Download Patterns in nature

Gill Sans Bold
Biology
Preliminary Course
Stage 6
Patterns in nature
Part 3: Cell theory and the light microscope
2
0
0
In
r2
e
b S
o
t
c NT
O
ng DM E
i
t
ra E N
o
rp A M
o
c
Gill Sans Bold
Contents
Introduction ............................................................................... 2
What is cell theory? ................................................................... 4
The historical development of the cell theory .....................................4
Evidence to support the cell theory .....................................................6
Problems with the cell theory...............................................................6
The microscope......................................................................... 7
The light microscope ............................................................................8
Cell organelles ........................................................................ 11
Using a light microscope....................................................................11
Summary................................................................................. 17
Suggested answers................................................................. 21
Exercises–Part 3 ..................................................................... 23
Part 3: Cell theory and the light microscope
1
Introduction
Organisms are made of cells which have similar structural characteristics.
In this part you will be given opportunities to learn to:
•
outline the historical development of the cell theory, in particular,
the contributions of Robert Hooke and Robert Brown
•
describe evidence to support the cell theory
•
discuss the significance of technological advances to developments
in cell theory
•
identify cell organelles seen with current light microscopes
In this part you will be given opportunities to:
•
use available evidence to assess the impact of technology, including
the development of the microscope on the development of the cell
theory
•
perform a first–hand investigation to gather first–hand information
using a light microscope to observe cells in plants and animals and
identify nucleus, cytoplasm, cell wall, chloroplast and vacuoles.
Extract from Biology Stage 6 Syllabus © Board of Studies NSW, originally
issued 1999. The most up-to-date version can be found on the Board's website
at http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html
This version November 2002.
2
Patterns in nature
Gill Sans Bold
There is a practical activity in this part that requires the use of a
microscope and the following equipment:
•
microscope lamp (if the microscope does not have one built into
the unit)
•
clean glass slides and coverslips
•
dropper
•
onion, moss or very soft fleshy plant stem
•
meat blood (obtained by collecting the liquid remaining after frozen
goods are defrosted)
•
iodine (or an antiseptic preparation containing iodine such as
Betadine®)
•
blade or small vegetable knife
•
cutting board.
Alternative exercises are provided if you do not have access to a
microscope.
Part 3: Cell theory and the light microscope
3
What is cell theory?
You have previously studied cells and you know that living things are
made of cells and that cells share similar structural characteristics.
But did you know that the existence of cells had to wait for the historical
development of the microscope for their discovery? To examine this
history you need to look at the contributions of a range of scientists
including, Robert Hooke and Robert Brown.
The historical development of the
cell theory
Before looking at the historical development of the cell theory it is
useful to be clear about what the cell theory means. The cell theory was
proposed by Scheilden and Swann in 1839 and added to by Virchow
in 1858.
In summary the cell theory states that:
•
cells are the basic units of life and reproduction
•
most organisms consist of a cell or cells and cell products
•
all cells come from pre–existing cells.
Most cells are so small that they are invisible to the naked eye.
This meant that the discovery of cells had to wait until there were
magnifiers to see them. The simplest magnifiers are single lenses such as
those used in a magnifying glass. The development of the cell theory
depended on the manufacture and use of lenses and magnifying devices
such as microscopes as well as the combined investigation efforts of
many scientists. This occurred over a period of 300 years.
4
Patterns in nature
Gill Sans Bold
Robert Hooke
In 1665 Robert Hooke, an English scientist, used a compound
microscope to observe the cellular nature of cork. (A compound
microscope consists of two or more lenses which are positioned inside a
tube.) Hooke’s microscope magnified objects about 270 times (x270)
their normal size.
Hooke's microscope.
Hooke's drawing of cork cells.
He called the structures seen in the cork, cells because they reminded
him of the small rooms (cells) that monks lived in.
Anton van Leewenhoek
A few years later in 1676 Anton van Leeuwenhoek
(lay–van–hook), a Dutchman made amazing
discoveries of unicellular organisms like bacteria.
Leeuwenhoek used, not compound microscopes,
but single lens microscopes. These magnified
about 250 times (x250).
Van Leeuwenhoek was a master lens grinder.
Unfortunately, he was extremely secretive about his
method of grinding lenses and didn’t pass on his
Van Leeuwenhoek's
knowledge and skills.
microscope.
Part 3: Cell theory and the light microscope
5
Robert Brown
Robert Brown (1773–1858) was a Scottish botanist who accompanied
Matthew Flinders to Australia in 1801. As well as identifying new
genera of plants in Australia, he is known for discovering Brownian
movement and the cell nucleus.
Evidence to support the cell theory
The main evidence for the cell theory occurred when the theory of
spontaneous generation was disproved. This theory stated that life arose
from non–living matter such as piles of rubbish and organisms such as
rats and flies were produced by rotting meat. Francesco Redi
(1626–1697) showed that maggots only arose from meat that flies had
visited and Louis Pasteur (1822–1888) showed that micro–organisms
only come from other micro–organisms. The work of these two
scientists convinced people that living things are made of cells and that
all cells come from pre–existing cells. This led to important changes in
hygiene and medical practices.
Problems with the cell theory
Since the time when the cell theory was proposed, viruses and prions
have been identified.
How do viruses and prions fit into the cell theory? They do not
structurally resemble cells. They are much smaller than cells. They do
not have a nucleus, cell membrane or cytoplasm.
Viruses consist mainly of a core of deoxyribose nucleic acid (DNA) and
a protein coat. Prions are an infectious protein particle that causes
various nervous diseases in mammals. Mad cow disease is an example
of a disease caused by a prion. Prions do not contain any genetic
material. While viruses often appear to be non–living, they can
reproduce when they are within a living cell. So, they pose a problem.
Are they living or non–living? Many people regard viruses as living, but
they differ from other living things in that they are not cellular.
Complete Exercise 3.1.
6
Patterns in nature
Gill Sans Bold
The microscope
The development of the cell theory was closely related to the
technological advances that occurred with the development of
improved lenses.
As well as the improvement in microscopes, other technological
advances have occurred. These include machines called microtomes that
are capable of cutting ultra–thin sections of material. Also the ability to
use different chemicals as staining agents. Some stains are taken up
selectively by different materials and can be used to identify chemicals
such as starch or different structures within the cell. Examples of
commonly used stains are iodine, toluidine blue and eosin.
When looking at microscopes there are two factors that are important,
magnification and resolution.
Magnification is the amount that the object is magnified or how much
larger the object appears. A light microscope magnifies up to 1000
times. The new generation of microscopes are the electron microscopes,
the first of which was built in 1933 by Ernst Ruska. The magnification
of an electron microscope is about one million times the normal size.
Resolution (resolving power) is the ability of a microscope to distinguish
two or more points together, as discrete objects. If the resolution of a
microscope is poor you can continue magnifying an object but it just
appears to be more blurred and bigger.
The magnification of an electron microscope can be up to one million
times with a resolving power of up to 0.0002 µm (micrometres).
The light microscope can only resolve up to 0.2 µm. This is about 500
times more than the human eye can resolve.
Part 3: Cell theory and the light microscope
7
The light microscope
The light microscope passes a beam of light through a specimen which is
magnified by the objective lens. The light then travels up the body of the
microscope and is then magnified again as it is passes through the lens in
the eyepiece.
Light microscope.
© Australian Key Centre for Microscopy.
In this activity you have to use available evidence to assess the impact of
technology, including the development of the microscope on the
development of the cell theory..
To do this look for information from a range of resources including popular
scientific journals, CD–ROMs and the Internet.
Illustrate the trends and patterns by creating a table such as the one below
that shows the development of the microscope. Look for a logical
progression of ideas as technological advances lead to the development of
the cell theory.
8
Patterns in nature
Gill Sans Bold
There are some useful starting points on the LMP Science Online site,
http://www.lmpc.edu.au/science.
Also try the interactive activities dealing with the microscope.
1590
Father and son team, Hans and Zacharias Janssen, construct the first
compound microscope.
1665
Robert Hooke using a compound microscope observed cork.
1672
Marcello Malpighi (1628–1699) discovered capillaries.
1676
Anton van Leeuwenhoek (1632–1723) described unicellular organisms.
His great skill was in producing outstanding lenses that had a better
resolution than any other lenses of the time.
1824
Rene Dutrochet (1776–1847) was he first to state that all animals and
plants are made of cells.
1831
Robert Brown (1773–1858) named the nucleus and found it was
present in both plants and animal cells.
1839
Mathias Schleiden (1804–1881) and Theodor Schwann (1810–1882)
were given credit for the cell theory.
1855
Rudolf Virchow (1821–1902) added the third statement of the cell
theory that all cells come from pre–existing cells.
1880
Walther Flemming (1843–1905) described cell division or mitosis from
observations on living and stained cells.
1932
Ernst Ruska built the first electron microscope.
Today
The best light microscopes today have a magnification of 1500x.
Electron microscopes can have a magnification of up to one million
times and high resolving power.
Part 3: Cell theory and the light microscope
9
1
List the main features of the cell theory.
• ____________________________________________________
• ____________________________________________________
• ____________________________________________________
2
Use the information in the table on the previous page to answer the
following questions.
a) Who observed that plant and animal cells have a nucleus?
_________________________________________________
b) Who are given credit for the cell theory?
_________________________________________________
c) Who stated that cells come from pre–existing cells?
_________________________________________________
3
Complete the following sentences.
a) Hooke’s compound microscope magnified objects about ____ X.
Van Leeuwenhoek’s microscopes magnified object about ___ X.
A modern compound microscope can magnify about ______ X.
b) The different microscopes were developed as a result of _____
changes.
c) One important improvement in microscopes has been that they
have increased the __________________________ of objects.
Check your answers.
Do Exercise 3.2 now.
10
Patterns in nature
Gill Sans Bold
Cell organelles
Within a cell there are structures that are common to many cells.
These are called organelles. All cells are held together as a unit by the
cell membrane (also called the plasma membrane). In plant cells, the cell
membrane is surrounded by a cell wall composed of cellulose.
Common organelles that you may have heard of include the nucleus,
mitochondria, chloroplasts and ribosomes. Some cell organelles are
visible using a light microscope; more are seen with an electron
microscope. You will look firstly at the organelles that are visible under
a light microscope.
Using a light microscope
The table below shows the cell structures that can be seen when using a
light microscope to examine plant and animal cells.
Animal cells
Plant cells
nucleus
nucleus
cell membrane
cell membrane
cytoplasm
cytoplasm
nuclear membrane
nuclear membrane
cell wall
chloroplasts
vacuole
Part 3: Cell theory and the light microscope
11
The cell wall and chloroplasts of some plant cells can be clearly seen.
The chloroplasts in plant cells show up clearly because they are green.
This is due to the presence of the green pigment, chlorophyll.
Vacuoles can be large in mature plant cells but very small in animal cells,
if it is present at all.
Observing plant and animal cells
The following activity will help you identify structures in some cells.
You will be observing plant and animal cells using a light microscope.
You can revise how to use and set up the microscope in the Science resource
book or if you have access to the Internet the following web site will provide
you with an address.
http://www.lmpc.edu.au/science.
If you do not have access to a microscope to carry out the activity, you
can use the light microscope photographs on the following pages to
answer the questions in this section.
Materials:
•
microscope
•
microscope lamp (if the microscope does not have one built into
the unit)
•
clean glass slides and coverslips
•
dropper
•
onion, moss or very soft fleshy plant stem
•
meat blood (obtained by collecting the liquid remaining after frozen
goods are defrosted)
•
iodine (or an antiseptic preparation containing iodine such as
Betadine®)
•
blade or small vegetable knife
•
cutting board.
Method:
1
Prepare a very thin section of specimen.
Use the blade to cut a very thin section of the plant material.
Caution: Always cut away from your fingers.
2
12
Prepare a wet mount of each specimen for investigation under the
microscope.
Patterns in nature
Gill Sans Bold
To make a wet mount use the dropper to place a drop of water onto
the clean glass slide. Place the thin section of plant material onto the
drop of water and gently lower the cover slip. If onion is being used,
a drop of iodine can be added to the slide to provide a greater
contrast. (Iodine will stain any starch present.) Gently lower a cover
slip using the techniques illustrated below.
The blood can be placed directly onto the slide and smeared thinly.
Lower the coverslip gently onto the smear.
Cut a thin section, place a drop of water on the slide,. add the section and then
cover with a coverslip.
3
Place the slide on the microscope stage and use the coarse focus to
move the objective as close as possible to the slide while watching
from the side. Then whilst looking through the eyepiece move the
coarse focus away from the slide until the image is focused.
If the image does not appear ecogni, then repeat the steps again
(more slowly), making sure that the objective is once again lowered
whilst watching from the side to ensure that the slide and objective
are not accidentally damaged.
For more detail rotate the objective stage so that the high power
objective is being used to bring your specimen into clear view.
If you have a fine focus knob it should only be used when looking
through the high power objective.
4
Draw a ecogni diagram of each of the specimens observed in the
spaces on the following pages. Label any of the structures you were
able to identify.
Remember when drawing diagrams:
•
use pencil for the drawing
•
make the diagrams at least a half page in size
•
use a heading and label any structures you ecognize.
Part 3: Cell theory and the light microscope
13
Photograph of human blood viewed through a light
microscope. The larger cell is a white blood cell
showing the lobed nucleus. The rest are red blood
cells with no nucleus. (Photo Jane West)
Drawing showing the biconcave shape
of red blood cells.
Your drawing of human blood cells.
14
Patterns in nature
Gill Sans Bold
Human cheek cells as seen through a light
microscope.(Photo Jane West)
Drawing of an animal cell as seen through a light
microscope.
Your drawing of animal cells.
Part 3: Cell theory and the light microscope
15
Photograph of a plant epidermis viewed through a
light microscope under high magnification. (Photo
Jane West)
A drawing of a plant cell. You may notice that it is difficult to
see the chloroplasts and vacuole in the photograph.
Your drawing of plant cells.
Do Exercise 3.3 now.
16
Patterns in nature
Gill Sans Bold
Summary
Use this self–correcting summary to check your knowledge of the cell
theory and the technology that has led to the discovery of cell organelles.
1
What are the three main generalisations in the cell theory?
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Which type/s of technology has made it possible for us to know that
cells exist?
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
Outline one improvement in microscopes following Hooke’s early
discoveries.
_____________________________________________________
_____________________________________________________
_____________________________________________________
4
Study the photograph of the light (optical) microscope on the next
page.
a) Identify the parts labelled and state a function of each part.
Do this by using a table with the three headings: Letter, Name of
part, and Function on your own paper. You may need to refer to
the Science resource book if you were unable to use a
microscope.
Part 3: Cell theory and the light microscope
17
Light microscope.
© Australian Key Centre for Microscopy.
b) Explain briefly how you would prepare a wet mount.
__________________________________________________
__________________________________________________
__________________________________________________
c) Name one chemical that can be used to stain cells.
__________________________________________________
d) Why are stains useful when examining cells using the
microscope?
__________________________________________________
__________________________________________________
__________________________________________________
18
Patterns in nature
Gill Sans Bold
e) Briefly outline the steps involved in observing a specimen using
the low power of a light (optical) microscope.
_________________________________________________
_________________________________________________
f)
What is the term used to describe the ability to see fine details
clearly?
_________________________________________________
Check your answers.
Part 3: Cell theory and the light microscope
19
20
Patterns in nature
Gill Sans Bold
Suggested answers
The microscope
1
Life and reproduction is not possible without cells.
Living things are made up of cells.
Cells are produced from other cells.
2
a) Robert Brown
b) Schleiden and Schwann
c) Rudolf Virchow
3
a) Hooke’s compound microscope magnified objects about 270 X.
Van Leeuwenhoek’s microscopes magnified object about 250 X.
A modern compound microscope can magnify about 1000 X.
b) The different microscopes were developed as a result of
technological changes.
c) One important improvement in microscopes has been that they
have increased the magnification of objects.
Summary
1
Cells are the basic units of life and reproduction.
Most organisms consist of a cell or the products of cells.
All cells come from pre–existing cells.
2
Magnifiers and microscopes make it possible for us to know that
cells exist.
3
The grinding of lenses for single lens microscopes.
Part 3: Cell theory and the light microscope
21
4
4
a)
Letter
Name
Function
e
eyepiece
magnification lens
o
objective
magnification lens
c
condenser
focuses light from mirror onto specimen
s
stage
support for specimen
l
light
light source
f
focus knob
moves the stage to focus image
b) A wet mount is made by placing a thin section of a specimen
into a drop of water on a clean slide. This is covered with a
coverslip. A stain may be added to the specimen if required.
c) Examples of stains are iodine, toluidine blue and eosin
d) Stains are useful because they make the image easier to see by
increasing the contrast between structures. They are also useful
because they can indicate the presence of different types of
compounds for example iodine turns blue–black in the presence
of starch.
e) Direct lamp or light source into microscope using the mirror.
Place slide on the stage.
Lower low power objective whilst looking from the side.
Look down eyepiece and slowly wind the coarse focus knob up,
until the image comes into focus.
f)
22
Resolution
Patterns in nature
Gill Sans Bold
Exercises – Part 3
Exercises 3.1 to 3.3
Name: _________________________________
Exercise 3.1: What is cell theory?
a)
What are the three main generalisations of the cell theory?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
b) What was the contribution of the following two scientists to the cell
theory.
Robert Hooke
_____________________________________________________
_____________________________________________________
_____________________________________________________
Robert Brown
_____________________________________________________
_____________________________________________________
_____________________________________________________
c)
Outline the theory of spontaneous generation. Name two scientists
whose work disproved the theory of spontaneous generation and
gave support to the cell theory.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 3: Cell theory and the light microscope
23
Exercise 3.2: The microscope
a)
Complete the sequence scaffold below highlighting the historical
developments in the establishment of technology in developing the
cell theory.
The first one has been done for you.
24
Date
Event
1590
Father and son team, Hans and Zacharias Janssen,
construct the first compound microscope .
Patterns in nature
Gill Sans Bold
1665
1676
circa 1900
circa 1933
b) The diagram above shows the development of the microscope.
These technological advances were important in the development of
the cell theory. Discuss the significance of technological advances
to developments in cell theory.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Exercise 3.3: Cell organelles
a)
Compare and contrast plant and animal cells. This means that you
must look at the similarities and differences between the two types
of cells.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 3: Cell theory and the light microscope
25
b) Draw a diagram in the space below of a generalised plant and animal
cell as seen through a light microscope, labeling the structures
identified.
26
Patterns in nature