Cellular Respiration: Structure, Function, and Pathways

Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells harvest chemical energy from organic molecules and use it to generate adenosine triphosphate (ATP), the primary energy currency of the cell. This process is essential for the survival of most organisms, including animals, plants, and many microorganisms.

• Definition: Cellular respiration is the set of metabolic reactions that convert biochemical energy from nutrients into ATP, releasing waste products.

• General Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$

• Main Purpose: To provide energy for cellular processes by breaking down glucose and other organic molecules.

• Major Fuel: Glucose is the primary fuel for most animals.

Structure of the Mitochondrion

Mitochondrial Anatomy and Function

The mitochondrion is the organelle where most stages of cellular respiration occur in eukaryotic cells. Its structure is specialized to maximize ATP production.

• Outer Membrane: Encloses the organelle and separates it from the cytosol.

• Inner Membrane: Highly folded into cristae, increasing surface area for reactions.

• Intermembrane Space: Space between the inner and outer membranes; important for proton gradient formation.

• Mitochondrial Matrix: The innermost compartment, containing enzymes for the citric acid cycle.

• ATP Synthase: Enzyme complex embedded in the inner membrane, responsible for synthesizing ATP.

Overview of Cellular Respiration Pathways

Main Stages of Cellular Respiration

Cellular respiration consists of three main stages, each with distinct locations and functions:

1. Glycolysis: Occurs in the cytosol; breaks down glucose into pyruvate.

2. Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose derivatives.

3. Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis): Occurs across the inner mitochondrial membrane; produces the majority of ATP.

Glycolysis

Pathway and Energy Yield

Glycolysis is the first step in cellular respiration, breaking down one molecule of glucose (6C) into two molecules of pyruvate (3C) in the cytosol.

• Location: Cytosol

• Inputs: 1 glucose, 2 NAD+, 2 ATP (investment), 4 ADP + 4 Pi

• Outputs: 2 pyruvate, 2 NADH, 2 ATP (net), 2 H2O

Stages of Glycolysis:

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

• Energy Payoff Phase: 4 ATP are produced (net gain of 2 ATP), and 2 NADH are generated.

Net Equation for Glycolysis:

$\text{Glucose} + 2\ \text{NAD}^+ + 2\ \text{ADP} + 2\ \text{P}_i \rightarrow 2\ \text{Pyruvate} + 2\ \text{NADH} + 2\ \text{ATP} + 2\ \text{H}_2\text{O}$

• Example: In muscle cells, glycolysis provides rapid ATP during intense exercise.

Pyruvate Oxidation

Conversion to Acetyl CoA

When oxygen is present, pyruvate enters the mitochondria and is oxidized to acetyl coenzyme A (acetyl CoA), which enters the citric acid cycle.

• Location: Mitochondrial matrix

• Inputs: 2 pyruvate, 2 NAD+, 2 CoA

• Outputs: 2 acetyl CoA, 2 NADH, 2 CO2

Equation:

$2\ \text{Pyruvate} + 2\ \text{NAD}^+ + 2\ \text{CoA} \rightarrow 2\ \text{Acetyl CoA} + 2\ \text{NADH} + 2\ \text{CO}_2$

Citric Acid Cycle (Krebs Cycle)

Pathway and Energy Yield

The citric acid cycle completes the breakdown of acetyl CoA, generating electron carriers and ATP.

• Location: Mitochondrial matrix

• Inputs (per glucose): 2 acetyl CoA, 6 NAD+, 2 FAD, 2 ADP + 2 Pi

• Outputs (per glucose): 4 CO2, 6 NADH, 2 FADH2, 2 ATP

Summary Table:

Input	Output
2 Acetyl CoA	4 CO2
6 NAD+	6 NADH
2 FAD	2 FADH2
2 ADP + 2 Pi	2 ATP

• Example: The citric acid cycle is central to aerobic metabolism in all aerobic organisms.

Oxidative Phosphorylation

Electron Transport Chain (ETC) and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration, producing the majority of ATP via the electron transport chain and chemiosmosis.

• Location: Inner mitochondrial membrane

• Components: ETC (series of protein complexes), ATP synthase

Electron Transport Chain (ETC)

• Electrons from NADH and FADH2 are transferred through a series of membrane proteins.

• As electrons move, protons (H+) are pumped into the intermembrane space, creating a proton gradient.

• Oxygen is the final electron acceptor, forming water:

$2\ \text{H}^+ + 2\ e^- + \frac{1}{2}\ \text{O}_2 \rightarrow \text{H}_2\text{O}$

• The ETC does not produce ATP directly but establishes the proton gradient for ATP synthesis.

Chemiosmosis and ATP Synthase

• ATP synthase uses the energy from the proton gradient to convert ADP and Pi into ATP.

• Protons flow down their gradient through ATP synthase, causing it to rotate and catalyze ATP formation.

• Produces about 26–28 ATP per glucose molecule.

Overall ATP Yield (per glucose):

Stage	ATP Produced
Glycolysis	2
Citric Acid Cycle	2
Oxidative Phosphorylation	26–28
Total	30–32

Respiration Without Oxygen

Anaerobic Respiration and Fermentation

Some organisms can generate ATP in the absence of oxygen through anaerobic respiration or fermentation.

• Anaerobic Respiration: Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate or nitrate); occurs in some prokaryotes.

• Fermentation: Generates ATP without an ETC; recycles NAD+ by transferring electrons to organic acceptors.

Types of Fermentation

• Alcohol Fermentation: Pyruvate is converted to ethanol; used by yeast and some bacteria.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; occurs in muscle cells during intense exercise.

Alcohol Fermentation Equation:

$\text{Glucose} \rightarrow 2\ \text{Ethanol} + 2\ \text{CO}_2 + 2\ \text{ATP}$

Lactic Acid Fermentation Equation:

$\text{Glucose} \rightarrow 2\ \text{Lactate} + 2\ \text{ATP}$

• Example: Human muscle cells use lactic acid fermentation when oxygen is scarce, causing a burning sensation due to lactate buildup.

Summary Table: ATP Yield from Cellular Respiration

Stage	Input	Output	ATP Produced
Glycolysis	1 Glucose	2 Pyruvate, 2 NADH	2
Pyruvate Oxidation	2 Pyruvate	2 Acetyl CoA, 2 NADH	0
Citric Acid Cycle	2 Acetyl CoA	4 CO2, 6 NADH, 2 FADH2	2
Oxidative Phosphorylation	10 NADH, 2 FADH2	H2O	26–28
Total			30–32

Practice Questions and Applications

Application of Cellular Respiration Concepts

• Oxygen's Role: Oxygen is the final electron acceptor in the ETC, essential for efficient ATP production.

• Proton Gradient: The ETC pumps protons into the intermembrane space, creating a gradient that drives ATP synthesis via chemiosmosis.

• Fermentation in Muscle Cells: When oxygen is limited, muscle cells switch to lactic acid fermentation to continue producing ATP.

Example: Fish deprived of oxygen after being caught may rely on fermentation to generate ATP for muscle activity, leading to lactic acid buildup.

Additional info: The permeability of mitochondrial membranes to protons (H+) can affect the proton gradient and thus the rate of ATP synthesis. Lower pH (higher H+ concentration) in the intermembrane space increases the gradient, enhancing ATP production up to an optimal point.