Cell Theory - HRSBSTAFF Home Page


IB Biology SL

2.1.1 Outline the cell theory.

2.1.2 Discuss the evidence for the cell theory.

The Cell: Hidden Kingdom

http://www.youtube.com/user/WhyEvol utionIsTrue#p/c/6/hScXxGeL0sA

Stephen Taylor’s page for BBC video link


Or try this if above does not work: http://www.youtube.com/watch?v=AeygTtDx2W8


People have known about the existence of cells for only the last 300 years or so.

The development of the first microscope, provided access to the micro-universe of the cell.

Early investigators discovered what you now take for granted: that all living things are made up of cells and that cells are the fundamental functional units of life.

An old idea...Spontaneous Generation

Prior to the nineteenth century people and scientists believed in spontaneous generation.

Spontaneous generation suggested that small living organisms could suddenly arise – poof!

Frogs and salamanders appeared suddenly from mud.

Mushrooms on logs

Cell theory (biogenesis)disproved the theory of spontaneous generation (abiogenesis).


Greek philosopher

(384-322 B.C.E.)

Classified all living matter into two categories – plants and animals

He writes that living organisms can arise spontaneously from nonliving matter.

Jan Baptista van Helmont


A firm believer in abiogenesis

“Mixing a dirty shirt with several wheat grains would produce adult mice after 21 days”

Robert Hooke

A new book,


, published in 1665 by

English scientist Robert Hooke (1635-1703), shows illustrations of once-living matter (the lining of tree bark) as observed with a compound microscope.

The magnification reveals empty room-like compartments. Hooke calls these compartments


Antony Van Leewenhoek

In 1666, Antony reads Hooke’s book

Designed his own microscopes to examine specimens for himself.

He designed simple, tiny microscopes

Francesco Redi


Develops controlled biological experiments.

Discovers that maggots do not appear in meat if flies cannot land on it.

Some scientists begin to doubt the theory of spontaneous generation.

The debate continues...

1673 – Loowenhoek writes papers where he describes that he sees with his microscope as

“animalcules” in standing water.

In 1683 he examines plaque from his own teeth. He observes “many very little living animalcules, very pretty a-moving” – The discovery of bacteria is made!

John Needham

1748 – English naturalist and priest

Designs an experiment to support the idea of spontaneous generation (next slide).

Boils meat broth and then seals it in a flask.

He leaves a second flask with boiled broth open.

Within days, both flasks are teeming with microorganisms.

Needham writes that his results support the theory of spontaneous generation.

Lazzaro Spallanzani


Learns of Needham’s experiment and repeats it only boils the broth much longer. The sealed flask doesn’t become cloudy days later.

Supporters of spontaneous generation claim that the boiling killed the vital principle (the thing in the air that is responsible for life to arise from non-living matter).




Support for and interest in science is very high.

Public lectures are popular.

Textbooks are developed by Jane Haldiman, in

1809 to help young people learn about science.

The terms “cell”, “cellular system” and “cell tissue” are used in these textbooks.

Advancements in glass making techniques

English manufacturers compete to produce the best microscope.

1830’s biology is evolving from a collection of assorted facts into and organized body of knowledge. Better optical theory leads to improved microscopes, which biologists use to study cells intensively.

Robert Brown

1831 – observes that all cells from a variety of backgrounds, seem to contain a dark region near the centre, known as the nucleus.

Matthias Jacob Schleiden

1838, a German botanist writes “All plants are made of cells”

Theodor Schwann

1810-1882 – writes “all animals are made up of cells”

Schwann also modifies and expands Schleiden’s work to say that “Cells are organisms, and entire animals and plants are collectives of these organisms.”


Many scientists still reject the conclusions of Brown,

Schleiden, and Schwann, even though these conclusions are based on repeated observations.

Aristotle’s influence, and the scientific and public support for spontaneous generation are still strong.

Some scientists argue that if plants are inferior to animals, as Aristotle taught, how could they have such similar structures.

TOK Connection...

What is a paradigm shift?

What does it take (or would it take) to change your mind about something?

What would it take to change popular belief today?

Rudolph Virchow

Writes in 1858, “Cells are the last link in a great

chain [that forms] tissues, organs, systems, and individuals…where a cell exists, there must have been a pre-existing cell…Throughout the whole series of living forms…there rules an eternal law

of continuous development.”

The Paris Academy of Science

1860, offers a prize to anyone who can settle the debate on spontaneous generation.

French biologist, Louis Pasteur (1822-1895) decides to take up the challenge (his experiment is on the next slide).

He disproves spontaneous generation and concludes that living organisms do not arise from non-living matter!

Finally...acceptance of Biogenesis

Took nearly 2000 years!

As a result of Pasteur’s experiment, all reputable scientists and much of the public accepted biogenesis by the end of the 19 th century.

Microscopes with the ability to magnify 2000x the size of the object, enable biologists to observe and identify cells and many of the smaller structures within them.

The Cell Theory

1. All living things are composed of cells

2. Cells are the basic units of all living things

3. All cells come from pre-existing cells

Evidence for the cell Theory

Through the process of scientific investigation much evidence has been collected to support the cell theory.

Living things have been examined and all have been found to consist of cells thus far.

However, like much of science there are some

exceptions to the rule and while they do not disprove the cell theory they do not fit into our idea of cells as small box-like structures with the same organelles inside each cell.

Exceptions to the cell theory

Mitochondria and chloroplasts have their own genetic material, and reproduce independently from the rest of the cell

Viruses are considered alive by some, yet they are not made up of cells. Viruses have many features of life, but by definition of the cell theory, they are not alive

Skeletal muscle and some fungal hyphae are not divided into cells but have a multinucleate cytoplasm.

Some tissues contain a lot of extra-cellular material like bones and teeth.

Despite these exceptions most living tissues are composed of cells. Cells can be removed from an organism and survive whereas smaller parts cannot so this is evidence that supports the theory that cells are the smallest unit of life.

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

Unicellular organisms consist of one cell only and that one cell must carry out all life functions for that organism.

This includes obtaining food and other nutrients, excreting wastes, and reproduction.

Unicellular organisms contain organelles that carry out the functions of life.

Paramecia and Amoebas are unicellular organisms

2.1.4Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using the appropriate SI unit.

molecules (1 nm), thickness of membranes (10 nm) viruses (100 nm), bacteria (1 µm) organelles (up to 10 µm) most Eukaryotic cells (10 µm to 100


Always keep in mind that cells are three dimensional in shape. Some organelles are so small we need an electron microscope to view them. These are sizes for the average or typical object.

Comparing Size...

1 nm = 1/1,000,000,000 of a meter, or . . .

0.000000001m, or . . .

1 billionth of a meter

1 µm = 1/1,000,000 of a meter, or . . .

0.000001m, or . . .

1 millionth of a meter

1 nm = 1/1,000 of a µm, or

1 µm = 1,000 nanometers

Therefore. . .

Comparing Size...

A 100 µm cell

A 10 µm organelle

A 1 µm bacteria

A 100 nm virus

A 10 nm membrane

A 1 nm molecule

10x larger than a. . .

10x larger than a. . .

10x larger than a. . .

10x larger than a. . .

10x larger than a. . .

Limits to cell size

Once cells reach a certain size they stop growing and divide. If a cell grew too large it would have many problems because its surface area to volume ration would become too small.

As the size of an object increases the ratio between surface are and volume decreases.

In cells, the rate at which materials can enter or leave a cell depends on the surface area of that cell while the rate at which those materials can be used or produced depends on the volume of that cell.

If cells become too large it becomes inefficient at exchanging materials with its environment.

Why are cells so small?

Now try to apply this idea by completing the worksheet...

Think about it...

Why are there specialized tissues and organs in multicellular organisms? How does this relate to cell size limits?

Imagine you were made up of one giant cell...

Would this be possible? Would it be efficient?

Fun in the Lab!

“Cell Size and Diffusion” – agar cubes and phenolphthalein!

You will be given the design of the lab

You are asked to hand in ‘data collection and processing’ as well as ‘conclusion’.

Emergent Properties

 can be defined as properties where the whole is more than the sum of their parts.

In other words, multicellular organisms can achieve more than the sum of what each cell could accomplish individually.

A good example of emergent properties in a multicellular organism would be the human brain. On their own, individual neurons (nerve cells) are not capable of thought but it is the interactions of all neurons that allow the brain to think.

A protein has properties/attributes that none of its component atoms have

Emergent properties are due to the arrangements and interactions of parts!

Cell Differentiation

A multicellular organism arises from a fertilized egg cell as the result of 3 processes:

 cell division, cell differentiation, morphogenesis

Cell division alone would produce only a great ball of identical cells

During development, cells undergo differentiation, becoming specialised in structure and function then organised into tissues and organs

Cell differentiation is directed by the genes of the cells.

All the cells contain the same genes but the cell only uses the ones it needs to follow its path of development – this is called differential expression of genes.

For example, the cells in your toes contain the genetic information in the form of genes to make the pigment colors for your eyes but the cell goes not express those genes.

Through the process of differentiation cells develop into blood cells, nerve cells, muscle cells etc.


This is the process that leads up to differentiation where the cell is irreversibly committed to its final date.

Ie – once a muscle cell has certain genes turned on or off and it begins its transition into a muscle cell, it cannot become a brain cell.

Differentiated cells are specialists!

Stem Cells

Stem cells retain the capacity to divide and have the ability to differentiate along different pathways.

Sources of stem cells are human embryos, umbilical cord of a new born baby, and some can be found in the adult body mostly in the bone marrow.

Stem cells are different from other cells in two main ways:

They are unspecialized or what is known as totipotent because they can become any type of cell due to the fact that they have not differentiated yet.

Stem cells are self-sustaining and they can perform mitotic cell division for long periods of time.

A stem cell has not yet expressed genes to specialise to a particular function. Under the right conditions stem cells can be induced to express particular genes and differentiate into a particular type of cell.

Animation on how stem cells are made

 http://www.dnalc.org/resources/animations/stemcells


Therapeutic Use of Stem Cells

Using stem cells to treat disease and repair tissues is often referred to as cell therapy.

We have been performing one type of cell therapy, bone marrow transplants, for over 40 years.

People who have leukemia can receive a bone marrow transplant which replaces their diseased bone marrow with healthy marrow and this will hopefully cure their disease.

Cell therapy is being used to create new skin grafts for burn victims.

It is also being used to grow new corneas for people suffering from failing eyesight

Outline a therapeutic use... Treating

Non-Hodgkins Lymphoma

1. patient requires heavy does of radiation and or chemotherapy. This will destroy health blood tissue as well as the diseased tissue.

2. Blood is filtered for the presence of peripheral stem cells. Cells in the general circulation that can still differentiate into different types of blood cell otherwise known as stem cells.

3. Bone marrow can be removed before treatment.

4. Chemotherapy supplies toxic drugs to kill the cancerous cells.

5. Radiation can be used to kill the cancerous cells.

6. Post radiation/ chemotherapy means that the patients healthy blood tissues were also destroyed by the treatment.

7. Healthy stem cells or marrow cells can be transplanted back to produce blood cells


Research into stem cells has only been going on for about 10 years and is a controversial area of research.

It is illegal in some countries to perform research on stem cells. Spain , Sweden , Denmark and the UK allow research using stem cells taken from embryos that are less than 14 days old.

The UK and Denmark are the only countries that will allow scientists to create embryos for research purposes.

President George Bush vetoed a bill in 2006 which would allow research using stem cells from destroyed embryos to be funded in the United States by federal money.

In 2009, President Barack Obama reversed the policy put in place by the Bush administration.

Obama reversing federal ban http://www.youtube.com/watch?v=U_W9cHBkt


Cell Culture – how it’s done!

 http://www.eurostemcell.org/films/cellculture/ESC_cellculture_en

New! Spinal cord surgery with stem cells

 http://www.youtube.com/watch?v=s19eFX5gYc



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

Photographs or drawings of images seen under a microscope are much larger than the actual size of the specimen so it is helpful to know how much larger the image is than the actual specimen.

The magnification should always be shown on a drawing or photograph.

Choose and measure an obvious length and measure it on the drawing. Measure the same length on the actual specimen. Calculate the magnification using the following formula:

Magnification = size of image size of specimen

Scale bars are usually added to a micrograph or drawing to show the actual size of the structures.

Magnification Practice –

See Handouts