AQA A Level Biology复习笔记3.2.3 Looking at the Gas Exchange under the Microscope

• Many biological structures are too small to be seen by the naked eye
• Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied
• For example, the movement of chromosomes during mitosis can be observed using a microscope

How optical microscopes work

• Light is directed through the thin layer of biological material that is supported on a glass slide
• This light is focused through several lenses so that an image is visible through the eyepiece
• The magnifying power of the microscope can be increased by rotating the higher power objective lens into place

Apparatus

• The key components of an optical microscope are:
  • The eyepiece lens
  • The objective lenses
  • The stage
  • The light source
  • The coarse and fine focus

   

• Other tools used:
  • Forceps
  • Scissors
  • Scalpel
  • Coverslip
  • Slides
  • Pipette

   

Image showing all the components of an optical microscope

Method

• Preparing a slide using a liquid specimen:
  • Add a few drops of the sample to the slide using a pipette
  • Cover the liquid/smear with a coverslip and gently press down to remove air bubbles
  • Wear gloves to ensure there is no cross-contamination of foreign cells

   

• Preparing a slide using a solid specimen:
  • Use scissors to cut a small sample of the tissue
  • Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps)
  • Some tissue samples need be treated with chemicals to kill/make the tissue rigid
  • Gently place a coverslip on top and press down to remove any air bubbles
  • A stain may be required to make the structures visible depending on the type of tissue being examined
  • Take care when using sharp objects and wear gloves to prevent the stain from dying your skin

   

• When using an optical microscope always start with the low power objective lens:
  • It is easier to find what you are looking for in the field of view
  • This helps to prevent damage to the lens or coverslip incase the stage has been raised too high

   

• Preventing the dehydration of tissue:
  • The thin layers of material placed on slides can dry up rapidly
  • Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration

   

• Unclear or blurry images:
  • Switch to the lower power objective lens and try using the coarse focus to get a clearer image
  • Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly
  • There could be cross-contamination with foreign cells or bodies

   

• Using a graticule to take measurements of cells:
  • A graticule is a small disc that has an engraved ruler
  • It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
  • As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
  • By using the two scales together the number of micrometers each graticule unit is worth can be worked out
  • After this is known the graticule can be used as a ruler in the field of view

   

The stage micrometer scale is used to find out how many micrometers each graticule unit represents

Limitations

• The size of cells or structures of tissues may appear inconsistent in different specimen slides
  • Cell structures are 3D and the different tissue samples will have been cut at different planes resulting in inconsistencies when viewed on a 2D slide

   

• Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
• The treatment of specimens when preparing slides could alter the structure of cells

• 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

Dicotyledonous leaves

• Plants often provide good specimens for microscopy
• A section of stained tissue from a dicotyledonous leaf is shown in the image below
• The different layers of tissue and cell types are clear
  • Waxy cuticle
  • Epidermal layers
  • Palisade mesophyll layer
  • Spongy mesophyll layer

   

• The stomata are visible with the guard cells on either side

Image showing a section of stained leaf