energy release (exam 3)

How Cells Make ATP

• Energy-Releasing Pathways

Energy Sources for Organisms

• Autotrophs: Organisms that produce their own organic molecules through photosynthesis.

• Heterotrophs: Organisms that live on organic compounds produced by other organisms.

• All organisms utilize cellular respiration to extract energy from organic molecules.

Cellular Respiration

Overview

• Cellular respiration involves a series of reactions that:

  • Are oxidations (loss of electrons).

  • Are dehydrogenations (loss of electrons accompanied by hydrogen).

  • Result in the loss of a hydrogen atom (1 electron, 1 proton).

Redox Reactions

• In redox reactions, electrons carry energy from one molecule to another.

• NAD+ is a key electron carrier:

  • Accepts 2 electrons and 1 proton to form NADH.

  • The reaction is reversible.

Types of Respiration

• Aerobic respiration: Final electron receptor is oxygen (O2) and is more common.

• Anaerobic respiration: Final electron acceptor is an inorganic molecule (not O2) and is less efficient.

• Fermentation: Final electron acceptor is an organic molecule.

Electron Pathways in Respiration

• During respiration, electrons are transferred through electron carriers to a final electron acceptor with the ultimate goal of producing ATP.

Stages of Aerobic Respiration

1. Glycolysis:

   • Glucose (6 carbons) splits into 2 pyruvate (3 carbons).

   • Does not require oxygen.

   • Produces 2 NADH and 2 ATP (net yield).

2. Pyruvate Oxidation

3. Krebs Cycle:

4. Electron Transport Chain & Chemiosmosis.

Glycolysis

Energy Investment Phase

• Splitting of glucose requires an investment of 2 ATP.

• Glucose is converted into glucose-6-phosphate, making it more reactive.

Energy Capture Phase

• Results in a net yield of 2 ATPs and 2 NADH:

  • Four ATPs and two NADH produced per glucose molecule.

Pyruvate Oxidation

• In the presence of oxygen, pyruvate undergoes oxidative decarboxylation to form Acetyl-CoA:

  • Releases CO2 and transfers electrons to NAD+ to form NADH.

Citric Acid Cycle (Krebs Cycle)

• For each glucose, 2 acetyl groups enter the cycle:

  • Each acetyl group combines with oxaloacetate to form citrate.

  • NADH and FADH2 are generated.

  • Total energy captured includes 2 ATP, 6 NADH, and 2 FADH2 per glucose.

Electron Transport Chain

• A series of membrane-bound electron carriers located in the inner mitochondrial membrane.

• Electrons from NADH and FADH2 are transferred through complexes in a series of redox reactions, leading to ATP synthesis via oxidative phosphorylation.

Chemiosmosis

• Protons (H+) accumulate in the intermembrane space and create a gradient.

• Protons flow back into the mitochondrial matrix through ATP synthase, which uses the gradient to synthesize ATP from ADP and Pi.

Summary of Aerobic Respiration Stages

• Glycolysis: Glucose is converted to pyruvate, producing a net of 2 ATP and 2 NADH.

• Formation of Acetyl CoA: Pyruvate is converted to Acetyl CoA with the release of CO2 and production of NADH.

• Citric Acid Cycle: Acetyl CoA is oxidized to CO2, yielding ATP, NADH, and FADH2.

• Electron Transport and Chemiosmosis: Electron carriers generate a proton gradient, leading to ATP synthesis.

Regulation of Respiration

• Regulated by feedback inhibition where ATP and citrate inhibit pathways to manage energy production.

Anaerobic Processes

Anaerobic Respiration

• Involves the use of inorganic molecules as the final electron acceptor, such as NO3 or SO4.

Fermentation

• Organic molecules function as the final electron acceptor, producing byproducts like ethanol or lactic acid without using the electron transport chain.

Comparison of Metabolic Pathways

• Aerobic vs. Anaerobic respiration and fermentation differ in electron acceptors and energy yields.

• Both pathways allow cells to generate ATP through substrate-level phosphorylation—anaerobic respiration uses a different final electron acceptor compared to aerobic respiration.

Catabolism of Macromolecules

• Carbohydrates are the main energy source.

• Proteins and Lipids can also be broken down into intermediates for use in metabolism.