Cellular Respiration: Core Concepts and Pathways

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Cellular Respiration

Core Concepts

1. Steps of a Metabolic Pathway

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. The process involves both catabolic and anabolic reactions, as illustrated below:

Catabolic reactions: Breakdown of molecules (e.g., glucose, fatty acids, amino acids) to release energy.
ATP synthesis: Energy released is used to make ATP, the cell’s main energy currency, by adding a phosphate to ADP.
ATP hydrolysis: When energy is needed, ATP is broken down in another catabolic reaction, releasing its stored energy.
Anabolic reactions: The energy from ATP powers biosynthetic processes (e.g., building glycogen, phospholipids, proteins).

In short: Catabolism breaks down food to release energy, which is used to make ATP. ATP then powers anabolism to build the cell’s needs.

2. Role of Enzymes & Coenzymes (NAD+, FAD)

Cellular respiration is a redox process: Involves oxidation (loss of electrons) and reduction (gain of electrons).
Enzymes: Catalyze each step, helping capture energy instead of losing it as heat.
Coenzymes: NAD+ and FAD act as electron carriers, shuttling electrons to the electron transport chain.

3. How ATP is Made

Substrate-level phosphorylation: Direct ATP production in glycolysis and the citric acid cycle.
Oxidative phosphorylation: ATP made indirectly via the electron transport chain (ETC) and ATP synthase.
ATP synthase: Uses the H+ gradient to power ATP production.
Most ATP comes from oxidative phosphorylation inside the mitochondria.

Types of Respiration

1. Aerobic Respiration

Aerobic respiration requires oxygen and consists of several stages, each occurring in specific cellular locations and producing distinct products.

Glycolysis (Cytoplasm)

Reaction: Glucose (6C) → 2 Pyruvate (3C)
Products: 2 ATP (net), 2 NADH
Notes: Anaerobic step but feeds into aerobic respiration if O2 is present.

Phases of Glycolysis:

Energy investment phase: ATP is used to phosphorylate glucose.
Energy payoff phase: ATP and NADH are produced, ending with two pyruvate molecules.

Net yield per glucose: 2 ATP and 2 NADH.

Pyruvate Oxidation (Mitochondrial Matrix)

Each pyruvate → Acetyl-CoA + CO2 + NADH
Links glycolysis to the citric acid cycle
No ATP made directly in this step

Citric Acid Cycle / Krebs Cycle (Mitochondrial Matrix)

Each Acetyl-CoA → 2 CO2 + 3 NADH + 1 FADH2 + 1 ATP
Per glucose (cycle turns twice): 2 ATP, 6 NADH, 2 FADH2, 4 CO2

Electron Transport Chain (ETC) and Oxidative Phosphorylation

NADH and FADH2 donate electrons to the ETC, which powers the production of ATP via ATP synthase.
Oxygen is the final electron acceptor, forming water.
Most ATP is generated in this stage.

Summary Table: ATP Yield from Aerobic Respiration (per Glucose)

Stage	ATP Produced (net)	NADH Produced	FADH2 Produced
Glycolysis	2	2	0
Pyruvate Oxidation	0	2	0
Citric Acid Cycle	2	6	2
Oxidative Phosphorylation	~26-28	-	-
Total	~30-32	-	-

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

Key Terms and Definitions

ATP (Adenosine Triphosphate): The primary energy carrier in cells.
NAD+ (Nicotinamide Adenine Dinucleotide): Electron carrier reduced to NADH during respiration.
FAD (Flavin Adenine Dinucleotide): Electron carrier reduced to FADH2 during the citric acid cycle.
Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.
Oxidative phosphorylation: ATP synthesis powered by the transfer of electrons through the ETC and the resulting proton gradient.
Redox reaction: Chemical reaction involving the transfer of electrons between two species.

Example: Overall Equation for Aerobic Cellular Respiration

The overall process of aerobic cellular respiration can be summarized by the following equation:

Additional info: This process is essential for providing energy to all eukaryotic cells and is tightly regulated to meet cellular energy demands.