Cell Structure

Cells are the basic functional unit of all life.

Ultrastructure of cells:

Animal cell:

The typical animal cell is shown in the diagram below. Important structures inside the typical animal cell include:

  • Cell membrane – controls what substances enter and leave the cell
  • Cytosol – medium in which all metabolic reactions occur
  • Nucleus – controls all activities of the cell
  • Nucleolus – makes RNA
  • Mitochondrion – carries out the reactions of respiration
  • Lysosome – destroys old, worn out cell organelles
  • Ribosome – makes proteins

Plant cell:

The typical plant cell is shown in the diagram below. Important structures inside the typical plant cell include:

  • Cell membrane – controls what substances enter and leave the cell
  • Cell wall – gives shape and support to the plant cell
  • Cytosol – medium in which all metabolic reactions occur
  • Nucleus – controls all activities of the cell
  • Nucleolus – makes RNA
  • Mitochondrion – carries out the reactions of respiration
  • Chloroplast – carries out the reactions of photosynthesis
  • Large central vacuole – stores food, water, minerals, vitamins, and wastes
  • Lysosome – destroys old, worn out cell organelles
  • Ribosome – makes protein

The Microscope

  • A microscope is used to view very small living organisms and cells
  • You must be familiar with and know how to use a light microscope

Two types of microscope you need to know for the Leaving Certificate are:

  • Light (compound) microscope: uses visible light, two or more lenses, and a specimen – usually stained to make structures more visible
  • Electron microscope: uses a beams of electrons (e-), a number of electromagnetic lenses (that focus and diverge the beam of e-), a piece of photographic film (like X-ray film), and a specimen

Parts of the light microscope and their functions:

  • Eyepiece lens: magnifies the image; closest to the observer’s eye
  • Objective lens: magnifies the image; closest to the specimen
  • Stage: holds the specimen (slide)
  • Diaphragm: controls the amount of light
  • Light source/mirror: sends light up through the stage and specimen
  • Fine/coarse focus wheels: make fine/large adjustments to the clarity of the image

Using a Light Microscope

  1. Ensure low-power lens (4X) is in position before placing specimen on stage
  2. Separate stage and objective as much as possible using coarse wheel before placing specimen on stage
  3. Adjust mirror or turn on light underneath stage
  4. Ensure diaphragm is fully open to allow light pass through
  5. Place specimen slide on stage so that specimen is directly above hole in stage
  6. Bring 4X close to specimen – it is easier and safer to do initial focusing using low-power lens than higher-power lenses
  7. Looking through eyepiece bring image into focus by turning the coarse and fine focus wheels towards you slowly
  8. Adjust diaphragm if necessary during focusing as image may be too bright to view specimen
  9. Once in focus, move slide around gently on stage to see different fields of view
  10. Change the objective to the 10X lens
  11. Draw a sketch of the field of view
  12. Change the objective to the 40X lens carefully as this lens may hit the slide and cause damage
  13. Refocus using fine focus wheel
  14. Draw a sketch of the field of view
  15. Once finished move the 4X lens back to the main position – only then remove slide

The Electron Microscope

  • Uses a beam of electrons to view specimens.
  • Produces magnifications of up to 1,000,000X.
An electron microscope

Prokaryotic and Eukaryotic Cells

  • Prokaryotic cells have a tough outer cell wall and have no membrane-bound nucleus nor membrane-bound organelles
  • Eukaryotic cells have a membrane-bound nucleus and organelles

Mandatory Experiment: to view animal and plant cells, stained and unstained, using the light microscope at 100X and 400X

Equipment:

  • Cotton wool buds (to obtain human cheek cells)
  • Onion
  • Microscope slides
  • Coverslips
  • Methylene blue (for animal cells)
  • Iodine (for plant cells)
  • Mounted needle
  • Tissue
  • Water
  • Light microscope

Method for viewing animal cells (human cheek cells):

  1. Using a cotton wool bud, rub the inside of your mouth and smear the bud onto a clean, dry glass slide.
  2. Place a drop of methylene blue on top of the smear and allow to soak in for 5 minutes.
  3. Tip the slide at an angle onto tissue paper to allow the excess stain to fall off.
  4. Add a drop of water to the stained smear.
  5. Lower a coverslip slowly using a mounted needle from a 45º angle to avoid trapping air bubbles.
  6. View slide under the light microscope at 40X, focus using the coarse focus wheel.
  7. View the cells under higher powers by focusing using the fine focus wheel.
  8. Sketch the visible fields of views at 100X and 400X.

Method for viewing plant cells (onion):

  1. Cut a piece of onion and remove a single layer of cells and place on the glass slide.
  2. Place a few drops of iodine onto the onion layer and allow to soak in for 5 minutes.
  3. Tip the slide at an angle onto tissue paper to allow the excess stain to fall off.
  4. Add a drop of water to the onion layer.
  5. Lower a coverslip slowly using a mounted needle from a 45º angle to avoid trapping air bubbles.
  6. View slide under the light microscope at 40X, focus using the coarse focus wheel.
  7. View the cells under higher powers by focusing using the fine focus wheel.
  8. Sketch the visible fields of views at 100X and 400X.
Unstained onion cells
Stained onion cells