IB Biology2 Cells

2.1 Cell Theory

Jason de Nys

All syllabus statements ©IBO 2007All images CC or public domain or link to original material.

http://www.flickr.com/photos/thejcb/4078621178/

32.1.1 Outline the cell theory.

Cells are the basic unit of life.

New cells are formed from other pre-existing cells.

Cells and cell products make up all of the structures in living things.

http://en.wikipedia.org/wiki/Cell_theory

KEYPOINTS

TOKWhat is the difference between a scientific theory and the more general use of the word “Theory”

2.1.2 Discuss the evidence for cell theory

http://commons.wikimedia.org/wiki/File:Microscope_de_HOOKE.png

1665 Englishman Robert Hooke examines cork under a compound microscope. Comes up with the term “cells” to describe what he sees.

1675 Dutchman Antonie van Leeuwenhoek discovers unicellular organisms.(A replica of his microscope at left)

http://commons.wikimedia.org/wiki/File:Leeuwenhoek_Microscope.pnghttp://commons.wikimedia.org/wiki/File:Yeast-Anton_van_Leeuwenhoek.jpg

His drawings of yeast

1837 German Botanist Mathias Schleiden posits that all plants are made of cells

http://en.wikipedia.org/wiki/File:Matthias_Jacob_Schleiden.jpg

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

1855 German doctor, pathologist and biologist Rudolf Virchow (A.K.A. the father of modern pathology)

He built on the work of others to come up with the statement: “every cell comes from another existing cell like it”

Omnis cellula e cellula

TOK

Virchow vehemently disagreed with another scientist about a theory. What was it? Find out why he was opposed. 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

2.1.3 State that unicellular organisms carry out all of the functions of life

http://www.flickr.com/photos/10451360@N00/284050321/

Micrococcus luteus

What are the functions of life?

Micrococcus luteus

The functions of life:Metabolism Nutrition Growth Reproduction Homeostasis Response to stimuli

2.1.4 Compare the relative size of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using appropriate SI units

Use the

10xrule

of

thumb

http://www.flickr.com/photos/sanna_nixi/799023133/

Molecules ≈ 1nmCell Membrane ≈ 10nm thickVirus ≈ 100nmBacteria ≈ 1μm (1000nm)Eukaryotic animal cell ≈ 10μm Eukaryotic plant cell ≈ 100μm

http://www.flickr.com/photos/rogerss1/3520043134/ http://click4biology.info/c4b/2/cell2.1.htm#size

Links to two visual comparisons of size

Of course, there are numerous egg-ceptions.For example, the yolk of an egg is a single animal cell

Thiomargarita namibiensis

Another exception, it is a sulfur metabolising bacterium found in the sediments on the sea floor.

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

2.1.5 Calculate linear magnification of drawings and the actual size of specimens in images of known magnification

50nm

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

𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑒𝑑 𝑠𝑖𝑧𝑒=𝑟𝑒𝑎𝑙 𝑠𝑖𝑧𝑒×𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛

∴𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛=𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑒𝑑𝑠𝑖𝑧𝑒 (𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 h𝑤𝑖𝑡 𝑟𝑢𝑙𝑒𝑟 )  𝑟𝑒𝑎𝑙𝑠𝑖𝑧𝑒 ( h𝑡 𝑒𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑛 h𝑡 𝑒 𝑠𝑐𝑎𝑙𝑒𝑏𝑎𝑟 )

e.g. Say the measurement I get is 2cm

http://commons.wikimedia.org/wiki/File:Bluetongue_virus.gif?uselang=en-gb

50nm

Calculate the size of the structure by measuring it with your ruler and dividing the measurement by the magnification.

Some practice calculations to doon the next few slides

http://commons.wikimedia.org/wiki/File:Neutrophil_with_anthrax_copy.jpg

5 μm

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 (measured 2.7cm*)

∴𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛=𝑚𝑎𝑔𝑛𝑖𝑓𝑖𝑒𝑑𝑠𝑖𝑧𝑒 (𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 h𝑤𝑖𝑡 𝑟𝑢𝑙𝑒𝑟 )  𝑟𝑒𝑎𝑙𝑠𝑖𝑧𝑒 ( h𝑡 𝑒𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑛 h𝑡 𝑒 𝑠𝑐𝑎𝑙𝑒𝑏𝑎𝑟 )

X

3.3cm*

*Measurements will vary depending on how big the image is that you are measuring

Bacterium

5 μm

8.5cm*

*Measurements will vary depending on how big the image is that you are measuring

Neutrophil

http://www.flickr.com/photos/ah_pao/2590017159/

Assume that the for the scale that can be seen, one ‘tick’ is 10

1) How big are the nuclei?

2) How wide is an average cell on it’s short axis?

2.7cm*

X

10 ticks are 100

Nucleus

4mm*

Width of cell

1cm*

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/

Think:

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

3m

3m

3m

1m

1m

1m

Ideal “Elephant” Surface Area = 3 x 3 x 6= 54m2

Volume = 3 x 3 x 3= 27m3

Ideal “Lion” Surface Area = 1 x 1 x 6= 6m2

Volume = 1 x 1 x 1= 1m3

SA: Volume RatioElephant 2:1Lion 6:1

The elephant has less surface area per unit of volume to dissipate heat than a lion.

Thus the elephant only has sparse hairs to avoid overheating.

Think: Where is this analogy going regarding cells?

What must get in?

What must get out?

http://www.flickr.com/photos/thejcb/5136606417/

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Cell p

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

cell

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.

2.1.7 State that multicellular organisms show emergent properties.

http://commons.wikimedia.org/wiki/File:Aristotle_Altemps_Inv8575.jpg

The whole is greater than the sum of its parts , and yeah…

I’m Aristotle

…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)

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/

2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialised functions by expressing some of their genes but not others.

http://commons.wikimedia.org/wiki/File:Karyotype_color_chromosomes_white_background.png

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.

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

2.1.9 State that stem cells retain the capacity to divide and have the ability to differentiate along different pathways

http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030234

1) Self-renewal: the ability to go through numerous cycles of cell division while maintaining the undifferentiated state.

Two things set stem cells apart from ‘regular cells’

Background: Human embryonic stem cells

http://www.flickr.com/photos/pfly/188629337/

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 totipotentor pluripotent to be able to give rise to any mature cell type

http://en.wikipedia.org/wiki/File:Stem_cells_diagram.png

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)

Some animations for your enjoyment:

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 cells2) Where do the stem cells used come from?

TOK Why is stem cell research controversial? On what basis do people object to it?

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

1cmSection through head of a femur

showing red and white marrow

http://en.wikipedia.org/wiki/File:Caput_femoris_cortex_medulla.jpg

http://learn.genetics.utah.edu/content/tech/stemcells/sctypes/

Further information:

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.

Perky Professor Poffenroth!Great short videos