7.
7. 1 Discovering cells
Figure 7.2
cells of life
Figure 7.3
Cork cells under a light
microscope stained with
a fluorescent stain (2005).
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One of the things that you did in the Thinking about was to write
down how many times each specimen that you looked at had been
magnified. Did you look at slides under a microscope? Microscopes
and other magnifiers contain lenses that allow light to pass through
them, but bend the light rays so that we can see things more clearly.
Cork cells seen
with a microscope
by Robert Hooke
in 1665.
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Figure 7.4
Different types of lenses carry out different functions.
Microscopes and magnification
The magnification of a microscope tells you how much bigger the
image is than the real object. If the microscope has a magnification
of ×400, then the image that you are looking at is 400 times bigger
than the actual object. (The symbol × in this case stands for
‘magnified by’).
To work out the magnification you must look at both the
eyepiece (occular) lens and the objective lens of the microscope. Each
has a number on it, such as ×10 or ×20. Multiplying these numbers
together gives you the total magnification of the microscope.
Optical
Microscope
Ocular lens
× 10
Objective lens
× 40
MagniÞcation
10 × 40 = ×400
Figure 7.5
Calculating the total magnification.
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work
S C I E N C E
Getting to know your microscope
EYEPIECE
OROCULARLENS
Light microscopes come in a variety of shapes
and sizes; however, all of them have the same
basic parts.
What to do
1. With a partner, collect a microscope. Make
A c t i v i t y 7 .2
COARSEFOCUSKNOB
MICROSCOPE
TUBE
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sure you carry it with two hands, one around
the arm and the other under the base; your
FINEFOCUSKNOB
teacher will show you how. Place your
OBJECTIVE
microscope on the bench away from the
LENS
edge. You will also need your diagram of a
STAGE
microscope with labels and questions from
Science@Work 7.1.
2. Look closely at your microscope and use
MIRROR
figure 7.16 to find each of the labelled parts.
Can you decide what each part is used for?
BASE
3. Table 7.2 contains a description and a function
for each part of the microscope but they are
Figure 7.1 6
all mixed up. Unscramble the information and
A standard light microscope. Microscopes
write the corrected table into your book.
that are used in schools may look a little
4. Compare your table with another group and
different from those used in laboratories,
discuss any differences.
but they essentially work the same way.
Description
eyepiece
(ocular) lens
Flat bottom surface.
objective lens
Flat surface to sit slides on, has
hole in its centre, may have clips.
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Name of part
focus knob
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Table 7. 2 Parts of a microscope
Function
Holds the slide in place, letting
light pass through it.
Adjusts the position of the lenses
so that the object can be seen
clearly: can be coarse or fine.
Iris like aperture which can be
adjusted to allow light through.
Light travels through it to the
eyepiece lens.
base
Knob that can be turned to move
the lenses.
Controls the amount of light
passing through the object.
mirror
The adjustable tube between the
eyepiece and objective lenses.
Supports the microscope.
stage
Lenses of different lengths which
can be positioned above the slide.
Reflects light up through the slide
into the lenses.
diaphragm
Round with a shiny surface.
Used to locate specimen, then
get different magnifications.
microscope
tube
Single lens closest to the eye.
Enables you to see magnified
specimen.
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S C I E N C E
EX P E R I M E N T
Using a microscope
Aim
7 .3
!
To become familiar with the use of a microscope.
Materials
•
•
•
•
light microscope
microscope lamp with filter
tweezers
clear plastic ruler
•
•
•
•
tissue
newspaper
cotton wool
fabric
• mini grid (optional)
• slides and cover slips
• hair
Part A: How big is it really?
Method
1. Collect a microscope and carry it with both hands to the workbench, placing it well away
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2.
3.
4.
from the edge.
Work out the magnification of your microscope when using the low and high power lenses.
Adjust your mirror so that an appropriate amount of light passes through the hole in the stage.
Place a clear plastic ruler on the stage of your microscope so that you can focus on the
millimetre scale. Once you have focused your microscope, the area that you can see is
called the ‘field of view’. It should appear as a round circle of light.
Using your low power objective lens, measure the width of your field of view. Note this
down in your book.
Using your high power objective lens, measure the width of your field of view. Note this
down in your book. How does it compare with your field of view using the low power
objective lens?
Extension
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Your teacher may have a ‘mini grid’ that you can use to estimate the size of what you see
under the microscope. Learn how to use the mini grid and then estimate the width of a
strand of your hair. If you don’t have a mini grid, remember the width of your field of view
measured above and use this to estimate the width of the hair. Compare your hair with
other people’s hair—is it thicker or thinner? Is blonde, red or dark hair thickest? Compare
curly and straight hair—is there a difference in the thickness?
Part B: Upside down and back-to-front
Method
1. Cut out a 1 cm square piece of newspaper. Make certain that it contains the letter ‘e’.
2. Place the print onto a microscope slide. Place a cover slip over the top of this.
3. Use the low power objective lens. While watching from the side, use the coarse adjustment
4.
5.
6.
7.
194
to lower the objective lens until it is just above the stage. Note: Pressing it down too far may
shatter the slide.
While looking through the eyepiece, carefully turn the coarse adjustment until the specimen
can be clearly seen.
Carefully use the fine adjustment so that you can see the details of your specimen as clearly
as possible.
Carefully move the slide until you have an ‘e’ in focus.
Pencil a sketch of what you see. Is it right side up, upside down or back-to-front?
How much of the field of view is covered by the ‘e’ at this magnification?
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8. In which direction did the paper under the microscope move when you moved the slide to
the left? What about when you moved it towards you?
9. Record the magnification that you are using.
Evaluation
What difficulties did you encounter while using the microscope?
How could you improve your microscope technique?
Extension
Look at some other specimens, such as tissue, cotton wool, coloured paper and fabric.
Sketch what you see and label the diagrams clearly. Remember to write down the
magnification that you are using.
7.
3. Create a T-chart covering the two main types
of microscope and list the main similarities
and differences between them.
e
4. Why was the invention of the electron
microscope so important? List its advantages
and disadvantages.
8. How many micrometres are there in
one millimetre? If a cell is 50 µm in diameter,
how big is it in millimetres? Would we be able
to see this cell without a microscope?
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2. List and describe ten things that scientists
have found out using microscopes.
Outline the steps you would follow to obtain
a clear focus using a light microscope on low
and high power.
9. What is the field of view of a microscope?
How can it be used to estimate the actual
size of objects placed under the microscope?
10. If our eyes can see things as small as 0.1 µm,
and one cell is one-tenth of this size, what is
the size of the cell in micrometres?
6. List three ways in which an image seen under
a microscope can differ from the original
specimen placed onto the stage.
11. Create a poster or multimedia presentation
covering the types of microscopes, when
they were invented and by whom.
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5. If you wanted a very detailed image of a cell,
which microscope would you use?
07
chapter
Why are scientists who study the structure
and function of cells called microbiologists?
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cells of life
questions 7.1
7.2 A closer look at cells
scif ile
There is no typical cell shape.
If we look at cells in the human
body we find column cells lining
the stomach; balloon-shaped
cells lining the bladder; red blood
cells shaped like doughnuts
in the blood stream; and flat
cells shaped like pancakes on
the surface of the skin. Some
bacteria cells are shaped like rods
and spirals. Guard cells in the
leaves of plants are shaped like
small sausages.
As microscopes developed over the years, scientists began to observe
plants and animals in detail. Everywhere they looked they found cells.
After careful observation and reading the work of other
scientists, two German scientists, Matthias Schleiden and Theodor
Schwann, developed the first two principles of the cell theory.
Cell theory
1. All living things are made up of cells.
2. Cells are the basic units of structure and function in living things.
Further work by Rudolf Virchow lead to the third principle.
3. All cells arise from pre-existing cells.
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in action
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viruses—are they ALIVe?
Viruses are able to infect us and take over
our cell functions so that they can trick us into
making more of them. Apart from getting sick,
we can't tell that they're in our bodies without
complex technology.
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Viruses don’t have cells like other organisms. This means
that they don’t have a nucleus, cell membrane or cytoplasm.
However, they do have some chemicals found only in living
cells. Viruses consist of a protein box that contains a strand of
nucleic acid. This strand contains the instructions for making
new viruses.
When they get into living cells, viruses behave as though
they are ‘alive’. When they are outside cells they don’t display
the characteristics of living things. Viruses are therefore not
described as living, because they can’t function independently.
A virus may lie dormant for many years until it comes into
contact with a living cell. Then it invades the cell and tricks it
into making many, many new viruses. This eventually kills the
host cell as it bursts open to release millions of new viruses
into the body. Human diseases caused by viruses include
influenza, acquired immune deficiency syndrome (AIDS),
chickenpox and measles.
DNA inside
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protein capsule
Figure 7. 2 2
This is the shape of a
common virus. Did you know
that they are measured
in nanometres (nm)?
A nanometre is
equivalent to a
millionth of a
millimetre.
legs to grip host cells
arequestions
they alive?
1.
How do viruses differ from animals and plants?
2. Are all viruses the same shape and size?
3. What features do viruses have in common with living things?
4. Do you think that viruses are living or non-living? Give reasons for your answer.
5. Create a poster or a PowerPoint presentation about one of the viruses mentioned above
and its effect on humans. Include information about the shape of the virus, the symptoms
of the disease, what can be done to prevent it and the treatments used.
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S C I E N C E
work
A c t i v i t y 7 .7
Once you have collected the worksheets from your teacher, cut out the cell parts.
Making your own animal and plant cell
Colour the cell parts so they will be easier to see. Paste the parts on the cell templates
provided. Label the animal cell, the plant cell, and all of the organelles in each.
questions 7. 2
1.
Explain the cell theory using a poem, rhyme
or role play.
7.
What does a nucleus do? What would
happen if it were damaged?
8. Write a short story or cartoon called ‘Journey
to the centre of the cell’. You should describe
what you see and how you move.
2. What do all living things have in common?
3. What is an organelle? Name five organelles
and state their function.
9. A biologist was studying the parts of a cell.
She removed the nucleus from a skin cell
with a very fine pipette, and found that the
cell died within a few hours. What does this
tell you about the importance of a nucleus?
Present your answer as a journal entry.
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4. Where are the proteins made in a cell?
5. What is cytoplasm and where is it found?
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6. What is the function of the cell membrane?
What do you think would happen to a cell if
its membrane burst?
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7.3 Cell specialisation
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Red blood cells do not have
a nucleus! They lose this just
before being released into the
bloodstream so that they can
be packed with oxygen-carrying
haemoglobin. This means that
they only live for about four or
five months and then die at a rate
of two million per second! In a
healthy body, new red blood cells
are made at the same rate.
200
Most plants and animals are made up of many different types
of cells. They have different shapes and sizes because they have
different jobs to do. Skin cells, for example, are a flattened pancakelike shape, whereas the cells lining your stomach are like house
bricks standing on their ends. Cells are quite complex things, and
they can perform a wide range of tasks including:
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ü
ü
ü
ü
ü
taking in nutrients and carrying out chemical reactions
producing waste products
making useful substances such as bone
reproducing by dividing in two
moving, for example, some special cells such as muscle cells
can contract, while sperm cells can ‘swim’
ü exchanging gases with their surroundings
ü capturing light energy from the sun, which is used to convert
carbon dioxide and water to sugar.
Does this list sound familiar?
It should! Anything an
organism can do is a
result of its cells.
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key
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Use the list of clues, the key terms and table 7.4
to help you find the words in the puzzle.
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Some ideas or rules about cells.
A very thin boundary around the nucleus of the cell.
A storage area of the cell that is full of fluid.
The basic building blocks of all living things
Many cells working together to do the same job.
Many tissues working together to do the same job.
Many organs working together to do the same job.
All the systems together.
A tool that helps us to see small things, such as cells, that we cannot see with our eyes.
How many times bigger the object looks than it really is. Strong microscopes magnify
objects many times.
A living, jelly-like material that makes up the inside of cells. It has lots of chemicals
and cell parts in it.
The control centre of the cell. It is a bit like the brain of the cell.
Small parts of the cell that make the energy for the cell to work.
A thin boundary around the cell that keeps things in cells and lets some things that
are needed into the cells.
Little round parts of cells that make chemicals called proteins. Proteins are needed
so that we can survive and grow.
Chemicals in the nucleus of the cell that carry all the information we get from our
parents.
A boundary around the cell that gives the plant cells a definite shape. Plant cell walls
help to give plants shape because plants do not have bones or skeletons like many
animals.
Green parts of plant cells that carry out the chemical reaction called photosynthesis.
Photosynthesis turns light energy from the sun into chemical energy (carbohydrates)
that animals can use for energy to survive when plants are eaten.
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cell division
cell membrane
cells
cell theory
cell wall
chlorophyll
chloroplast
cilia
cytoplasm
magnification
microscope
nucleus
organ
organelles
photosynthesis
stomata
system
tissue
vacuole
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KEY TERMS
07
Chapter review
•
•
•
•
•
•
•
•
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review questions
1.
Copy and complete the following table:
Cell feature
Plant cells
6. Match each of the following cell names with one of
the diagrams below. Make a concept map covering
their purposes in the human body.
Animal cells
cell wall
goblet cell, motor nerve cell, muscle cell,
white blood cell, brain cell, egg cell
cytoplasm
cell membrane
A
B
C
D
E
F
large vacuole
nucleus
How could a microscope be used to help solve
a crime? Present your answer as a news report
or poster.
8. Why would there be many mitochondria in nerve
and muscle cells? What is their purpose?
9. What are guard cells? What do they do? What are
their two main stages? Draw diagrams to represent
these stages.
10. Some scientists (called cytologists) study life
processes by growing and observing cells in a
laboratory. Find out how cell cultures are maintained
and what scientists can learn by studying them.
Present your answer in a format of your choice.
7.
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2. A student was using a light microscope to observe
some cells. She was using a ×5 ocular and a
×40 objective lens. How many times is the cell
magnified? If she changed to a ×100 objective,
would she see fewer or more cells? Explain your
answer.
3. If a cell is 50 µm wide, how many cells would be
needed to span a distance of 4 cm?
4. Choose the correct word to complete
the following sentences.
(a) The part that controls what a cell does is the
cytoplasm/membrane/nucleus/vacuole.
(b) The process that takes place in a chloroplast is
called growth/photosynthesis/cell reproduction/
movement.
5. Decide if the following statements are true or false.
Rewrite the false statements to make them true.
(a) Animal cells have cell walls.
(b) A virus can only be seen with an electron
microscope.
(c) The average cell is 1 mm across.
(d) Cells reproduce by dividing in two.
(e) Light microscopes can magnify 3000 times.
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chloroplasts
reflection cells
complete
1. Complete the Mission that you began at the start of this chapter and present it to the class.
Ask the class for feedback on your model. Make any improvements suggested by the class.
2. Imagine that you are one of the following:
• a skin cell on someone’s face
• a red blood cell
• a muscle cell in a leg
• a motor nerve cell in the spinal cord
• a photosynthesising cell in a leaf
• a guard cell in a leaf
• a transport cell in a plant stem.
Find out about your chosen cell, and then write an illustrated story,
role play or diary entry about a day in your life.
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