3 - Cells

• Magnification

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

  1. Magnification = size of image / size of real object

  2. Size of real object = size of image / magnification

  3. Size of image = size of real object x magnification
• Resolution

  The minimum distance apart that two objects can be in order for them to appear as separate items
• Cell fractionation

  The process where cells are broken up and the different organelles they contain are separated out
• Cell fractionation steps

  1. Homogenisation

  2. Filtration

  3. Ultracentrifugation
• Homogenisation

  • Cells are broken up to release organelles
  • The solution should be cold to reduce enzyme activity
  • The solution should be isotonic to prevent organelles from bursting or shrivelling
  • The solution should be buffered to maintain pH and prevent damaging organelles
• Filtration

  • Pass homogenate through gauze to remove cell and tissue debris
• Ultracentrifugation

  1. Filtrate poured into centrifuge tube

  2. Tube of filtrate is placed in the centrifuge and spun at a slow speed

  3. The heaviest organelles, the nuclei, are forced to the bottom of the tube

  4. The fluid at the top of the tube (supernatant) is removed, leaving just the sediment of nuclei

  5. The supernatant is transferred to another tube and spun in the centrifuge at a faster speed

  6. The next heaviest organelles, the mitochondria, are forced to the bottom of the tube

  7. The process is continued in this way so that, at each increase in speed, the next heaviest organelle is sedimented and separated out
• Organelles in order of decreasing density

  • Nucleus
  • Chloroplasts
  • Mitochondria
  • Lysosomes
  • Endoplasmic Reticulum
  • Ribosomes
• Electron microscope

  Exposes specimens to a beam of electrons instead of light
• Electron microscope

  • Very short wavelength = high resolving power
  • Negatively charged = beam can be focussed using an electromagnet
  • Require a vacuum – to prevent air particles interfering with electrons
• Transmission electron microscope (TEM)

  1. The condenser electromagnet focuses beam of electrons

  2. The beam of electrons is transmitted through a thin specimen

  3. The denser parts of the specimen absorb more electrons = darker image
• Resolving power of TEM

  0.1nm although this cannot always be achieved in practice
• Limitations of TEM

  • Difficulties preparing the specimen limit the resolution that can be achieved
  • A higher energy electron beam is required, and this may destroy the specimen
• Disadvantages of TEM

  • The whole system must be in a vacuum and therefore living specimens cannot be observed
  • A complex 'staining' process is required and even the image is not in colour
  • The specimen must be extremely thin
  • The image may contain artefacts
• TEM image

  The electrons must be extremely thin to allow electrons to penetrate. Therefore, the image is 2D
• Scanning electron microscope (SEM)

  1. A beam of electrons focuses onto the surface of the specimen from above

  2. Scan the beam back and forth across specimen

  3. The electrons are knocked off and scattered in a specific pattern

  4. The CRT detects the scattered electrons which produces a 3-D image
• Resolving power of SEM

  Lower than TEM, around 20nm
• Main limitations of TEMs and SEMs

  • Whole system requires vacuum – therefore no living specimens
  • Requires complex staining
  • Images may contain artifacts (damaged specimen)
• Additional limitations of TEM

  • Requires extremely thin specimen to transmit electrons resulting in a flat 2-D image
  • Lower resolution due to difficulty in preparing specimen & high energy beam may destroy specimen
• Eyepiece graticule

  A glass disc that is placed in the eyepiece of a microscope, with a scale etched on it
• Eyepiece graticule

  • 10mm long
  • Divided into 100 subdivisions
  • Visible when looking down the eyepiece of the microscope
• The scale on the eyepiece graticule cannot be used directly to measure the size of objects under a microscope's objective lens because each objective lens will magnify to a different degree
• Calibrating an eyepiece graticule

  1. Find the length (mm) of the stage graticule

  2. Calculate the value of each division

  3. Align eyepiece graticule with stage graticule

  4. Work out what 10 units on the stage graticule are equivalent to on the eyepiece graticule scale

  5. Divide the stage micrometre unit by the number found above to calculate the value of each eyepiece graticule unit
• Nuclear envelope

  A double membrane that surrounds the nucleus. It controls the entry and exit of materials in and out of the nucleus and contains the reactions taking place within it.
• Nuclear pores

  Allow the passage of large molecules out of the nucleus.
• Nucleoplasm

  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. It manufactures ribosomal RNA and assembles the ribosomes.
• Nucleus

  • Site of DNA replication and transcription (making mRNA)
  • Contains the genetic code for each cell
• Endoplasmic Reticulum (ER)


  A 3-dimensional system of sheet like membranes, spreading through the cytoplasm of the cells. It is continuous with the outer nuclear membrane. The membranes enclose a network of tubules and flattened sacs called cisternae.
• Rough endoplasmic reticulum

  Has ribosomes present on the outer surfaces of the membranes.
• Rough endoplasmic reticulum

  • To provide a large surface area for the synthesis of proteins and glycoproteins
  • To provide a pathway for the transport of materials, especially proteins, throughout the cell
• Smooth endoplasmic reticulum

  Lacks ribosomes on its surface and is often more tubular in appearance.
• Cell variation

  Cells all have the same genes, but genes are switched on (expressed) and off in different cells
• Cell variation

  • It is not just the shape of cells that varies, but also the number of organelles
• Smooth endoplasmic reticulum

  • To synthesis, store and transport lipids
  • To synthesise, store and transport carbohydrates
• Cell specialisation

  A specialised cell is a cell that performs a specific function. The structure of the cell helps it to carry out this function
• Cell specialisation

  • The cells of a multicellular organism have evolved to become more and more suited to one specialised function. These cells are adapted to their own function and perform it more effectively
  • As a result, the whole organisms' functions efficiently
• Golgi apparatus and vesicles

  It consists of a stack of membranes that make up flattened sacs, or cisternae, with small rounded hollow structures called vesicles.