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Cellular Respiration: Energy Flow and Metabolic Pathways in Cells

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Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the oxygen-dependent process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the main energy currency of the cell. This process involves a series of metabolic pathways that break down glucose and other fuels, releasing energy that is captured in ATP.

  • Definition: Cellular respiration is a set of metabolic reactions that convert biochemical energy from nutrients into ATP, releasing waste products.

  • General Equation:

  • Key Steps: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle (Krebs Cycle), and Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis).

  • Location: In eukaryotes, most steps occur in the mitochondria; glycolysis occurs in the cytoplasm.

Energy and Matter Flow in Living Systems

Energy flows through living systems via a series of transformations, while matter cycles through these systems in the form of molecules.

  • Energy Flow: Begins with sunlight captured by producers (plants, algae, some bacteria) through photosynthesis, then transferred to consumers via food chains.

  • Matter Cycling: Elements such as carbon, oxygen, and hydrogen cycle through living organisms via metabolic processes.

Photosynthesis and Respiration: Linked Processes

Photosynthesis and cellular respiration are interconnected through the cycling of glucose and energy.

  • Photosynthesis: Converts CO2 and H2O into sugars and O2 using sunlight (occurs in chloroplasts).

  • Respiration: Breaks down sugars with O2 to produce CO2, H2O, and ATP (occurs in mitochondria).

  • Energy Transformation: Sunlight energy is stored in chemical bonds during photosynthesis and released during respiration.

Pathways and Production of ATP

The breakdown of organic molecules is exergonic, releasing energy that is used to synthesize ATP. The main pathways include glycolysis, fermentation, the citric acid cycle, and oxidative phosphorylation.

  • Glycolysis: Anaerobic process in the cytoplasm; splits glucose into two pyruvate molecules, producing 2 ATP and 2 NADH.

  • Fermentation: Occurs in the absence of oxygen; regenerates NAD+ and produces lactic acid (animals) or ethanol (yeast).

  • Aerobic Respiration: In the presence of oxygen, pyruvate enters mitochondria for the citric acid cycle and oxidative phosphorylation, yielding up to 36 ATP per glucose.

Redox Reactions in Cellular Respiration

Cellular respiration involves oxidation-reduction (redox) reactions, where electrons are transferred from one molecule to another, allowing energy to be captured in ATP.

  • Oxidation: Loss of electrons (or hydrogen atoms); glucose is oxidized to CO2.

  • Reduction: Gain of electrons (or hydrogen atoms); O2 is reduced to H2O.

  • Electron Carriers: NAD+ and FAD accept electrons and become NADH and FADH2, which transport electrons to the electron transport chain.

Stages of Cellular Respiration

  • Glycolysis: Occurs in the cytosol; breaks down glucose into 2 pyruvate, 2 ATP (net), and 2 NADH.

  • Pyruvate Oxidation: Pyruvate is converted to Acetyl CoA, producing NADH and releasing CO2.

  • Citric Acid Cycle (Krebs Cycle): Completes the breakdown of Acetyl CoA, generating 3 NADH, 1 FADH2, 1 ATP (per turn), and releasing CO2.

  • Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; uses NADH and FADH2 to generate a proton gradient that drives ATP synthesis via ATP synthase.

ATP Yield from Cellular Respiration

  • Total ATP: Up to 36 ATP molecules per glucose in eukaryotes (actual yield may vary).

  • Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP during glycolysis and the citric acid cycle.

  • Oxidative Phosphorylation: Majority of ATP produced via the electron transport chain and chemiosmosis.

Energy Flow in Ecosystems

Energy flows from producers to consumers in a food chain, with energy lost as heat at each trophic level.

Trophic Level

Energy (kcal/m2/yr)

Example

Primary Producers

20,000

Plants

Primary Consumers

2,000

Herbivores

Secondary Consumers

200

Carnivores

Tertiary Consumers

20

Top Carnivores

Quaternary Consumers

2

Predators

Comparison of Photosynthesis and Respiration

Process

Inputs

Outputs

Location

Photosynthesis

CO2, H2O, Light

O2, Sugars

Chloroplasts

Respiration

Sugars, O2

CO2, H2O, ATP

Mitochondria

Key Terms

  • ATP (Adenosine Triphosphate): The primary energy carrier in cells.

  • Glycolysis: The first step in glucose breakdown, occurring in the cytoplasm.

  • Citric Acid Cycle (Krebs Cycle): A series of reactions in the mitochondrial matrix that further oxidizes Acetyl CoA.

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

  • Electron Transport Chain (ETC): A series of protein complexes in the inner mitochondrial membrane that transfer electrons and pump protons to create a gradient for ATP synthesis.

  • Fermentation: An anaerobic process that allows glycolysis to continue by regenerating NAD+.

Example: ATP Production from One Glucose Molecule

  • Glycolysis: 2 ATP (net)

  • Citric Acid Cycle: 2 ATP

  • Oxidative Phosphorylation: ~32 ATP

  • Total: Up to 36 ATP per glucose

Additional info:

  • Some prokaryotes can perform cellular respiration using electron acceptors other than oxygen (e.g., nitrate or sulfate).

  • Feedback inhibition regulates cellular respiration, with ATP acting as an allosteric inhibitor of key enzymes.

  • Glycogen is the main storage form of glucose in animals, which can be broken down to release glucose when needed.

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