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24/10/2010
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
Topic 2.1 Cell Theory
A. Allen
IB Biology HL
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
History of the Debate
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.
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
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.
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…History of the Debate
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
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
History
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.
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.
2.1.1. Outline the cell theory.
The cell theory states:
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).
1. All living things are composed of one or
more cells.
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2.1.1. Outline the cell theory.
2. Cells are the basic units of structure
and function in living things.
2.1.2. Discuss the current evidence that supports the cell theory.
2.1.1. Outline the cell theory.
3. All cells come from preexisting cells.
2.1.2 Discuss the current evidence that supports the cell theory.
Evidence of 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.
2.1.7 Define emergent properties.
Emergent properties
• 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.
• What other examples can you think of by looking at the
diagram below?
• The following are some exceptions to the general
cell structure:
– 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.
2.1.7 Define emergent properties.
…Emergent properties
• Multicellular organisms show emergent properties
when the cells work together to achieve more than
what one cell on its own can achieve (unicellular).
• 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.
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2.1.4 Compare the relative sizes of molecules, cell membrane thickness viruses, bacteria, organelles and cells, using the appropriate SI unit
Size of Cells and Other relevant structures
•
•
•
•
•
•
molecules (1 nm),
thickness of membranes (10 nm)
viruses (100 nm),
bacteria (1 µm)
organelles (up to 10 µm)
most cells (up to 100 µm)
Don’t forget: all of these structures are in 3D space
…Size of Cells and Other relevant structures
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. . .
2.1.6 Explain the importance of the surface area to volume ratio as a factor limiting cell size
…Size of Cells and Other relevant structures
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
10x larger
10x larger
10x larger
10x larger
than a. . .
than a. . .
than a. . .
than a. . .
than a. . .
Why are Cells Small?
• 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 ratio 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.
2.1.6 Explain the importance of the surface area to volume ratio as a factor limiting cell size
Why are Cells Small?
Size of Cube-Shaped
organism
Surface Area
Volume
Surface: Volume
ratio
1 cm x 1 cm x 1 cm
10 cm x 10 cm x 10 cm
100 cm x 100 cm x 100 cm
1.
2.
3.
Questions
When the cube dimensions increase by a factor of 10 (from 1 cm to 10 cm),
what is the factor of increase for the surface area? The volume?
Consider a unicellular organism that exchanges gasses through its surface.
Based on what you have learned, infer why there are size restrictions on
unicellular organisms.
Why do you tend to see specialized tissues and organs in multicellular
organisms? Think homeostasis! HINT: larger organisms have an increased
requirement for nutrients etc.
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