Cellular Metabolism, ATP, and Glycolysis: Study Notes

Metabolism and ATP

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within a cell, enabling it to maintain life, grow, and reproduce. These reactions are organized into metabolic pathways, which can be broadly classified as catabolic or anabolic.

• Metabolism: The sum of all biochemical reactions in a cell.

• Catabolism: Breakdown reactions that release energy by decomposing complex molecules into simpler ones.

• Anabolism: Synthetic reactions that consume energy to build complex molecules from simpler ones.

• Example: The breakdown of glucose during cellular respiration (catabolism) and the synthesis of proteins from amino acids (anabolism).

Linking Energetics to Metabolism

Metabolic reactions are governed by the principles of thermodynamics, particularly entropy and free energy changes.

• Catabolism:

  • Increases entropy ($S$)

  • Exergonic (releases energy)

  • Spontaneous

• Anabolism:

  • Decreases entropy ($S$)

  • Endergonic (requires energy)

  • Nonspontaneous

• Equation: The change in free energy ($riangle G$) determines spontaneity: $riangle G = riangle H - T riangle S$

ATP: Structure, Bonds, and Energetics

ATP Structure and Bond Types

Adenosine triphosphate (ATP) is the primary energy currency of the cell. Its structure includes three phosphate groups linked by high-energy bonds.

• Phosphoanhydride bonds: Bonds between adjacent phosphate groups in ATP; these are high-energy bonds.

• Phosphoester bond: Bond between the first phosphate and the ribose sugar.

• ATP hydrolysis: The terminal (last) phosphate is cleaved, releasing energy.

• Standard free energy change: $riangle G^ ext{°'} = -7.3 ext{ kcal/mol}$ for ATP hydrolysis.

Why ATP Hydrolysis is Exergonic

ATP hydrolysis is highly exergonic due to several factors:

• Charge repulsion: The three phosphate groups are negatively charged and repel each other, making the molecule unstable.

• Resonance stabilization: The products of hydrolysis (ADP and inorganic phosphate) are more stabilized by resonance than ATP itself.

• Increased entropy: Hydrolysis converts one molecule (ATP) into two (ADP + Pi), increasing disorder.

Comparison of Phosphate Bonds

ATP contains both phosphoanhydride and phosphoester bonds, but only the phosphoanhydride bonds are considered high-energy.

Bond Type	Location	Energy Released (kcal/mol)	Resonance Stabilization
Phosphoanhydride	Between phosphate groups	-7.3 (ATP/ADP), -3.6 (AMP)	Both products stabilized
Phosphoester	First phosphate to ribose	Lower energy	Only one product stabilized

NAD+ and Redox Reactions

Role of NAD+ in Redox Reactions

Nicotinamide adenine dinucleotide (NAD+) is a key electron carrier in cellular metabolism, participating in redox reactions.

• Oxidation: Loss of electrons (and often hydrogen atoms); molecule becomes more positive.

• Reduction: Gain of electrons (and hydrogen atoms); molecule becomes more negative.

• Mnemonic: "OIL RIG" — Oxidation Is Loss, Reduction Is Gain.

• Dehydrogenases: Enzymes that catalyze the removal of hydrogen atoms during oxidation.

• NAD+ reduction: NAD+ + 2H → NADH + H+

• Example: During glycolysis, NAD+ is reduced to NADH as glucose is oxidized.

Organisms and Oxygen Requirements

Classification Based on Oxygen Use

Organisms are classified by their oxygen requirements, which affects their metabolic pathways.

Type	Oxygen Requirement	Examples
Obligate aerobes	Require oxygen	Most animals, some bacteria
Obligate anaerobes	Oxygen is toxic	Some bacteria, archaea
Facultative anaerobes (Additional info)	Can use oxygen but also grow without it	Yeast, some bacteria

Glycolysis: Pathway and Bioenergetics

Overview of Glycolysis

Glycolysis is a universal metabolic pathway that converts glucose into pyruvate, generating ATP and NADH. It occurs in the cytosol and does not require oxygen.

• Location: Cytosol (some organisms have glycosomes)

• Reactants: Glucose (6 carbons)

• Products: 2 pyruvate (3 carbons each), 2 NADH, 2 net ATP

• Phases: Three main phases: Preparation/Cleavage, Oxidation/ATP Generation, Pyruvate Formation/ATP Generation

Phase 1: Preparation and Cleavage

This phase involves the investment of ATP to phosphorylate glucose and rearrange it for subsequent breakdown.

• Step 1: Glucose is phosphorylated to glucose-6-phosphate (G6P) by hexokinase; ATP is consumed.

• Step 2: G6P is isomerized to fructose-6-phosphate (F6P).

• Step 3: F6P is phosphorylated to fructose-1,6-bisphosphate (F16BP) by phosphofructokinase; another ATP is consumed.

• Step 4: F16BP is split into two 3-carbon molecules (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate).

• Equation: $ext{Glucose} + 2 ext{ATP} ightarrow 2 ext{G3P} + 2 ext{ADP}$

Phase 2: Oxidation and ATP Generation

In this phase, the 3-carbon molecules are oxidized, generating NADH and ATP via substrate-level phosphorylation.

• Step 1: Glyceraldehyde-3-phosphate is oxidized; NAD+ is reduced to NADH.

• Step 2: Addition of inorganic phosphate forms 1,3-bisphosphoglycerate (high-energy intermediate).

• Step 3: Substrate-level phosphorylation transfers phosphate to ADP, forming ATP.

• Equation: $ext{G3P} + NAD^+ + P_i ightarrow 1,3 ext{-BPG} + NADH + H^+$

Phase 3: Pyruvate Formation and ATP Generation

The final steps rearrange the intermediates and generate additional ATP and pyruvate.

• Step 1: Rearrangement forms phosphoenolpyruvate (PEP), a high-energy intermediate.

• Step 2: Substrate-level phosphorylation transfers phosphate from PEP to ADP, forming ATP and pyruvate.

• Equation: $ext{PEP} + ADP ightarrow ext{Pyruvate} + ATP$

Glycolysis Summary Table

Step	ATP Used	ATP Produced	NADH Produced	End Product
Preparation/Cleavage	2	0	0	2 G3P
Oxidation/ATP Generation	0	2	2	2 1,3-BPG
Pyruvate Formation/ATP Generation	0	2	0	2 Pyruvate
Net Total	2	4	2	2 Pyruvate

• Net ATP: 2 (4 produced - 2 used)

• Net NADH: 2

• Net Pyruvate: 2

• Overall free energy change: $riangle G^ ext{°'} = -20 ext{ kcal/mol}$

Summary

• Metabolism includes both catabolic (breakdown) and anabolic (synthesis) reactions.

• ATP is a central intermediate-energy compound with high-energy phosphoanhydride bonds.

• NAD+ is reduced to NADH during redox reactions, carrying electrons and energy.

• Glycolysis is a key metabolic pathway, generating ATP and NADH from glucose without requiring oxygen.

Additional info: Facultative anaerobes can switch between aerobic and anaerobic metabolism depending on oxygen availability.