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

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):
- Using a cotton wool bud, rub the inside of your mouth and smear the bud onto a clean, dry glass slide.
- Place a drop of methylene blue on top of the smear and allow to soak in for 5 minutes.
- Tip the slide at an angle onto tissue paper to allow the excess stain to fall off.
- Add a drop of water to the stained smear.
- Lower a coverslip slowly using a mounted needle from a 45º angle to avoid trapping air bubbles.
- View slide under the light microscope at 40X, focus using the coarse focus wheel.
- View the cells under higher powers by focusing using the fine focus wheel.
- Sketch the visible fields of views at 100X and 400X.
Method for viewing plant cells (onion):
- Cut a piece of onion and remove a single layer of cells and place on the glass slide.
- Place a few drops of iodine onto the onion layer and allow to soak in for 5 minutes.
- Tip the slide at an angle onto tissue paper to allow the excess stain to fall off.
- Add a drop of water to the onion layer.
- Lower a coverslip slowly using a mounted needle from a 45º angle to avoid trapping air bubbles.
- View slide under the light microscope at 40X, focus using the coarse focus wheel.
- View the cells under higher powers by focusing using the fine focus wheel.
- Sketch the visible fields of views at 100X and 400X.
