Cellular Respiration and Fermentation: Pathways for Energy Harvest in Cells

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is essential for the survival of most organisms and involves four main stages: glycolysis, pyruvate processing, the citric acid cycle, and electron transport with oxidative phosphorylation.

• 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: Acetyl CoA is oxidized to CO2, generating NADH and FADH2.

• Electron Transport and Oxidative Phosphorylation: Electrons from NADH and FADH2 move through the electron transport chain (ETC), creating a proton gradient used to synthesize ATP.

Catabolic and Anabolic Pathways

Cellular respiration is interconnected with other metabolic pathways. Cells can use carbohydrates, fats, and proteins as substrates for ATP production. Catabolic pathways break down these macromolecules, while anabolic pathways use intermediates to synthesize macromolecules.

• Fats: Broken down into glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA).

• Proteins: Broken into amino acids; amino groups are removed, and carbon skeletons enter glycolysis or the citric acid cycle.

• Anabolic Use: Intermediates from glycolysis and the citric acid cycle are used to synthesize nucleotides, amino acids, fatty acids, and glucose.

Glycolysis

Pathway and Regulation

Glycolysis is a sequence of 10 enzyme-catalyzed reactions occurring in the cytosol. It consists of an energy investment phase (uses 2 ATP) and an energy payoff phase (produces 4 ATP and 2 NADH), resulting in a net gain of 2 ATP and 2 NADH per glucose molecule.

• Substrate-level phosphorylation: ATP is produced directly by transferring a phosphate group to ADP from a phosphorylated substrate.

• Regulation: Glycolysis is regulated by feedback inhibition, primarily at the enzyme phosphofructokinase, which is inhibited by high ATP levels.

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.
4	Fructose-bis-phosphate aldolase	Cleaves fructose-1,6-bisphosphate into two 3-carbon sugars.
5	Triose phosphate isomerase	Converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate (G3P).
6	Glyceraldehyde-3-phosphate dehydrogenase	Oxidizes G3P, producing NADH and 1,3-bisphosphoglycerate.
7	Phosphoglycerate kinase	Transfers phosphate to ADP, forming ATP and 3-phosphoglycerate.
8	Phosphoglycerate mutase	Rearranges 3-phosphoglycerate to 2-phosphoglycerate.
9	Enolase	Removes water, forming phosphoenolpyruvate.
10	Pyruvate kinase	Transfers phosphate to ADP, forming ATP and pyruvate.

Pyruvate Processing

Conversion to Acetyl CoA

Pyruvate produced by glycolysis is transported into mitochondria (in eukaryotes) and converted to acetyl CoA by the enzyme pyruvate dehydrogenase. This process produces NADH and releases CO2.

• Location: Mitochondrial matrix (eukaryotes), cytosol (prokaryotes).

• Regulation: Pyruvate dehydrogenase is inhibited by high levels of NADH, acetyl CoA, or ATP (feedback inhibition).

The Citric Acid Cycle (Krebs Cycle)

Oxidation of Acetyl CoA

The citric acid cycle completes the oxidation of glucose by converting acetyl CoA into CO2. It generates NADH, FADH2, and ATP (or GTP). The cycle turns twice for each glucose molecule.

• Location: Mitochondrial matrix (eukaryotes), cytosol (prokaryotes).

• Regulation: Controlled by feedback inhibition; high ATP or NADH slows the cycle.

Table: The Reactions of the Citric Acid Cycle

Step	Enzyme	Reaction
1	Citrate synthase	Acetyl group from acetyl CoA + oxaloacetate → citrate.
2	Aconitase	Citrate → isocitrate (via water removal/addition).
3	Isocitrate dehydrogenase	Isocitrate oxidation, NADH and CO2 produced.
4	α-Ketoglutarate dehydrogenase	α-Ketoglutarate oxidation, NADH and CO2 produced, forms succinyl CoA.
5	Succinyl-CoA synthetase	Succinyl CoA → succinate, ATP (or GTP) produced.
6	Succinate dehydrogenase	Succinate oxidation, FADH2 produced, forms fumarate.
7	Fumarase	Fumarate → malate (water addition).
8	Malate dehydrogenase	Malate oxidation, NADH produced, regenerates oxaloacetate.

Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain (ETC)

The ETC is a series of protein complexes (I-IV) and mobile carriers (ubiquinone/Q and cytochrome c) embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through the chain, ultimately reducing oxygen to water. The energy released pumps protons into the intermembrane space, creating a proton gradient.

• Complex I: NADH dehydrogenase oxidizes NADH, pumps protons.

• Complex II: Succinate dehydrogenase oxidizes FADH2.

• Complex III: Cytochrome c reductase transfers electrons to cytochrome c.

• Complex IV: Cytochrome c oxidase reduces O2 to H2O.

Table: Components and Reactions of the Electron Transport Chain

ETC Component	Descriptive Name	Reaction
Complex I	NADH dehydrogenase	Oxidizes NADH, reduces Q, pumps 4 H+.
Complex II	Succinate dehydrogenase	Oxidizes FADH2, reduces Q.
Q	Ubiquinone	Mobile carrier, shuttles electrons between complexes.
Complex III	Cytochrome c reductase	Oxidizes Q, reduces cytochrome c, pumps 4 H+.
Cytochrome c	Cytochrome c	Mobile carrier, shuttles electrons to complex IV.
Complex IV	Cytochrome c oxidase	Reduces O2 to H2O, pumps 2 H+.

ATP Synthase and Chemiosmosis

ATP synthase is a multi-subunit enzyme complex that synthesizes ATP as protons flow down their gradient from the intermembrane space into the mitochondrial matrix. This process, called chemiosmosis, couples the proton-motive force to ATP synthesis.

• F0 unit: Membrane-bound proton channel.

• F1 unit: Catalytic knob that synthesizes ATP from ADP and Pi.

ATP Yield

Oxidative phosphorylation produces the majority of ATP during cellular respiration. The theoretical maximum yield is about 29 ATP per glucose molecule.

Aerobic vs. Anaerobic Respiration

Electron Acceptors

Oxygen is the most efficient terminal electron acceptor due to its high electronegativity, allowing for maximal ATP yield (aerobic respiration). Some prokaryotes use alternative acceptors such as nitrate or sulfate (anaerobic respiration), resulting in lower ATP yields.

Fermentation

Fermentation Pathways

When oxygen or another terminal electron acceptor is unavailable, cells use fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. Fermentation is less efficient than cellular respiration, producing only 2 ATP per glucose.

• Lactic Acid Fermentation: Pyruvate accepts electrons from NADH, forming lactate (in muscle cells).

• Alcohol Fermentation: Pyruvate is converted to acetaldehyde and CO2; acetaldehyde accepts electrons from NADH, forming ethanol (in yeast).

• Facultative Anaerobes: Organisms that can switch between aerobic respiration and fermentation depending on oxygen availability.

Key Equations

• Overall equation for aerobic cellular respiration: $\mathrm{C_6H_{12}O_6} + 6\ \mathrm{O_2} \rightarrow 6\ \mathrm{CO_2} + 6\ \mathrm{H_2O} + \text{energy (ATP + heat)}$

• Net ATP yield per glucose (theoretical maximum): $2\ \text{(glycolysis)} + 2\ \text{(citric acid cycle)} + 25\ \text{(oxidative phosphorylation)} = 29\ \text{ATP}$

Additional info: The actual ATP yield in cells may be lower due to leaky membranes and the use of the proton gradient for other processes. The regulation of cellular respiration ensures metabolic homeostasis and adapts to the cell's energy needs.