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Difficulty: HardGaseous Exchange and Cellular Respiration

In insects, gaseous exchange occurs through a network of internal air tubes called tracheae that terminate in fine, fluid-filled tracheoles in direct contact with active tissues. During intense muscular exertion, metabolic activity leads to lactic acid accumulation within muscle cells, significantly raising their internal solute concentration. Which of the following best describes the physiological effect of this osmotic shift on gas exchange at the tracheoles?

  1. Water moves by osmosis from the tracheoles into the muscle cells, drawing air deeper into the tracheoles and increasing the surface area available for gaseous diffusion.Answer
  2. B
    Water is osmotically drawn from muscle cells into the tracheoles, expanding the fluid volume to dissolve and flush out excess carbon dioxide through hydrostatic pressure.
  3. C
    The increased cellular acidity causes spiracles to lock completely shut, forcing the insect to rely exclusively on anaerobic cutaneous respiration.
  4. D
    The fluid volume in the tracheoles remains constant while lactic acid chemically converts oxygen gas into bicarbonate ions within the tracheal fluid.

Answer

Water moves by osmosis from the tracheoles into the muscle cells, drawing air deeper into the tracheoles and increasing the surface area available for gaseous diffusion.
During vigorous muscle contraction, anaerobic respiration generates lactic acid within the tissue cells. This increases the internal solute concentration of the muscle cells. Consequently, water moves by osmosis from the fluid-filled tracheole tips into the muscle cells. As the liquid recedes, air travels deeper into the tracheoles, bringing oxygen closer to the mitochondria and speeding up gaseous exchange because oxygen diffuses much faster in air than in liquid.

Step-by-Step Solution

1
Analyze the resting state of tracheoles
At rest, the terminal ends of tracheoles contain liquid, which limits the rate of oxygen diffusion because gases diffuse much slower through liquids than through air.
Understanding baseline physiological conditions is necessary to determine the direction of physical shifts.
2
Determine the osmotic change during intense exertion
Anaerobic respiration in active muscles produces lactic acid, increasing the solute concentration (hypertonicity) inside the muscle cells relative to the tracheole fluid.
Water moves down its water potential gradient toward areas of higher solute concentration.
3
Evaluate the movement of water and its effect on gas exchange
Water moves out of the tracheoles and into the muscle cells by osmosis. As the fluid level drops, air extends further down the tracheoles closer to the cell membranes, dramatically reducing diffusion distance and expanding the surface area for direct gas-phase diffusion.
Gaseous diffusion in air is thousands of times faster than dissolved gas diffusion in liquid.

Key Concept

Insect Tracheal Adaptation and Osmotic Control of Tracheole Fluid
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Topics

Gaseous Exchange and Cellular Respiration

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