Cellular Respiration and Mitochondria: Key Concepts and Processes

Cellular Respiration and Mitochondria

Parts of a Mitochondrion

The mitochondrion is a double-membraned organelle known as the "powerhouse of the cell" because it produces most of the cell's ATP through cellular respiration.

• Outer Membrane: Smooth membrane that encloses the entire organelle.

• Inner Membrane: Folded into structures called cristae, increasing surface area for reactions.

• Intermembrane Space: The space between the outer and inner membranes.

• Matrix: The innermost compartment, containing enzymes for the Krebs cycle and mitochondrial DNA.

• Cristae: Folds of the inner membrane where the electron transport chain occurs.

Aerobic Respiration vs. Anaerobic Respiration

Aerobic respiration requires oxygen and produces more ATP, while anaerobic respiration occurs without oxygen and yields less ATP.

• Aerobic Respiration: Occurs in mitochondria, uses oxygen, produces up to 36-38 ATP per glucose.

• Anaerobic Respiration: Occurs in cytoplasm, does not use oxygen, produces 2 ATP per glucose.

Comparison Table:

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen Required?	Yes	No
Location	Mitochondria	Cytoplasm
ATP Yield (per glucose)	36-38	2
End Products	CO2, H2O, ATP	Lactic acid or ethanol, ATP

Alcoholic and Lactic Acid Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue by regenerating NAD+.

• Alcoholic Fermentation: Occurs in yeast and some bacteria; converts pyruvate to ethanol and CO2.

• Lactic Acid Fermentation: Occurs in animal muscle cells and some bacteria; converts pyruvate to lactic acid.

Example: Muscle cells perform lactic acid fermentation during intense exercise when oxygen is scarce.

Inputs and Outputs in the Stages of Cellular Respiration

Cellular respiration consists of several stages, each with specific inputs and outputs.

• Glycolysis: Occurs in cytoplasm. Input: Glucose, 2 ATP, 2 NAD+ Output: 2 Pyruvate, 4 ATP (net gain 2 ATP), 2 NADH

• Krebs Cycle (Citric Acid Cycle): Occurs in mitochondrial matrix. Input: 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP Output: 4 CO2, 6 NADH, 2 FADH2, 2 ATP

• Electron Transport Chain (ETC) & Oxidative Phosphorylation: Occurs in inner mitochondrial membrane. Input: NADH, FADH2, O2 Output: H2O, ~32-34 ATP

What is Glycolysis, Krebs Cycle, Electron Transport Chain, and Oxidative Phosphorylation?

• Glycolysis: The breakdown of glucose into two molecules of pyruvate, producing ATP and NADH.

• Krebs Cycle: A series of reactions that further break down pyruvate derivatives, generating NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): A sequence of proteins in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, pumping protons to create a gradient.

• Oxidative Phosphorylation: The process of ATP synthesis powered by the flow of protons back into the matrix through ATP synthase.

Where is Each Process?

• Glycolysis: Cytoplasm

• Krebs Cycle: Mitochondrial matrix

• Electron Transport Chain & Oxidative Phosphorylation: Inner mitochondrial membrane (cristae)

• Fermentation: Cytoplasm

Why Do the Hydrogens Move Through the ATP Synthase?

During the electron transport chain, protons (H+) are pumped into the intermembrane space, creating a proton gradient. The only way for protons to return to the matrix is through ATP synthase, a protein complex. As protons flow through ATP synthase, the energy released drives the conversion of ADP and inorganic phosphate (Pi) into ATP.

Equation:

$\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

How Much ATP is Made in Each Step?

• Glycolysis: 2 ATP (net gain)

• Krebs Cycle: 2 ATP (per glucose)

• Electron Transport Chain & Oxidative Phosphorylation: ~32-34 ATP (per glucose)

• Total (Aerobic Respiration): 36-38 ATP per glucose molecule

• Anaerobic Respiration (Fermentation): 2 ATP per glucose (from glycolysis only)

Summary Table: ATP Yield by Stage

Stage	ATP Produced (per glucose)
Glycolysis	2
Krebs Cycle	2
Electron Transport Chain & Oxidative Phosphorylation	32-34
Total (Aerobic)	36-38
Total (Anaerobic/Fermentation)	2

Additional info: The actual ATP yield can vary depending on the cell type and conditions. Some textbooks report a range of 30-38 ATP per glucose due to variations in shuttle mechanisms and membrane permeability.