This is a PowerPoint presentation for Topic 2 in the Edexcel Biology B A Level course that starts in 2015. This is a free sample, the full PowerPoint presentation is available to purchase here: https://sellfy.com/MrExham

1. A Level Biology
MAKING SENSE OF
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2. Copyright © 2015 Henry Exham
• Now you have learnt about the common
biological molecules we can begin to put these
together and make the basic units of life . .
.cells!
• In this section you will learn the structure of
the different types of cells and the technology
used to observe them.
• You will learn about how cells divide and how
plants and animals can reproduce.
• You will also cover viruses which are a bit
strange as they are not made of cells.
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Cells, Viruses and Reproduction of Living Things
Introduction

3. Cells, Viruses and Reproduction of Living Things
MAKING SENSE OF
2.1 Cell structure and function
2.2 Viruses
2.3 Eukaryotic cell cycle and division
2.4 Sexual reproduction in mammals
2.5 Sexual reproduction in plants
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4. Copyright © 2015 Henry Exham
• What is the cell theory?
• Can you recall the magnification and
resolution of light and electron microscopes?
• Why are samples stained in microscopy?
• What is the ultrastructure of eukaryotic cells
• How are organisms organised?
• What is the ultrastructure of prokaryotic cells?
• What is the difference between gram positive
and gram negative cell walls?
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2.1 Cell structure and function
Objective Questions

5. Copyright © 2015 Henry Exham
• The smallest unit of life is one cell big.
• This means that cells are the key to life.
• A single cell has everything required for life.
• If we look inside cells we can see that they are
compartmentalised into different sections
which each have their own role.
• In this section we will look at the structure of
cells and also how to observe them using
microscopes.
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2.1 Cell structure and function
Introduction

6. Copyright © 2015 Henry Exham
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2.1 Cell structure and function
Observing cells
• In 1665 a man called Robert Hooke built this.
• It was the first microscope.
• He looked at many things down his
microscope and drew them in great detail.
• For example this amazing flea!

7. Copyright © 2015 Henry Exham
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2.1 Cell structure and function
Observing cells
• Hooke also looked at some cork under his microscope
and saw this structure.
• He thought the
compartments looked like the
small cells that monks lived
in.
• Therefore he called them
cells and that is where the
term came from.

8. Copyright © 2015 Henry Exham
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2.1 Cell structure and function
Observing cells
• After plenty more research it was finally realised
that cells were the basic unit of life.
• Schleiden and Schwann coined the cell theory in
1839 which stated that all living organisms are
made of one or more cells and the cell is the
most basic unit of life.

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2.1 Cell structure and function
Observing cells
• Microscopes are now much more advanced
and a huge amount has been discovered
about how cells work.
• There are two types of microscope that you
should know about.
• They have various pros and cons that we will
review.
Light
Microscope
Electron
Microscope

10. Copyright © 2015 Henry Exham
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2.1 Cell structure and function
Observing cells
• The microscopes can magnify the image by this many
times.
Light Microscope Electron Microscope
X1500 X500,000

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2.1 Cell structure and function
Observing cells
• However, it is also about how much detail they can show.
• This is measured by the resolving power or resolution of
the microscope.
• The resolution tells you the limit that two objects are
seen as two objects, any closer and it would appear as
one.
• This is a measure of detail.
Low resolution High resolution

12. Copyright © 2015 Henry Exham
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2.1 Cell structure and function
Observing cells
• The resolving power of these microscopes:
• 1nm = 0.000,001mm
Light Microscope Electron Microscope
200nm 0.5nm

13. Copyright © 2015 Henry Exham
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2.1 Cell structure and function
Observing cells
Light Microscope
1. Light enters here, either by a built in lamp or
a light shone on the mirror.
2. The light is reflected up through specimen which is
mounted on a slide and placed on the stage.
3. It then passes up through one of the objective lenses.
There are usually three different objective lenses to choose
from.
4. It then passes through the eyepiece lens where the image
is magnified again before entering the eye.

14. Copyright © 2015 Henry Exham
• To work out the total magnification you just multiply
the objective lens with the eyepiece lens.
• In order to prepare a slide for viewing on a light
microscope you need to make sure the sample is thin
enough for light to pass through.
• Most biological matter, especially when cut in a thin
slice, is transparent so usually a stain is added to
highlight certain structures.
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2.1 Cell structure and function
Observing cells
Light Microscope
Eyepiece lens magnification x Objective lens magnification = Total magnification
x10 x x40 = x400

15. Copyright © 2015 Henry Exham
• Here are some common stains.
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2.1 Cell structure and function
Observing cells
Light Microscope
Haematoxylin and Eosin Stain Methylene Blue Iodine
Stains nuclei purple and
cytoplasm pink
Stains nuclei blue in animal
cells
Stains starch granules in
plant cells blue/black

16. Copyright © 2015 Henry Exham
• Good
– Cheap, accessible and portable
– Can look at live cells with little preparation
– For example blood samples and sperm
• Bad
– Limited in terms of magnification (X1500) and
resolution (200nm).
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2.1 Cell structure and function
Observing cells
Light Microscope

17. Copyright © 2015 Henry Exham
• The electron microscope works in a similar
principle to the light microscope, but a beam
of electrons is used instead of a beam of light.
• The electron beam has a much smaller
wavelength so can be magnified much more
and has a higher resolution.
• The beam is focused by magnets not lenses.
• There are two types the transmission electron
microscope (TEM) and the scanning electron
microscope (SEM). 17
2.1 Cell structure and function
Observing cells
Electron Microscope

18. Copyright © 2015 Henry Exham
• The TEM produces the images at the highest
magnification.
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2.1 Cell structure and function
Observing cells
Electron Microscope

19. Copyright © 2015 Henry Exham
• But the SEM produces very cool 3D images.
• For example this is different types of pollen!
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2.1 Cell structure and function
Observing cells
Electron Microscope

20. Copyright © 2015 Henry Exham
• Or these bacteria
• The colour is added after by computer, the
microscope can only produce black and white
images.
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2.1 Cell structure and function
Observing cells
Electron Microscope

21. Copyright © 2015 Henry Exham
• Good
– Massive magnification (X500,000)
– Massive resolution (0.5nm)
• Bad
– You have to examine specimens in a vacuum so they
have to be dead and dried of all water.
– Very expensive
– Large
– Very specialised skills required to prepare samples
– Artefacts very likely
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2.1 Cell structure and function
Observing cells
Electron Microscope

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2.1 Cell structure and function
Observing cells
• You must be able to calculate the magnification
or actual size from a micrograph (picture from
a microscope).
• To do this use the IAM triangle.
I
A M
Image size
Actual size Magnification

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2.1 Cell structure and function
Observing cells
• For example, what is the actual length of this
bacterium?
x2000
I
A M
Image size
Actual size Magnification
Actual size = Image size/magnification
1. You are given
magnification so just
need to measure the
image size using a
ruler.
2. It measures 4.5cm
3. Therefore actual size is
4.5/2000 = 0.00225cm
4. This is not in very
useful units so best to
convert it into μm
5. Therefore the answer
is 2.25μm

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2.1 Cell structure and function
Observing cells
• For these questions you will need to be able to
convert numbers between units with ease.
Unit Symbol Metric
Equivalent
Standard Form
Metre M 1m 1m
Centimetre cm 0.01m 1x10-2m
Millimetre mm 0.001m 1x10-3m
Micrometre μm 0.000001m 1x10-6m
Nanometre nm 0.000000001m 1x10-9m

25. Copyright © 2015 Henry Exham
• For core practical 2 you need to be
able to use a light microscope with a
graticule.
• A graticule is a simple scale fitted into
the eyepiece that you can use along
with a micrometer to measure the
samples that you are looking at.
• Then you need to be able to draw
some cells.
• This sounds simple enough but there
are rules about how you should draw
in biology.
• This practical is covered in full detail in
a separate resource on the core
practicals.
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2.1 Cell structure
Core Practical 2

26. Copyright © 2015 Henry Exham
• All cells can be split into two categories.
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2.1 Cell structure and function
Eukaryotic cells
Prokaryotic Cells Eukaryotic Cells
• The major difference is that prokaryotes don’t
have a nucleus whereas eukaryotes do.
• We will look at eukaryote cell structure first.

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