Cellular Respiration and Fermentation: Study Guide

Cellular Respiration and Fermentation

Overview and Connection to Photosynthesis

Cellular respiration and photosynthesis are two fundamental metabolic processes in living organisms. While photosynthesis captures energy from sunlight to produce glucose, cellular respiration breaks down glucose to release energy for cellular activities.

• Photosynthesis: Converts carbon dioxide and water into glucose and oxygen using sunlight.

• Cellular Respiration: Uses glucose and oxygen to produce carbon dioxide, water, and ATP (energy).

• Connection: The products of photosynthesis (glucose and oxygen) are the reactants for cellular respiration.

• Equation for Cellular Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

Stages of Cellular Respiration

Cellular respiration consists of three main metabolic stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation (electron transport chain).

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate, producing ATP and NADH.

• Citric Acid Cycle: Takes place in the mitochondrial matrix; oxidizes pyruvate to CO2, generating NADH and FADH2.

• Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; uses NADH and FADH2 to drive ATP synthesis via the electron transport chain.

• Total ATP Yield: Up to 32 molecules of ATP per glucose (actual yield may vary).

Redox Reactions in Cellular Respiration

Redox reactions involve the transfer of electrons between molecules, which is central to energy extraction in respiration.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Oxidizing Agent: Accepts electrons (is reduced).

• Reducing Agent: Donates electrons (is oxidized).

• Example: In respiration, glucose is oxidized and oxygen is reduced.

ATP Production and Energy Transfer

ATP (adenosine triphosphate) is the main energy currency of the cell, produced during cellular respiration.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP during glycolysis and the citric acid cycle.

• Oxidative phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

• Electron Carriers: NAD+ and FAD transport electrons to the electron transport chain.

Electron Transport Chain and Chemiosmosis

The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane that transfer electrons and pump protons to create a gradient.

• Location: Inner mitochondrial membrane (eukaryotes); plasma membrane (prokaryotes).

• Function: Transfers electrons from NADH and FADH2 to oxygen, forming water.

• Proton Gradient: Protons are pumped into the intermembrane space, creating an electrochemical gradient.

• ATP Synthase: Uses the proton gradient to synthesize ATP from ADP and inorganic phosphate.

• Equation for ATP Synthesis: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm and does not require oxygen.

• Meaning: "Splitting of sugar"

• Phases: Energy investment phase and energy payoff phase.

• Products: 2 pyruvate, 2 ATP, and 2 NADH per glucose.

• Can occur without oxygen: Yes, but leads to fermentation if oxygen is absent.

Fermentation and Anaerobic Respiration

When oxygen is not available, cells can undergo fermentation or anaerobic respiration to produce ATP.

• Alcohol Fermentation: Converts pyruvate to ethanol and CO2 (yeast).

• Lactic Acid Fermentation: Converts pyruvate to lactic acid (muscle cells, some bacteria).

• Anaerobic Respiration: Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., nitrate).

Type	End Product	Organisms
Alcohol Fermentation	Ethanol, CO2	Yeast, some bacteria
Lactic Acid Fermentation	Lactic acid	Muscle cells, some bacteria

Obligate vs. Facultative Anaerobes

• Obligate Anaerobes: Cannot survive in the presence of oxygen.

• Facultative Anaerobes: Can survive with or without oxygen (e.g., yeast, many bacteria).

Evolutionary Significance of Glycolysis

Glycolysis is an ancient metabolic pathway, present in nearly all organisms, suggesting it evolved early in the history of life.

• Significance: Provides a mechanism for ATP production without oxygen.

• Location: Cytoplasm, does not require membrane-bound organelles.

Comparing Carbohydrates and Fats as Fuels

• Fats: Yield more ATP per gram than carbohydrates due to more reduced carbon atoms.

• Carbohydrates: Easier to metabolize quickly but provide less energy per gram.

• Features making fats better fuels: Higher energy density, more C-H bonds for oxidation.

Additional info:

• ATP synthase is a molecular machine that synthesizes ATP using the energy from the proton gradient.

• Variability in ATP yield per glucose is due to differences in shuttle mechanisms and membrane permeability.