2.1.1. Cell structure

• Magnification is the ratio of image size to object size (size of image/size of object).
• Resolution is the ability to distinguish between two objects that are close together as separate entities, allows to see more detail
• Light microscopes have a magnification of 1000-2000x, and a resolution of 50-200 nm. They are used for viewing cells and tissues.

  Light microscopes are cheap and easy to use and allow us to see living things, but their resolution is limited.
• Scanning electron microscopes have a magnification of 50,000-500,000x, a resolution of 0.4-20 nm, and are used for viewing the surface of cells and organelles, and providing depth in three dimensional images.
• Transmission electron microscopes  have a magnification of 300,000-1,000,000x, a resolution of 0.05-1.0 nm, and are used for detailing organelle ultrastructure.
• Scanning electron and transmission electron microscopes have better resolution than a light microscope, which means it is worth magnifying the image more as the image will show more detail. Scanning electron microscopes also:• give 3D images with depth of field• are good for viewing surfaces
• Electron microscopes are large and very expensive. They require trained operativesThe sample must be dried out and is therefore dead. This may affect the shape of the features seen (called an artefact).

  The image is in black and white, but colours may be added later by computer graphics. These are called false colour electron micrographs.
• Laser scanning confocal microscopes have a magnification of 1000-2000x, a resolution of 50-200 nm, and are used to produce three dimensional images with good depth selection.Laser scanning confocal microscope can also see living things and have the advantage that they can focus at a specific depth so the image is not confused by other components that are not in focus. It relies on a computer to piece together all the information from the dots of light created by the lasers. This means that the image is an interpretation rather than a real-life image.
• Most cell components are colourless and hard to see. Staining is the application of coloured stains to the tissue or cells. Staining:

  1. makes objects visible in light microscopes
  2. increases contrast so that the object can be seen more clearly
  3. is often specific to certain tissues or organelles. For example, acetic orcein stains chromosomes dark red, eosin stains cytoplasm, Sudan red stains lipids, and iodine in potassium iodide solution stains the cellulose in plant cell walls yellow and starch granules blue/black
• In an electron microscope, the stains are actually heavy metals or similar atoms that reflect or absorb the electrons.
• Flagella is a large extension of the cell surface membrane containing microtubules (in eukaryotes). Its able to beat to enable locomotion or move fluids
• Golgi apparatus is responsible for modifying proteins made in ribosomes by repackaging them into vesicles for secretion. It often acts as a carbohydrate group.
• Lysosomes are small vacuoles containing hydrolytic or lytic enzymes.
• Mitochondria is the site of aerobic respiration for ATP synthesis
• Nucleus, nucleolus (assembles ribosomes) and nuclear envelope (separates genetic material from cytoplasm) contains genetic material (chromosomes) and controls cell activity. The nuclear pores allow molecules of mRNA to pass from the nucleus to the ribosomes in the cytoplasm.
• Ribosomes are the site of protein synthesis
• Rough endoplasmic reticulum holds many of the ribosomes and provides a large surface area for protein synthesis.
• Smooth endoplasmic reticulum is associated with the synthesis, storage and transport of lipids and carbohydrates.
• Cell wall provides structural support and surrounds plant cells.
• Centrioles are involved in the organisation of microtubules that make up the cytoskeleton. They form the spindle fibres used to move chromosomes in nuclear division.
• Chloroplasts are the site of photosynthesis.
• Cilia is small hair-like extensions of cell surface membrane containing microtubules that are able to move whole organisms or fluid (mucus) across a surface
• Cytoskeleton is a network of microtubules and microfilaments that _

  • provides mechanical strength, support and structure for the cell.
  • maintains the cell shape and is used in some cells to change the shape.
  • enables movement of organelles/ vesicles inside the cell
  • enables movement of the whole cell/ enocytosis/ exocytosis/ phagocytosis/ cytokinesis
  • holds organelles in position
  • formation/ movement of cilia/ flagella
• Prokaryote

  • small, 10um long 1-2um wide
  • no nucleus and no membrane bound organelles
  • 70s ribosomes (18nm)
  • single loop of DNA, no histones
• Eukaryote

  • large, 10um diameter
  • nucleus present and membrane bound organelles
  • 80s ribosomes, 22nm
  • DNA associated with histone proteins
• What piece of equipment do you need to make an accurate measurement of the length of something being viewed under a microscope?

  eyepiece graticule
• Explain how to measure a cell's organelle

  • use eyepiece graticule
  • calibrate the graticule using a stage micrometre
  • measure in graticule units
  • take repeat measurements and calculate a mean diameter in epu
  • use calibrated epu to calculate the measurement
• When preparing tissue as a sample

  • use a sharp blade so the slide is thin enough and individual cells will be visible/ resolution is high
  • have a method for slicing pieces of tissue (e.g. with a microtome)
  • select the thinnest slices to ensure maximum light can penetrate sample
  • use a wet mount to prevent dehydration/ distortion of tissue
  • squash slides so its easier to see individual cells/ allows light to penetrate tissue more easily
• Benefits of staining

  • contrast is higher
  • more (internal) structures visible
  • some (named) organelles/ components more visible as they bind to stain
  • clearer image can be obtained
• Gram positive

  • thick cell wall made of peptidoglycan
  • retain crystal violet dye
• Gram negative

  • thin peptidoglycan layer that is sandwiched between an inner membrane and a bacterial outer membrane
  • pink
• Procedures for gram staining

  1. crystal violet staining, all purple
  2. iodine treatment, all purple
  3. alcohol decolourising, G+ purple G- colourless
  4. safranin counter-staining, G+ purple G- red/pink

  helps in medical diagnosis of bacteria
• Plant cells

  • divide via cytokinesis
  • cellulose cell wall
• Yeast

  • divide via budding
  • nucleus and ribosomes present
  • chitin cell wall
• Bacterium

  • divide via binary fission
  • no nucleus
  • peptidoglycan cell wall
  • ribosomes present
• Evidence for endosymbiosis theory in mitochondria

  • length/ size similar to bacterium
  • contain circular DNA
  • contain 70s ribosomes
  • (may) have plasmids
  • double membrane
• Why can early eukaryotes grow more quickly than cells that did not possess mitochondria?

  can respire aerobically> this produces more ATP> atp needed for active transport/ cell division/ protein synthesis/ DNA replication> more ATP allows for faster metabolic processes/ reactions
• Path of proteins exported from a cell


  rer> ga> secretory vesicle
• Vesicles are used to transport proteins to the cell surface membrane and for exocytosis
• Describe how genes are prepared and secreted by cells after translation has taken place

  transport vesicle from RER, modification (conversion into a glycoprotein) at the the golgi apparatus also packaged into secretory vesicle, vesicles move along the cytoskeleton using motor proteins/ microtubules, vesicle fuses with cell surface/ plasma membrane, secretion occurs by exocytosis