Biomedical Engineering Reference
In-Depth Information
of these reactions is presented in Fig. 10.16 . The major categories are (1) fueling reactions:
degradation of nutrients, (II) biosynthesis of small molecules (amino acids, nucleotides),
(III) polymerization reactions: biosynthesis of large molecules, and (IV) assembling reac-
tions. These reactions take place in the cell simultaneously. As a result of metabolic reac-
tions, end products are formed and released from the cells. These end products (organic
acids, amino acids, antibiotics) are often valuable products for human and animal
consumption.
Energy in biological systems is primarily stored and transferred via ATP, which contains
high-energy phosphate bonds. The active form of ATP is complexed with Mg 2 þ . The stan-
dard free-energy charge for the hydrolysis of ATP is 30.5 kJ/mol. The actual free-energy
release in the cell may be substantially higher because the concentration of ATP is often
much greater than that for ADP.
4 ; DG 0 ¼30:5
ATP
þ
H
O
ADP
þ
H
PO
kJ
=
mol
(10.35)
%
2
3
Figure 10.17 shows the chemical structural changes of Eqn (10.35) . Biological energy is stored
in ATP by reversing this reaction to form ATP from ADP and H 3 PO 4 . Similarly, ADP disso-
ciates to release energy.
4 ; DG 0 ¼30:5
þ
þ
=
(10.36)
ADP
H
O
%
AMP
H
PO
kJ
mol
2
3
Class III
Or
Polymerization
Reactions
Class II
Or
Biosynthesis
Reactions
Class I
Or
Fueling
Reactions
Biomass
PO 4 3-
DNA
2-4%
RNA
10-20%
Proteins
50-60%
Peptidoglycan
Nucleotides
Chromosome
Ribosomes
Soluble
Amino acids
PO 4 3-
ATP
Carbon
skeletons
Glucose
Glucose
Hexosamines
Envelope 20-30%
CO 2 ,
“waste”
organics
Lipopoly-
saccharides
NH +
Fatty acids
SO 4 2-
Lipids
Storage
Glycogen
5%
Sugars
Proteins
CO 2 , “waste” organics
NH +
SO 2-
Metabolic byproducts
Metabolic byproducts
FIGURE 10.16 Schematic diagram of reactions and processes involved in cellular growth and product
formation in a bacterial cell. Substrates are taken up by the cells by transport processes and converted into
precursor metabolites via fueling reactions. The precursor metabolites (nucleotides, amino acids, hexosamines,
fatty acids, and sugars) are converted to building blocks that are polymerized to macromolecules. Finally,
macromolecules are assembled into cellular structures like membranes, organelles etc. that make up the
functioning cell. Precursor metabolites and building blocks may be secreted to the extracellular medium as
metabolites or they may serve as precursors for metabolites that are secreted. The cell may also secrete certain
macromolecules d primarily proteins that can act as hydrolytic enzymes, but some cells may also secrete
polysaccharides
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