How Cells Harvest Energy

Introduction

This chapter explores how cells obtain and utilize energy from organic molecules through cellular respiration. The main metabolic pathways discussed include glycolysis, pyruvate oxidation, the Krebs cycle, and the electron transport chain. Understanding these processes is fundamental to general biology, as they explain how living organisms convert food into usable energy.

Organisms Obtain Organic Molecules

Autotrophs and Heterotrophs

Organisms are classified based on how they obtain organic molecules necessary for energy and growth.

• Autotrophs: These are "self-feeders" such as plants, algae, and some bacteria. They use solar energy to produce organic materials through photosynthesis.

• Heterotrophs: These include all other living organisms that must consume organic materials produced by autotrophs. They are "fed by others" and rely on ingesting organic molecules for energy.

Example: Plants synthesize glucose from sunlight, while animals eat plants (or other animals) to obtain organic molecules.

Overview of Respiration

Cellular Respiration

Both autotrophs and heterotrophs harvest energy from organic materials using similar metabolic pathways. Cellular respiration is the process by which cells oxidize organic compounds to extract energy stored in chemical bonds.

• Definition: Cellular respiration is the oxidation of organic compounds (such as glucose) to produce energy in the form of ATP.

• General Equation for Respiration:

Change in free energy: kcal/mole of glucose

Example: The breakdown of glucose during cellular respiration releases energy that cells use to perform work.

Oxidation and Reduction in Respiration

Redox Reactions

Cellular respiration involves a series of oxidation-reduction (redox) reactions, where electrons are transferred between molecules, releasing energy.

• Oxidation: Loss of electrons from a molecule; the molecule becomes oxidized.

• Reduction: Gain of electrons by a molecule; the molecule becomes reduced.

• Dehydrogenation: In respiration, oxidation reactions often involve the removal of hydrogen atoms (each consisting of one proton and one electron).

Example: In the oxidation of glucose, glucose loses electrons and is oxidized, while oxygen gains electrons and is reduced.

Electron Carriers and Energy Transfer

NAD+ and NADH

During cellular respiration, electrons (often accompanied by protons as hydrogen atoms) are transferred from glucose to electron carriers such as NAD+ (nicotinamide adenine dinucleotide).

• NAD+ accepts electrons and is reduced to NADH.

• NADH stores energy that can be used to synthesize ATP.

• Electron carriers shuttle electrons to the electron transport chain, where the energy is used to produce ATP.

Example: NAD+ is reduced to NADH during glycolysis and the Krebs cycle, then NADH donates electrons to the electron transport chain.

Mechanisms for ATP Synthesis

Substrate-Level and Oxidative Phosphorylation

Cells synthesize ATP through two main mechanisms:

• Substrate-level phosphorylation: A phosphate group is directly transferred from a substrate molecule to ADP, forming ATP.

• Oxidative phosphorylation: Energy from electrons transferred through the electron transport chain is used to phosphorylate ADP to ATP. This process occurs in eukaryotes and aerobic prokaryotes.

ATP: The energy currency of the cell, used to drive endergonic (energy-requiring) reactions.

Stages of Aerobic Respiration

Four Main Stages

The complete oxidation of glucose in aerobic respiration proceeds through four major stages:

1. Glycolysis: The breakdown of glucose into pyruvate, producing ATP and NADH.

2. Pyruvate Oxidation: Conversion of pyruvate into acetyl-CoA, generating NADH and releasing CO2.

3. Krebs Cycle (Citric Acid Cycle): Acetyl-CoA is oxidized, producing ATP, NADH, FADH2, and CO2.

4. Electron Transport Chain & Chemiosmosis: Electrons from NADH and FADH2 are transferred through protein complexes, driving ATP synthesis via chemiosmosis.

Note: Students are not expected to memorize every detailed step of glycolysis and the citric acid cycle, but should understand the overall process, key intermediates, and main energy-producing steps, including important enzymes involved.

Final Electron Acceptors

Types of Respiration

The final electron acceptor in respiration determines the type of process:

Example: In human muscle cells under low oxygen, pyruvate acts as the final electron acceptor in lactic acid fermentation.

Review Questions

Sample Questions

• When a substance is oxidized, it loses electrons.

• When a substance is reduced, it gains electrons.

Additional info: The above review questions reinforce the definitions of oxidation and reduction in biological systems.