Chapter 6: Harvesting Chemical Energy – Study Guide

Harvesting Chemical Energy

Major Themes and Learning Objectives

This chapter explores how living organisms acquire and utilize energy through a series of chemical reactions. These metabolic pathways have evolved to enable biological work and are central to life on Earth.

Key Concepts and Definitions

Metabolic Intermediates and Pathways

• Metabolic Intermediate (Intermediate): A compound formed in one step of a metabolic pathway and used in the next step.

• Substrate-Level Phosphorylation: The direct transfer of a phosphate group to ADP from a phosphorylated intermediate, producing ATP.

• Oxidative Phosphorylation: The production of ATP using energy derived from the redox reactions of an electron transport chain.

• Chemiosmosis: The movement of ions (usually H+) across a selectively permeable membrane, down their electrochemical gradient, coupled to ATP synthesis.

• Electron Transport Chain (ETC) / Electron Transport System (ETS): A series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions, coupled with the transfer of protons across a membrane.

• ATP Synthase: An enzyme that synthesizes ATP from ADP and inorganic phosphate, powered by the flow of protons across a membrane.

Energy Sources in Living Organisms

• Autotrophs: Organisms that produce their own food from inorganic substances (e.g., plants via photosynthesis).

• Heterotrophs: Organisms that obtain energy by consuming organic molecules produced by other organisms (e.g., animals, fungi).

• Comparison: Autotrophs use light or inorganic chemicals as energy sources, while heterotrophs rely on organic compounds.

Oxidation-Reduction (Redox) Reactions

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Role in Cells: Redox reactions transfer energy by moving electrons from one molecule to another, essential for cellular respiration.

• Example: In cellular respiration, glucose is oxidized and oxygen is reduced.

Summary Reaction of Cellular Respiration

• Overall Equation:

$\mathrm{C_6H_{12}O_6 + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O + \text{energy (ATP + heat)}}$

• Reactants: Glucose (C6H12O6), Oxygen (O2)

• Products: Carbon dioxide (CO2), Water (H2O), ATP

• Cellular Sites: Glycolysis (cytoplasm), Pyruvate oxidation and Krebs cycle (mitochondrial matrix), ETC and chemiosmosis (inner mitochondrial membrane)

• Oxidized: Glucose

• Reduced: Oxygen

Electron Carriers in Metabolism

• NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons and becomes NADH.

• FAD (Flavin adenine dinucleotide): Accepts electrons and becomes FADH2.

• Role: Shuttle electrons to the ETC for ATP production.

Stages of Cellular Respiration

1. Glycolysis

• Location: Cytoplasm

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

• Products: 2 Pyruvate, 2 NADH, 2 ATP (net)

2. Pyruvate Oxidation (Transition Step)

• Location: Mitochondrial matrix

• Reactants: 2 Pyruvate, 2 NAD+, 2 CoA

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

3. Krebs Cycle (Citric Acid Cycle)

• Location: Mitochondrial matrix

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

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

4. Electron Transport Chain and Chemiosmosis

• Location: Inner mitochondrial membrane

• Reactants: NADH, FADH2, O2, ADP, Pi

• Products: NAD+, FAD, H2O, ATP

• Process: Electrons from NADH and FADH2 pass through the ETC, pumping protons to create a gradient. ATP synthase uses this gradient to produce ATP (chemiosmosis).

ATP Yield per Glucose

Stage	ATP Produced (per glucose)
Glycolysis	2
Pyruvate Oxidation	0
Krebs Cycle	2
ETC & Chemiosmosis	~26-28
Total	~30-32

Note: The actual ATP yield is often lower due to leaky membranes and energy costs for transporting molecules.

Fermentation

• Role: Allows ATP production in the absence of oxygen by regenerating NAD+ from NADH.

• Location: Cytoplasm

• Reactants: Glucose

• Products: 2 ATP (from glycolysis), various end products (e.g., lactate, ethanol, CO2)

• Conditions: Occurs when oxygen is scarce or absent.

Fermentation vs. Cellular Respiration

Feature	Fermentation	Cellular Respiration
Oxygen Required?	No	Yes
ATP Yield (per glucose)	2	~30-32
End Products	Lactate or ethanol + CO2	CO2 + H2O
Electron Acceptor	Organic molecule	O2

Alternative Energy Sources

• Polysaccharides: Broken down to glucose and enter glycolysis.

• Lipids: Glycerol enters glycolysis; fatty acids are converted to acetyl-CoA (beta-oxidation) and enter the Krebs cycle.

• Proteins: Amino acids are deaminated and converted to intermediates of glycolysis or the Krebs cycle.

Example: During fasting, the body uses stored fats, which are converted to acetyl-CoA and oxidized for energy.

Additional info: The efficiency of cellular respiration is about 34%, with the rest of the energy lost as heat. This heat helps maintain body temperature in endothermic organisms.