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• 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 theoryAnton 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.

HistoryRobert 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