Practical: Dissection of Gas Exchange Surfaces in Fish & Insects (OCR A Level Biology)

• Dissections are a vital part of scientific research
• They allow for the internal structures of organs to be examined so that theories can be made about how they function
• There are ethical concerns surrounding dissections
  • People worry about how the animals for dissections are raised and killed
  • It goes against the religious beliefs of some individuals

• The biological specimen used for dissection should be from a reputable source and should be disposed of in the correct manner
• If multiple specimens are being dissected then they should be taken from individual organisms of the same species and roughly the same age

Apparatus

• Scissors
• Scalpel
• Tweezers / Forceps
• Dissection board
• Paper towels
• Biological specimen
• Pins

Method

• A lab coat, gloves and eye protection should be worn
  • To avoid contamination with biological material (which could cause an allergic reaction)

• Place the specimen on the dissecting board
• Use the tools to access the desired structure
  • When using the scalpel cut away from your body and keep your fingers far from the blade to reduce the chance of cutting yourself
  • Scissors can be used for cutting large sections of tissue (cuts do not need to be precise)
  • A scalpel enable finer, more precise cutting and needs to be sharp to ensure this

• Use pins to move the other sections of the specimen aside to leave the desired structure exposed

Limitations

• It can be hard to see some of the smaller, finer structures within organs
• The specimens do not reflect how the tissue would look in a living organism
• If only a single specimen is dissected then anomalies found within that specimen may be ignored or glossed over

Dissection of gas exchange surfaces

• The main structures of the gas exchange systems in mammals and fish can be revealed in dissections
• The much smaller gas exchange systems of organisms such as insects can be more difficult to examine by dissection

Mammalian lungs

• The key structures that can be seen from a dissection of mammalian lungs are shown in the image below
  • Trachea
  • Bronchi
  • Bronchioles

• The smaller structures such as the alveoli can be hard to distinguish in a dissected lung

Image showing the visible structures of the lungs after dissection

Bony fish gills

• The key structures that can be seen from a dissection of fish gills are shown in the image below
  • Gill arch
  • Filaments

• The smaller structures such as the lamellae can be hard to distinguish in a dissected fish

Image showing the visible structures of the gills after dissection

Insect tracheal system

• Due to the small size of insect tracheal systems specialised equipment and skills are sometimes required to dissect them
• Microscopes are also needed to observe the structures

Gas exchange under the microscope

• The gas exchange surfaces of different organisms can be observed using microscopes
• They often appear very different in photomicrographs than they do in the diagrams found in textbooks
• It is important to be able to identify the gas exchange surface and the key structures present

Mammal gas exchange

• A section of stained lung tissue can be seen in the image below
• The alveoli are of different sizes and shapes
  • This is because they are no longer inflated as they would be in a living lung

• The nuclei are shown as dark dots
• Blood vessels can found in between the alveoli
• Sometimes white blood cells are present in tissue samples

Image showing a section of stained lung tissue

Fish

• A section of stained fish gills taken from a dogfish are shown in the image below
• The gill arch resembles a backbone for the gills
• The different filaments are shown with many of the lamellae visible

Image showing a section of fish gills taken from a dogfish

Insect

• As insects are very small obtaining a clear image of their gas exchange system can be difficult
• Electron microscopes can take clear images of the spiracle structures found on the surface of insects, like the one shown below

Image showing a spiracle found in the wall of a caterpillar