2. Fish spend a lot of energy removing oxygen from water. Which one of the
following is the most plausible reason?
a. The CO2 content of their tissues is much higher than that of terrestrial animals
b. Fish haemoglobin is much less efficient than mammalian haemoglobin
c. Water has low oxygen content, a large amount must be pumped through gills
d. Fish have a reduced body temperature which slows down reaction rates
Fish expend significant energy extracting oxygen from water due to its low dissolved oxygen content, requiring them to pump large volumes through their gills. The correct answer is option c: Water has low oxygen content, a large amount must be pumped through gills.
Option Analysis
a. The CO2 content of their tissues is much higher than that of terrestrial animals
Fish maintain lower blood CO2 levels (around 2-3 torr) than air-breathers due to efficient CO2 excretion into water via gills, not higher tissue CO2 driving energy use. This option is incorrect as it misrepresents CO2 dynamics.
b. Fish haemoglobin is much less efficient than mammalian haemoglobin
Fish hemoglobins show adaptations like the Root effect for oxygen unloading in tissues and can extract 75-80% of available oxygen via countercurrent exchange, comparable or superior in aquatic contexts to mammalian hemoglobin. Fish Hb autooxidizes faster but efficiency for O2 binding/release suits low-oxygen water; this is not the primary energy cost.
c. Water has low oxygen content, a large amount must be pumped through gills (Correct)
Air holds ~200,000-300 mg/L O2, while water contains just 4-8 mg/L (30-50 times less), demanding fish pump 10-20x more medium volume over gills via buccal-opercular pumping or ram ventilation. This ventilation consumes 10-20% of metabolic energy, far exceeding lung costs in terrestrial animals.
d. Fish have a reduced body temperature which slows down reaction rates
Most fish are ectotherms with temperatures matching water (often 10-25°C), but lower temperature would reduce, not increase, metabolic demand and pumping energy; respiration rates rise with temperature. This option reverses the effect.
Fish spend substantial energy removing oxygen from water primarily because water holds far less dissolved oxygen (4-8 ppm) than air (200,000+ ppm), forcing them to pump massive volumes through gills for respiration. This CSIR NET Life Sciences question tests understanding of aquatic respiration challenges versus terrestrial breathing.
Fish Gills vs Lungs: Energy Cost Breakdown
| Aspect | Fish Gills | Mammalian Lungs |
|---|---|---|
| O2 Availability | 4-8 mg/L water | 200,000 ppm air |
| Extraction Efficiency | 75-80% via countercurrent | ~25% per breath |
| Ventilation Energy | 10-20% metabolism (high volume pumping) | <5% metabolism |
| Mechanism | Buccal pump/ram ventilation | Diaphragm expansion |
Gills excel at extraction but demand constant water flow, unlike passive air diffusion in lungs.
CSIR NET Implications
For exams like CSIR NET, recognize low water O2 solubility drives ventilatory costs, not hemoglobin flaws or CO2/temperature issues. Key: Compare media O2 content quantitatively.
