Energy & Enzymes: Cellular Respiration and Metabolism (Chapter 7 Study Guide)

Energy & Enzymes: Cellular Respiration and Metabolism

Overview of Biological Energy and Thermodynamics

Biological systems grow and change by processes based on chemical transformation pathways, governed by the laws of thermodynamics. This chapter focuses on how cells obtain, transform, and utilize energy, particularly through cellular respiration and related metabolic pathways.

• Energy: The capacity to do work or cause change. In biological systems, energy is stored in chemical bonds and transferred via metabolic pathways.

• Thermodynamics: The study of energy transformations. The first law states that energy cannot be created or destroyed, only transformed. The second law states that every energy transfer increases the entropy (disorder) of the universe.

• Metabolism: The sum of all chemical reactions in a cell, including catabolic (breaking down molecules to release energy) and anabolic (building molecules using energy) pathways.

Photosynthesis and Cellular Respiration

Relationship Between Photosynthesis and Respiration

Photosynthesis and cellular respiration are interconnected processes. Photosynthesis captures energy from sunlight to produce glucose and oxygen, while cellular respiration breaks down glucose to produce ATP, the energy currency of the cell.

• Photosynthesis: Occurs in chloroplasts of plants and algae; converts light energy, water, and CO2 into glucose and O2.

• Cellular Respiration: Occurs in mitochondria; converts glucose and O2 into ATP, CO2, and water.

• Equation for Cellular Respiration:

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

• Catabolic Pathways: Break down macromolecules to release energy (e.g., glycolysis, Krebs cycle).

• Anabolic Pathways: Use energy to build complex molecules (e.g., photosynthesis).

ATP and Energy Coupling

ATP: The Energy Currency

ATP (adenosine triphosphate) stores and transfers energy within cells. Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate (Pi).

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Energy Coupling: The use of exergonic (energy-releasing) reactions to drive endergonic (energy-consuming) reactions.

• Example: The synthesis of glutamine from glutamate and ammonia is coupled to ATP hydrolysis.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of redox (reduction-oxidation) reactions, where electrons are transferred from one molecule to another.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron Carriers: NAD+ and FAD are key electron carriers that shuttle electrons to the electron transport chain.

Stages of Cellular Respiration

1. Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down one glucose molecule into two pyruvate molecules, producing a small amount of ATP and NADH.

• Inputs: Glucose, 2 NAD+, 2 ADP, 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP (net gain)

• Does not require oxygen (anaerobic).

2. Pyruvate Oxidation

Pyruvate is transported into the mitochondria and converted to acetyl-CoA, producing NADH and releasing CO2.

• Inputs: 2 Pyruvate, 2 NAD+, 2 CoA

• Outputs: 2 Acetyl-CoA, 2 NADH, 2 CO2

3. Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose by oxidizing acetyl-CoA to CO2. It generates NADH, FADH2, and ATP.

• Inputs: 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP, 2 Pi

• Outputs: 4 CO2, 6 NADH, 2 FADH2, 2 ATP

• Main Purpose: To supply high-energy electrons to the electron transport chain.

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is a series of protein complexes in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through the chain, powering the pumping of protons (H+) to create a proton gradient. ATP synthase uses this gradient to produce ATP.

• Inputs: NADH, FADH2, O2

• Outputs: ATP, H2O

• Oxidative Phosphorylation: The production of ATP using energy derived from the redox reactions of the ETC.

• Chemiosmosis: The movement of protons down their gradient through ATP synthase, driving ATP production.

Summary Table: ATP Yield from Cellular Respiration

Stage	Substrate-level Phosphorylation	Oxidative Phosphorylation	Total ATP
Glycolysis	2	~5	~7
Pyruvate Oxidation	0	~5	~5
Krebs Cycle	2	~15	~17
Total	4	~25	~29

Additional info: Actual ATP yield may vary depending on cell type and conditions.

Fermentation

Purpose and Types

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ from NADH, enabling glycolysis to continue.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate (e.g., in muscle cells).

• Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2 (e.g., in yeast).

• Produces much less ATP than aerobic respiration.

Electron Carriers: NAD+ and FAD

Roles in Cellular Respiration

• NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons and becomes NADH, which donates electrons to the ETC.

• FAD (Flavin adenine dinucleotide): Accepts electrons to become FADH2, also donating electrons to the ETC.

• Comparison Table:

Carrier	Reduced Form	ATP Yield (per molecule)
NAD+	NADH	~2.5
FAD	FADH2	~1.5

Case Study: Metabolic Poisons and Cellular Respiration

Effects of Poisons (e.g., DNP, Rotenone) on Respiration

Certain chemicals can disrupt cellular respiration by inhibiting components of the electron transport chain or uncoupling oxidative phosphorylation.

• DNP (2,4-dinitrophenol): Uncouples oxidative phosphorylation by allowing protons to leak across the mitochondrial membrane, reducing ATP production and increasing heat.

• Rotenone: Inhibits electron transfer in the ETC, preventing ATP synthesis.

• Symptoms: Decreased ATP, increased heat production, possible cell death.

Check Your Understanding

• Describe the major steps of cellular respiration, including the inputs and outputs.

• Explain the purpose of cellular respiration and why it is done in multiple steps.

• Describe the differences between substrate-level and oxidative phosphorylation.

• Describe the purpose of NAD+ and FAD in cellular respiration.

• Describe the steps in the electron transport chain and chemiosmosis.

Example Application: If a poison blocks the electron transport chain, ATP production drops, and cells may switch to fermentation for energy, resulting in lactic acid buildup.

Additional info: For more details, refer to textbook sections on metabolism, energy, and cellular respiration.