Prokaryotic Cells

Prokaryotic Cells

1. Its cells lack a nucleus

   • The DNA is free floating in a circle

2. e.g. bacteria cell

   • Cell wall is made up of murein

3. Cell wall

   • Functions
     • Excludes substances
     • Protects against mechanical damage and osmotic lysis

4. Cell surface membrane

   • This is inside the cell wall
     • It encloses the cytoplasm
   • Function
     • Controls entry and exit of chemicals
       • It has a permeable layer

5. Ribosomes

   • Size
     • 70S in prokaryotic cells
   • Function
     • Protein synthesis

6. Circular strand of DNA

   • (DNA is still a double helix)
   • It is not associated with proteins
     • i.e. not wrapped around protein
   • Found in central area of the cell called the nucleoid
   • Function:
     • Stores genetic information for replication of bacterial cells

7. Microscopy

   • Maginifcation
     • This is how many times bigger the image is than the actual object
     • Magnification = size of image/size of real object
   • Resolution
     • This is the minimum distance apart two objects can be in order to appear as separate items
     • The human eye has a resolution of 0.2mm
     • The light microscope has a resolving power of 0.2 micro metres
     • The best electron microscope has a resolution of 0.1nm
   • The light microscope
     • Specimens must be thin sections of prepared tissue that has been stained with coloured pigments
       • Or they can be small live specimens like Daphnia
       • Sections must be thin to allow light to pass through and to make sure you are only looking at a single layer of cell
       • They must be stained as sections are often transparent
     • A beam of light is shone through the specimen through a series of lenses resulting in a magnified image
     • Very little intracellular detail can be seen
     • The eyepiece graticule
       • Usually around 10mm long with 100 sub divisions
       • Stage micrometer
         • This is usually 1mm long it goes on the specimen stage
       • Calibrating the eyepiece graticule
         • Because the scale remains constant no matter what magnification you're on, you must change the eyepiece graticule
   • The electron microscope
     • This uses a beam of electrons rather than light
       • The electron beam has a shorter wavelength than light which means the resolving power is higher
     • Transmission electron microscope (TEM)
       • Ultra thin sections needed, must be stained and heavy metals are needed
       • Where electrons pass thought the specimen the image appears bright and when the electrons are absorbed the image is dark
       • A near vacuum is required
         • Specimen must be dead
       • 2D black and white images
     • Scanning Electron Microscope (SEM)
       • Electrons do not penetrate the specimen
         • The beam is passed to and fro over the specimen surface in a regular pattern
           • Electrons are scattered depending on the contours
       • Produces a 3D image
         • Black and white
       • Resolution is lower than that of the TEM
   • Artefacts
     • Due to all of the processes the specimens must undergo under microscopes, images may contain objects that shouldn't be there
     • An artefact may be a break in a membrane, empty spaces in cells
       • In the TEM heavy metal stains are used, so granular deposits may be seen
     • It is therefore not possible to be sure what we see on an image is the actual natural specimen

8. Some prokaryotic cells have

   • Plasmid
     • Small circular strand of DNA
     • Function:
       • Gives genes that aid survival
         • e.g. they can reproduce with antibiotic resistance
     • Used extensively as vectors
   • Capsule
     • This is mucilaginous slime around the outside of the cell wall
     • Functions:
       • It stops bacteria being deteced
         • This protected them from other cells
       • It helps bacteria stick together for protection
   • Flagellum
     • Function
       • Used for locomotion
         • Makes bacteria move

9. Viruses

   • This is an acellular, non living particle
   • Contains nucleic acids such as DNA or RNA as genetic material enclosed in a protein coat or capsid
     • Capsid or envelope have attachment proteins
       • These allow the virus to identify and attach to a host cell
   • Can only multiply inside a living host cell
   • May be surrounded by a living host cell

Images and files in the map