Looking at the Gas Exchange under the Microscope (AQA A Level Biology)

Apparatus & Techniques: Using an Optical 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

Gas Exchange Under a 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

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