium (abiotic phase). Passive transport (diffusion) requires
a positive concentration gradient, i.e. C SE > C SI . However, active transport (protein-facilitated

transport) can appear to be against the concentration gradient, due to the difference in
binding coefficients, or the saturation constants, K SE and K SI .
Cellular metabolism starts with the transport of substrates into the cell and is concerned
with two primary functions: catabolism and anabolism. A summary of the metabolism is
shown in Table 10.8 . Catabolism involves the degradation of a substrate to more highly