Cellular Respiration Overview

Introduction to Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells harvest energy from organic molecules, primarily glucose, to produce adenosine triphosphate (ATP). This process occurs in several stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. The overall reaction for aerobic respiration is:

Equation:

Key Points:

• Reactants: Glucose and oxygen

• Products: Carbon dioxide, water, and ATP

• ATP Yield: Approximately 30-32 ATP molecules per glucose

ATP and Electron Carriers

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) stores and transfers energy within cells. It is often compared to a rechargeable battery, as it cycles between ATP (charged) and ADP (uncharged) forms.

ATP is regenerated from ADP and inorganic phosphate () using energy derived from food metabolism:

NAD+/NADH: Electron Carriers

NAD+ (nicotinamide adenine dinucleotide) is a crucial electron carrier in cellular respiration. It cycles between oxidized (NAD+) and reduced (NADH) forms:

• NAD+ (oxidized): Accepts electrons and is reduced to NADH.

• NADH (reduced): Donates electrons to the electron transport chain, becoming oxidized back to NAD+.

Role: NAD+ shuttles electrons from catabolic reactions to the electron transport chain, facilitating ATP production.

Stages of Cellular Respiration

Overview of the Four Stages

Cellular respiration consists of four main stages, each occurring in specific cellular locations:

Glycolysis

Overview and Location

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It involves the breakdown of one glucose molecule (6 carbons) into two molecules of pyruvate (3 carbons each) through a series of ten enzyme-catalyzed steps.

Key Features:

• Starting molecule: Glucose

• End products: 2 Pyruvate, 2 ATP (net), 2 NADH

• Phases: Energy investment (steps 1-5) and energy payoff (steps 6-10)

• Oxygen requirement: Anaerobic (does not require oxygen)

• Regulation: Controlled by phosphofructokinase (allosteric enzyme)

Energy Investment Phase (Steps 1-5)

During this phase, the cell uses 2 ATP molecules to phosphorylate glucose and its intermediates, preparing them for subsequent breakdown.

• Key enzyme: Phosphofructokinase (regulatory step)

• Splitting: Aldolase splits fructose-1,6-bisphosphate into two 3-carbon molecules

Energy Payoff Phase (Steps 6-10)

In this phase, energy is harvested as 4 ATP and 2 NADH are produced per glucose molecule. The net gain is 2 ATP (since 2 were used in the investment phase).

• Redox reactions: Glyceraldehyde-3-phosphate (G3P) is oxidized, reducing NAD+ to NADH

• ATP formation: Substrate-level phosphorylation transfers phosphate groups to ADP

Summary Table: Glycolysis

Pyruvate Oxidation

Overview and Location

Pyruvate oxidation, also known as the link reaction, connects glycolysis to the citric acid cycle. It occurs in the mitochondrial matrix when oxygen is present. Each pyruvate is converted into acetyl-CoA, producing NADH and CO2.

• Reactants: Pyruvate, NAD+, Coenzyme A

• Products (per pyruvate): 1 Acetyl-CoA, 1 NADH, 1 CO2

• ATP production: None directly

Key Steps of Pyruvate Oxidation

• Decarboxylation: Removal of one carbon as CO2

• Redox reaction: Remaining 2-carbon fragment is oxidized, reducing NAD+ to NADH

• Formation of Acetyl-CoA: The 2-carbon acetyl group is attached to coenzyme A

Citric Acid Cycle (Krebs Cycle)

Overview and Location

The citric acid cycle, also known as the Krebs cycle or TCA cycle, completes the oxidation of organic molecules. It occurs in the mitochondrial matrix and processes each acetyl-CoA to produce NADH, FADH2, ATP, and CO2.

• Reactants: Acetyl-CoA, oxaloacetate

• Products (per acetyl-CoA): 3 NADH, 1 FADH2, 1 ATP, 2 CO2

• ATP production: 1 ATP per cycle (by substrate-level phosphorylation)

• Cycle: Oxaloacetate is regenerated at the end of each cycle

Key Steps of the Citric Acid Cycle

• Carbon dioxide release: Steps 3 and 4 (decarboxylation)

• Redox reactions: NAD+ reduced to NADH (steps 3, 4, 8); FAD reduced to FADH2 (step 6)

• ATP formation: Step 5 (substrate-level phosphorylation)

• Regeneration of oxaloacetate: Step 8

Key Terminology

• Cellular respiration: The process of extracting energy from organic molecules to produce ATP

• Glycolysis: The breakdown of glucose to pyruvate

• Pyruvate oxidation: Conversion of pyruvate to acetyl-CoA

• Citric acid cycle: Series of reactions that completes the oxidation of acetyl-CoA

• ATP: Main energy currency of the cell

• Phosphorylation: Addition of a phosphate group to a molecule

• Redox reactions: Chemical reactions involving the transfer of electrons

• Electron carriers: Molecules like NADH and FADH2 that transport electrons

• Allosteric regulation: Regulation of enzyme activity by binding of molecules at sites other than the active site