Microbial Metabolism: Study Guide and Review

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Microbial Metabolism

Overview

Microbial metabolism encompasses all chemical reactions occurring within microbial cells. These reactions are organized into pathways that either break down molecules to release energy (catabolism) or build complex molecules using energy (anabolism). Understanding microbial metabolism is essential for grasping how microbes obtain, transfer, store, and utilize energy for growth and survival.

Anabolic and Catabolic Reactions

Definitions and Energy Relationships

Catabolic reactions: Break large molecules into smaller ones, releasing usable energy. These are generally exergonic (energy-releasing).
Anabolic reactions: Build large molecules from smaller ones, requiring an input of energy. These are generally endergonic (energy-consuming).
Catabolic means "cut apart"; anabolic means "assemble".
Energy released by catabolism is often captured in ATP and used to power anabolism.

Examples:

Catabolic: Glucose breakdown, beta oxidation, protein catabolism
Anabolic: Protein synthesis, DNA synthesis, lipid synthesis

Key Terms:

Exergonic reaction: Releases free energy; products have less free energy than reactants.
Endergonic reaction: Requires energy input; products have more free energy than reactants.

ATP: The Reusable Energy Currency

Structure and Function

ATP (Adenosine Triphosphate): Consists of adenine, ribose, and three phosphate groups.
Provides immediate energy for cellular work (e.g., active transport, flagellar movement, biosynthesis).
Cells continually use and regenerate ATP; it is not stored in large amounts.

ATP Energy Cycle:

ATP + H2O → ADP + Pi + energy (hydrolysis releases energy)
ADP + Pi + energy → ATP (phosphorylation stores energy)

Phosphorylation: Transfer of a phosphate group to another molecule, often making it more reactive.

ATP is like a rechargeable battery: ATP is the charged form; ADP is the lower-energy form.

Three Ways Cells Produce ATP

Method	How ATP is Produced	Where it Appears
Substrate-level phosphorylation	Enzyme directly transfers phosphate to ADP	Glycolysis, Krebs cycle
Oxidative phosphorylation	Electron transport chain creates proton gradient powering ATP synthase	Aerobic/anaerobic respiration
Photophosphorylation	Light-driven electron transport creates proton gradient powering ATP synthase	Photosynthesis

Enzyme Function and Regulation

Enzyme Properties

Enzymes: Biological catalysts (usually proteins) that speed up reactions by lowering activation energy.
Not permanently consumed; can be reused.
Substrate: The reactant on which an enzyme acts.
Active site: Region where substrate binds and reaction occurs.
Specificity: Active site fits particular substrates due to shape and chemistry.
Induced fit: Substrate binding causes a shape change that improves fit.
Activation energy: Energy required to start a chemical reaction.

Factors Affecting Enzyme Activity

Temperature: Activity increases toward an optimum; excessive heat denatures enzyme, low temperature slows activity.
pH: Each enzyme has an optimum pH; extremes can denature or alter the active site.
Substrate concentration: More substrate increases rate until all active sites are occupied (saturation point).
Enzyme concentration: More enzyme increases rate if substrate is sufficient.

Enzyme Inhibition

Feature	Competitive Inhibition	Noncompetitive Inhibition
Binding location	Active site	Allosteric site
Main effect	Blocks substrate binding	Changes enzyme shape and active-site function
Competition with substrate	Yes	No
Effect of more substrate	May overcome inhibition	Usually does not overcome inhibition

Feedback inhibition: Final product of a pathway inhibits an early enzyme, conserving energy and resources.

Cellular Respiration and Fermentation

Pathways and ATP Yield

Both begin with glycolysis.
Respiration: Continues through additional pathways (transition reaction, Krebs cycle, electron transport chain).
Fermentation: Stops after glycolysis; regenerates NAD+ without an electron transport chain.

Major Steps of Cellular Respiration

Step	Major Event	Major Products per Glucose
1. Glycolysis	Glucose splits into two pyruvate	2 pyruvate, 2 net ATP, 2 NADH
2. Transition reaction	Pyruvate becomes acetyl CoA	2 acetyl CoA, 2 NADH, 2 CO2
3. Krebs cycle	Acetyl CoA oxidized; carbon leaves as CO2	4 CO2, 6 NADH, 2 FADH2, 2 ATP
4. Electron transport and chemiosmosis	Electrons power proton pumping; ATP synthase uses gradient	Most ATP; water (aerobic)

Electron Transport and Chemiosmosis

Located in the cytoplasmic membrane (prokaryotes).
NADH and FADH2 donate electrons to the chain.
Electron energy pumps H+ across the membrane, creating a proton motive force.
H+ flows through ATP synthase, generating ATP.
Final electron acceptor receives electrons at the end of the chain.

Traditional ATP Yield in Prokaryotes

Stage	ATP per Glucose
Glycolysis	2
Krebs cycle	2
Electron transport chain	~34
Total (maximum)	Up to 38

Note: Actual ATP yield varies by organism and conditions.

Fermentation

Uses glycolysis only; does not use Krebs cycle or electron transport chain.
Uses an organic molecule as the final electron acceptor.
Regenerates NAD+ so glycolysis can continue.
Net yield: 2 ATP per glucose.
End products retain much energy (e.g., lactic acid, ethanol).

Respiration vs. Fermentation Comparison

Feature	Cellular Respiration	Fermentation
Shared pathway	Glycolysis	Glycolysis
Krebs cycle	Yes	No
Electron transport chain	Yes	No
Final electron acceptor	O2 or other inorganic	Organic molecule
Glucose breakdown	Complete	Incomplete
Net ATP per glucose	Up to 38	2

Electron Carriers and Final Electron Acceptors

NAD+, FAD, and Electron Flow

NAD+ and FAD are recyclable electron carriers.
Accept electrons during nutrient breakdown, becoming NADH and FADH2.
Deliver electrons to other reactions or the electron transport chain.
After donating electrons, revert to oxidized forms for reuse.

OIL RIG: Oxidation Is Loss; Reduction Is Gain (of electrons).

Electron Carriers Table

Carrier	Oxidized Form	Reduced Form	Function
NAD+	NAD+	NADH	Accepts/carries high-energy electrons and H
FAD	FAD	FADH2	Accepts/carries high-energy electrons and H

Final Electron Acceptors

Process	Final Electron Acceptor	Result
Aerobic respiration	Oxygen	Forms water; highest ATP yield
Anaerobic respiration	Other inorganic (e.g., nitrate, sulfate)	Lower ATP yield
Fermentation	Organic molecule	Regenerates NAD+; no electron transport chain

Facultative anaerobes can switch to fermentation when oxygen is unavailable, allowing glycolysis to continue by regenerating NAD+.

Photosynthesis and Chlorophyll

Purpose and Mechanism

Photosynthesis: Captures light energy and converts it to chemical energy.
Produces ATP and reducing power (NADPH), which are used to fix CO2 into organic molecules.

Role of Chlorophyll:

Pigment that absorbs light energy, exciting electrons.
Energized electrons enter a photosynthetic electron transport chain.
Chlorophyll reflects green light, giving many photosynthetic organisms their color.

Stages of Photosynthesis

Stage	What Happens	Major Products/Results
Light-dependent reactions	Chlorophyll absorbs light; electrons move through ETC	ATP, NADPH, O2 (in oxygenic photosynthesis)
Light-independent reactions (Calvin cycle)	ATP and NADPH used to fix CO2	Organic molecules (carbohydrates)

Note: Light-independent reactions do not require darkness; they do not directly absorb light.

Catabolism of Different Nutrients

Entry of Various Molecules into Energy Pathways

Microbes can catabolize carbohydrates, lipids, proteins, and nucleic acids.
Breakdown products enter glycolysis, transition reaction, or Krebs cycle.

Nutrient Catabolism Table

Nutrient	Initial Breakdown Products	Entry into Energy Metabolism
Carbohydrates	Simple sugars	Enter glycolysis
Lipids	Glycerol, fatty acids	Glycerol to glycolysis; fatty acids to acetyl CoA via beta oxidation
Proteins	Amino acids	After deamination, carbon skeletons enter glycolysis or Krebs cycle
Nucleic acids	Pentose sugars, phosphate, nitrogenous bases	Sugars to carbohydrate pathways; other parts recycled/degraded

Beta oxidation: Removes two-carbon units from fatty acids, producing acetyl CoA and electron carriers.
Deamination: Removes amino group from amino acids; carbon skeleton used for energy.
Fatty acids yield large amounts of ATP due to many high-energy electrons.
Catabolic pathways converge at pyruvate, acetyl CoA, or Krebs-cycle intermediates.

High Yield Exam Review

Catabolism: breaks down molecules, exergonic.
Anabolism: builds molecules, endergonic.
ATP hydrolysis releases energy:
Enzymes lower activation energy; bind specific substrates at active sites.
Competitive inhibitors bind active site; noncompetitive inhibitors bind elsewhere.
Glycolysis: 2 pyruvate, 2 NADH, 2 net ATP per glucose.
Aerobic respiration: O2 as final electron acceptor.
Anaerobic respiration: electron transport chain with non-oxygen acceptor.
Fermentation: regenerates NAD+, yields 2 ATP per glucose.
Traditional maximum aerobic yield (prokaryotes): up to 38 ATP per glucose.
NADH and FADH2 deliver electrons; NAD+ and FAD are regenerated.
Chlorophyll absorbs light energy, energizes electrons.
Carbohydrates, lipids, proteins, nucleic acids all feed energy pathways.

Quick Self-Check

Which type of reaction builds larger molecules and requires energy? Anabolic and endergonic.
What molecule is the immediate reusable energy currency of the cell? ATP.
Where does a competitive inhibitor bind? At the active site.
What pathway is shared by respiration and fermentation? Glycolysis.
What is the net ATP yield of fermentation per glucose? 2 ATP.
What is the final electron acceptor in aerobic respiration? Oxygen.
Why must NAD+ be regenerated? To accept more electrons in glycolysis and other oxidation reactions.
What pigment captures light energy during photosynthesis? Chlorophyll.
What process converts fatty acids to acetyl CoA? Beta oxidation.
What must be removed before many amino acids enter energy pathways? The amino group (deamination).