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Cellular Respiration

Glycolysis • Krebs Cycle • Electron Transport • ATP

Agenda

1. What cellular respiration is
Overall purpose and equation
2. Mitochondria structure
Matrix, membranes, cristae
3. Glycolysis
Glucose split into pyruvate
4. Krebs cycle
Carbon dioxide and electron carriers
5. Electron transport system
Proton gradient and ATP synthase
6. Aerobic vs anaerobic
Role of oxygen and fermentation

Big Picture

  • Cellular respiration releases energy from glucose and stores much of it in ATP.
  • Its major stages are glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation.
  • Glycolysis produces a net gain of 2 ATP and 2 NADH per glucose, while later mitochondrial stages generate much more ATP.
Overall flow
Glucose → Pyruvate
Pyruvate → Acetyl-CoA
Acetyl-CoA → Krebs cycle
NADH/FADH2 → Electron transport chain
ADP + Pi → ATP

Mitochondrion Structure

  • Mitochondria have an outer membrane, an inner membrane, an intermembrane space, and a matrix.
  • NCBI explains that the inner membrane folds into cristae, which greatly increase surface area for oxidative phosphorylation.
  • The matrix contains enzymes for pyruvate oxidation and the Krebs cycle.
Labeled mitochondrion diagram

ATP, ADP, and Energy Transfer

ATP

ATP is the cell’s main energy currency and releases usable energy when its terminal phosphate bond is broken.

ADP

ADP has one less phosphate than ATP and can be recharged back into ATP by phosphorylation.

Cycle

Respiration couples energy from glucose breakdown to the conversion of ADP + Pi into ATP.

Glycolysis

  • Glycolysis occurs in the cytosol and splits one 6-carbon glucose into two 3-carbon pyruvate molecules.
  • Glycolysis has a net yield of 2 ATP and 2 NADH per glucose.
  • This step can occur with or without oxygen.
Glycolysis pathway diagram

After Glycolysis

Pyruvate Oxidation

Each pyruvate enters the mitochondrial matrix and is converted to acetyl-CoA, producing carbon dioxide and NADH.

Why It Matters

Acetyl-CoA is the 2-carbon molecule that feeds into the Krebs cycle and connects glycolysis to the mitochondrial stages.

Krebs Cycle

  • The Krebs cycle, also called the citric acid cycle, takes place in the mitochondrial matrix.
  • It oxidizes acetyl-CoA and releases carbon dioxide.
  • It generates ATP, NADH, and FADH2 for later ATP production.
Krebs cycle diagram

NADH and “NADHase” Context

NADH

NAD+ accepts electrons and hydrogen to become NADH. NADH then transports high-energy electrons to the electron transport system.

NADH Oxidation

In class use, “NADHase” usually refers to enzymes that oxidize NADH back to NAD+; in respiration, this oxidation occurs as NADH donates electrons into the mitochondrial electron transport chain.

Electron Transport System

  • The electron transport chain is embedded in the inner mitochondrial membrane.
  • Electrons from NADH and FADH2 move through carriers, and the released energy pumps protons across the membrane.
  • This creates the proton gradient used by ATP synthase.

Oxygen and ATP Synthase

Final Electron Acceptor

Oxygen accepts electrons at the end of the chain and combines with hydrogen ions to form water.

Chemiosmosis

The proton gradient stores potential energy across the inner membrane.

ATP Synthase

As protons flow back through ATP synthase, ADP is phosphorylated to ATP.

ATP Yield by Stage

Glycolysis2 ATP net
Krebs cycle2 ATP per glucose
Oxidative phosphorylationMost ATP

Aerobic Respiration

With Oxygen

Aerobic respiration uses oxygen as the terminal electron acceptor and allows the electron transport chain to keep running.

High ATP Yield

Because oxidative phosphorylation continues, aerobic respiration produces much more ATP than fermentation pathways.

Anaerobic Respiration / Fermentation

  • When oxygen is unavailable, cells cannot keep aerobic electron transport running.
  • StatPearls explains that pyruvate can be reduced to lactate so NADH is oxidized back to NAD+, allowing glycolysis to continue.
  • This pathway is much lower in ATP yield and typically gives only 2 ATP per glucose from glycolysis.

Key Takeaways

Glycolysis begins in the cytosol, the Krebs cycle and electron transport steps occur in mitochondria, NADH delivers electrons, oxygen is the final electron acceptor in aerobic respiration, and ATP is the main usable energy product of the pathway.

Thank you

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