CELL THEORY NOTES

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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
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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

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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 didnt 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.

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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.

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History of the Debate Contd

• 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 Contd

• 4. Lazzaro Spallazani (1800)- retried Needhams
  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.

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History of the Debate Contd

• 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.

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Beginnings of a cell theory

• Anton Van Leeuwenhoek
• The microscope was invented by Anton Van
  Leeuwenhoek, a Dutch biologist in the early
  1600s. Leeuwenhoeks invention allowed him to
  see tiny living organisms in droplets of water.

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History

• Robert Hook
• Became interested in Van Leeuwenhoeks 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.

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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).

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The cell theory states

• 1. All living things are composed of one or more
  cells.

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2. Cells are the basic units of structure and
function in living things.
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• 3. All cells come from preexisting cells.

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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.

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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.

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Organelles

• Specialized structures found inside. Each
  organelle carries out a special function to keep
  the cell alive. eg. nucleus, chloroplast,
  lysosome. etc.

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2 Basic Cell Types

• Prokaryotes

Eukaryotes