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Introduction to Microscopic Work

HOW DOES IT WORK?                                                                             

The cell is the basic unit of all living things. When cells join together to take on a specialized function within a larger organism, they form a tissue. All organisms are made up of at least one cell.  Large organisms, such as humans, are made up of trillions of cells.  Animal and plant cells share similar characteristics as well as differences. In this lab you will observe these similarities and differences under a microscope. You will be looking at cells from both plants and animals.

Experiment 1: Observing plant cells (Onion).                                 

Time allocation: 45 Min

Students will use a microscope to view cells in a wet suspension. A wet suspension is a sample that is viewed in a liquid medium pressed between the microscope slide and the cover slip, the tissue is still living. This allows us the opportunity to see the cell in a living environment. At first the structure of the tissue will be viewed at 4 x magnification then at 10 x magnification enabling students to view the major structure of the cells.  At 40 x magnification students will be able to note some of the components of individual cells. In this way students are able to compare cells and their differences.

Experiment 2: Observing animal cells (Human cheek swab).            

Time allocation: 45 Min

The cells lining of your mouth is constantly being replaced. The old cells that are ready to slough off can easily be collected. Use a toothpick to scrape cells from the inside of your cheek. Prepare a wet-mound slide be sure to use the methyl-blue stain as well. The methyl-blue stain makes the cell components more visible.

SCIENTIFIC RULES ABOUT DRAWING

  • Use a sharp HB pencil or 0,5mm clutch pencil with HB lead.
  • Use single, solid, thin continuous lines, when drawing scientific diagrams.
  • No shading or colouring of any parts of a scientific diagram.
  • Make sure that the shape, parts as well as the parts are in relation to each other.
  • Drawings of biological specimens such as cells should have a scale.
  • Draw label lines with a ruler, with pen, from the middle of the structure that must be labelled.
  • No crossing of label lines.
  • Label lines underneath each other.
  • Start labels on the right hand side of diagram, if necessary labels may be placed on the left hand side.
  • All labels horizontal, next to label lines.

SECTIONAL VIEWS OF CELLS

Longitudinal – the representation of an object as it would appear if cut by the vertical plane passing through the longest axis of the object.

Cross section – a section made by a plane cutting anything at angles to the longest axis.

Cells under a microscope:

 

Cells are very small, so to be able to see them we need to use a microscope or look at photographs taken under a microscope. These photographs are called micro-graphs.

Relative size of cells:

 

The smallest objects we can see with our eyes can be measured in millimeters. What we see under the light microscope is measure in units called micrometers (μ). A micrometer is one thousandth of a millimeter. Structures viewed under an electron microscope are measured in nanometers.

How to determine the actual size of a structure:

We can work out the size of cells or smaller structures from a micro-graph by using the magnification of the microscope when the photo was taken, and the scale line on the micro-graph.

The transmission micro-graph of the mitochondrion, above, was taken at a magnification of 64 000 x.

We can calculate the actual length, from point A to point B of the mitochondrion by using the magnification or using the scale line.

  • The mitochondrion is magnified 64 000 x.
  • Measure the length from A to B using your ruler in millimeter.
  • Now multiply the measure length by 1 000 to convert the length into micrometers
The actual length of the mitochondrion = The measured length in micrometresThe magnification

The scale line shows the length of 1 μ on the photo.

  • Measure the length of the scale line.
  • The measured length represents 1μ.
  • Now measure the length of the mitochondrion in mm.
The actual length =

 

Measured length (mm)Length of scale line (mm)

How to make a wet mount:

  • Cut a lowercase letter. preferably an ‘e’, from the newspaper
  • Place it onto the center of a clean slide
  • Put a drop of water on the top of the letter using a dropper pipette
  • If too much water is added, the cover slip will “float” creating a water layer that is too thick
  • If too little water is added, the specimen may be crushed or dry out too quickly
  • Place the edge of a cover slip against the water and with a pencil gently lower the cover slip over the letter
  • Placing the cover slip in the manner prevents air bubbles from forming underneath the cover slip.

 

Observation: Plant Cells and Animal Tissue

EXPERIMENT 1: OBSERVING PLANT CELLS.

AIM:     

  Time allocation: 45 Min

Observing plant cells using a light microscope.

APPARATUS:

  • Compound microscope
  • Scalpel
  • Forceps
  • Pipette Dropper
  • Iodine Solution
  • Microscope slides
  • Cover slips
  • Tissue paper
  • Onion
  • Water

METHOD:

  1. Peel the delicate transparent tissue from the inner surface of a piece of onion using forceps, it should peel off easily. This tissue will be as thin and flexible as plastic wrap.
  2. Make a wet mount by placing the onion tissue, unwrinkled, in a small drop of water on a glass slide.
  3. Add one small drop of iodine stain onto the onion tissue and cover with a cover slip.
  4. Press down with tissue paper to securely seal the sample in place.
  5. Examine the onion epidermis at 4 x, 10 x and 40 x Magnification and Try to identify the parts of the onion cell. Look for the nucleus, cytoplasm and cell wall.

OBSERVATION:

Experiment 1: Observing plant cells

EXPERIMENT 2: OBSERVING ANIMAL CELLS (HUMAN CHEEK SWAB).

Aim:

Time allocation: 45 Min

Observing animal cells using a light microscope.

Apparatus:

  • Compound microscope
  • Microscope slides
  • Cover slips
  • 1% Methylene blue stain
  • Toothpick
  • Glass beaker with water

METHOD:

 

  1. Using a toothpick, take a swab of tissue from the inside of the subject’s mouth.
  2. Place a drop of water on a clean microscope slide using the pipette dropper.
  3. Stir the material on the toothpick in the drop of water on the slide
  4. Add one drop of methylene blue stain to the microscope slide, which helps makes the cell components more visible.
  5. Examine the slide at 4 x, 10 x and 40 x Magnification. Try to identify the parts of the cell. Look for the nucleus, cytoplasm and cell wall.

  1. OBSERVATION:

    Experiment 2: Observing animal cells (human cheek swab).

PRECAUTIONS:

  • When working with the cover slips act with caution as they break easily.
  • Cut the onion on a hard surface.
  • Always cut in a direction away from your body.
  • Students must not share toothpicks.
  • The cheeks should be scraped gently avoiding any injury.
  • When working with the wet mount experiment students must wear gloves, to prevent the spread of diseases.
  • Place the samples in a sink before dying to prevent bench top stains.
  • When working with burner never leave a flame unattended.
  • Chemical safety – Methylene Blue solution:
    • Caution consumption of chemical by an allergic individual must be dealt with by a medical professional.
    • The chemical is primarily used as stain, if spilt may stain clothing or surface, try cleaning surface with rubbing alcohol Methylene Blue Stain.
    • If ingested, do not induce vomiting, loosen clothing, if person is not breathing perform first aid and call for emergency medical attention.
    • If spilled rinse surface off until there is no trace left of the stain on surface.
    • Chemical can be disposed through normal sewerage methods.

Basic Care instructions for proper handling of a Microscope:

A Microscope is a Fragile Piece of Equipment please adhere to the following precautions on how to best use the Microscope without damaging it:

  • If the Microscope is inside a carry case, ensure it is on a flat surface before you proceed to open it.
  • Always carry a Microscope with BOTH hands, one must be Under the base of the Microscope and the other should be holding the microscope at the Neck (The middle section of the microscope). DO NOT hold it by the Eyepiece or Objective lens or Stage area.
  • Dont directly touch any of the optical lenses with your fingers, pens, pencils or other objects/liquids.
  • Don’t attempt to clean the lenses with tissue paper or any type of cloth, a microfiber cloth used for normal glasses can be used to clean it, otherwise lens paper is a special type of paper that can be used to clean an optical lens.
  • Everytime you use the microscope there is a cardinal rule to be followed as you wil ALWAYS start on the Lowest Magnification with the stage at its HIGHEST POINT, then move on to the Medium and High Powered lenses while adjusting the stage height.
  • You WILL BREAK the slide being observed if you adjust the stage too close to the lens you are magnifying it under, this usually happens when we are using the Highest Power Lens so ensure you move away from the lens and check where the stage height is in relation to the Objective lens being used.
  • Call your teacher to assist you if you do not know where the Fine, Coarse adjustment knobs or Light source knob are, as it is better to ask for assistance rather than to damage the microscope.
  • If anything is broken inside the microscope DO NOT attempt to dismantle or fix the microscope yourself as it’s internal components are very fragile and can be further damaged in this manner.