Cellular Respiration: How Cells Harvest Chemical Energy

Chapter 6: How Cells Harvest Chemical Energy

Big Ideas and Overview

This chapter explores how cells obtain energy from organic molecules, focusing on the processes of cellular respiration and fermentation. It also discusses the connections between metabolic pathways and the role of photosynthesis in providing energy for life.

• Cellular Respiration: Aerobic harvesting of energy from organic molecules.

• Fermentation: Anaerobic harvesting of energy.

• Stages of Cellular Respiration: Glycolysis, Citric Acid Cycle, and Oxidative Phosphorylation.

• Connections Between Metabolic Pathways: Integration of carbohydrate, fat, and protein metabolism.

Photosynthesis and Cellular Respiration Provide Energy for Life

Energy Flow in Ecosystems

All living organisms require energy to survive. In nearly all ecosystems, the ultimate source of energy is the sun. Photosynthesis and cellular respiration are complementary processes that enable the flow of energy through living systems.

• Photosynthesis:

  • Uses sunlight to convert carbon dioxide (CO2) and water (H2O) into organic molecules (such as glucose).

  • Releases oxygen (O2) as a byproduct.

  • Equation: $6CO_2 + 6H_2O + ext{light energy} ightarrow C_6H_{12}O_6 + 6O_2$

• Cellular Respiration:

  • Consumes oxygen (O2) to break down organic molecules, producing carbon dioxide (CO2) and water (H2O).

  • Captures released energy as ATP.

  • Equation: $C_6H_{12}O_6 + 6O_2 ightarrow 6CO_2 + 6H_2O + ext{ATP}$

Key Point: Both plant and animal cells perform cellular respiration, while only plant cells (and some protists and bacteria) perform photosynthesis.

Stages of Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a multi-step process that converts the chemical energy in glucose into ATP, the energy currency of the cell. It occurs in three main stages:

1. Glycolysis (in the cytosol):

   • Breaks down glucose into two molecules of pyruvate.

   • Produces a net gain of 2 ATP and 2 NADH.

   • Equation: $ext{Glucose} ightarrow 2 ext{Pyruvate} + 2 ext{ATP} + 2 ext{NADH}$

2. Pyruvate Oxidation and Citric Acid Cycle (in mitochondria):

   • Pyruvate is converted to acetyl CoA, which enters the citric acid cycle.

   • Each turn of the cycle releases 2 CO2, produces 3 NADH, 1 FADH2, and 1 ATP.

3. Oxidative Phosphorylation (in mitochondria):

   • Includes electron transport and chemiosmosis.

   • NADH and FADH2 donate electrons to the electron transport chain.

   • Most ATP is produced in this stage (up to 28 ATP per glucose).

   • Oxygen acts as the final electron acceptor, forming water.

Redox Reactions in Cellular Respiration

Electron Transfer and Energy Capture

Cellular respiration involves a series of redox (reduction-oxidation) reactions, where electrons are transferred from fuel molecules to electron carriers and ultimately to oxygen.

• Oxidation: Loss of electrons (OIL: Oxidation Is Losing).

• Reduction: Gain of electrons (RIG: Reduction Is Gaining).

• NAD+: An electron carrier that becomes reduced to NADH when it accepts electrons and a proton.

• Electron Transport Chain: NADH and FADH2 pass electrons through a series of proteins, releasing energy used to pump protons and generate ATP.

Glycolysis: Splitting Glucose

Energy Investment and Payoff Phases

Glycolysis consists of two phases: the energy investment phase and the energy payoff phase.

• Energy Investment Phase:

  • 2 ATP are used to phosphorylate glucose.

  • Glucose is split into two three-carbon sugars.

• Energy Payoff Phase:

  • Oxidation of G3P yields 2 NADH.

  • Substrate-level phosphorylation yields 4 ATP.

  • Net products: 2 pyruvate, 2 ATP, 2 NADH.

Substrate-level phosphorylation: Direct transfer of a phosphate group from an intermediate to ADP, forming ATP.

Citric Acid Cycle (Krebs Cycle)

Completing the Breakdown of Glucose

The citric acid cycle completes the oxidation of organic molecules, generating electron carriers and releasing carbon dioxide.

• Each acetyl CoA (from pyruvate) enters the cycle.

• For each turn:

  • 2 CO2 are released.

  • 3 NADH and 1 FADH2 are produced.

  • 1 ATP is generated by substrate-level phosphorylation.

Oxidative Phosphorylation

Electron Transport and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration, where most ATP is produced.

• Electrons from NADH and FADH2 are transferred to the electron transport chain.

• Energy from electron transfer pumps protons (H+) into the intermembrane space.

• ATP synthase uses the proton gradient to synthesize ATP (chemiosmosis).

• Oxygen is the final electron acceptor, forming water.

Fermentation: Anaerobic Harvesting of Energy

ATP Production Without Oxygen

Fermentation allows cells to produce ATP in the absence of oxygen by recycling NAD+ for glycolysis.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2.

• Occurs in muscle cells, yeasts, and certain bacteria.

Evolutionary Significance of Glycolysis

Ancient Pathway

Glycolysis is a universal metabolic pathway found in nearly all organisms. It does not require oxygen or membrane-bound organelles, suggesting it evolved early in the history of life.

Connections Between Metabolic Pathways

Integration of Carbohydrate, Fat, and Protein Metabolism

Cells can use carbohydrates, fats, and proteins as fuel for cellular respiration. Intermediates from these pathways are also used for biosynthesis of other organic molecules.

• Carbohydrates: Sucrose, starch, and other sugars are broken down to glucose.

• Fats: Fatty acids are converted to acetyl CoA.

• Proteins: Amino acids are deaminated and enter the citric acid cycle.

Summary Table: ATP Yield from Cellular Respiration

The following table summarizes the ATP yield from one molecule of glucose:

Stage	ATP Produced (per glucose)
Glycolysis	2 (net)
Citric Acid Cycle	2
Oxidative Phosphorylation	Up to 28
Total	Up to 32

Additional info: Brown fat in humans can produce heat without making ATP, which may have implications for obesity research. Metabolic pathways are regulated by feedback inhibition to maintain homeostasis.