B1.1 - Cell structures

• Eukaryotes


  Organisms made from complex cells called eukaryotic cells (e.g. all animals and plants)
• Prokaryotes


  Organisms made from smaller and simpler cells called prokaryotic cells (e.g. bacteria)
• Both eukaryotic and prokaryotic cells contain sub-cellular structures (parts of cells that each have a specific function)
• Animal Cell


  • Nucleus - contains DNA (genetic material) in the form of chromosomes that controls the cell's activities
  • Cytoplasm - gel-like substance where most of the chemical reactions happen
  • Mitochondria - site of cellular respiration and contain the enzymes needed for the reactions involved
  • Cell membrane - holds the cell together and controls what goes in and out, contains receptor molecules for cell communication
• Plant Cell- same as animal cell except it also has:


  • Rigid cell wall made of cellulose, gives support for the cell
  • Chloroplasts where photosynthesis occurs, contain chlorophyll
• Prokaryotic Cell


  • Chromosomal DNA (one long circular chromosome) controls the cell's activities and replication, floats free in the cytoplasm
  • Plasmids - small loops of extra DNA that aren't part of the chromosome, contain genes for things like drug resistance, can be passed between bacteria
  • Cell membrane - controls what goes in and out, supported by a cell wall
• Microscopes


  Use lenses to magnify images
• Microscopes


  • Increase the resolution of an image
  • Resolution is how well a microscope distinguishes between two points that are close together
• Light microscopes


  • Let us see things like nuclei and chloroplasts
  • Light microscopes were invented in 1590s
• Electron microscopes


  • Invented in 1930s
  • Let us see much smaller things in more detail like the internal structure of mitochondria
  • Have allowed us to have a much greater understanding of sub-cellular structures
  • Only electron microscopes will let us see things as tiny as plasmids or viruses
• Transmission electron microscopes (TEMs)


  • Have a higher magnification and resolution than light microscopes
  • They're not portable
  • They're expensive
  • It's a complicated process to prepare specimens for use
  • They can't be used to look at living tissue, unlike light microscopes
• Eyepiece lens


  Looked through to see the image and also magnifies the image
• Objective lens


  Magnifies the image. Usually there are three different objective lenses (e.g. x4, x10 and ×40)
• Stage


  • Supports the slide
• Clip


  • Holds the slide in place
• Handle


  • To carry the microscope with
• Lamp


  • Shines light through the slide so the image can be seen more easily
• Focusing knobs


  • Move the stage up and down to bring the image into focus
• Specimen Preparation

  1. Specimen needs to let light through it, to be able to see it clearly

  2. Take a thin slice if it is a thick specimen

  3. Use pipette to put a drop of water or mountant on a clean slide

  4. Use tweezers to place specimen on slide

  5. Add a drop of stain if specimen is transparent or colourless to make it easier to see.

  6. Place a cover slip on the specimen

  7. Lower the cover slip onto the slide and press down gently to remove air bubbles
• Using a microscope to view a specimen

  1. Clip the slide containing the specimen onto the stage

  2. Select the lowest-powered objective lens

  3. Use the coarse adjustment knob to move the stage up to just below the objective lens

  4. Move the stage downwards until the specimen is just about in focus

  5. Adjust the focus with the fine adjustment knob until you get a clear image

  6. Swap to a higher-powered objective lens and refocus if needed

  7. Produce a scientific drawing of the specimen
• Total magnification =

  eyepiece lens magnification x objective lens magnification
• Magnification =

  image size / real size (must both be same units)
• Light microscope:

  A) eyepiece lens

  B) eyepiece

  C) handle

  D) objective lenses

  E) coarse adjustment

  F) fine adjustment

  G) focusing knobs

  H) clip

  I) stage

  J) lamp

  
• DNA:

  A) strands

  B) base

  C) cross links

  D) bases

  
• Nucleotide:

  A) phosphate

  B) sugar

  C) base

  
• DNA:

  A) monomer

  B) polymer