Cellular Respiration: Mechanisms and Pathways

Cellular Respiration

Overview of Cellular Respiration in Plants and Other Organisms

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP. In plants, energy processing involves both photosynthesis and cellular respiration, with chloroplasts and mitochondria playing key roles.

• Photosynthesis occurs in chloroplasts, using sunlight, CO2, and water to produce glucose and O2.

• Cellular respiration occurs in mitochondria, breaking down glucose in the presence of O2 to produce ATP and heat.

• Photosynthesis is also carried out by algae and some prokaryotes.

• Cellular respiration is a universal process in eukaryotes and many prokaryotes.

Redox Reactions in Cellular Respiration

Redox reactions (Reduction and Oxidation) are fundamental to cellular respiration, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Electron transfer always involves both a donor and an acceptor.

• NAD+ (Nicotinamide Adenine Dinucleotide) acts as an electron carrier, becoming NADH when reduced.

• Example: When organic fuel is oxidized, NAD+ gains electrons and a proton to become NADH.

Electron Transport Chain (ETC)

The Electron Transport Chain is a series of protein complexes and carriers embedded in the mitochondrial inner membrane, facilitating electron transfer and ATP production.

• NADH donates electrons to the ETC, which are ultimately transferred to oxygen, forming water.

• Each transfer is a redox (exergonic) reaction, releasing energy for ATP synthesis.

• ETC drives cellular respiration and other energy-requiring processes.

Chemiosmosis, Brown Fat, and Beige Cells

Chemiosmosis couples the electron transport chain to ATP synthesis by creating a proton gradient across the mitochondrial membrane.

• H+ ions are pumped into the intermembrane space, creating a concentration gradient (proton gradient).

• The energy stored in this gradient is used by ATP synthase to produce ATP.

• Brown fat is metabolically active tissue with mitochondria that generate heat instead of ATP, especially in response to cold.

• Some white fat cells can convert to beige cells, which may help burn excess fat.

The Stages of Cellular Respiration

Cellular respiration consists of three main stages: Glycolysis, Pyruvate Oxidation and Citric Acid Cycle, and Oxidative Phosphorylation.

Glycolysis

• Occurs in the cytoplasm; involves 9 enzyme-catalyzed steps.

• Splits 6-carbon glucose into two 3-carbon pyruvate molecules.

• Produces 2 NADH and 2 ATP (via substrate-level phosphorylation).

• Has two phases:

  • Energy investment phase: Glucose is converted to glyceraldehyde 3-phosphate (G3P), using ATP.

  • Energy payoff phase: G3P is converted to pyruvate, producing ATP and NADH.

• ATP produced is immediately usable; NADH stores energy for later use in the ETC.

Pyruvate Oxidation and Citric Acid Cycle (TCA/Krebs Cycle)

• Pyruvate (from glycolysis) enters mitochondria and is oxidized.

• One carbon is released as CO2; remaining 2-carbon fragment is oxidized, reducing NAD+ to NADH.

• Coenzyme A is added, forming Acetyl-CoA.

• Acetyl-CoA enters the Citric Acid Cycle, generating:

  • 3 NADH

  • 1 FADH2

  • 2 CO2

  • 1 ATP per Acetyl-CoA

Oxidative Phosphorylation

• Largest ATP yield; over 90% of cellular ATP is produced here.

• H+ ions are actively transported across the membrane (chemiosmosis), creating a proton gradient.

• ETC protein complexes (I-IV), Coenzyme Q, and Cytochrome C transfer electrons from NADH and FADH2 to O2.

• ATP synthase uses the proton gradient to synthesize ATP from ADP and phosphate.

Key Equation for Cellular Respiration:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP}$

Fermentation

Fermentation is an anaerobic process that allows ATP production without oxygen, using glycolysis and subsequent reactions to regenerate NAD+.

Lactic Acid Fermentation

• Pyruvate is reduced to lactate; NADH is oxidized to NAD+.

• Occurs in muscle cells during intense activity and in lactic acid bacteria (used in dairy, soy, sauerkraut).

• Lactate is converted back to pyruvate in the liver for continued respiration.

Ethanol Fermentation

• Pyruvate is reduced to ethanol; NADH is oxidized to NAD+.

• Occurs in yeast and some bacteria under anaerobic conditions; CO2 is released.

• Fermentation is not reversible.

• Obligate anaerobes: Require anaerobic conditions; poisoned by O2.

• Facultative anaerobes: Can switch between fermentation and oxidative phosphorylation depending on O2 availability.

Summary Table: Stages of Cellular Respiration

Stage	Main Location	Key Reactants	Key Products	ATP Yield
Glycolysis	Cytoplasm	Glucose, NAD+, ADP	Pyruvate, NADH, ATP	2
Pyruvate Oxidation & Citric Acid Cycle	Mitochondrial Matrix	Pyruvate, NAD+, FAD, ADP	CO2, NADH, FADH2, ATP	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	NADH, FADH2, O2, ADP	ATP, H2O	~28

Additional info: The total ATP yield from cellular respiration is typically about 32 ATP per glucose molecule, though actual yield may vary depending on cell type and conditions.