Oxidative phosphorylation II

Oxidative Phosphorylation II

The video explains how metabolites and high-energy electrons move across the inner mitochondrial membrane, how ATP and ADP are exchanged, and how the cell’s demand for ATP regulates oxidative phosphorylation. Because the inner membrane is not freely permeable to small molecules, these movements depend on specific transporters and shuttles.

NADH electron shuttles

Cytoplasmic NADH cannot cross the inner mitochondrial membrane. Cells use different shuttles to transfer its electrons into mitochondria while regenerating cytoplasmic NAD⁺ so glycolysis can continue.

Glycerol 3-phosphate shuttle

This shuttle is emphasized for muscle cells:

1. Cytoplasmic NADH transfers its electrons to dihydroxyacetone phosphate (DHAP), converting it to glycerol 3-phosphate and regenerating NAD⁺.
2. Mitochondrial glycerol 3-phosphate dehydrogenase oxidizes glycerol 3-phosphate, transferring the electrons to its FAD cofactor.
3. FAD passes the electrons to ubiquinone (Q), forming QH₂.
4. QH₂ carries the electrons into the electron transport chain at complex III.

Malate–aspartate shuttle

This more complex shuttle is emphasized for heart and liver cells. It uses two membrane transporters and four enzymes. The instructor notes that the key point is the overall electron transfer, rather than memorizing every enzyme.

1. In the cytosol, oxaloacetate accepts electrons from NADH and becomes malate, regenerating cytoplasmic NAD⁺.
2. Malate enters the mitochondrial matrix, where it is oxidized back to oxaloacetate and produces mitochondrial NADH.
3. Because oxaloacetate cannot cross the membrane directly, it is converted to aspartate through a transamination reaction.
4. Transporters exchange malate for α-ketoglutarate and glutamate for aspartate, allowing the shuttle intermediates to cycle.

Both shuttles move electrons into the mitochondrion for use by the electron transport chain, but they do so in different ways.

ADP and ATP transport

Mitochondria need ADP to make ATP and must export the ATP they produce. The ATP/ADP translocase exchanges cytoplasmic ADP for matrix ATP:

• The transporter binds ADP on the cytoplasmic side, changes shape, and releases it into the matrix.
• It then binds ATP in the matrix, changes shape again, and releases ATP to the cytoplasm.
• The exchange is influenced by the proton-motive force, including the membrane’s electrical potential. The video notes that ATP’s negative charge helps favor its movement toward the more positively charged side.

The ATP/ADP translocase, a phosphate carrier, and ATP synthase are described as being associated in a larger complex called the ATP synthasome. Their close arrangement supports coordinated ATP production and transport.

ATP yield and regulation

• The video gives a total of 30 ATP per fully oxidized glucose, with 26 ATP attributed to oxidative phosphorylation and the remainder to substrate-level phosphorylation.
• During fermentation, the video states that only 2 ATP per glucose are generated overall, because NADH electrons are used to regenerate NAD⁺ rather than support oxidative phosphorylation.
• The rate of oxidative phosphorylation depends on the cell’s need for ATP. This regulation is called acceptor control or respiratory control:
  • When ADP is abundant, ATP synthase can produce ATP, and electron flow through the electron transport chain proceeds.
  • When ADP is scarce, oxidative phosphorylation slows.
• The video connects this regulation to the cell’s energy charge, including the balance of ATP with ADP and ATP with AMP.

The lesson ends with four membrane-transport practice problems for viewers to try.

Speakers and sources featured

• Sparks Chemistry instructor/narrator: The only speaking source in the subtitles.
• Diagrams and practice problems: Visual teaching materials referred to by the instructor; no other speakers are identified.