Metabolism and Cellular Respiration: Chapter 9 Study Notes

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Metabolism

Overview of Metabolism

Metabolism encompasses all the chemical reactions that occur within an organism, enabling it to maintain life. These reactions are organized into metabolic pathways, which can be classified as either anabolic or catabolic.

Anabolic pathways: Synthesize larger molecules from smaller components, often requiring energy input in the form of ATP.
Catabolic pathways: Break down molecules, harvesting stored chemical energy to produce ATP.
Endergonic reactions: Require energy input (e.g., synthesis of macromolecules).
Exergonic reactions: Release energy (e.g., breakdown of glucose).

Example: The synthesis of proteins from amino acids is an anabolic, endergonic process, while the breakdown of glucose during cellular respiration is catabolic and exergonic.

Redox Reactions

Reduction and Oxidation in Biological Systems

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules, playing a central role in energy transformations within cells.

Reduction: Gain of electrons (and potential energy); often involves the gain of hydrogen (hydrogenation).
Oxidation: Loss of electrons (and potential energy); often involves the loss of hydrogen (dehydrogenation).
Reduced compounds in cells tend to have many C-H bonds, which possess high potential energy.
Oxidized compounds tend to have many C-O bonds, which have lower potential energy due to oxygen's high electronegativity.

Example: During cellular respiration, glucose (C6H12O6) is oxidized to carbon dioxide (CO2), and oxygen (O2) is reduced to water (H2O).

Equation:

Electron Carriers

Electron carriers are molecules that can accept or donate electrons (and hydrogen ions), facilitating redox reactions in metabolism.

Common electron carriers: NAD+, FAD, NADP+
NAD+ is the oxidized form; NADH is the reduced form.
Electron carriers shuttle electrons between metabolic pathways, enabling ATP production.

Example: NAD+ + 2e- + 2H+ → NADH + H+

Cellular Respiration

Purpose and Overview

Cellular respiration is the process by which cells convert the energy stored in chemical bonds of energy-rich molecules (such as glucose) into ATP, which is usable for life processes.

Main goal: To produce ATP from organic molecules.
Overall equation:
Energy is released gradually through a series of controlled redox reactions, not all at once.

Major Steps of Cellular Respiration

Cellular respiration consists of four main steps, each with distinct locations, reactants, and products:

Glycolysis (cytoplasm): Breakdown of glucose into pyruvate; produces ATP and NADH.
Pyruvate oxidation (mitochondrial matrix): Conversion of pyruvate to acetyl CoA; produces NADH and CO2.
Citric acid cycle (Krebs cycle) (mitochondrial matrix): Oxidation of acetyl CoA; produces ATP, NADH, FADH2, and CO2.
Electron transport chain and chemiosmosis (inner mitochondrial membrane): Uses NADH and FADH2 to generate a proton gradient, driving ATP synthesis via oxidative phosphorylation.

Glycolysis

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

Investment phase: Uses 2 ATP to split glucose into two 3-carbon molecules (G3P).
Payoff phase: Produces 4 ATP (net gain of 2 ATP) and 2 NADH per glucose.
Substrate-level phosphorylation: ATP is produced by direct transfer of a phosphate group from a substrate to ADP.

Net yield per glucose: 2 ATP, 2 NADH, 2 pyruvate

Equation:

Regulation of Glycolysis

Glycolysis is regulated by feedback inhibition. High levels of ATP inhibit the enzyme phosphofructokinase-1 (PFK-1), slowing down glycolysis when energy is abundant.

PFK-1: Key regulatory enzyme; inhibited by ATP.
When ATP levels are high, glycolysis slows down.

Fate of Pyruvate

After glycolysis, pyruvate can follow different pathways depending on oxygen availability:

Aerobic conditions: Pyruvate is transported into mitochondria and oxidized to acetyl CoA for entry into the citric acid cycle.
Anaerobic conditions: Pyruvate undergoes fermentation in the cytoplasm.

Lactic Acid Fermentation

Occurs in muscle cells and some bacteria when oxygen is scarce.
Pyruvate accepts electrons from NADH, forming lactate and regenerating NAD+.
Allows glycolysis to continue producing ATP.

Alcohol Fermentation

Occurs in yeast and some bacteria.
Pyruvate is converted to ethanol and CO2, regenerating NAD+.

Pyruvate Oxidation

Pyruvate is converted to acetyl CoA by the enzyme complex pyruvate dehydrogenase (PDH) in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes).

Produces NADH and CO2.
Regulated by feedback inhibition: High levels of ATP, NADH, or acetyl CoA inhibit PDH.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of glucose, generating ATP, NADH, FADH2, and CO2.

Occurs in the mitochondrial matrix (eukaryotes) or cytoplasm (prokaryotes).
Regulated by feedback inhibition at multiple steps.
Per glucose: 2 ATP, 6 NADH, 2 FADH2, 4 CO2

Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) is a series of protein complexes in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through the ETC, releasing energy used to pump protons and create a proton gradient.

Oxygen: Final electron acceptor; forms water.
ATP synthase: Enzyme that uses the proton gradient to synthesize ATP from ADP and Pi (chemiosmosis).
Oxidative phosphorylation: Most ATP (~25-27 per glucose) is produced by this process.

Summary Table: Cellular Respiration Steps

Step	Location	Reactants	Products	ATP Yield	Regulation
Glycolysis	Cytoplasm	Glucose, NAD+, ADP	Pyruvate, NADH, ATP	2 (net)	PFK-1 inhibited by ATP
Pyruvate Oxidation	Mitochondrial matrix	Pyruvate, NAD+, CoA	Acetyl CoA, NADH, CO2	0	PDH inhibited by ATP, NADH, acetyl CoA
Citric Acid Cycle	Mitochondrial matrix	Acetyl CoA, NAD+, FAD, ADP	CO2, NADH, FADH2, ATP	2	Multiple enzymes inhibited by ATP, NADH
ETC & Chemiosmosis	Inner mitochondrial membrane	NADH, FADH2, O2, ADP	ATP, H2O, NAD+, FAD	~25-27	Oxygen availability

Key Terms and Concepts

Metabolism: All chemical reactions in an organism.
ATP (Adenosine Triphosphate): Main energy currency of the cell.
Redox Reaction: Electron transfer between molecules.
Electron Carrier: Molecule that transports electrons (e.g., NAD+, FAD).
Substrate-level phosphorylation: Direct transfer of phosphate to ADP.
Oxidative phosphorylation: ATP synthesis driven by electron transport and chemiosmosis.
Feedback inhibition: Regulation of metabolic pathways by products (e.g., ATP).

Example Questions (from notes)

When a compound is oxidized, it generally loses potential energy.
During photosynthesis, the transfer of electrons from water to the electron transport chain yields oxygen; the molecule receiving electrons does so in a reduction reaction.
Ethanol produced during fermentation contains energy because it still contains C-H bonds that can be oxidized.

Additional info: These notes are expanded with academic context and definitions to ensure completeness and clarity for exam preparation.