Microbiology Chapter 7: Microbial Metabolism

Microbial Metabolism

• Microbial metabolism is a complex system of interwoven pathways coordinated by an intricate, multilayered regulatory network.
• Microbes use various metabolic strategies to obtain energy and nutrients required for viability and reproduction.
• There are two types of metabolic reactions: catabolic and anabolic reactions.

Metabolic Pathways

• Cellular processes, such as building or breaking down complex molecules, occur through series of stepwise, interconnected chemical reactions called metabolic pathways.
• Anabolism refers to endergonic metabolic pathways involved in biosynthesis, converting simple molecular building blocks into more complex molecules, and fueled by the use of cellular energy.

Energy and ATP

• Energy is required to fuel anabolic reactions.
• Exergonic reactions provide energy for endergonic reactions.
• ATP is produced through cellular respiration and fermentation.

Fermentation

• Fermentation is a metabolic process that produces ATP with a low ATP yield.
• Fermentation uses an organic molecule as a final electron acceptor to regenerate NAD⁺ from NADH.
• Fermentation does not involve an electron transport system, and no ATP is made by the fermentation process directly.

Respiration

• Respiration is a metabolic process that produces ATP with a high ATP yield.
• Cellular respiration begins when electrons are transferred from NADH and FADH₂ to a final inorganic electron acceptor (either oxygen in aerobic respiration or non-oxygen inorganic molecules in anaerobic respiration).

Glycolysis

• Glycolysis is the first step in the breakdown of glucose, resulting in the formation of ATP, NADH, and two pyruvate molecules.
• Glycolysis catalyzes the conversion of glucose to pyruvic acid (pyruvate).
• Glycolysis releases energy to yield 2 ATP per glucose and transfers high-energy electrons (+H) to NAD⁺ to yield 2 NADH.

Catabolism of Carbohydrates, Lipids, and Proteins

• Catabolic pathways for carbohydrates, lipids, and proteins eventually connect into glycolysis and the Krebs cycle.
• Microbes can degrade a wide variety of carbon sources, including lipids and proteins.

Photosynthesis and the Importance of Light

• Photosynthesis is the biochemical process by which phototrophic organisms convert solar energy (sunlight) into chemical energy.
• Microbial photosynthesis is a significant supplier of chemical energy, fueling many diverse ecosystems.

Biogeochemical Cycles

• Energy flows directionally through ecosystems, entering as sunlight for phototrophs or as inorganic molecules for chemoautotrophs.
• The six most common elements associated with organic molecules (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur) take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath earth's surface.

Enzymes

• Almost all biochemical reactions are catalyzed by a specific enzyme: proteins that accelerate the rate of a reaction without being changed themselves.
• Enzymes lower the activation energy (Ea) and provide a way to control or regulate biochemical reactions.

Control of Enzyme Activity

• Biochemical reactions can be controlled by changes in enzyme activity, which can be influenced by:
  1. Changes in the amount of enzyme or substrate.
  2. Changes in temperature, pH, or salt.
  3. Availability of any necessary cofactors.
  4. Effect of inhibitors.

Principles of Energy Generation

• All organisms depend on autotrophs, which can produce organic molecules from CO2, an inorganic carbon source.
• The source of energy for autotrophic processes can be:
  1. Light: photoautotrophs that carry out photosynthesis.
  2. Chemical: chemoautotrophs that use various molecules as a source of high-energy electrons.