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• Light Microscopes

  Cells range in size:

  • animal cells are between 0.01 mm – 0.05 mm
  • plant cells are between 0.01 mm – 0.10 mm

  The human eye can see objects as small as around 0.05 mm. A microscope is required to see cells in any detail.

  Microscopes magnify the image of a biological specimen so that it appears larger. The type of microscope used in a school laboratory is a compound microscope.
• Light microscope

  
• Calculating the magnification of the microscope

  The compound microscope uses two lenses to magnify the specimen – the eyepiece and an objective lens.

  In most microscopes, there is a choice of objective lenses to use. Magnification can be varied according to the size of the specimen and the level of detail required.

  The magnification of a lens is shown by a multiplication sign followed by the amount the lens magnifies, eg ×10.
• Calculating the magnification of an image

  Microscopes use lenses to magnify the image of a biological specimen so that it appears larger.

  The formula to calculate magnification is:

  magnification = size of image

  -------------

  real size of object

  
• Cell size

  Most animal and plant cells are 0.01 – 0.10 mm in size. The smallest thing seen with the naked eye is about 0.05 mm.

  For all cells we need a microscope to see them in any detail.

  The best unit to measure most cells is the micrometre, symbol μm.

  For some sub-cellular structures, for instance ribosomes, or organisms such as viruses, it’s best to use a smaller unit – the nanometre, symbol nm.
• One metre can be broken down into the following measurements:

  
• Standard form

  When writing and working with very large or very small numbers, we use standard form.

  Standard form shows the size of numbers as powers of ten.
• Using standard form for large numbers

  • A population of 120 000 000 microorganisms could be written as 1.2 × 108.
  • This number can be written as 120 000 000.0.
  • If the decimal place is moved eight spaces to the left we get 1.2.
  • So we put x 108 after 1.2 to show this.
  • Because the original number is greater than one metre the minus sign before the 8 is not needed.
  • It makes a very large number easier to write down.
• Using standard form for small numbers

  • A red blood cell's diameter of 7 μm or 0.000007 m could be written as 7 × 10-6 m.
  • This number can be written as 0.000007.
  • If we move the decimal place six spaces to the right we get 7.0
  • So we put x 10-6 after 7 to show this.
  • Because the original number is less than one metre we put a minus sign before the 6.
  • It makes a very small number easier to write down.
• Calculating the magnification of a cell:

  In a book, a micrograph of the cell measured 100 mm.

  The real size of the cell shown is 0.05 mm.

  To calculate the magnification:

  𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛=100𝑚𝑚0.05𝑚𝑚=2000

  It’s important to work in the same units when calculating magnification. Sizes of most cells are given in micrometres, symbol μm.

  
• To calculate magnification using the same formula in micrometres, convert the measurement of the cell above from mm into micrometres:

  Cell measurement = 100 mm

  1 mm = 1000 μm

  100 mm = 100 x 1000 μm

  100 mm = 100 000 μm

  The real size of the cell above in micrometres is 50 μm.

  The magnification of the image:

  𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛=100000𝜇𝑚50𝜇𝑚=2000

  From this we know that the image has been magnified 2000 times.
• Microscopes


  Instruments that use lenses to magnify small objects
• Development of light microscopes

  1. 1590s - Dutch spectacle makers Janssen experimented with putting lenses in tubes, made the first compound microscope

  2. 1650 - British scientist, Robert Hooke observed and drew cells using a compound microscope

  3. Late 1600s - Dutch scientist Antonie van Leeuwenhoek constructed a microscope with a single spherical lens

  4. 1800s - Optical quality of lenses increased, microscopes similar to today's
• No-one knows who first invented the microscope
• Light microscopes


  • Magnification has increased over time
  • Maximum magnification around ×1500
• Janssen's microscopes magnified from ×3 to ×9
• Leeuwenhoek's microscope magnified up to ×275
• Magnification

  Not the only factor that's important when viewing cells
• Detail that can be seen

  Also important when viewing cells
• Resolution or resolving power

  The ability to see two points as two points, rather than merged into one
• Digital photo enlarged


  Over a certain size, you won't be able to see any more detail, it will just become blurry
• Resolution of a light microscope

  Around 0.2 μm, or 200 nm
• Light microscope cannot distinguish two points closer than 200 nm</b>
• Nanometre (nm)


  One billionth of a metre, written as 1 × 10-9m
• Electron microscope


  Uses a beam of electrons instead of light rays
• Types of electron microscope

  • Scanning electron microscope (SEM)
  • Transmission electron microscope (TEM)
• Scanning electron microscope (SEM)

  • Has a large field of view so can be used to examine the surface structure of specimens
  • Often used at lower magnifications
• Transmission electron microscope (TEM)

  • Used to examine thin slices or sections of cells or tissues
  • Has a maximum magnification of around ×1 000 000
  • Images can be enlarged beyond that photographically
  • Limit of resolution is now less than 1 nm
• The TEM has revealed structures in cells that are not visible with the light microscope
• Animal cells

  Almost all animals and plants are made up of cells.

  Animal cells have a basic structure. Below the basic structure is shown in the same animal cell, on the left viewed with the light microscope, and on the right with the transmission electron microscope.

  Mitochondria are visible with the light microscope but can’t be seen in detail. Ribosomes are only visible with the electron microscope.

  
• Cytoplasm


  A jelly-like material that contains dissolved nutrients and salts and structures called organelles. It is where many of the chemical reactions happen.
• Nucleus


  Contains genetic material, including DNA, which controls the cell's activities.
• Cell membrane


  Its structure is permeable to some substances but not to others. It therefore controls the movement of substances in and out of the cell.
• Mitochondria


  Organelles that contain the enzymes for respiration, and where most energy is released in respiration.
• Ribosomes


  Tiny structures where protein synthesis occurs.
• Most cells are specialised and are adapted for their function.
• Animals and plants therefore consist of many different types of cell working together.
• Plant cell


  Basic structure of a plant cell as viewed with light microscope and transmission electron microscope
• Animal and plant cells have certain structures in common
• Cytoplasm


  Jelly-like material that contains dissolved nutrients and salts and structures called organelles, where many chemical reactions happen