Microbiology, 2021

288 8 • Microbial Metabolism
typically depend on organic molecules for energy, growth, and reproduction. Prokaryotes, on the other hand,
can metabolize a wide range of organic as well as inorganic matter, from complex organic molecules like
cellulose to inorganic molecules and ions such as atmospheric nitrogen (N 2 ), molecular hydrogen (H 2 ), sulfide
(S 2− ), manganese (II) ions (Mn 2+ ), ferrous iron (Fe 2+ ), and ferric iron (Fe 3+ ), to name a few. By metabolizing such
substances, microbes chemically convert them to other forms. In some cases, microbial metabolism produces
chemicals that can be harmful to other organisms; in others, it produces substances that are essential to the
metabolism and survival of other life forms (Figure 8.1).
8.1 Energy, Matter, and Enzymes
Learning Objectives
By the end of this section, you will be able to:
• Define and describe metabolism
• Compare and contrast autotrophs and heterotrophs
• Describe the importance of oxidation-reduction reactions in metabolism
• Describe why ATP, FAD, NAD + , and NADP + are important in a cell
• Identify the structure and structural components of an enzyme
• Describe the differences between competitive and noncompetitive enzyme inhibitors
Clinical Focus
Part 1
Hannah is a 15-month-old girl from Washington state. She is spending the summer in Gambia, where her
parents are working for a nongovernmental organization. About 3 weeks after her arrival in Gambia,
Hannah’s appetite began to diminish and her parents noticed that she seemed unusually sluggish,
fatigued, and confused. She also seemed very irritable when she was outdoors, especially during the day.
When she began vomiting, her parents figured she had caught a 24-hour virus, but when her symptoms
persisted, they took her to a clinic. The local physician noticed that Hannah’s reflexes seemed abnormally
slow, and when he examined her eyes with a light, she seemed unusually light sensitive. She also seemed to
be experiencing a stiff neck.
• What are some possible causes of Hannah’s symptoms?
Jump to the next Clinical Focus box.
The term used to describe all of the chemical reactions inside a cell is metabolism (Figure 8.2). Cellular
processes such as the building or breaking down of complex molecules occur through series of stepwise,
interconnected chemical reactions called metabolic pathways. Reactions that are spontaneous and release
energy are exergonic reactions, whereas endergonic reactions require energy to proceed. The term
anabolism refers to those endergonic metabolic pathways involved in biosynthesis, converting simple
molecular building blocks into more complex molecules, and fueled by the use of cellular energy. Conversely,
the term catabolism refers to exergonic pathways that break down complex molecules into simpler ones.
Molecular energy stored in the bonds of complex molecules is released in catabolic pathways and harvested in
such a way that it can be used to produce high-energy molecules, which are used to drive anabolic pathways.
Thus, in terms of energy and molecules, cells are continually balancing catabolism with anabolism.
Access for free at openstax.org.