Cell structure

• The cell is the basic unit of life
• Cells are not visible to the naked eye and their structure is only apparent when seen under a microscope
• Microscopes


  Instruments that produce a magnified image of an object
• Magnification


  How many times bigger the image is when compared to the object
• Resolution


  The minimum distance apart that two objects can be in order for them to appear as separate items
• The resolving power of a light microscope is about 0.2 μm
• The beam of electrons in the electron microscope can distinguish between two objects only 0.1 nm apart
• Cell fractionation


  1. Cells are broken up

  2. Different organelles are separated out
• Ultracentrifugation


  1. Filtered homogenate is spun

  2. Heaviest organelles sediment

  3. Supernatant is removed

  4. Next heaviest organelles are spun again
• Before cell fractionation, the tissue is placed in a cold, buffered solution of the same water potential as the tissue
• Cold buffered solution


  • Reduces enzyme activity
  • Prevents organelles bursting or shrinking
  • Maintains pH
• Units of length


  • kilometre
  • metre
  • millimetre
  • micrometre
  • nanometre
• Increasing the magnification does not always increase the resolution
• The electron microscope has a high resolving power due to the short wavelength of the electron beam
• Transmission Electron Microscope (TEM)

  Produces a 2-D image by passing electrons through a thin specimen
• Scanning Electron Microscope (SEM)


  Produces a 3-D image by scanning the surface of the specimen
• The resolving power of the TEM is 0.1 nm
• The main limitations of the TEM include the need for a vacuum and the requirement for extremely thin specimens
• Artefacts may appear on photomicrographs but are not part of the natural specimen
• The SEM does not require specimens to be extremely thin
• The series of photomicrographs produced is a slow and complicated process
• Scanning Electron Microscope (SEM)

  A microscope that directs a beam of electrons onto the surface of the specimen from above
• Transmission Electron Microscope (TEM)


  A microscope that penetrates specimens from below
• Scanning Electron Microscope (SEM)

  • Specimens need not be extremely thin
  • Produces 3-D images by computer analysis
  • Lower resolving power than TEM
• The basic SEM has a lower resolving power than a TEM, around 20 nm, but is still ten times better than a light microscope
• Electrons do not penetrate in SEM
• Calibrating an eyepiece graticule

  1. Use a stage micrometer

  2. Line up scales

  3. Calculate length of divisions

  4. Record results for future use
• Eyepiece graticule


  A glass disc placed in the eyepiece of a microscope with a scale etched on it
• The scale on the eyepiece graticule cannot be used directly to measure the size of objects under a microscope's objective lens
• The nuclear envelope is a double membrane that surrounds the nucleus
• Nucleus


  • Contains hereditary material
  • Controls cell activities
  • Typically spherical and between 10 and 20 μm in diameter
• Nuclear pores allow the passage of large molecules, such as messenger RNA, out of the nucleus
• There are typically around 3000 pores in each nucleus, each 40-100 nm in diameter
• Nucleoplasm


  The granular, jelly-like material that makes up the bulk of the nucleus
• Chromosomes


  Consist of protein-bound, linear DNA
• Nucleolus


  A small spherical region within the nucleoplasm that manufactures ribosomal RNA and assembles ribosomes
• Mitochondria


  • Usually rod-shaped
  • 1-10 μm in length
  • Sites of aerobic respiration
• Mitochondria are responsible for the production of the energy-carrier molecule, ATP
• Functions of the nucleus


  • Control centre of the cell
  • Retain genetic material
  • Manufacture ribosomal RNA and ribosomes
• Chloroplasts are the organelles that carry out photosynthesis