Cellular Respiration: Pathways, Redox Reactions, and Glycolysis

Cellular Respiration: Introduction and Overview

Chemical Energy and Cellular Work

Cellular respiration is the process by which animal and plant cells break down organic molecules to release energy, primarily in the form of ATP. This process occurs in the mitochondria and is essential for powering cellular activities.

• Organic molecules (such as glucose) are broken down, releasing energy.

• The energy from these molecules is transformed into ATP, the cell's main energy currency.

• Some energy is lost as heat, in accordance with the second law of thermodynamics.

Reactants and Products of Photosynthesis and Respiration

Photosynthesis and cellular respiration are complementary processes in the energy cycle of living organisms.

• Photosynthesis: Reactants: CO2, H2O, and light Products: C6H12O6 (glucose) and O2

• Respiration: Reactants: C6H12O6 and O2 Products: CO2, H2O, and ATP

• These processes are nearly the perfect inverse of each other.

Catabolic Pathways and ATP Production

Types of Catabolic Pathways

Catabolic pathways break down organic molecules to release energy. There are several types:

• Fermentation: Partial breakdown of organic molecules without O2.

• Aerobic respiration: Complete breakdown using O2; most efficient.

• Anaerobic respiration: Similar to aerobic, but uses other compounds instead of O2 as the final electron acceptor.

Cellular Respiration Equation

Cellular respiration is often traced using glucose as the primary fuel:

• General equation: $C_6H_{12}O_6 + 6 O_2 ightarrow 6 CO_2 + 6 H_2O + ext{Energy (ATP + heat)}$

• ATP links catabolism to cellular work, acting as a chemical drive shaft.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox reactions involve the transfer of electrons, which releases energy used to synthesize ATP.

• Oxidation: Loss of electrons; substance becomes more positive.

• Reduction: Gain of electrons; substance becomes more negative.

• NAD+ is reduced to NADH.

• FADH2 is oxidized to FAD: $FADH_2 ightarrow FAD + 2e^- + 2H^+$

Examples of Redox Reactions

• Inorganic Example: $Na + Cl ightarrow Na^+ + Cl^-$ Sodium is oxidized (loses electron), chlorine is reduced (gains electron).

• Generalized Example: $Xe^- + Y ightarrow X + Ye^-$ Xe is oxidized, Y is reduced.

Redox Agents

• Reducing agent: Electron donor (e.g., sodium).

• Oxidizing agent: Electron acceptor (e.g., chlorine).

Redox in Covalent Bonds

Some redox reactions involve changes in electron sharing rather than complete transfer, such as the reaction between methane and oxygen:

• $CH_4 + 2 O_2 ightarrow CO_2 + 2 H_2O + ext{Energy}$

• Methane is the reducing agent, oxygen is the oxidizing agent.

Energy Release in Redox Reactions

• Electrons moving from less electronegative to more electronegative atoms release chemical energy.

Stages of Cellular Respiration

Overview of Stages

Cellular respiration consists of three main stages:

1. Glycolysis: Occurs in the cytosol; breaks down glucose (6C) into two pyruvate (3C).

2. Pyruvate Oxidation and Citric Acid Cycle: Pyruvate is oxidized to acetyl CoA, which enters the citric acid cycle (Krebs cycle) in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes).

3. Oxidative Phosphorylation: Electrons from NADH and FADH2 are transferred through the electron transport chain, driving ATP synthesis via chemiosmosis.

Electron Carriers

• NAD+ and FAD are reduced to NADH and FADH2 during glycolysis and the citric acid cycle.

• These carriers transfer electrons to the electron transport chain.

ATP Production by Stage

• Glycolysis: 2 net ATP

• Citric Acid Cycle: 2 ATP

• Oxidative Phosphorylation: 28 ATP

• Total: Up to 32 ATP per glucose molecule

Substrate-Level Phosphorylation

Some ATP is produced by substrate-level phosphorylation, where an enzyme transfers a phosphate group directly from a substrate to ADP.

ATP vs. Glucose for Cellular Work

• ATP is more practical for cellular work due to its rapid breakdown and direct energy release.

• Glucose requires multiple enzymatic steps before its energy can be extracted.

Glycolysis: Pathway and Phases

Overview of Glycolysis

Glycolysis is the process of splitting glucose into two pyruvate molecules. It occurs in the cytosol and is the first step in cellular respiration.

• Consists of 10 steps, grouped into two phases:

• Energy Investment Phase: 2 ATP are used to phosphorylate glucose.

• Energy Payoff Phase: 4 ATP and 2 NADH are produced.

• Net yield: 2 ATP, 2 NADH, and 2 pyruvate per glucose.

Energy Input and Output of Glycolysis

Phase	Inputs	Outputs
Energy Investment	Glucose, 2 ATP	2 ADP + 2 Pi
Energy Payoff	4 ADP + 4 Pi, 2 NAD+, 4 e-, 4 H+	4 ATP, 2 NADH, 2 pyruvate, 2 H2O
Net	Glucose	2 pyruvate, 2 ATP, 2 NADH, 2 H2O

Energy Investment Phase Details

• Two ATP are used to add phosphate groups to glucose, forming glucose 6-phosphate and then fructose 6-phosphate.

• Fructose 1,6-bisphosphate is split into two 3-carbon sugars: glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

• G3P is a key intermediate also found in the Calvin cycle of photosynthesis.

Energy Payoff Phase Details

• Each G3P is oxidized by NAD+ to produce NADH.

• Substrate-level phosphorylation generates ATP as phosphate groups are transferred to ADP.

• Final product is pyruvate.

Summary of Glycolysis

• Occurs in the cytosol of both eukaryotes and prokaryotes.

• Does not require oxygen.

• Provides precursors for further energy extraction in cellular respiration.

Example: Glycolysis Net Equation

• $ext{Glucose} + 2 NAD^+ + 2 ADP + 2 P_i ightarrow 2 ext{Pyruvate} + 2 NADH + 2 ATP + 2 H_2O + 2 H^+$

Additional info:

• Glycolysis is a universal pathway, indicating its evolutionary significance.

• Each step is catalyzed by a specific enzyme, ensuring precise control and regulation.