MICROSCOPES - The School for Excellence | Who We · PDF fileInteresting differences in the powers of resolution between light and electron microscopes ... Complete the following table

The School For Excellence 2016 Summer School – Year 11 Biology – Book 1 Page 9

MICROSCOPES Two main types of microscopes exist: The light and electron microscopes. The light microscope increases the ability to see minute objects using lens systems which magnify images of specimens using light.

Light microscopes widely used in schools contain two lenses, the _________________

and _____________________ lenses, and are referred to as compound microscopes. • Total magnification of the image is determined by multiplying the individual eyepiece

and objective lens magnification values. • All images are presented in two dimensions.

A typical light microscope


The light microscope is composed of numerous parts. Complete the following table outlining microscope parts and their functions:

Microscope Part(s) Function Stage

Provide variation in image magnification

Coarse focus

Fine focussing of the image, particularly under high magnification

Iris diaphragm

Condenser To adjust the view under a light microscope you can increase the magnification, use the fine focus adjustor or adjust the light using the iris diaphragm or condenser. Stains Particular stains improve the visibility of structures within a cell. Unfortunately, the cell dies with their application. E.g. Iodine: _____________________________________________________________

Methylene Blue: _____________________________________________________ Electron Microscopes The electron microscope uses beams of electrons to reveal complex internal structures of cells, or to magnify the surface textures of specimens. • Transmission electron microscope (TEM): is used to reveal the internal detail of

stained dead cells. • Scanning electron microscope (SEM): provides detailed images of surfaces of dead

cells or specimens. Resolution is a measure of the image detail or clarity of an image produced by a particular microscope, i.e. the minimum distance required to distinguish between two lines. The human eye has a minimum resolution of 0.1 mm. Interesting differences in the powers of resolution between light and electron microscopes exist.


Complete the following table of comparison between light and electron microscopes:

Light Microscope Electron Microscope

Used to Create Image

Light Electrons

Magnification

Resolution

200 nm 0.2 nm

Specimens

Living or dead, true colour, staining if desired

Thickness

Thin – fixation not required Very thin – fixation required

• Fluorescence microscopes detail chemicals in cells, such as DNA.

(Campbell NA & JB Reece, 2002)


Determining the size of a cell: • Measure width/diameter of field of view (FOV). • Estimate no. of cells that fit across FOV. • Divide FOV by number of cells that would fit across FOV.

At 100x magnification, field of View might be 1.5 mm. That would equal 1500 µm. • The number of this type of cell that would fit across the FOV would be about four. • Therefore the approximate width of one cell would be 1500 µm/4 and would be 375 µm.

SHAPES OF CELLS Cells are all three-dimensional and can vary in shape and size (nearly all are microscopic). Being microscopic ensures that a cell has a high surface area (cell membrane) to volume ratio. This results in the cell being efficient at exchanging substances. When viewing cells with a microscope, the view seen is two dimensional and only a section of the cell.

Field of View (FOV)

Diameter of FOV

Cell


QUESTION 1 Draw how a plant cell in the shape of a cylinder will appear if sliced in the directions as shown:

(a) (b) (c)

A

B

C


When viewing a cell under the microscope, one section may contain a nucleus while another section from the same cell may not. Biologists often take many sections in the one plane from a cell to determine what is inside the cell. EXAMPLE (a) (b) (c)

A B C


SURFACE AREA TO VOLUME RATIO In determining the surface area : volume of an organism or cell, its surface area needs to be compared to its volume. Surface area (SA) refers to the area of the outer covering of the cell or organism. Volume (V) refers to the amount of space taken up by the cell or organism. The ratio is achieved by calculating the total SA and dividing it by its total Volume. • A smaller organism/cell will have a greater SA:V compared to a larger one. • In terms of a cell, SA:V refers to the number of units of cell membrane (SA) that are

available to each unit of volume (cytoplasm). • The SA:V determines how quickly substances, or heat energy, can be

absorbed or released from a cell or organism. For example:

Size of object Surface Area Volume SA:V ratio

Rate of Diffusion

1 cm sided cube 6 cm² 1 cm³ 6.0 : 1

Quick

10 cm sided cube 600 cm² 1000 cm³ 0.6 : 1

Slow

Hence, as an object increases in size, its SA:V decreases.

The shape can also affect the SA:V ratio.

For example:

Shape of Container Surface area Volume SA:V ratio

Rate of Diffusion

Sheet of paper (100 x 100 x 0.1 cm)

10,040 cm² 1,000 cm³ 20.4 : 1 Rapid

Cube (10 x 10 x 10 cm)

600 cm² 1,000 cm³ 0.6 : 1 Slow

Sphere (6.2 cm radius)

483 cm² 1,000 cm³ 0.5 : 1 Slow


Consider the following three shapes:

large SA:V very large SA:V low SA:V

• The SA:V of an object/cell increases as its shape becomes flatter. A sphere has the

lowest SA:V of all shapes. • The SA:V of an object/cell increases as its size decreases. Hence, by being

microscopic in size, cells ensure that they gain nutrients and excrete wastes efficiently.

TYPES OF CELLS All cells can be divided into two main groups: prokaryotic and eukaryotic cells. Any persistent structure that has a particular function within either cell type is referred to as an organelle (e.g. nucleus, ribosomes etc.). Organelles do not have to be membrane-bound, e.g. ribosomes.

PROKARYOTIC CELLS Prokaryotic organisms were the first to evolve on Earth. All prokaryotes are primitive in structure, and smaller than eukaryotic cells. The following list includes features of prokaryotic cells: • No membrane-bound nucleus. A circular DNA molecule floats freely in the cytoplasm,

and is not bound by a membrane. Most also possess smaller rings of DNA (plasmids). • No membrane bound organelles are present. • Free floating ribosomes are required for protein synthesis. • Can be photosynthetic (cyanobacteria) as they can contain free floating chlorophyll. • Divide by binary fission. • Examples are bacteria and cyanobacteria (blue-green algae).



EUKARYOTIC CELLS Eukaryotic cells are thought to have evolved from prokaryotic cells hundreds of millions of years ago. They possess membrane-bound organelles, such as nuclei, in addition to their non-membrane-bound organelles, such as ribosomes. Eukaryotic cells: • Possess a distinct nucleus enclosed in a double membrane. • Contain membrane-bound organelles. • (e.g. mitochondria, golgi apparatus, etc.) • Possess ribosomes. • Found in animal, plant, fungi and protoctist kingdoms. • Divide by mitosis. • In multicellular organisms cells communicate by chemical means, and perform

cytoplasmic streaming.

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THE FIVE KINGDOMS Animalia: Includes all vertebrate (e.g. mammals, reptiles etc.) and invertebrate animals

(e.g. insects, jellyfish etc.). All members are multicellular eukaryotic heterotrophs (must consume organic nutrients to survive).

Plantae: Includes flowering and non-flowering plants. All members are multicellular,

eukaryotic autotrophs (i.e. synthesise their own organic nutrients). Fungi: Includes all unicellular and multicellular fungi. All are eukaryotic heterotrophs

with cell walls of chitin. Protista: Unicellular eukaryotic organisms. Monera: Consists of all known prokaryotic cells.

All living organisms are classified into five Kingdoms. Each kingdom has particular cellular characteristics:

KINGDOM

Monera Protista Fungi Plants Animals

Examples

Distinguishing Features

* unicellular * prokaryotic

* unicellular * eukaryotic

* uni/multi-cellular * heterotrophs * chitin cell wall

*multicellular * autotrophs

* multicellular * heterotrophs * motile

Tick the boxes below when the feature listed on the left is part of a cell representative of the kingdom.

Cytoplasm Nucleus

Cell Membrane Cellulose Cell Wall

Chitin Cell Wall Other Cell Wall


Animalia: • Includes all vertebrate (e.g. mammals, reptiles) and invertebrate (e.g. insects, jellyfish)

animals. • All members are eukaryotic. • Heterotrophic and motile. Plantae: • Includes flowering and non-flowering plants. • All are eukaryotic (e.g. Banksias, mosses). • Cellulose cell walls. • Autotrophs. Fungi: • Two main groups: Unicellular fungi (e.g. yeast) and multicellular fungi with thread-like

hyphae (e.g. mushrooms). • All are eukaryotic. • Chitin cell walls.

• Heterotrophic Protista: • Unicellular organisms. • Aquatic. • All are eukaryotic (e.g. Amoeba, Paramecium, Euglena). Monera: • Simplest kingdom (has been divided by many

scientists into more kingdoms). • Prokaryotic cells (e.g. bacteria and cyanobacteria).

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MICROSCOPES - The School for Excellence | Who We · PDF fileInteresting differences in the powers of resolution between light and electron microscopes ... Complete the following table

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