U N I T U N I T Chapter 1Cells: discovery and exploration Chapter 2Structure and function of cells Chapter 3Composition of cells Chapter 4Cell replication U N I T Y A N D D I V E R S I T Y 1 AREA OF STUDY 1 Cells in action
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KEY KNOWLEDGE This chapter is designed to enable students to:
appreciate the historical development of microscopy techniques
investigate current and emerging technologies in light and electron microscopy
understand the importance of technological advances to our knowledge of lifeforms and cells.
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Figure 1.1 Examination of high-resolution three-dimensional brilliant fluorescence images is now possiblewith current stereomicroscopes such as this SteREOLumar.V12 manufactured by Carl Zeiss Pty Ltd. Thestereomicroscope has lenses specially developed for use withfluorescence and its operation is completely motorised. Thefocus of an object can be rapidly set and precisely reproducedwith the use of human interface panels (HIP). A system control
panel (SyCoP) is designed for use by either a right- orleft-handed person and combines joystick, buttons and atouch screen — a design similar to a computer mouse sothat the operator can control the microscope while stillviewing through the eyepiece. In this chapter, we willdiscuss significant historical developments in microscopytechniques and the latest advancements in microscopetechnologies.
Life on Earth … and beyond?Is there (or was there ever) life on Mars?
At the turn of the twentieth century, an American astronomer, Percival Lowell
(1855–1916), drew maps of the surface of planet Mars that showed intricate
patterns of linear structures that he called canals. He argued that these canals
were not natural features but were artificial con-
structions produced by intelligent life. Figure 1.2ashows Lowell’s drawings of canals on Mars. Figure
1.2b shows a typical area on the surface of Mars as
revealed by the Viking Lander in 1976. Definitely
no canals! Definitely no evidence of life, intelligent
or otherwise!
The Viking Lander carried instruments to test for the existence of living organ-
isms on Mars (note the trenches dug by the soil retrieval scoop in figure 1.2b),
but the results of the tests were inconclusive.
Then, in 1996, sensational headlines worldwide publicised the claim by NASA
scientists that life once existed on Mars. This claim was based on studies of a
meteorite that originated from that planet. The evidence included the presence of
tiny structures within the meteorite (see figure 1.3) that were said to be fossilised
microbes (tiny living organisms). However, other scientists disputed this claimand argued that these microbe-like structures could be produced by chemical
reactions. Again, the evidence for life on Mars was inconclusive.
Another development occurred in January 2004
when two Rovers landed on the surface of Mars to
study its rocks and minerals. Data from these Rovers
provided evidence that liquid water once existed on
Mars. We know that liquid water is essential for life.
We also know that microbes can survive in extreme
environments on Earth, such as in rocks deep below
ground, in ice-sealed lakes, in glaciers high on moun-
tains and in cold dry valleys of Antarctica. Based on
these facts, it remains possible that life does or did
exist on Mars.There are plans to launch a Mars Science Labor-
atory from Earth to Mars in December 2009. This
mobile laboratory will search for evidence of life — past or present — on Mars
using instruments that can detect organic compounds, such as proteins and amino
acids, that are made only by living organisms.
Scientists expect that, if life exists now or existed in the past on Mars, this
extraterrestrial life will be like the microbes that live today in extreme environ-
ments on Earth. Microbes, like all living things, are organised into microscopic
‘compartments’ known as cells. Each microbe typically consists of just one cell
and the internal contents of each cell are separated from the external environment
ODD FACT
In July 1976,when the Viking Lander reachedthe surface of Mars, it becamethe first spacecraft to land onthe surface of another planet.
Figure 1.3 Scanning electronmicrograph image of part of a meteorite(known as ALH84001) from the surfaceof Mars that landed in the Antarctic.While the elongated structures look likemicrobes (tiny living organisms) theyare not universally accepted as beingfossilised microbes.
(a)
Figure 1.2 (a) Canals on Mars based on observations made fromEarth by Lowell in the early 1900s, and (b) the surface of Mars asrevealed by the Viking Lander in 1976. Can you suggest a possiblereason for Lowell’s observations being flawed?
by a membrane boundary. The strongest direct evidence for past or present extra-
terrestrial microbial life on Mars would be the discovery of structures that can
without any doubt be identified as cells.
Let us now look in more detail at the historical development of ideas and tech-
nological advances that have contributed to our knowledge and understanding of
life forms, and their living compartments or cells.
Cells and microscopes:an introductionCells are the basic structural and functional units of all living things (figure 1.4).
Although most cells are too small to be seen with the unaided eye, microscopes
give enlarged images of cells and the structures they contain, and make it possible
for us to examine cells with great detail.
Cell type Example Size
animal frog egg
human egg (note the
relative size of sperm)
human white blood cell
human red blood cell
1500 Mm
200 Mm
25 Mm
8 Mm
fungus yeast cell 5 Mm
bacteria Staphylococcus
(causes infections
such as boils)
Diplococcus pneumoniae
(causes pneumonia)
Treponema pallidum
(spiral — causes
syphilis)
1 Mm
0.1 Mm
0.3 Mm wide
and
10 Mm long
plant epidermal leaf
cell
200–400 Mm
The development of microscopes over the centuries has depended on the
development of glass, then on glass being made into lenses, the development of
different kinds of lenses and their assembly to form microscopes.
Figure 1.4 Most human cellstypically range in diameter from about8 to 25 micrometres (Mm) or 0.008 to0.025 mm. In comparison, a hair from
a man’s beard is about 200 Mm(0.2 mm) wide. Typical bacterial cellsrange from 0.1–1.5Mm and giantamoeba are about 1000 Mm wide.Note the sizes of various kinds of cells(1 mm = 1000 Mm). Cells are not drawnto scale.
The increase in our understanding of cells has paralleled:
• the improvements in and development of new kinds of microscopes
• the variety of different techniques available, including stains, sectioning and
using different kinds of light.
The advanced microscopes of today have a history dating back more than
three thousand years to when the first glass was made by Phoenician sailors. A
summary of some of the important steps in the development of the microscope
and our understanding of cells is presented in table 1.1.
Table 1.1 Some important
milestones in the development ofmicroscopes
Date or period Person and development
> 3000 years ago Glass beads first made by Phoenician sailors, who were from an area now known as Lebanon
250 BC–AD 100 In China, the first recorded uses of optical lenses
AD 79
(excavated 1748)People of Pompeii used glass-crystal lenses
1200–1250 Robert Grosseteste, Bishop of Lincoln, UK, made a primitive but functional magnifying glass.
1590sDutch lens-makers, Hans Janssen and his son Zacharias, used two lenses to develop the first compound
microscope, called ‘telescope’ by some writers.
1605–1619Cornelius Drebbel (1572–1633), a Dutch/English inventor of many scientific instruments, developed a
machine for grinding lenses and improved the quality of compound microscopes.
1605–1614
Galileo Galilei (1564–1642), an Italian, refined the Janssen microscope into a high-quality astronomical
telescope. He also further developed the microscope and may have been the first to examine and describe
living tissue. He described the cuticle of a fly as being covered in fur.
between 1605–
1610
Galileo was a prominent member of the Accademia dei Lincei (Academy of the Lynx) that introduced the
term ‘microscopio’ — a lens for the examination of very small objects.
1665Englishman Robert Hooke (1635–1703) published Micrographia. He describes ‘cells’ in a piece of cork
(page 6 and figure 1.5) and draws many cell types. Hooke’s microscope could magnify 14–42 times.
1674Antony van Leeuwenhoek (1632–1723), a Dutch cloth merchant, built a microscope with a magnifyingrange from 50 to 300 times. He was the first to make descriptive drawings of protozoa, bacteria,
spermatozoa and red blood cells (page 6 and figure 1.6, page 7).
1733Englishman Chester Moor Hall used lenses made of different kinds of glass to invent the achromatic lensthat removed many of the optical distortions of previous lenses.
1738German Johann Lieberkuhn added a metal reflector to the microscope to increase light falling on aspecimen.
1831Robert Brown (1773–1858), a Scottish botanist and naturalist, described the nucleus in orchid cells (figure
1.7, page 7 and page 8).
1838Two Germans, botanist Matthias Schleiden (1804–1881) and zoologist Theodor Schwann (1810–1882),
suggested that cells are the basic structural units of all plant and animal matter.
1851 Binocular microscope (viewing with two eyes) constructed by Professor Riddell
1878Germans Ernst Abby and Carl Zeiss produced improved oil-immersion microscope lenses that significantly
increased the ability to magnify cells (figure 1.10, page 11).
1931–1933 German Ernst Ruska developed the electron lens and used several to make the first electron microscope.
1936 Swedish Torbjorn Oskar Caspersson used an ultraviolet microscope to study cells.
1938 Dutch Fritz Zernike built the first phase contrast microscope enabling examination of transparent cells andmicro-organisms without the need to stain or kill.
1955 Marvin Minsky of the USA invented the confocal scanning microscope.
1969Scientists in Holland, Britain and America developed confocal laser scanning microscopy. AmericansPaul Davidovits and David Egger announced they were able to ‘optically section’ thin slices of three-
dimensional specimens such as a cell (as in figure 1.15, page 12).
2003PlasDIC is a special form of a differential interference contrast microscope in which special prisms areused to reveal high-resolution, three-dimensional details of a specimen illuminated with non-polarised light
(figures 1.21 and 1.22 on page 16).
2004Laser scanning microscopy LSM 5 LIVE scans living cells at speeds of up to 1010 frames per second.
Allows a better understanding of cellular processes and study into cellular interaction mechanisms.
Figure 1.6 (a) The simple microscope built by Leeuwenhoek.The specimen was placed on the tip of a pin that acted as aspecimen holder. The lens, only two millimetres wide, was groundout of a quartz crystal and was fitted into a hole in a metal plate.
The instrument was held up to the eye and the specimen viewedthrough the lens. (b) Some of the ‘little animacules’ seen byLeeuwenhoek. These were various bacteria. (c) Algal and othercells from pond water as drawn by Leeuwenhoek
Figure 1.7 (a) Wax medallionof Robert Brown, made in 1852(b) One of the microscopes used byBrown in his observations of pollenand other plant cells. Note themirror (closest to the base) and thefine-adjustment knob for movementof the specimen platform above it.
Leeuwenhoek built over 50 simple microscopes to examine material from dif-
ferent sources. Figure 1.6c shows Leeuwenhoek’s drawings of some of the algaland other cells he observed in pond water. More than 150 years later, in 1831, Robert Brown (1773–1858), a Scottish
botanist (see figure 1.7, page 7), was involved in a dispute about how pollination
and fertilisation occurred in plants. During his studies with orchids, on 13 June
1831 he made a note that:
It appears that each cell has … on its inner side a spherule or at least orbicular
corpuscle …
Brown called this structure the nucleus of a cell. Others, including Leeuwenhoek,
had observed nuclei but Brown was the first to introduce the concept of a nucle-
ated cell as the unit of structure in plants. Brown had no idea about the importance
of the nucleus and had some doubt about whether each cell needed one.
Recognising the pattern: the Cell TheoryBy the early 1800s, the accepted idea was that plants and animals were composedof globules, called cells, and formless material. Brown had enhanced this idea bydescribing nuclei in cells of orchid plants. These views were to be extended bytwo German biologists. In 1838, a German botanist, Matthias Schleiden (1804–1881), suggested thatcells were the basic structural unit of all plant matter. A German zoologist, TheodorSchwann (1810–1882), independently proposed that animals were aggregates ofcells arranged according to a definite law. In sharing their ideas over dinner inOctober 1838, the two biologists came to recognise that both plant and animal
tissues have a cellular organisation. Nearly 200 years after Hooke first describedcells, the basic structural pattern of living things was finally recognised. The recognition that all kinds of living things share a common structural unit— the cell — provided the foundation of one of the major unifying themes ofbiology. All living things are composed of cells and substances produced by cellsor developed out of cells. Because of this unity of structure, results of studies of
cells from one type of organism can be used to make predictions about cells fromother kinds of organisms. Schwann wrote in 1839:
The elementary parts of all tissues are formed of cells in an analogous,
though very diversified manner, so that it may be asserted, that there is one
universal principle of development for the elementary parts of organisms,
however different, and that this principle is the formation of cells.
This basic idea arising from the work of Schwann and Schleiden, pub-
lished in 1839, is known as the Cell Theory:
All living things consist of one or more organised structures that are called
cells or of products of cells.
Cells are the basic functional unit of life.
A German doctor, Rudolf Virchow (1821–1902) added to the under-
standing of cells by providing a new answer to the question: How are new
living things produced? Past answers to this question included spon-
taneous generation, the idea that living things could arise from non-
living matter or dead matter. Another idea was that living things developed
from globules that gathered to form a compact mass and then became
organised into cells.
In 1858, Virchow challenged these old ideas with his concept of biogenesis
(from bio = life; genesis = origin, creation). He proposed that new cells come
from existing cells, and in one of his famous lectures said:
ODD FACT
Schleiden was initiallyeducated as a barrister. Because
of his lack of success in thisprofession, he attempted
suicide, shooting himself inthe forehead, but recovered.
Schleiden then turned tothe study of natural science
and medicine and became aprofessor of botany.
Figure 1.8 According to one theory,
living things could arise from dead
matter; for example, leaves could
become birds or fish, depending on
where they fell.
ODD FACT
Robert Brown wasthe botanist who accompanied
Sir Joseph Banks on the
Investigator , when CaptainMatthew Flinders charted the
southern coast of Australia from1801 to 1805. Brown and Bankscollected nearly 4000 specimens
of different species of plants,most of them unknown to Western
science at the time. Brown alsodiscovered molecular movement,now called ‘Brownian movement’.
… no development of any kind begins de novo [from new] … Where a cell arises,
there a cell must have previously existed just as an animal can spring only from
an animal, a plant only from a plant … No developed tissue can be traced either
to any large or small simple element, unless it be a cell.
Virchow’s contribution extended the Cell Theory to include the basic concept:
New cells are produced from existing cells.
In 1862, the French biologist, Louis Pasteur (1822–1895) carried out experi-
ments that conclusively disproved the old idea of spontaneous generation, and
supported the view that new cells are produced by existing cells.
Life span of cellsCells of a multicellular organism do not necessarily live as long as the organism
itself. Some cells have a relatively short life and are constantly being replaced.
The average life spans of some human cells are as follows:
• stomach cells 2 days
• mature sperm cells 2–3 days
• skin cells 20–35 days
• red blood cells about 120 days.
A person can make a blood donation because the cells removed can be replaced
by new cells. Skin can be taken from one part of a person’s body and grafted onto
another area where the skin tissue has been completely destroyed. Skin cells
from the undamaged area will reproduce to replace the cells removed.
In contrast, other types of cell, such as brain cells, have long life spans and
are not replaced during a person’s lifetime. If brain tissue is damaged, most cell
types in the brain cannot reproduce to replace the damaged cells.
ODD FACT
‘Spray-on skin’cells are now used in thetreatment of some burns
(see chapter 4, page 77).
Cells were first identified and named by Hooke in 1665.The nucleus in a cell was identified and named by Brown in 1831.The Cell Theory arose in the mid-1800s.The Cell Theory recognises that all living things are composed of one ormore cells and that new cells are produced by existing cells.The life span of cells in a multicellular organism varies.The unit of measure often used in relation to cell size is the micrometre (Mm).
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KEY IDEAS
1 Suggest why cells were not discovered by the Greek physician Hippocrates(died 357 BC).
2 Who is credited with the discovery of the basic building block of livingorganisms?
3 Who is credited with the discovery of the cell nucleus? 4 What was the important contribution by Schleiden and Schwann to biology? 5 Identify one commonplace idea about the origin of living things before
Virchow. 6 Which person is more likely to have permanent damage after an accident:
person A who survives after blood loss or person B who survives after someloss of brain tissue? Explain.
7 How many micrometres (Mm) are there in a millimetre (mm)?
as near-field scanning opticalmicroscopy (NSOM), was
developed for viewing cellsand other objects. The
technique allows organellesthat are too small to be resolved
with a normal light microscopeto be seen.
Tools for viewing cellsWith few exceptions, individual cells typically are too small to be seen with an
unaided eye. Because of this, the study of cells has depended on the use of instru-
ments, such as microscopes. There are many different kinds of microscopes but
they can be broadly divided into two groups, light and electron.
Light microscopesHooke needed a light microscope to see the dead cells present in cork. Light
microscopes (LMs) increase the ability of the human eye to see tiny objects.
LMs can reveal objects such as the unicellular organism in figure 1.11a that
are too small to be seen, or details that are too minute to be resolved with an
unaided human eye. LMs use visible light that illuminates and passes through a
specimen. When tissues are examined using an LM, a slice of tissue just a few
cells thick is viewed. The tissue is usually stained with a dye (see page 11) to
make it more visible.
Simple light microscope
Light microscopes (LM) use glass lenses. Early LMs with only one lens, like thekind used by Hooke and van Leeuwenhoek, are called simple light microscopes.
They are similar to a magnifying glass.
Compound light microscopeMicroscopes with at least two sets of lenses are called compound light micro-
scopes (CLM). Most compound light microscopes have several objective lenses,
each of a different magnification (see figure 1.9). The amount of magnification
you obtain when using a light microscope, that is, how large the object appears,
depends on the magnification powers of both the objective lenses and eyepiece
(ocular) lenses you use. The magnification you obtain of an object is calculated by
multiplying the magnification (power) of
the objective lens by the magnificationof the ocular lens you use.
The highest magnifications are
obtained with the use of an oil immer-
sion objective lens. Light usually
travels in a straight line through a partic-
ular medium. As light passes from one
medium to a different medium, the rays
change direction — they are refracted.
Hence, as light passes through a glass
slide holding a specimen and into air
above, rays are refracted and there is a
reduction of light entering the objective
lens (figure 1.10). A reduction of lightreduces the clarity of an image. With an
Figure 1.10 Note that the use of oil (with the samerefractive index as glass) between the specimen andthe objective lens increases the amount of light passingthrough the optical system of the microscope by reducingrefraction. Higher magnification objective lenses can beused and a clearer image of the specimen is obtained.
Characteristics of the lenses also influence a microscope’s
resolution. Resolution is the ability to see two points that are
close together as two separate points. Our eyes have limitedresolving power: they may interpret two small spots that are
close together as a single blurred spot. We use microscopes to
resolve things that our eyes are unable to see; with an appro-
priate microscope we can distinguish the two small spots. But
microscopes also have a limit to their resolving power. A poor
quality microscope might simply magnify the blur we see into
a larger blur. The wavelength of light used, as well as the char-
acteristics of the lenses, influence the size of an object that can
be resolved with a microscope. The smaller the wavelength of
light used, the smaller the size discernible. Standard light micro-
scopes use visible light.
Cells are virtually colourless and hence are difficult to seeunder a standard LM. Staining is required. Groups of cells are
also cut into thin slices before staining. These treatments nec-
essarily kill cells and often distort cell features. During the
twentieth century, other kinds of microscopes and techniques
as described below were developed for viewing and analysing
cells.
Phase contrast microscopeThe phase contrast microscope is a modified compound light
microscope (CLM) that was developed to observe unstained,
intact living cells (figure 1.11). These microscopes use the fact
that different parts of a cell transmit and change the direction oflight to varying degrees and enhance that difference. The image
produced has highly contrasting bright and dark areas.
Fluorescence microscopeAnother kind of CLM is the fluorescence microscope, which
uses ultraviolet (UV) light to reveal compounds that have been
stained with fluorescent dyes that bind to particular compounds
in a cell. The colour of fluorescence depends on the particular
fluorescent stain being used and the nature of the compound to
which it is attached (see figure 1.12).
Figure 1.11 (a) Image of Paramecium, a unicellular organism,as seen with a light microscope(b) Same type of cell as seen witha phase contrast microscope
Immersion oil
Condenser
lens
Glass
slide
Air
Light
As light moves
from glass into
air it is refracted
away from the
vertical and hence
away from the
objective lens
Objective
Figure 1.12 Cancerous breast cells viewed with afluorescence microscope after staining for the presence ofvimentin (green) and keratin (red)
Electron microscopesTransmission electron microscopeIn the 1930s, the transmission electron microscope (TEM) was developed.
Instead of light, a beam of electrons with a much shorter wavelength passes
through and is used to illuminate specimens. Instead of glass lenses that control
the passage of light rays in LMs, a TEM has a series of electromagnets that each
create an electromagnetic field to control the path of the electron beam. Figure1.18 shows a comparison between the internal structure of a light microscope and
a transmission electron microscope. Note the similarities; note the differences.
TEMs have a much greater resolving power than light microscopes (see table
1.2) because of the short wavelengths of electron beams. TEMs have revealed the
presence of many kinds of cell organelles and have shown the complex internal
structure that exists within cells. (See pages 33 and 34, figures 2.14b and 2.16a,
which show part of the internal structure of a cell as seen with a TEM.)
Human eye LM TEM
smallest resolvable
separation distance
0.1 mm
(100 Mm)
0.000 2 mm
(0.2 Mm)
0.000 000 5 mm
(0.0005 Mm)
source of illumination light rays light rays electron beam
Scanning electron microscopeThe scanning electron microscope was released in 1965. This instrument is
able to provide detailed images of surfaces (see figure 1.19). An electron gun
produces an electron beam that is focused onto one spot on the surface of a
specimen and is then scanned back and forth along the specimen’s surface. The
surface releases another set of electrons from the specimen and these form an
image on a small fluorescent screen. Depending on their size, whole organisms
can be scanned (figure 1.19).
Figure 1.18 (a) Optical system ofa light microscope (LM). The light
source is visible light and glasslenses (gl) produce an image thatcan be detected by an eye or otherappropriate receptor such as a camera.(b) Optical system of a transmissionelectron microscope (TEM). A tungstenfilament emits a beam of electronswhich is controlled by a series ofelectromagnetic lenses (el). In thisfigure, the orientation of the TEMsystem has been reversed to allowdirect comparison of its componentswith those of a light microscope. Inreality, the filament is at the top and
the viewing screen at the bottom soa TEM resembles an inverted lightmicroscope.
Table 1.2 Comparison of thehuman eye, light microscope (LM)and transmission electron microscope(TEM). The smaller the separation
distance between two objects, thelarger the resolving power of theinstrument being used.
Although electron microscopes have greater resolving power than light micro-
scopes, they can be used only with dead cells or organisms. The current ability of
modern light microscopes, such as the confocal microscope, to allow the detailed
study of living cells and identification and location of specific molecules in a
cell make light microscopes more appropriate for some settings in spite of their
reduced resolution. The size of the cell or organism under examination is also
important in the choice of instrument. Figure 1.20 outlines the limits of use of the
unaided eye, light microscopes and electron microscopes.
Figure 1.19 Scanningelectron micrograph of the pincushion millipede, Phryssonatus
novaehollandiae. Note (a) the plates and hairs along thedorsal surface and (b) the pairsof legs and hairs visible on theventral surface of the same
organism. The adults of thisspecies grow to about 4 mm inlength and are abundant in thesand and soils of Victoria.
A logarithmic scale is one in which
each marked unit moving up the scale is 10 times larger than the
next. This contrasts with a linear
scale in which each marked unit is
the same size as the next.
Figure 1.20 The arrows on theright-hand side indicate the rangesover which viewing is possible withan unaided eye, light microscope andelectron microscope. The logarithmicscale indicates the size of organism,cell or cell part visible by theparticular tools. 1 mm 1000 Mm;1 Mm 1000 nm.
◊ Length of some nerve and muscle cells e.g. from giraffe neck
Recent developments incurrent systemsLight microscopesLight microscopes have now been in use for centuries. As new technologies
and materials became available, the power and capabilities of microscopes havechanged significantly. Development continues. Computers have facilitated the
use of many microscopes. Other advancements include modification of existing
lens systems and automation in cells examination. We will consider two of these
Various types of microscopes can be used to examine cells.
Light microscopes (LMs) reveal details about the arrangement of cells andthe internal structure of cells.
Compound light microscopes (CLMs) have at least two sets of lenses:
objective lenses and ocular (or eyepiece) lenses.Cells are often stained with one or more dyes to make their variouscomponents easier to see.
Phase contrast microscopes allow the study of unstained living cells.
Fluorescent microscopes reveal details of chemical substances present.
Confocal microscopes use lasers to produce a sharply in-focus image of athin layer of a specimen.
Electron microscopes use beams of electrons instead of beams of light.
Transmission electron microscopes (TEMs) reveal fine detail of theinternal structure of cells.
Scanning electron microscopes (SEMs) reveal details of cell surfaces.
Technological advances involving equipment and stains associated withmicroscopy continue to be developed.
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KEY IDEAS
8 If you had a choice of any kind of light microscope, identify, giving areason, the most appropriate one for viewing the following:a a living amoeba
b a section of stained plant tissue
c the transfer of a nucleus from one cell into another. 9 True or false? Briefly explain your choice.
a All kinds of light microscopes use visible light to illuminate objects.b If the objective lens of a light microscope has a 5 magnification and
its ocular lens is 10, then the magnification obtained of an objectbeing viewed is 15.
c The use of an oil immersion lens increases the magnification capabilityof a microscope.
10 If you had a choice of any kind of electron microscope, identify, giving areason, the most appropriate one for viewing the following:a the surface of a layer of cellsb a section of brain tissue
c a small insect about 1 mm long.11 True or false? Briefly explain your choice.a Electron microscopes can be used to view living and non-living
tissues.b The resolving power of a TEM is greater than that of a LM.c TEMs and SEMs are equally appropriate to use for viewing minute