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Biology
Preliminary Course
Stage 6
Patterns in nature
Part 4: Electron microscope and cell organelles
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Contents
Introduction................................................................................ 2
The electron microscope ........................................................... 3
Using an electron microscope .............................................................4
Structure and function..........................................................................5
Tissues, organs and organ systems ........................................ 14
Summary ................................................................................. 15
Suggested answers ................................................................. 17
Exercises–Part 4 ..................................................................... 21
Part 4: Electron microscope and cell organelles
1
Introduction
Plants and animals have specialised structures to obtain nutrients from
their environment. You may recall that plants and animals obtain
nutrients differently. Plants rely on the Sun to manufacture food by a
process called photosynthesis. Plants are autotrophic organisms.
Animals cannot manufacture their own food; they consume or eat other
organisms in order to gain the nutrients they require for life processes.
Animals are heterotrophic organisms.
Plants and animals have specialised cells, tissues and organs to obtain the
nutrition they require and carry out their body processes. Some of these
will be investigated in this part.
In this part you will be given opportunities to learn to:
•
identify cell organelles seen with an electron microscope
•
describe the relationship between the structure of cell organelles and
their function
•
identify some examples that demonstrate the structural and
functional relationships between cells, tissues, organs and organ
systems in multicellular organinsms
In this part you will be given opportunities to:
•
process information from secondary sources to analyse electron
micrographs of cells and identify mitochondria, chloroplasts, Golgi
bodies, lysosomes, endoplasmic reticulum and cell membranes.
Extracts 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/index.html.
This version November 2002.
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The electron microscopes
There are two main types of electron microscopes:
•
the transmission electron microscope and
•
the scanning electron microscope.
Unlike light microscopes that use a beam of light passing through the
specimen electron microscopes use a beam of electrons.
The transmission electron microscope uses the electrons that pass
through very thin specimens, to show detailed images of internal
structures. The scanning electron microscope produces images of the
surface features of objects, often coated with a very thin layer of metal
atoms to enhance the image.
One disadvantage of using electron microscopes is that the preparation of
specimens is very expensive. For example, the specimen must be kept in
a vacuum to avoid scattering the electron beam or it must be fixed in
heavy metal compounds eg. gold. This means that it is not possible to
view live specimens, also a disadvantage. It is unclear whether such
harsh treatment of specimens might actually distort the true nature of the
structure of the cells.
An electron microscope
Part 4: Electron microscope and cell organelles
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The advantage of the electron microscope is that the magnification
(x 1 million) and the resolution (0.0002 micrometre) are very high.
Do Exercise 4.1 now.
Using an electron microscope
Under a light microscope there is a limit to the organelles that are visible
even with the latest technology. The story is different however, under an
electron microscope where there are many more organelles visible.
The table below lists the cell structures that are visible with an
electron microscope.
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Cell structure
Animal cells
Plant cells
nucleus
present
present
nucleolus
present
present
cell membrane
present
present
cytoplasm
present
present
nuclear membrane
present
present
mitochondria
present
present
Golgi bodies
present
present
ribosomes
present
present
endoplasmic reticulum
present
present
lysosomes
present
absent
cell wall
absent
present
chloroplasts
absent
present
vacuole
absent
present
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Structure and function
The nucleus
The nucleus is usually large and spherical. The nucleus is enclosed by a
double membrane. This membrane has pores that allow fairly large
molecules to move in and out of the nucleus.
The nucleus often contains a nucleolus which is involved in the
manufacture of proteins in the cell. (Refer to the electron micrograph
below showing the nucleus and other organelles.)
Nucleus in a rat intestinalwall. (g—Golgi bodies, m—mitochondrion,
nu—nucleolus, n—nucleus, rer—rough endoplasmic reticulum)
© Australian Key Centre for Microscopy.
The nucleus controls the activities of the cell. It does this largely by
controlling the formation of proteins in the cell. The nucleus contains the
chromosomes, which carry the genes. Genes are units of inheritance and
determine which types of proteins are formed.
Part 4: Electron microscope and cell organelles
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What sort of structural feature/s does the nucleus have that makes it suited
for its function?
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Plastids and chloroplasts
Plastids are oval–shaped organelles. Some store substances such as food
made by plants eg. starch. Other plastids contain pigments such as the
green pigment, chlorophyll. Plastids that contain chlorophyll are
called chloroplasts.
Chloroplast in soybean leaf. (g — granum, r — ribosomes, s — starch grains)
© Australian Key Centre for Microscopy.
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Look at the three dimensional drawing of a chloroplast below.
Note that it has, like a nucleus, a double membrane. Within a chloroplast
is a membrane known as the lamella. In some areas, the lamella is
densely packed into grana (singular granum). These resemble stacks of
coins and increase the surface area. The grana are embedded in a
colourless substance called the stroma.
ring of DNA
double membrane
starch grain
stroma
lamella
ribosome
stack of grana
(thylakoids)
Photosynthesis occurs in chloroplasts. So, essentially, the chloroplasts
are the food–making organelles of plants. The chlorophyll is contained
in the grana. It is the chlorophyll that absorbs light energy which makes
the process of photosynthesis possible.
Describe how the structure of a plastid such as a chloroplast assists the
process of photosynthesis in plant cells.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
The cell membrane
The cell membrane, also known as the plasma membrane, surrounds all
cells. It is a physical barrier but it is thin and contains pores that allow
selected materials to pass through. In plant cells the cell membrane is
surrounded and protected by the cell wall.
The function of the cell membrane, apart from keeping the cell together,
is to regulate the flow of substances into and out of the cell.
The membrane is described as selectively permeable. This means it
allows some substances to pass through while stopping other substances.
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Describe how the structure of the cell membrane enables it to carry out the
function described here.
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
A summary
Following is a summary of important points so far. Fill in the missing
words as you read this summary.
1
The _____________ is a large spherical organelle that controls the
activities of the cell. This organelle is surrounded by a
_____________ membrane that has _____________ in it.
2
Chloroplasts are one type of _____________. They are green coloured
because they contain the green pigment, _____________. Like the
nucleus, the chloroplast is surrounded by a _____________ membrane.
Chloroplasts absorb _____________ energy, which drives the
food–making process called _____________.
3
The cell or plasma membrane keeps the cell intact. It allows some
substances to enter and leave while preventing others from entering or
leaving. It is therefore described as _____________ permeable.
Check your answers.
Mitochondria
Cells may contain several or hundreds of mitochondria. The more active
a cell is, the more mitochondria there are.
1
Make a prediction to explain why there are more mitochondria in an
active cell than one not involved in activity.
_____________________________________________________
_____________________________________________________ .
A mitochondrion is surrounded by a double membrane.
Look at the photomicrograph on the next page, which shows some
mitochondria. You can see that the inner membrane is highly folded.
Folded membranes like this provide a very large surface within a
small space.
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Mitochondria in unicellular algae.
© Australian Key Centre for Microscopy.
Mitochondria are the organelles in which the final stages of cellular
respiration occur. They are the organelles in which the energy produced
is released by cellular respiration. The wastes from this process, carbon
dioxide and water, are also formed in these structures. Mitochondria are
often referred to as the powerhouses of the cell because they are the
organelles which provide energy for the cell.
2
How does the structure of a mitochondrion support its function?
_____________________________________________________
_____________________________________________________
Check your answers.
Golgi bodies
A Golgi body was first observed and identified as an organelle by
Camillo Golgi (1844–1926), an Italian histologist. Hence, Golgi is
always spelt with a capital letter. We are inclined to get the impression
that these not so well known organelles were discovered when the
electron microscope came into use. This is not so.
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The Golgi body was actually discovered when Golgi used an optical
microscope and a dye containing a silver compound. There was
originally much argument as to whether or not it was a new organelle or
simply a product of the staining technique he used. The matter was not
resolved for about 60 years when electron microscopes verified
Golgi’s claims.
Find the Golgi body in the electron micrograph below. Note that the
Golgi body consists of stacks of membranes which bulge out in places to
travel through the cell.
Small Golgi body in a grass leaf cell. (g — Golgi body, rer — rough
endoplasmic reticulum, cw — cell wall)
© Australian Key Centre for Microscopy.
The Golgi body has been found in nearly all types of cells, but is
particularly abundant in cells that secrete substances eg. the
salivary glands.
The Golgi apparatus can absorb amino acids and sugars and use them to
synthesise more complex proteins and carbohydrates. These chemicals
are contained in the vesicles, which become detached from the Golgi
apparatus. These vesicles appear to move away from the Golgi apparatus
across the cytoplasm to the cell membrane. Finally, the small vesicles
fuse with the cell membrane and the protein–carbohydrate substances are
discharged (secreted) from the cell.
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So, the Golgi body prepares and secretes various chemicals for use either
within or outside the cell.
How is the function of a Golgi body supported by its structure?
_________________________________________________________
_________________________________________________________
Check your answer.
Ribosomes
Compared to the nucleus, mitochondria, chloroplasts and Golgi body,
ribosomes are very small indeed. They are tiny, spherical organelles
which are found throughout the cell.
Ribosomes are found within other organelles such as the nucleus and
chloroplasts. They are also found attached to a membrane system known
as the endoplasmic reticulum.
Rough endoplasmic reticulum in rat intestine. (Arrowheads indicate ribosomes
attached to the surface of the rough endoplasmic reticulum).
© Australian Key Centre for Microscopy.
Part 4: Electron microscope and cell organelles
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Ribosomes are the organelles where proteins are made. They are most
numerous in cells that produce proteins. The small size of ribosomes
gives a high surface area to volume ratio.
How does the structure of a ribosome enable it to produce proteins?
_________________________________________________________
_________________________________________________________
Check your answer.
Endoplasmic reticulum
The endoplasmic reticulum is a network of membranes that run through
the cytoplasm. The rough endoplasmic reticulum looks rough because
there are ribosomes attached to it. Smooth endoplasmic reticulum has no
ribosomes on it. The endoplasmic reticulum is an intricate combination
of canals.
Rough endoplasmic reticulum in mouse intestine. Parallel sheets of rough
endoplasmic reticulum can be seen surrounding the nucleus. (n — nucleus,
nu— nucleolus, m — mitochondria, rer — rough endoplasmic reticulum)
© Australian Key Centre for Microscopy.
The endoplasmic reticulum forms a complex system of canals or
channels along which substances are transported throughout the cell.
Smooth endoplasmic reticulum is involved in the formation of lipids
(fats). It also helps inactivate some drugs.
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How does the structure of endoplasmic reticulum relate to its function?
_________________________________________________________
_________________________________________________________
Check your answer.
Lysosomes
Lysosomes were first described in the 1950s. They are smaller than
mitochondria and are enclosed by single membranes. The membrane
does not permit the movement of enzymes from within and is capable of
resisting their digestive action. Lysosomes are more commonly found in
animal cells.
The lysosome is thought to contain enzymes that take in and break down
older cell organelles. If a lysosome should rupture (break) the enzymes
would break down and destroy the cell.
How is the structure of a lysosome relevant for the function it performs?
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
You will need to look carefully at the micrographs throughout the module
and familiarise yourself with them, as you may be required to identify such
pictures/diagrams at a later stage. Use other secondary sources of electron
micrographs to identify mitochondria, chloroplasts, Golgi body, lysosomes,
endoplasmic reticulum and cell membranes. Use the electron micrographs
on the previous pages and the LMP Science webpage for more sources of
pictures. Make sure you can identify the above cell organelles.
Complete Exercise 4.2.
Part 4: Electron microscope and cell organelles
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Tissues, organs and organ systems
Cells are not a disorganised mass of matter. Instead cells are grouped
together to form tissues, tissues make up organs and organs make up
organ systems. The table below summarises this information.
Definition
Animal example
Plant example
tissue
a group of cells of the same type
with the same function
blood
phloem
organ
part of an animal or plant forming a
structural and functional unit which
is made up of one or more tissues
heart
leaf
organ system
a group of organs that function
together as a unit
blood system
vascular system
Explain the difference between tissues, organs and organ systems.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Do Exercise 4.3 to complete this part.
In the next part you will go on to investigate how specialised structures
are used by organisms to obtain nutrients from their surroundings.
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Summary
Self–correcting summary.
1
a) What are two advantages of using electron microscopes over
light microscopes?
_________________________________________________
_________________________________________________
b) In what way is the use of the electron microscope limited?
_________________________________________________
c) Which organelles would you be able to see using an electron
microscope that were not visible using a compound or light
microscope?
_________________________________________________
2
Complete the summary of cell organelle structure and function
following by filling in the missing words.
a) Mitochondria (singular ________________ ) are organelles with
a double ________________. The inner membrane is highly
________________ producing a large surface area.
Mitochondria are described as the powerhouses of cells because
they provide cells with ________________ to do work. The
final stages of the energy–releasing change called
________________ occur in these organelles.
b) A Golgi body consists of ________________. These
membranes bulge out to form ________________. These
vesicles contain various chemicals which are used whether
within the cell or ________________ by the cell for use
elsewhere.
c) ________________ are small, spherical structures which are the
site of protein synthesis.
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d) The endoplasmic reticulum is a network of ________________
which form canals or passageways. The canals
________________ a variety of substances throughout the cell.
Endoplasmic reticulum with ribosomes attached is called
________________ endoplasmic reticulum.
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Suggested answers
The nucleus
The nucleolus is involved in the formation of proteins which controls the
activities of the cell. Substances are able to move in and out of the cell
through the pores in the nuclear membrane.
Plastids and chloroplasts
The stack of grana in chloroplasts increases the surface area available
within the chloroplast for photosynthesis.
Chloroplasts contain chlorophyll which absorbs light energy and makes
photosynthesis possible.
The cell membrane
The cell membrane is a physical barrier, it is thin and contains tiny pores
to enable the diffusion of matter across it.
A summary
1
The nucleus is a large spherical organelle which controls the
activities of the cell. This organelle is surrounded by a double
membrane that has pores in it.
2
Chloroplasts are one type of plastid. They are green coloured
because they contain the green pigment, chlorophyll. Like the
nucleus, the chloroplast is surrounded by a double membrane.
Chloroplasts absorb light energy, which drives the food–making
process called photosynthesis.
3
The cell or plasma membrane keeps the cell intact. It allows some
substances to enter and leave while preventing others from entering
Part 4: Electron microscope and cell organelles
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or leaving. It is therefore described as selectively (or semi)
permeable.
Mitochondria
1
Active cells require large amounts of energy. Energy in cells is
provided by the process of respiration. Mitochondria might be the
site where respiration occurs, therefore providing the energy needed.
2
The highly folded inner membrane increases the surface area for
respiration. The large surface area provided by a folded membrane
provides more places for respiration to occur. So, respiration can
occur on a large–scale.
Golgi body
The Golgi consist of stacks of membranes which increase the surface
area for diffusion and synthesis of complex molecules. The vesicles
function is transporting these molecules to the cell membrane.
Ribosomes
The large surface area to volume ratio enables efficient movement of
materials across the ribosome’s membrane.
Endoplasmic reticulum
The large surface area enables the endoplasmic reticulum to efficiently
transport materials around the cell.
Lysosome
The semipermeable nature of the membrane prevents the enzymes
escaping into the cell and destroying it.
Tissues, organs and organ systems
Cells are fundamental units that make up a living thing. Similar cells
work together to form tissues and different tissues work together in
an organ.
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Summary
1
a) Greater magnification and resolution
b) Specimens are killed in the process of preparation for viewing
with an electron microscope. No guarantee that the resulting
images are true representations of the original living tissue. The
cost of electron microscope technology is very high.
c) Nucleolus, Golgi, endoplasmic reticulum (rough and smooth),
lysosome, mitochondria, ribosomes.
2
a) Mitochondria (singular mitochondrion) are organelles with a
double membrane. The inner membrane is highly folded
producing a large surface area. Mitochondria are described as
the powerhouses of cells because they provide cells with energy
to do work. The final stages of the energy–releasing change
called respiration occur in these organelles.
b) A Golgi body consists of membranes. These membranes bulge
out to form vesicles. These vesicles contain various chemicals
which are used whether within the cell or secreted by the cell for
use elsewhere.
c) Ribosomes are small, spherical structures which are the site of
protein synthesis.
d) The endoplasmic reticulum is a network of membranes, which
form canals or passageways. The canals transport a variety of
substances throughout the cell. Endoplasmic reticulum with
ribosomes attached is called rough endoplasmic reticulum.
Part 4: Electron microscope and cell organelles
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Exercises – Part 4
Exercises 4.1 to 4.3
Name: _________________________________
Exercise 4.1: The electron microscope
a)
Identify two types of electron microscopes.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
b)
Describe two disadvantages of using an electron microscope.
_____________________________________________________
____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
c)
What magnification and resolution are available with an electron
microscope?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 4: Electron microscope and cell organelles
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Exercise 4.2: Observing plant and animal cells
The following diagrams show the structure of a generalised plant and
animal cell, as viewed with an electron microscope.
cytoplasm
Golgi apparatus
plasma membrane
cell wall
ribosomes
chloroplast
vacuole
nucleus
nucleolus
mitochondrion
endoplasmic reticulum
Plant cell (as seen with an electron microscope).
mitochondrion
nucleus
Golgi apparatus
nucleolus
smooth
endoplasmic
reticulum
plasma membrane
lysosome
rough
endoplasmic
reticulum
Animal cell (as seen with an electron microscope).
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a) Which structures are you able to see with an electron
microscope that are not possible to see with a light microscope?
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
b) Identify differences between plant and animal cells.
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
c) You are a biologist working in a research laboratory comparing
plant and animal cells. Prepare a comparison of the two types of
cells, their structure and function.
You may find it more convenient to put this information into a
table. A suggested table with headings is provided here for you.
You may select headings of your own.
For each organelle state how the structure enables it to carry out
its specific function.
Part 4: Electron microscope and cell organelles
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Organelle structure
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Function
Found in
animal cells
Found in
plant cells
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Exercise 4.3: Tissues, organs and organ systems
a)
Define the following terms
tissues _______________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
organs _______________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
organ systems _________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
b) Organ systems in multicellular organisms supply the needs of cells.
For example, the circulatory system transports oxygen from the
lungs to the cells. Outline how one other organ system supplies the
needs of cells.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 4: Electron microscope and cell organelles
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