The cell theory states that cells are the basic unit of life, new cells are formed from existing cells, and cells and their products make up living organisms. Evidence for the cell theory came from early microscope observations by Hooke, van Leeuwenhoek, and others showing the presence of cells. Schleiden and Schwann combined these observations with their own work to propose the first two principles of the cell theory. Virchow later added the third principle that cells only arise from pre-existing cells. Unicellular organisms carry out all the functions of life, while multicellular organisms show emergent properties and cell differentiation allows specialization of function.

1. IB Biology
2 Cells
2.1 Cell Theory
All syllabus statements ©IBO 2007
All images CC or public domain or link to original material.
Jason de Nys
http://www.flickr.com/photos/thejcb/4078621178/

2. 2.1.1 Outline the cell theory.
Cells are the basic unit of life.
New cells are
formed from other
pre-existing cells.
KEY
POINTS Cells and cell products make up all of
the structures in living things.
What is the difference between a
TOK scientific theory and the more general
use of the word “Theory”
http://en.wikipedia.org/wiki/Cell_theory

3. 2.1.2 Discuss the evidence for cell theory
1665 Englishman Robert Hooke
examines cork under a compound
microscope. Comes up with the
term “cells” to
describe what
he sees.
http://commons.wikimedia.org/wiki/File:Microscope_de_HOOKE.png

4. 1675 Dutchman Antonie van
Leeuwenhoek discovers unicellular
organisms.
(A replica of his microscope at left)
His drawings of yeast
http://commons.wikimedia.org/wiki/File:Leeuwenhoek_Microscope.png
http://commons.wikimedia.org/wiki/File:Yeast-Anton_van_Leeuwenhoek.jpg

5. 1837 German Botanist
Mathias Schleiden posits
that all plants are made of
cells
http://en.wikipedia.org/wiki/File:Matthias_Jacob_Schleiden.jpg

6. 1839 German physiologist
Theodor Schwann, after a
lovely dinner with his mate
Schleiden and a chat about
nuclei, realised that animals
were comprised of cells too
and stated: “All living things
are composed of cells and
cell products”
He was also responsible for the discovery of Schwann cells in the PNS, pepsin in
the gut, the fact that yeast is organic… and he made up the word ‘metabolism’.
What a legend! Or, as they say in German, legende!
http://en.wikipedia.org/wiki/File:Schwann_Theodore.jpg

7. 1855 German
doctor, pathologist and
biologist Rudolf Virchow
(A.K.A. the father of
modern pathology)
Omnis cellula
He built on the work of e cellula
others to come up with the
statement: “every cell Virchow vehemently disagreed with another
scientist about a theory. What was it? Find out
comes from another why he was opposed.
existing cell like it” TOK Comment on how modern day celebrities and
scientists “weigh in” on scientific fields
in which they may not be experts.
Hint: Google Jenny McCarthy and Lord
Monckton for starters
http://en.wikipedia.org/wiki/File:Rudolf_Virchow.jpg

8. 2.1.3 State that unicellular organisms carry out all of the functions of life
What are the functions
of life?
Micrococcus
luteus
http://www.flickr.com/photos/10451360@N00/284050321/

9. The functions of life:
Metabolism
Nutrition
Growth
Reproduction
Micrococcus
Homeostasis
luteus Response to
stimuli

10. 2.1.4 Compare the relative size of molecules, cell membrane thickness, viruses, bacteria,
organelles and cells, using appropriate SI units
Use the
10x
rule
of
thumb
http://www.flickr.com/photos/sanna_nixi/799023133/

11. Molecules ≈ 1nm
Cell Membrane ≈ 10nm thick
Virus ≈ 100nm
Bacteria ≈ 1μm (1000nm)
Eukaryotic animal cell ≈ 10μm
Eukaryotic plant cell ≈ 100μm
Of course, there are
numerous egg-ceptions.
For example,
the yolk of an
egg is a single
Links to two visual animal cell
comparisons of size
http://www.flickr.com/photos/rogerss1/3520043134/
http://click4biology.info/c4b/2/cell2.1.htm#size

12. Another exception, it is a
sulfur metabolising
bacterium found in the
sediments on the sea floor.
Thiomargarita namibiensis
Specimens have been
found at up to 0.75mm
long, which is visible to the
naked eye!
http://en.wikipedia.org/wiki/File:Sulphide_bacteria_crop.jpg

13. 2.1.5 Calculate linear magnification of drawings and the actual size of specimens in images of
known magnification
Using a scale bar:
The image at right is of a virus-like
particle. The bar is 50nm long.
Use a ruler to measure the scale bar
and thus calculate the magnification
50nm
e.g. Say the measurement I get is 2cm
http://commons.wikimedia.org/wiki/File:Bluetongue_virus.gif?uselang=en-gb

14. Calculate the size of the structure by
measuring it with your ruler and dividing
the measurement by the magnification.
50nm
Some practice calculations to do
on the next few slides

15. What is the
magnification?
1) How long is one of
the rust-coloured
anthrax bacteria?
2) What is the size of
the yellow cell (a
neutrophil) at it’s
widest point?
You can measure on
the screen with a
ruler. 5 μm
http://commons.wikimedia.org/wiki/File:Neutrophil_with_anthrax_copy.jpg

16. Bacterium
5 μm (measured 2.7cm*)
*Measurements will vary depending on how big the image is that you are measuring

17. Neutrophil
5 μm
*Measurements will vary depending on how big the image is that you are measuring

18. 1) How big are the
nuclei?
2) How wide is an
average cell on it’s
short axis?
http://www.flickr.com/photos/ah_pao/2590017159/

20. 2.1.6 Explain the importance of the surface area to volume ratio as a factor limiting cell size
What does it have to do with elephants?
http://www.flickr.com/photos/artbandito/67829361/

21. Think:
Why is it that elephants aren’t furry, but other
animals that live in the same environment, like
lions and zebras, are furry?

22. 3m
1m
3m 1m
1m
3m
Ideal “Elephant” Ideal “Lion”
Surface Area SA: Volume Ratio Surface Area
=3x3x6 Elephant 2:1 =1x1x6
= 54m2 Lion 6:1 = 6m2
Volume Volume
=3x3x3 The elephant has less surface area per unit =1x1x1
= 27m3 of volume to dissipate heat than a lion. = 1m3
Thus the elephant only has sparse hairs to
avoid overheating.
Think: Where is this analogy going regarding cells?

23. What must get in?
What must get out?
http://www.flickr.com/photos/thejcb/5136606417/

30. If a cell is too large,
the SA:Volume ratio is
too small for diffusion
to accommodate the
requirements of the
cell

31. Cells can get around this problem by
growing projections, having a flattened
form, or being long and thin.
Multicellular organisms have developed
circulatory systems to deliver nutrients
and oxygen and remove wastes.
Exchange structures with large surface
areas, such as the lungs and the
gut, have evolved.

32. 2.1.7 State that multicellular organisms show emergent properties.
http://commons.wikimedia.org/wiki/File:Aristotle_Altemps_Inv8575.jpg

33. The whole is greater
than the sum of its
parts , and yeah…
I’m Aristotle

34. …Individual atoms can be combined to
form molecules such as polypeptide chains, which in
turn fold and refold to form proteins, which in turn
create even more complex structures.
These proteins, assuming their functional status
from their spatial conformation, interact together
and with other molecules to achieve higher
biological functions and eventually create
an organism. (Wikipedia)

35. Individually, these cardiac
muscle cells can’t do much.
Together they make cardiac
muscle tissue that beats in
time to a pacemaker impulse.
Cardiac muscle tissue plus
valves plus arteries and veins
makes the heart, an organ that
pumps blood.
http://www.flickr.com/photos/akay/244989836/

36. 2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialised functions
by expressing some of their genes but not others.
Every cell contains a copy of every
gene possessed by an organism
(at some stage of the cell’s life)
But only certain genes are turned on
So, for example, the cells in your kidney do
not produce the pigments in your skin cells
and the cells in your fingers don’t produce the
insulin that cells in your pancreas can make.
http://commons.wikimedia.org/wiki/File:Karyotype_color_chromosomes_white_background.png

37. The genes that aren’t expressed are
more tightly coiled than the genes
that are expressed.
Heterochromatin, the more tightly
coiled DNA, appears darker under
an electron microscope than
euchromatin, the loosely coiled
DNA.
More on coiling and transcription in 3.3 and 3.5
http://en.wikipedia.org/wiki/File:Diagram_human_cell_nucleus.svg

38. 2.1.9 State that stem cells retain the capacity to divide and have the ability to differentiate along
different pathways
Two things set stem cells apart from ‘regular cells’
1) Self-renewal:
the ability to go through
numerous cycles of cell
division while maintaining
the undifferentiated state.
Background: Human embryonic stem cells
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030234

39. 2) Potency:
Stem cells are
undifferentiated and
have the capacity
to differentiate down
different paths into
specialised cell types.
This requires stem cells
to be either totipotent
or pluripotent to be
able to give rise to any
mature cell type
http://www.flickr.com/photos/pfly/188629337/

40. The morula just after fertilisation
is comprised of totipotent cells
that can differentiate into
anything
At the blastocyst
stage the inner
cells are
pluripotent and
can differentiate
into almost any
cells
(The outer layer of the
blastocyst goes on to
form the placenta)
http://en.wikipedia.org/wiki/File:Stem_cells_diagram.png

41. Some animations
for your
enjoyment:

42. 2.1.10 Outline one therapeutic use of stem cells
Take a few minutes to do your own research:
1) Find out about a therapeutic use of stem cells
2) Where do the stem cells used come from?

43. Section through head of a femur
1cm showing red and white marrow
Adult stem cells have been
used for many years to
treat leukemia through
bone marrow transplants.
The bone marrow contains
cells that differentiate into
the different types of
blood cell more
Why is stem cell research
TOK controversial? On what basis do
people object to it?
http://en.wikipedia.org/wiki/File:Caput_femoris_cortex_medulla.jpg

45. Further information:
Perky Professor Poffenroth!
Great short videos
Amazing work by Stephen Taylor with more
detail and extension. Use it to add to your
notes, contains more practice questions for
calculating actual size.