Download THE CELL THEORY The Cell Theory More on Cells…

9/29/2013
Definitions
The Importance of
Theory in Biology
An Introduction to Some Big Ideas
Refer to text pgs. 169-173; 368-386
The Cell Theory
There are three parts to the Cell Theory:
• A law is a description of natural
phenomena.
• A hypothesis is a testable
suggested explanation for a set of
observations.
• A scientific theory is a hypothesis
that is so well supported by
evidence that it is generally
considered true. It explains the
natural phenomena.
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• Organisms may be:
– unicellular (made up of one cell)
OR
1) Cells are the basic units of structure
and function of all organisms.
2) All cells come from pre-existing cells
(one cell divides into two cells during
mitosis).
3) All organisms are made up of one or
more cells.
ANOTHER BIG IDEA:
NATURAL SELECTION
• For a long time people didn’t
know how organisms changed
over time (evolved).
• There was plenty of evidence of
evolution, but no one could
suggest a mechanism for how it
worked.
• Many scientists had some ideas,
but we will just look at two of the
most popular ideas at the time.
– multicellular (consisting of many
specialized cells)
• Cells may be many different shapes and
sizes.
• Cells contain organelles: structures that
perform various functions within the
cell.
Lamarck’s Hypothesis
• A French scientist named Lamarck
had an idea that organisms gained or
lost certain organs or traits with use
or disuse and those traits (or lack
thereof) were passed onto the next
generation.
• So if an organism used a certain body
part a lot, it would develop a lot and
the organism’s offspring would show
similar development.
A BIG IDEA: THE CELL THEORY
There were several important events
that helped develop the Cell Theory.
Below is an example of such an event:
• Two scientists (Hooke and
Leeuwenhoek) in the mid-1600s
independently used microscopes to
make discoveries about the microscopic
world.
• Without these discoveries and those
like them cells might not have been
discovered for a long time.
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Cells are either prokaryotic or
eukaryotic.
•Prokaryotes have no nucleus and few
organelles; eukaryotes have a nucleus and
many different organelles.
•Prokaryotes are small; eukaryotes are
much bigger.
•Prokaryotes are mostly unicellular;
eukaryotes can be unicellular but many
are multicellular.
•Prokaryotes were the first type of life on
Earth; eukaryotes came later and probably
developed from merging prokaryotes.
Lamarck’s Hypothesis
• What do you think of this idea?
It was popular in Lamarck’s time
as it seemed to explain some
characteristics of living things.
• There were, however, some
problems with the hypothesis.
Can you think of any?
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Voyage of the Beagle
• In the 1831 a ship called the Beagle
needed a naturalist (as many ships
of the day did).
• The position was taken by a young
man named Charles Darwin.
• Among the many places that the
Beagle visited over the next few
years Darwin was able to see
patterns in the diversity of living
things.
Voyage of the Beagle
• During the expedition the ship went to
the Galapagos Islands. It was there that
Darwin noticed that some very similar
finches on different islands had slight
differences.
• The birds’ bills were perfectly adapted
to the type of food the finch ate –
seeds, insects, plants, etc.
• He got the idea that the finches started
out with a common ancestor, but
changed over time as they adapted to
their surroundings on new islands.
Ideas That Influenced Darwin
• Geological events could shape the Earth
over millions of years.
• His findings in the Galapagos and other
areas, especially with finches and
tortoises.
• Books on economics and human
population growth. One book suggested
that if the population growth stayed the
same there would not be enough
resources for everyone.
The Theory of Natural Selection
Darwin published a book called On The Origin
of Species. This landmark work outlined his
theory:
1) Individual organisms differ, and some of
this variation is inherited.
2) Organisms produce more offspring that
can survive.
3) Individual organisms have different
advantages/disadvantages.
4) Competition for limited resources leads
to “survival of the fittest.”
5) Species today are descended with
modification from successful ancestors.
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