Aerobicity Presentation 6
Biology 3020: Microbiology for Non-Science Majors
Aerobicity
Definition:
Aerobicity refers to two key aspects regarding the relationship of organisms with oxygen:
The ability of a cell to tolerate the presence of oxygen.
The utilization of oxygen by the cell.
Oxygen and Organisms
General Perspectives:
Contrary to common misconceptions, not all organisms utilize oxygen:
Many organisms either do not require oxygen or find it toxic.
Plants do not use oxygen.
Most microorganisms do not utilize oxygen.
All animals depend on oxygen, raising questions about its functional significance.
Tolerance to Oxygen:
Tolerance to oxygen raises queries about what it entails physiologically for the organism.
Properties of Oxygen
Reactivity:
Oxygen is characterized as a corrosive or poisonous gas due to its reactivity.
In the atmosphere, oxygen can spontaneously decompose into reactive byproducts called superoxides.
Reactive ions resembling superoxides can be identified: $ ext{O}_2$, $ ext{O}^{*}$, $ ext{O}^+$.
Superoxides
Definition:
Superoxides are reactive derivatives of oxygen that can cause significant damage to chemical bonds within biological molecules.
Mechanism of Damage:
Oxygen's electronegativity allows it to extract electrons from various atoms, thus leading to bond destruction.
Resulting molecules from the destruction of bonds signify that superoxides remain intact post-interaction, perpetuating oxidative stress in cells.
Existence in Oxygen-Rich Environments:
Superoxides co-exist whenever oxygen is present in an organism's environment.
Enzymatic Defense Mechanisms
Addressing Superoxides:
Organisms must produce specific enzymes to mitigate the harmful effects of superoxides.
1. Superoxide Dismutase (SOD)
2. Catalase
Superoxide Dismutase (SOD)
Function:
Catalyzes the reaction of superoxides with hydrogen ions, converting them into peroxides:
ext{Superoxide}: ext{ } ext{O}2 + 2 ext{H}^+ ightarrow ext{H}2 ext{O}_2
Role of Peroxides:
Peroxides generated can serve as both a reactive agent and a method of neutralization for various cellular objectives.
Catalase
Function:
Converts peroxides into harmless byproducts, specifically water and oxygen gas:
ext{Catalase: } ext{H}2 ext{O}2
ightarrow 2 ext{H}2 ext{O} + ext{O}2
Understanding Aerobicity in Context
Key Question: Instead of merely asking if an organism can tolerate oxygen, a more pertinent query is whether an organism produces Superoxide Dismutase and Catalase, as the absence of these enzymes renders oxygen toxic to the organism.
Oxygen's Role in Cellular Respiration
Primary Function:
For organisms using oxygen, its primary role revolves around cellular respiration, which includes cellular processes that yield ATP, the energy currency of cells.
Cellular respiration incorporates various reactions alongside oxygen consumption.
Phases of Cellular Respiration
Glycolysis:
Defined as the "breaking of sugar"; involved in the breakdown of carbohydrates (inputs vary based on the organism).
Location: Occurs within the cytoplasm.
Outputs: Produces pyruvate through a series of enzyme-mediated reactions:
Requires an initial investment of energy (ATP).
Also yields some ATP and NADH as a byproduct.
Processes:
Phase 1: Energy Investment (utilization of ATP)
Phase 2: Energy Payoff (generation of ATP and NADH)
Summary Equation:
ext{Glucose} + 2 ext{ADP} + 2 ext{NAD}^+
ightarrow 2 ext{Pyruvate} + 2 ext{NADH} + 2 ext{ATP}
Kreb's Cycle (TCA Cycle):
Also known as the Tricarboxylic Acid Cycle or Citric Acid Cycle.
Input: Acetyl CoA derived from the modification of pyruvate.
Output: Generates NADH and other electron carriers.
Location:
In prokaryotes, it occurs just inside the cell membrane.
In eukaryotes, it occurs in the mitochondrial matrix.
Electron Transport Chain:
Organized within a membrane structure;
Input: Consumes NADH and other electron carriers generated by earlier phases;
Output: Produces ATP via a pumping mechanism based on electron transfer across the membrane; resulting in a concentration gradient driving ATP synthesis.
Specifics:
Occurs across either prokaryotic cell membranes or the inner mitochondrial membrane in eukaryotes.
Components: Include protein complexes known as Cytochrome complexes, which act as electron acceptors and facilitate ATP generation through chemiosmosis.
Oxidative Phosphorylation
Function of Oxygen:
In aerobic organisms, oxygen acts as the final electron acceptor during the electron transport chain, which is crucial for completing the process of electron donation from NADH.
Role of Cytochrome Oxidase:
It catalyzes the conversion of electrons originating from NADH in conjunction with protons to form water:
2 e^{-} + 2 ext{H}^{+} + ext{O}2 ightarrow ext{H}2 ext{O}
Classification of Organisms Based on Aerobicity
Aerobes (obligate):
Must produce Superoxide Dismutase and Catalase; utilize oxygen as a final electron acceptor.
Anaerobes (obligate):
Do not produce Superoxide Dismutase or Catalase; do not utilize oxygen as a final electron acceptor, thus finding it toxic.
Facultative Anaerobes:
Can produce Superoxide Dismutase and Catalase; do use oxygen as final electron acceptor when available but can also survive without it.
Aerotolerant Anaerobes:
Produce Superoxide Dismutase and Catalase; do not use oxygen as final electron acceptor but can tolerate its presence.
Microaerophiles:
Do not produce Superoxide Dismutase or Catalase; require low levels of oxygen for some metabolic processes, but higher levels are harmful.
Method of Assessing Aerobicity in Microbial Cultures
Agar Deep Method:
Agar is placed in a wide test tube and subjected to sterilization by autoclaving, which expels gases including oxygen.
Upon cooling, the agar solidifies, and slow re-infiltration of oxygen creates an oxygen gradient.
Interpretation of Growth Patterns:
No growth near the surface: Obligate anaerobe.
Growth throughout agar: Facultative anaerobe.
Growth only at the top: Obligate aerobe.