Chapter 6: Cell Structure
Leaving Certificate Biology
Higher Level
The Microscope
• A microscope is used to view very small
living organisms and cells
• “Be familiar with and know how to use a
light microscope”
The Microscope
•
Two types of microscope you need to know
for the Leaving Certificate are:
1. Light (compound) microscope: uses visible light, two
or more lenses, and a specimen - usually stained to
make structures more visible
2. Transmission 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
The Light Microscope
• Eyepiece: magnifies the image; closest
to the observer’s eye
• Objective: magnifies the image; closest
to the specimen
• Turret: holds the objective lenses
• Stage: holds the specimen (slide)
• Clips: holds the slide in place
• 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
The Electron Microscope
• The beam of e- from tungsten filament travels through
specimen and onto photographic film
• Everything in the microscope is held in a vacuum
• The specimen must be coated with a heavy metal and must
also be very thin as e- cannot penetrate thick specimens
• Electromagnetic lenses focus the high-velocity e- beam as it
hits the specimen and diverges the beam as it approaches
the photographic film - thereby magnifying the image amount of divergence dictates what the magnification will be
• If the divergence of the beam is too big the magnification
will be such that we would lose good resolution – thereby
producing a blurry image
• The maximum resolution of the electron microscope is:
0.1 nanometre (nm) = 0.0001 mm = 1/10,000th mm
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
Using a Light Microscope (cont.)
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
Animal Cell Structure
Animal Cell Structure and Function
• Cell Membrane: contains the contents of the cell
and controls what enters and leaves the cell
• Cytoplasm: liquid of cell – consists of cytosol
and cell organelles and is where chemical
reactions occur
• Nucleus: contains the DNA in the form of either
chromosomes or chromatin and controls all of
the activities of the cell
• Nucleolus: where ribosomes are made
• Nuclear pores: allow movement of chemical
substances in and out of nucleus
Animal Cell Structure and Function
• Nuclear envelope: double phospholipid
membrane keeping the DNA separate from the
cytoplasm
• Chromatin: form DNA takes when the cell is not
dividing - elongated chromosomes
• DNA: deoxyribonucleic acid - organised into
chromosomes; where genes are located
• Mitochondrion: site of respiration - supplies
energy for metabolism
• Ribosome: site of protein synthesis
Plant Cell Structure
Plant Cell Structure and Function
• Cell wall: made of cellulose; gives shape and
support to plant cell; permeable to water, salts and
minerals; prevents cell from bursting; protects cell
from bacteria and viruses
• Chloroplast: contains green pigment chlorophyll;
responsible for photosynthesis
• Vacuole: large, fluid-filled (sap) sac that takes up
the majority the plant cell and is separated from the
cytoplasm by a membrane called the tonoplast.
Water, food (glucose, amino acids, etc), minerals,
ions and wastes are stored here
Cell
1
membrane
Nucleus
2
3 Nucleolus
Chromatin
4
Nuclear
5 pore
6
Nuclear
envelope
9
Ribosomes
Cytoplasm 8
7 Mitochondrion
Cell membrane 1
12
Cell wall
2
Nucleus
3 Nucleolus
4 Chromatin
Nuclear
5 envelope
11
Central
vacuole
6
Nuclear
pore
10
Ribosome
9
Cytoplasm
7
Mitochondrion
Chloroplast
8
Prokaryotic and Eukaryotic Cells
• Two major kinds of cells - prokaryotic and
eukaryotic - can be distinguished based on
their structural organisation
• Prokaryotic cells have a tough outer cell
wall and have no membrane-bound
nucleus nor membrane-bound organelles
• Eukaryotic cells have a membranebound nucleus and organelles
Mandatory Experiment: To
prepare and examine one
animal cell and one plant
cell, unstained and stained
using the light microscope
at 100X and 400X
To prepare and examine an animal cell
• Apparatus/Chemicals:
– Cotton buds; microscope slides; coverslips; methylene
blue; needle; tissue; water; light microscope
• Method:
– Scrape the inside of your mouth with cotton bud once
and immediately smear on the centre of clean glass
slide - do not let smear dry
– Immediately add a few drops of methylene blue - leave
5 min
– Blot excess stain with tissue and add a drop of water
To prepare and examine an animal cell
• Method (continued):
– Place coverslip over smear by lowering slowly from a
45º angle using the needle
Coverslip
Glass slide
Water droplet with
methylene blue
Cheek cell smear
– Prepare another smear slide in same way but without
stain
– Examine both slides under microscope using correct
method - at 100X and 400X
To prepare and examine an animal cell
• Results:
– Images are visible stained and unstained cheek cells at
both 100X and 400X
– Based on your
experience of viewing
cells at 100X and 400X,
at what magnification
do you think this image
was obtained?
?X
• Conclusion:
– Cells can be examined in detail using the light
microscope
– Cell numbers, structure, and organelles can be better
examined by staining the cells
To prepare and examine a plant cell
• Apparatus/Chemicals:
– Microscope slides; coverslips; iodine; needle; blades;
tissue; water; light microscope
• Method:
– Using the blade carefully incise out shallow and small
squares from an internal leaf of an onion
– Separate a single-celled layer of tissue from one of the
squares
– Place this epidermal strip onto the glass slide
– Add few drops of iodine - leave 5 min
– Blot excess iodine with tissue and add drop of water
To prepare and examine a plant cell
• Method (continued):
– Place coverslip over epidermal strip by lowering slowly
from a 45º angle using the needle
Mounted
needle
Coverslip
Glass slide
Water droplet
Onion layer
– Prepare another slide in same way but without iodine
– Examine both slides under microscope using correct
method - at 100X and 400X
To prepare and examine a plant cell
• Results:
– Images are visible of stained and unstained onion cells at
both 100X and 400X
– Based on your
experience of viewing
cells at 100X and 400X,
at what magnification
do you think this image
?X
was obtained?
?X
• Conclusion:
– Cells can be examined in detail using the light microscope
– Cell numbers, structure, and organelles can be better
examined by staining the cells
To prepare and examine an animal cell
• Safety:
– Care must be taken with the slides and coverslips. They
are broken very easily - broken glass should be reported
immediately to the teacher and/or laboratory technician.
– Use a clean cotton wool buds (and not your finger) to
obtain a smear of cells.
– Extreme care must be taken when using the blade to
obtain the epidermal strip of onion cells - a wooden
chopping board should be used to minimise the risk of
the onion slipping whilst cutting it
– Wear latex or nitrile gloves when handling the methylene
blue and iodine solutions - they easily stain the skin and
their effects on humans are unknown so care must be
taken (especially with substances that stain nuclei of
living cells as they are potential carcinogens).
– Lab coats must be worn to protect clothing