Cellular Respiration Overview

Overview

This lecture explains aerobic cellular respiration in eukaryotic cells, focusing on how cells generate ATP, the primary energy currency, through a multi-step process involving glycolysis, the Krebs Cycle, and the electron transport chain.

ATP and Cellular Energy

• ATP (adenosine triphosphate) is the energy currency used by cells for vital processes.
• Both prokaryotic and eukaryotic cells must produce ATP for survival.
• Eukaryotic cells perform aerobic cellular respiration within mitochondria to make ATP.

Aerobic Cellular Respiration Overview

• The main goal of aerobic cellular respiration is to convert glucose into ATP.
• The general equation involves glucose and oxygen as reactants, producing carbon dioxide, water, and ATP as products.
• The process is similar but not opposite to photosynthesis; plants conduct both processes.

Step 1: Glycolysis

• Glycolysis occurs in the cytoplasm and does not require oxygen (anaerobic).
• Glucose is converted into 2 pyruvate, yielding a net gain of 2 ATP and 2 NADH.
• NADH acts as an electron carrier for later steps.

Intermediate Step: Pyruvate to Acetyl CoA

• Pyruvate is transported into the mitochondrial matrix.
• Pyruvate is oxidized into 2 acetyl CoA, releasing carbon dioxide and producing 2 NADH.

Step 2: Krebs Cycle (Citric Acid Cycle)

• The Krebs Cycle occurs in the mitochondrial matrix and requires oxygen (aerobic).
• Each acetyl CoA is processed, producing 2 ATP, 6 NADH, 2 FADH2, and releasing carbon dioxide.
• FADH2 is another electron carrier similar to NADH.

Step 3: Electron Transport Chain & Chemiosmosis

• Occurs on the inner mitochondrial membrane and is strictly aerobic.
• NADH and FADH2 transfer electrons to protein complexes, creating a proton gradient.
• Protons flow through ATP synthase, powering the synthesis of ATP from ADP.
• Oxygen accepts electrons at the end, forming water as a byproduct.
• This step creates the majority of ATP (range is 26-34 ATP per glucose).

Total ATP Yield & Variability

• The total net ATP produced per glucose molecule ranges from 30-38, depending on various factors.
• The exact ATP number can vary due to differences in cellular conditions and proton gradients.

Alternative Pathways & Importance

• In the absence of oxygen, some cells use fermentation, which is less efficient than aerobic respiration.
• Inhibiting any step, such as with cyanide, can halt ATP production and be lethal.
• Mitochondrial function is crucial; research on mitochondrial diseases continues to grow.

Key Terms & Definitions

• ATP (Adenosine Triphosphate) — Energy-carrying molecule used by cells.
• Glycolysis — Breakdown of glucose to pyruvate in cytoplasm, producing ATP and NADH.
• Pyruvate — Product of glycolysis; intermediate for further energy production.
• Acetyl CoA — Molecule entering the Krebs Cycle after pyruvate oxidation.
• Krebs Cycle (Citric Acid Cycle) — Series of reactions in mitochondria producing NADH, FADH2, ATP, and CO₂.
• NADH/FADH2 — Electron carriers transferring high-energy electrons to the electron transport chain.
• Electron Transport Chain — Mitochondrial process creating proton gradient and majority of ATP.
• ATP Synthase — Enzyme making ATP using proton gradient.
• Fermentation — Anaerobic process generating ATP without oxygen.

Action Items / Next Steps

• Review steps of cellular respiration and understand their inputs/outputs.
• Read further about the Citric Acid Cycle and proton gradients as suggested.
• Explore video or resources on fermentation for comparison.