AGR130: Cellular Respiration Notes

AGR130: Biology in Agriculture - Cellular Respiration

Overview of Respiration

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• Respiration involves breaking down glucose into smaller molecules to capture the energy stored in chemical bonds.

• The overall reaction is: C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + \text{Chemical energy}

• This process uses glucose and oxygen to produce carbon dioxide, water, and energy.

Processes of Respiration

1. Glycolysis

   • Glucose is broken down into pyruvate (a 3-carbon molecule).

   • Produces some ATP.

2. Pyruvate Processing

   • Pyruvate is prepared for the citric acid cycle.

3. Citric Acid Cycle

   • Also known as the Krebs cycle.

   • Produces NADH and FADH2 for oxidative phosphorylation, and some ATP.

4. Oxidative Phosphorylation

   • Involves the electron transport chain.

   • Produces a large amount of ATP.

Energy Molecules: ATP, NADH, and FADH2

• ATP (Adenosine Triphosphate) is the primary energy currency of the cell.

• NADH and FADH2 are electron carriers that provide electrons for the electron transport chain.

• Phosphorylation reactions add phosphate groups to molecules, while redox reactions involve the transfer of electrons.

Redox Reactions

• Redox reactions involve reduction and oxidation, which always occur together.

• Oxidation involves the loss of electrons (or hydrogen) or the gain of oxygen.

• Reduction involves the gain of electrons (or hydrogen) or the loss of oxygen.

  • Oxidation: - Hydrogen (H), - electron (e-), + Oxygen (O)

  • Reduction: + Hydrogen (H), + electron (e-), - Oxygen (O)

Location of Respiration

• Glycolysis: Occurs in the cytoplasm.

• Pyruvate processing and Citric Acid Cycle: Take place in the mitochondrial matrix.

• Electron Transport Chain and Chemiosmosis/Oxidative Phosphorylation: Occur in the inner mitochondrial membrane (cristae).

Glycolysis

• Two Phases:

  1. Glucose Activation

  2. Energy Harvest

• Net Output per Glucose:

  • 2 ATP

  • 2 NADH

  • 2 Pyruvate

• Primitive Energy Recovery:

  • Occurs in nearly all organisms.

  • Does not require O2 directly.

  • Occurs in the cell cytosol.

Fermentation

• Occurs when glycolysis cannot proceed due to a lack of NAD+ under anaerobic conditions.

• It is an alternative to the Krebs cycle to replenish NAD+ for glycolysis.

• Produces toxic by-products and has very low efficiency.

• Examples include lactic acid fermentation and ethanol fermentation.

Preparation of Pyruvate

• Convert 3-carbon pyruvate to 2-carbon acetyl CoA.

• Net Output per Glucose:

  • 2 NADH

  • 2 Acetyl CoA

Citric Acid Cycle (Krebs Cycle)

• A series of enzyme-catalyzed reactions that take place in the mitochondrial matrix.

• Acetyl CoA combines with a four-carbon acceptor molecule (oxaloacetate) to form a six-carbon molecule (citrate).

• Through a series of reactions, two carbon atoms are lost as CO_2, and ATP, NADH, and FADH2 are produced.

• The four-carbon acceptor molecule is regenerated.

• Summary of Krebs cycle:

  • 3 NAD^+ are converted to 3 NADH

  • FAD is converted to FADH_2

  • Acetyl CoA (2C) and the cycle releases CO_2

  • ATP is generated from ADP

• Net Output per Glucose:

  • 6 NADH

  • 2 FADH2

  • 2 ATP

  • 8 CO_2

Oxidative Phosphorylation

• Captures the energy in NADH & FADH2.

• Takes place across the inner mitochondrial membrane.

• Chemical energy in NADH and FADH2 from the Krebs cycle is converted to ATP.

• Involves the electron transport chain.

• Uses oxygen as the final electron acceptor (oxidative).

• ADP is converted to ATP (phosphorylation).

Electron Transport Chain

• Electrons are passed down a chain, setting up an H^+ gradient.

• O2 is the final electron acceptor, completing the reaction: C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H_2O

ATP Synthase

• Utilizes the H^+ gradient set up by the electron transport chain.

• ATP synthase is an enzyme complex that phosphorylates ADP to ATP.

• Total Energy Yield per Glucose: 36-38 ATP.

Summary of Respiration

• Respiration is the breakdown of glucose into available energy.

• Four main processes:

  • Glycolysis occurs in the cytosol.

  • Pyruvate preparation and the citric acid cycle occur in the mitochondrial matrix.

  • Oxidative phosphorylation occurs in the cristae membranes.

  • ATP accumulates in the matrix.

• Overall inputs: 1 glucose, 6 O_2, ADP.

• Overall outputs: 36-38 ATP, 6 CO2, 6 H2O