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CELL THEORY
NOTES
The most exciting phrase to hear in science, the
one that heralds the most discoveries, is not
"Eureka!" (I found it!) but "That's funny..."
~Isaac Asimov
The Scientific Endeavour
• ‘The purpose of scientific enquiry is not to compile an
inventory of factual information, nor to build up a
totalitarian world picture of Natural Laws in which
every element that is not compulsory is forbidden.
We should think of it rather as a logically articulated
structure of justifiable beliefs about nature. It begins
as a story about a Possible World - a story which we
invent and criticize and modify as we go along, so
that it ends by being, as nearly as we can make it,
a story about real life.’
~Peter Medawar
Biogenesis vs. Abiogenesis
• Early scientists thought that some living things could arise
from nonliving things
- eg. frogs could come from mud, flies from rotting
meat, plants from the dried out mud of ponds, etc.
• We call this process “abiogenesis” (also called
spontaneous generation).
• They didn’t know about microscopic life such as bacteria,
or even how many organisms reproduced.
• Biogenesis - the theory that states that only living things
can give rise to other living things. This is the theory we
accept today as true.
History of the Debate
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Many scientists, over time, contributed to the debate. Some were:
1. Aristotle – in 334 BC, he stated that living organisms can arise
spontaneously from nonliving matter.
2. Francesco Redi (1660) – challenged the idea of abiogenesis. Many
people believed that rotting meat produced maggots. Redi observed
the maggots longer than anyone else had and saw them enter a
cocoon stage and later emerge as flies. He recalled flies on the rotting
meat so set out to prove that maggots come from flies and are part of
their life cycle.
He used proper scientific methods and performed a controlled
experiment in which some containers of fresh meat were left opened
and other containers of fresh meat were left covered. Flies could not
land on the covered meat and no maggots appeared on that meat.
Flies could land on the uncovered meat and in time maggots appeared.
Many of those that believed in abiogenesis refused to accept his
evidence.
History of the Debate Cont’d
• 3. John Needham (1748) – believed in abiogenesis.
New research had shown the existence of
microorganisms in water and that boiling water killed
them. Needham boiled a meat broth. He placed the
boiled broth in two flasks. One he left opened, the
other he sealed. Microorganisms appeared in both
flasks, allowing Needham to claim that the
microorganisms had come from the broth since the
original ones had been killed
• He did not realize that he needed to boil the broth
longer since bacteria can survive boiling for longer
than 10 minutes.
History of the Debate Cont’d
• 4. Lazzaro Spallazani (1800)- retried Needham’s experiment but
recognized that Needham had not boiled his broth long enough
to kill all microorganisms. Needham boiled the same kind of
broth for over 1 hour. He then left one container open and
sealed others by melting the glass necks shut to get an airtight
seal. No microorganisms appeared in the sealed flasks but did
appear in the open ones. When he broke the seals on the
sealed flasks, microorganisms appeared in them in hours.
Scientists opposing Spallazani argued that he had destroyed
some “active principle” in the air of the flask by boiling the broth
for too long.
History of the Debate Cont’d
• 5. Louis Pasteur (1861) – performed an experiment that
convinced people once and for all that biogenesis was the
correct theory and that abiogenesis was false. He placed broth
in long - necked flasks. He then bent the necks of the flasks into
an S - shaped tube. Pasteur then heated the flasks long
enough to kill any microorganisms present. The curve of the
flask prevented any microorganisms from entering the flasks but
allowed air to enter into the flask. No one could object that the
“active principle” in the air was kept out of the flasks. No
microorganisms grew in the flasks but when Pasteur broke off
the S - shaped necks of some of the flasks, microorganisms
appeared in these flasks.
Beginnings of a cell theory
Anton Van Leeuwenhoek
• The microscope was
invented by Anton Van
Leeuwenhoek, a Dutch
biologist in the early
1600’s. Leeuwenhoek’s
invention allowed him to
see tiny living organisms in
droplets of water.
History
Robert Hook
• Became interested in Van
Leeuwenhoek’s microscope and used
one to look at pieces of cork.
• He could see that it was composed of
thousands of tiny chambers. He called
these chambers “cells” since they
reminded him of the small rooms called
cells in a monastery.
• His discovery was significant since it
opened up the study of cells.
Over the next 300 years…
• Robert Brown – observed that many cells had
a dark structure near the center of the cell,
which we now call the nucleus (1833).
• Matthias Schleiden – stated that all plants are
made of cells. (1838)
• Theodor Schwann – discovered that all
animals are made of cells too (1839).
• Rudolf Virchow – stated that all cells arise
from the division of preexisting cells (1855).
• Janet Plowe – demonstrated that the cell
membrane is a physical structure, not just an
interface between two liquids (1931).
The cell theory states:
1. All living things are composed of one or
more cells.
2. Cells are the basic units of structure
and function in living things.
3. All cells come from preexisting cells.
Nucleus
• The part of the cell which controls all
of the cell functions, including
reproduction and heredity. The
nucleus is the control centre of a
cell. A “true” or proper nucleus has
a protective wrapping around it
called a nuclear membrane.
Prokaryotic vs. Eukaryotic
• Organisms which have a true
nucleus with a nuclear membrane are
said to be eukaryotic.
• Organisms which do not have a
nuclear membrane or a “true”
nucleus are said to be prokaryotic.
• Nuclear membrane - a protective
“wrapping” or barrier around the
nucleus.
Organelles
• Specialized structures found inside.
Each organelle carries out a special
function to keep the cell alive. eg.
nucleus, chloroplast, lysosome. etc.
2 Basic Cell Types
Prokaryotes
Eukaryotes