Cellular Respiration and Fermentation: An Overview of Energy Harvesting in Cells

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Cellular Respiration and Fermentation

Introduction to Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells harvest energy from high-energy molecules, such as glucose, to produce ATP, the universal energy currency of the cell. This process is essential for sustaining life, as ATP fuels most cellular work. The energy released from glucose is used to add a phosphate group to ADP, forming ATP.

Life requires energy to maintain order and perform work.
ATP is unstable and must be continually produced by cells.
Cells obtain glucose from photosynthesis (plants) or food (other organisms).
Glucose is stored as glycogen (animals) or starch (plants).

Overview of Cellular Respiration

Cellular respiration consists of four main processes, each contributing to the stepwise extraction of energy from glucose:

Glycolysis: Glucose (6C) is split into two molecules of pyruvate (3C each).
Pyruvate Processing: Each pyruvate is oxidized to form acetyl CoA (2C).
Citric Acid Cycle (Krebs Cycle): Acetyl CoA is oxidized to CO2.
Electron Transport and Oxidative Phosphorylation: Electrons move through a transport chain, creating a proton gradient used to synthesize ATP.

These processes occur in the cytosol and mitochondria of eukaryotes, or the cytosol and plasma membrane of prokaryotes.

Redox Reactions in Cellular Respiration

Cellular respiration involves a series of redox reactions (oxidation-reduction reactions):

Oxidation: Loss of electrons (e.g., glucose is oxidized to CO2).
Reduction: Gain of electrons (e.g., NAD+ is reduced to NADH).
Energy released during these reactions is used to synthesize ATP.

Catabolic and Anabolic Pathways

Catabolic pathways: Breakdown molecules to harvest energy for ATP production (e.g., carbohydrates, fats, proteins).
Anabolic pathways: Use molecules from carbohydrate metabolism to synthesize macromolecules.

Step 1: Glycolysis

Overview of Glycolysis

Glycolysis is a sequence of 10 enzyme-catalyzed reactions that occur in the cytosol. It splits one glucose molecule into two pyruvate molecules, producing a net gain of ATP and NADH.

Energy investment phase (steps 1-5): Uses 2 ATP to phosphorylate glucose and its intermediates.
Energy payoff phase (steps 6-10): Produces 4 ATP (net gain 2 ATP) and 2 NADH.
Net products: 2 pyruvate, 2 ATP, 2 NADH per glucose.

Summary Table: The Reactions of Glycolysis

Step	Enzyme	Reaction
1	Hexokinase	Uses ATP to phosphorylate glucose, increasing its potential energy.
2	Phosphoglucose isomerase	Converts glucose-6-phosphate to fructose-6-phosphate.
3	Phosphofructokinase	Uses ATP to phosphorylate fructose-6-phosphate (key regulatory step).
4	Aldolase	Cleaves fructose-1,6-bisphosphate into two 3-carbon sugars.
5	Triose phosphate isomerase	Converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.
6	Glyceraldehyde-3-phosphate dehydrogenase	Oxidizes G3P, reduces NAD+ to NADH, adds phosphate.
7	Phosphoglycerate kinase	Transfers phosphate to ADP, forming ATP (substrate-level phosphorylation).
8	Phosphoglycerate mutase	Rearranges phosphate group.
9	Enolase	Removes water, forming phosphoenolpyruvate (PEP).
10	Pyruvate kinase	Transfers phosphate to ADP, forming ATP and pyruvate.

Regulation of Glycolysis

Regulated by feedback inhibition at phosphofructokinase (step 3).
High ATP levels inhibit the enzyme by binding to a regulatory site, slowing glycolysis.

Step 2: Pyruvate Processing

Conversion of Pyruvate to Acetyl CoA

Pyruvate produced in glycolysis is transported into mitochondria (eukaryotes) or remains in the cytosol (prokaryotes). It is processed by the pyruvate dehydrogenase complex:

One carbon is released as CO2 (waste product).
NAD+ is reduced to NADH.
The remaining two-carbon unit is attached to coenzyme A, forming acetyl CoA.

Regulation occurs via feedback inhibition: when products accumulate, pyruvate dehydrogenase is phosphorylated and inhibited.

Step 3: Citric Acid Cycle (Krebs Cycle)

Oxidation of Acetyl CoA

The citric acid cycle completes the oxidation of glucose derivatives. Each acetyl CoA enters the cycle and is oxidized to two CO2 molecules. The cycle occurs in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes).

Each turn of the cycle produces: 3 NADH, 1 FADH2, 1 ATP (or GTP).
Since two acetyl CoA are produced per glucose, totals per glucose: 6 NADH, 2 FADH2, 2 ATP.

Summary Table: The Reactions of the Citric Acid Cycle

Step	Enzyme	Reaction
1	Citrate synthase	Transfers acetyl group to oxaloacetate, forming citrate.
2	Aconitase	Converts citrate to isocitrate.
3	Isocitrate dehydrogenase	Oxidizes isocitrate, reduces NAD+ to NADH, releases CO2.
4	α-Ketoglutarate dehydrogenase	Oxidizes α-ketoglutarate, reduces NAD+ to NADH, releases CO2.
5	Succinyl-CoA synthetase	Converts succinyl-CoA to succinate, produces ATP (or GTP).
6	Succinate dehydrogenase	Oxidizes succinate, reduces FAD to FADH2.
7	Fumarase	Converts fumarate to malate.
8	Malate dehydrogenase	Oxidizes malate, reduces NAD+ to NADH, regenerates oxaloacetate.

Regulation of the Citric Acid Cycle

Regulated by feedback inhibition at multiple steps.
High ATP or NADH levels inhibit key enzymes, slowing the cycle.
Low ATP or NADH levels increase reaction rates.

Step 4: Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain (ETC)

The ETC is a series of protein complexes and molecules embedded in the inner mitochondrial membrane. It oxidizes NADH and FADH2, transferring electrons to oxygen (the final electron acceptor), forming water.

As electrons move through the chain, protons (H+) are pumped into the intermembrane space, creating a proton gradient (proton-motive force).
Key components include protein complexes and ubiquinone (coenzyme Q).
Redox potential increases along the chain; energy is released in small steps.

ATP Synthesis by Chemiosmosis

ATP synthase uses the proton gradient to drive the synthesis of ATP from ADP and Pi.
This process is called oxidative phosphorylation.
Most ATP from glucose oxidation is produced in this step.

Equation for Cellular Respiration

The overall equation for aerobic cellular respiration is:

Fermentation

Fermentation as an Alternative Pathway

When oxygen is not available, cells can regenerate NAD+ through fermentation, allowing glycolysis to continue producing ATP by substrate-level phosphorylation.

Fermentation is less efficient than cellular respiration (2 ATP per glucose vs. ~38 ATP per glucose).
Types of fermentation include lactic acid fermentation (produces lactate) and alcohol fermentation (produces ethanol and CO2).
Some organisms are facultative anaerobes, switching between fermentation and aerobic respiration depending on oxygen availability.

Comparison Table: Aerobic Respiration vs. Fermentation

Process	Final Electron Acceptor	ATP Yield (per glucose)	End Products
Aerobic Respiration	O2	~38	CO2, H2O
Fermentation	Organic molecule (e.g., pyruvate or acetaldehyde)	2	Lactate or ethanol + CO2

Summary of ATP Yield

Glycolysis: 2 ATP (net), 2 NADH
Pyruvate Processing: 2 NADH
Citric Acid Cycle: 2 ATP, 6 NADH, 2 FADH2
Electron Transport Chain: ~34 ATP (from NADH and FADH2)
Total (aerobic respiration): ~38 ATP per glucose (theoretical maximum)

Key Questions for Each Step

What goes in?
What comes out?
What happens to the energy that is released?
Where does each step occur?
How is it regulated?

Example: During intense exercise, muscle cells may switch to lactic acid fermentation when oxygen is scarce, producing lactate and allowing glycolysis to continue.

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