Cells

• Cell Theory


  Cells are the basic structural unit of all living organisms
• Cell Theory


  Until microscopes became powerful enough to view individual cells, no-one knew for certain what living organisms were made from
• Cell Theory


  Robert Hooke came up with the term "cells" in the 1660's after examining the structure of cork
• Cell Theory


  Matthias Schleiden and Theodor Schwann came up with the idea that all living organisms are made of cells in 1837
• Cell Theory


  The cell theory is a unifying concept in biology (meaning it is universally accepted)
• Main ideas of the cell theory


  • All living organisms are made up of one or more cells
  • Cells are the basic functional unit (i.e. the basic unit of structure and organisation) in living organisms
  • New cells are produced from pre-existing cells
• Although cells vary in size and shape they all are surrounded by a membrane, contain genetic material, and have chemical reactions occurring within the cell that are catalysed by enzymes
• Deductive reasoning


  An approach where one progresses from general ideas to specific conclusions
• Based on cell theory, a newly discovered organism can be predicted to consist of one or more cells
• Cytology


  The branch of biology which focuses on cell theory
• Optical (light) microscopes


  An invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied

  
• Key components of an optical (light) microscope


  • Eyepiece lens
  • Objective lenses
  • Stage
  • Light source
  • Coarse and fine focus

  
• Other tools that may be used


  • Forceps
  • Scissors
  • Scalpel
  • Coverslip
  • Slides
  • Pipette
• Preparing a temporary mount slide using a liquid specimen


  1. Add a few drops of the sample to the slide using a pipette

  2. Cover the liquid/smear with a coverslip and gently press down to remove air bubbles

  3. Wear gloves to ensure there is no cross-contamination of foreign cells
• Preparing a temporary mount slide using a solid specimen


  1. Use scissors to cut a small sample of the tissue

  2. 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)

  3. Some tissue samples need be treated with chemicals to kill / make the tissue rigid

  4. A stain may be required to make the structures visible depending on the type of tissue being examined

  5. Gently place a coverslip on top and press down to remove any air bubbles

  6. Take care when using sharp objects and wear gloves to prevent the stain from dying your skin
• Using an optical microscope


  1. Place the microscope slide on the stage, fix in place using the stage clips (ensure the microscope is plugged in and on)

  2. Always start with the low power objective lens

  3. Move the coarse focusing knob until the specimen comes into focus, then use the fine focusing knob to sharpen the focus

  4. To examine the whole slide, move it carefully with your hands (or if using a binocular microscope use the stage adjusting knobs)

  5. Carefully move to a higher objective lens (10X and 40X), but do not move the stage down

  6. At the higher objective powers only use the fine focusing knob

  
• Graticule


  A small disc that has an engraved scale, it can be placed into the eyepiece of a microscope to act as a ruler in the field of view

  
• Stage micrometer


  A scale engraved on a microscope slide, used to calibrate the graticule
• Magnification


  How many times bigger the image of a specimen observed is in comparison to the actual (real-life) size of the specimen
• Micrometer (μm)


  A unit of measurement used for the size of cells and cellular structures
• Nanometer (nm)


  A unit of measurement used for the size of cellular structures, smaller than micrometers
• Producing a scale bar


  1. Use the eyepiece graticule and stage micrometer to calculate the distance between two markings on the eyepiece graticule, this is the graticule unit

  2. Measure the length of the specimen using the eyepiece graticule which will be in graticule units

  3. Determine the length of the specimen in micrometers by multiplying the number of graticule units by the length of each unit

  4. Draw your specimen and measure the length of your drawing in mm, your scale bar should be 20% of the length of your specimen drawing

  5. Add the actual length your scale bar represents underneath your scale bar

  
• Using a scale bar


  1. Use a ruler to measure the length of the scale bar in millimetres (mm)

  2. Convert this measurement into the same units as the number on the scale bar

  3. Insert these numbers into the magnification formula
• Magnification


  Tells you how many times bigger the image produced by the microscope is than the real-life object you are viewing
• Resolution


  The ability to distinguish between objects that are close together (i.e. the ability to see two structures that are very close)
• Data can be collected about cell and organelle sizes
• Qualitative data


  Non-numerical data such as colour and presence of structures which can also be determined using microscopes
• Making observations and taking measurements form the basis for developing new hypotheses in Biology
• Magnification


  How many times bigger the image produced by the microscope is than the real-life object you are viewing
• Resolution


  The ability to distinguish between objects that are close together (i.e. the ability to see two structures that are very close together as two separate structures)
• Types of microscopes


  • Optical microscopes (light microscopes)
  • Electron microscopes
• Optical (light) microscopes


  • Use light to form an image
  • Have a maximum resolution of around 0.2 micrometres (µm) or 200 nm
  • Can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts
  • Cannot be used to observe smaller organelles such as ribosomes, the endoplasmic reticulum or lysosomes
  • Have a maximum useful magnification of about ×1500

  
• Electron microscopes


  • Use electrons to form an image
  • Have a maximum resolution of around 0.0002 µm or 0.2 nm (i.e. around 1000 times greater than that of optical microscopes)
  • Can be used to observe small organelles such as ribosomes, the endoplasmic reticulum or lysosomes
  • Have a maximum useful magnification of about ×1,500,000

  
• Types of electron microscopes


  • Transmission electron microscopes (TEMs)
  • Scanning electron microscopes (SEMs)
• Transmission electron microscopes (TEMs)


  • Use electromagnets to focus a beam of electrons
  • The beam of electrons is transmitted through the specimen
  • Denser parts of the specimen absorb more electrons, making these denser parts appear darker on the final image produced
  • Give high-resolution images (more detail)
  • Can only be used with very thin specimens or thin sections of the object being observed
  • Cannot be used to observe live specimens
  • Do not produce a colour image

  
• Scanning electron microscopes (SEMs)


  • Scan a beam of electrons across the specimen
  • The beam bounces off the surface of the specimen and the electrons are detected, forming an image
  • Can produce three-dimensional images that show the surface of specimens
  • Can be used on thick or 3-D specimens
  • Allow the external, 3-D structure of specimens to be observed
  • Give lower resolution images (less detail) than TEMs
  • Cannot be used to observe live specimens
  • Do not produce a colour image

  
• The resolving power of an electron microscope is much greater than that of the light microscope, as structures much smaller than the wavelength of light will interfere with a beam of electrons
• Learn the difference between resolution and magnification! Also, learn how the light and electron microscope differ in terms of resolution and magnification.

  
• The microscope has undergone many developments since the first one used in the 1600s by Robert Hooke
• Optical (light) microscope developments


  • Condenser lenses have been developed to direct light from the light source through the specimen
  • Use of fluorescent stains and immunofluorescence can be used to view cellular structures such as RNA