Cellular Respiration

Definition: Cellular respiration is a multi-step metabolic pathway in which organic molecules, such as glucose, are disassembled in a controlled manner by a series of enzymes to eventually form ATP (Adenosine Triphosphate).
Stages: There are four main stages of cellular respiration:
  - Glycolysis
  - Intermediate Stage
  - Citric Acid Cycle (Krebs Cycle)
  - Electron Transport System (ETS)
Locations:
- Glycolysis occurs in the cytosol.
- Intermediate Stage, Citric Acid Cycle, and Electron Transport System occur in the mitochondria.

Questions:
  - What is cellular respiration?
  - What are the four stages?
  - What do we start with in the whole process?
  - What do we end with in the whole process?
Answers:
  - Multi-step metabolic pathway leading to ATP formation from glucose.
  - Stages: Glycolysis, Intermediate Stage, Krebs Cycle, ETS
  - Starts with glucose.
  - Ends with ATP.

Definition: The first stage of cellular respiration; breaks down glucose.
Location: Occurs in the cytosol.
Process: Two main phases:
  - Energy Investment Phase:
    - Consumes 2 ATP and converts glucose to fructose 1,6-bisphosphate.
  - Energy Generation Phase:
    - Produces a net gain of 2 ATP.
    - Produces 2 NADH and 2 pyruvate.
Net Output: 2 ATP, 2 NADH, 2 Pyruvate

Questions:
  - Is glycolysis aerobic or anaerobic?
  - Where does this occur?
  - How many net ATP are created?
  - What is created at the end of glycolysis?
Answers:
  - Anaerobic.
  - Occurs within the cytosol.
  - 2 ATP.
  - 2 ATP and pyruvate are produced.

Oxygen Availability:
  - Sufficient O2: Cells proceed through all four stages of cellular respiration, producing approximately 30 ATP.
  - Insufficient O2: Cells switch to fermentation, leading to lactic acid production, skipping the full cellular respiration process.
  - Lactic Acid Fermentation:
    - Occurs in the cytoplasm.
    - Converts pyruvate into lactate while regenerating NAD+ necessary for glycolysis.

Input: Pyruvate, NAD+, Coenzyme A.
Output: Acetyl Coenzyme A (Acetyl CoA), NADH, and CO₂.
Function: Converts pyruvate into acetyl CoA, which then enters the Citric Acid Cycle.

Input: Acetyl CoA, four-carbon acceptor molecule (oxaloacetate).
Process:
  - Two Acetyl CoA molecules combine with oxaloacetate to form citrate.
  - A series of reactions leads to two CO₂ being produced for each Acetyl CoA.
  - Outputs include:
    - 2 ATP
    - 8 NADH
    - 2 FADH2
Recycling: Oxaloacetate is regenerated for the cycle to continue.
Energy Carrier: Involves reduced electron carriers NADH and FADH2 which will be used in the Electron Transport System (ETS).

Definition: Employs oxidative phosphorylation to produce ATP via a series of electron transfer reactions.
Key Components:
- Electron Transport Chain (ETC)
- Chemiosmosis
Process:
  - Electrons from NADH and FADH2 are passed through complexes in the inner mitochondrial membrane.
  - H ext{+} ions are pumped into the intermembrane space, creating a gradient.
  - ATP Synthase uses this gradient to convert ADP + Pi into ATP.
  - Final electron acceptor is O2, which combines with electrons and H+ to form water (H2O).
Output: Approximately 32 ATP produced.

Questions:
  - Which stage creates the most ATP?
  - What is another name for the ETS?
  - Where do the Intermediate Stage, Krebs Cycle, and ETS take place?
  - What is the enzyme that creates ATP in the ETS?
  - Does aerobic or anaerobic respiration create the most ATP?
Answers:
  - Electron Transport System.
  - Oxidative Phosphorylation.
  - Inside the mitochondria.
  - ATP synthase.
  - Aerobic respiration generates significantly more ATP compared to anaerobic respiration.