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Difficulty: HardGaseous Exchange and Cellular Respiration

During mammalian aerobic respiration, inhaled oxygen in the pulmonary alveoli must travel through multiple anatomical compartments and fluid media to serve as a terminal electron acceptor in tissue cellular respiration. What is the correct sequential path taken by an oxygen molecule from alveolar air space to its final metabolic reduction inside a muscle cell mitochondrion?

  1. 1Diffusion across the alveolar epithelium and pulmonary capillary endothelium into blood plasma.
  2. 2Reversible binding to the heme iron moiety (Fe2+Fe^{2+}) of hemoglobin inside red blood cells.
  3. 3Transport through pulmonary veins to the left side of the heart for systemic arterial delivery.
  4. 4Unbinding from hemoglobin and diffusion across tissue capillary walls into cell cytoplasm.
  5. 5Reduction by electrons and protons at the inner mitochondrial membrane to form water (H2OH_2O).

Answer

The correct sequential order begins with oxygen diffusing across the alveolar and capillary endothelial walls into blood plasma, followed by binding to hemoglobin in red blood cells, circulatory transport through pulmonary veins to the left heart and systemic arterial system, dissociation and diffusion across tissue capillaries into cell cytoplasm, and ultimately its reduction to water at the inner mitochondrial membrane.
Inhaled oxygen moves from the alveolar air space across the thin alveolar epithelial membrane and pulmonary capillary endothelium into blood plasma. It then passes into red blood cells where it binds reversibly to the heme iron of hemoglobin. This oxygenated blood travels via pulmonary veins into the left side of the heart, which pumps it into systemic arterial circulation. Upon reaching systemic capillaries in active tissues, the low partial pressure of oxygen induces dissociation from hemoglobin, allowing oxygen to diffuse into tissue interstitial fluid and cell cytoplasm. Finally, oxygen diffuses into the mitochondrial matrix and inner mitochondrial membrane, acting as the terminal electron acceptor in oxidative phosphorylation to produce water.

Step-by-Step Solution

1
Identify the primary site of external gas exchange across the respiratory membrane.
Oxygen moves out of the alveolar lumen, passing sequentially through the alveolar epithelial cell layer, basement membrane, and endothelial cell layer into blood plasma.
Diffusion occurs passively from a region of high partial pressure of oxygen (PO2≈104 mmHgPO_2 \approx 104\text{ mmHg}) in the alveoli to lower partial pressure in deoxygenated capillary blood.
2
Determine how oxygen is bound for bulk transport in blood.
Dissolved oxygen in plasma moves across erythrocyte cell membranes and binds reversibly to the ferrous iron (Fe2+Fe^{2+}) center of hemoglobin.
Hemoglobin binding allows the blood to transport significantly higher volumes of oxygen than dissolved plasma alone.
3
Trace the macro-circulatory movement of oxygenated blood.
Oxygenated blood flows from pulmonary capillaries into pulmonary veins, entering the left atrium, passing to the left ventricle, and being propelled into the systemic arterial tree.
Pulmonary veins carry oxygen-rich blood back to the heart to provide hydraulic pressure for systemic tissue distribution.
4
Analyze the mechanism of oxygen delivery to metabolizing tissue cells.
In systemic capillaries, low tissue PO2PO_2 promotes oxygen dissociation from hemoglobin; free oxygen diffuses across the capillary wall, through interstitial fluid, and across the plasma membrane into cytosol.
Active tissue metabolism continuously consumes oxygen, creating a steep concentration gradient favoring unloading.
5
Identify the final intracellular biochemical sink for oxygen.
Oxygen diffuses into mitochondria, reaching the inner mitochondrial membrane where it accepts electrons from Complex IV (cytochrome c oxidase) and combines with protons (H+H^+) to yield water (H2OH_2O).
Oxygen acts as the ultimate electron acceptor in oxidative phosphorylation, enabling the continued flow of electrons along the electron transport chain.

Key Concept

Respiratory gas exchange pathway and cellular oxygen delivery in mammalian physiological respiration
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Topics

Gaseous Exchange and Cellular Respiration

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