Accounting of Energy Molecules & Oxidative Phosphorylation part 5

The overall goal is to harvest energy from glucose to produce ATP through glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation.

A six-carbon glucose molecule is split into two pyruvate molecules.
Net yield: 2 ATP molecules (through substrate-level phosphorylation).
2 NADH molecules are produced.
NADH produced in glycolysis cannot directly cross the inner mitochondrial membrane.
Electrons from NADH are transferred to electron shuttles.
Electron shuttles reduce either NAD+ to NADH or FAD to FADH2, depending on the shuttle.

Each pyruvate molecule loses one carbon, forming a two-carbon acetyl group that attaches to Coenzyme A (CoA), making Acetyl CoA.
2 NADH molecules are generated.

Acetyl CoA feeds into the cycle. The CoA is released, and the acetyl group combines with oxaloacetate.
2 ATP molecules are produced directly.
The cycle generates NADH and FADH2 (reduced forms of NAD+ and FAD).
NADH and FADH2 carry high-energy electrons to the electron transport chain.

The electron transport chain (ETC) is located in the inner mitochondrial membrane.
Electrons from NADH and FADH2 are transferred through the ETC.
Electron transfer is coupled with proton pumping, creating a high proton concentration in the intermembrane space compared to the matrix.
The proton gradient drives ATP synthase, which produces the bulk of ATP.
The number of protons pumped per electron from NADH and FADH2 is not precisely defined, but NADH generally contributes to more proton pumping than FADH2.
The majority of ATP is generated through the electron transport chain and ATP synthase.

Oxygen enters the mitochondria to act as the final electron acceptor in the ETC.
Carbon dioxide (CO2) generated during the citric acid cycle and pyruvate oxidation exits the mitochondria.
Fatty acids can be converted into acetyl groups and enter the citric acid cycle, providing energy from fats.

NADH is oxidized to NAD+ when it donates electrons to the first complex of the ETC.
Three of the ETC complexes pump protons across the inner membrane during electron transfer, building the proton gradient.
Protons flow down their concentration gradient through ATP synthase, driving ATP production.
ATP is transported out of the mitochondria into the cytosol to power cellular processes.
The cell maintains an ATP concentration about 10 times higher than ADP.
ATP is hydrolyzed to ADP and a phosphate group, releasing energy.
ADP and phosphate are transported back into the mitochondrial matrix for ATP regeneration by ATP synthase.
A typical ATP molecule is shuttled out of and back into the mitochondrion for recharging more than once per minute.

Fermentation occurs when oxygen is insufficient to serve as the final electron acceptor in the electron transport chain.
Without oxygen, the electron carriers (NADH and FADH2) cannot release their electrons, leading to a backup in the citric acid cycle and a halt in ATP production via oxidative phosphorylation.
Glycolysis does not require oxygen and can continue even when oxygen is limited.
Glycolysis requires NAD+ to continue operating (specifically in reaction 6, where glyceraldehyde-3-phosphate is converted).
Fermentation regenerates NAD+ from NADH, allowing glycolysis to continue producing a small amount of ATP through substrate-level phosphorylation.

In human muscle cells, lactic acid fermentation occurs when oxygen is scarce.
The enzyme lactate dehydrogenase transfers electrons from NADH to pyruvate, producing lactate and regenerating NAD+.

Yeast performs alcohol fermentation, producing ethanol and CO2.
Pyruvate is converted to acetaldehyde, releasing CO2.
Acetaldehyde accepts electrons from NADH, forming ethanol and regenerating NAD+.

Glycolysis is an interconnected pathway where molecules can be shunted off to build other molecules.
Catabolic pathways funnel electrons from various organic molecules into cellular respiration.

Proteins can be broken down into amino acids.
Amino groups are removed (deamination).
The remaining molecules are converted into pyruvate, acetyl CoA, or other intermediates of the citric acid cycle.
Fats are a major energy source.
Glycerol can be converted into glyceraldehyde-3-phosphate (an intermediate in glycolysis).
Fatty acid tails are broken down into two-carbon acetyl groups, which enter the citric acid cycle.
Fatty acids yield more ATP than glucose due to the large number of acetyl groups that can be generated from their long hydrocarbon tails.

The body uses small molecules (from food or intermediates in glycolysis and the citric acid cycle) to build larger substances.

Knowing the initial reactants and final products of each stage is important for understanding cellular respiration.