Gas Exchange in Insects & Fish

Gas Exchange in Insects & Fish

1. Single-Celled Organisms

   • Microscopic size = Very easy for gas exchange!
   • SA/V ratio = Massive
   • Diffusion is across only 1 cell membrane = v.little distance to travel
   • Take in oxygen and get rid of CO2 easily
   • If there is a cell wall, no barrier to gas exchange as completely permeable

2. Insects

   • Multi-celled organisms
     • Cannot rely on diffusion for gas ex.
       • Surfaces that exchange gasses also facilitate water loss = Problem for insects
   • Covered in waxy coating to help stop water loss, but it's fairly impermeable to gases
   • Tracheal System developed for gas exchange
     • Tiny holes called spiracles
       • Spiracles lead to tubes Tracheae
         • Tubes lines with Chitin - impermeable to gasses & adds structural strength
         • Splits off into smaller TRACHEOLES = gas exchange surfaces. Not lines with chitin.
           • Loads of tracheoles = huge surface area for gas exchange
             • Tracheole ends filled with water
               • When insect is active, water is withdrawn for more efficient diffusion
           • Located near respiring tissues for efficient diffusion
     • Some insects have valve systems that control the size of the spiracles to control water loss
       • Valves only open when respiration rate & need for Oxygen is high and excess CO2 is detected
       • e.g. locusts

3. Larger insects

   • Have trachea can expand and contract called air sacs
     • Move with expansion and contraction of the abdomen and thorax
       • Gas exchange system completely independent from circulatory system

4. Aquatic Insects & Larvae

   • Larvae = immature insect
   • Adaptations to allow gas exchange
     • Some larvae have gill like structures on abdomen
       • Situated near openings to tracheae
       • Have no working spiracles
       • Provide massive surface area for gas exchange
       • Continually move to provide continuous flow of oxygenated water around them
     • Some larvae have elongated breathing tubes
       • Connect to tracheal system and act as snorkels to get oxygen from the air
     • Adult insects: Some beetles attach air bubble to abdomen for oxygen when diving. Others trap oxygen in the hairs on their legs
   • Larvae in poorly oxygenated water have haemoglobin in their body fluids, which has a much higher affinity to oxygen than human hgb.

5. Bony Fish

   • Diffusion not possible
     • Relatively large in size
     • Waterproof skin
   • Internal Gills deveoped to absorb oxygen from surrounding water
     • Gills provide large surface area for gas exch. Very short diffusion distance; high conc. gradient

6. Gill Structure

   • Internal, located behind the head.
   • Made of gill filaments, stacked very closley together.
     • Each filament covered with gill lamellae, at right angles to filaments to offer maximum surface area.
   • How fish perform gas exchange:
     • Take in water through their mouth & direct over gills
       • Gills open and water is expelled from the body
         • Gas exchange takes place as water moves over the gills and blood flows through the gill lamellae in the opposite direction
           • Steep concentration gradient maintained by current flow.

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