Download Development of cell theory Long ago, people believed in a theory

Development of cell theory
Long ago, people believed in a theory called abiogenesis (spontaneous generation). This theory
said that non-living things could be changed into living things. For example, since flies were
always seen on rotting meat, it was thought that flies came from rotting meat.
In 1668, Francesco Redi performed an experiment to show that maggots did not come from
rotting meat.
He placed meat in open jars and closed jars.
As the meat rotted, flies appeared on the meat in the open jars but not on the meat in the closed
jars.
This suggested that the theory of abiogenesis was wrong.
Not everyone trusted Redi’s results.
Many more experiments were performed before the theory of abiogenesis was considered false.
Three of the key experiments were performed by: John Needham, Lazzaro Spallanzani and Louis
Pasteur
In the late 1600’s scientists started to use microscopes to observe tiny living creatures called
microorganisms.
The scientists that followed Redi could now include these tiny creatures when looking for the
presence of living things.
In the mid-1700.s, John Needham tried to disprove Redi’s work.
He heated sealed bottles of gravy to kill any microscopic organisms present and checking them
to see if they reappeared.
The sealed bottles of gravy did grow microscopic organisms.
This supported the theory of abiogenesis, not Redi’s theory.
Lazzaro Spallanzani believed that Needham did not heat the gravy long enough when he did his
experiment.
Spallanzani repeated Needham’s work, but boiled the gravy longer than Needham. Spallanzani
then sealed one flask and left the other flask open.
He only detected microorganisms in the open flask.
This did not support abiogenesis, it did support Redi’s theory.
Critics argued that the lack of air in the sealed flasks was preventing growth in Spallanzani’s
experiment.
In 1864, Louis Pasteur designed a flask to allow air to enter but not microorganisms. Using the
new flask, he followed the same procedure as Spallanzani.
He allowed his experiment to run for one year and still did not observe microorganisms in the
flask.
After one year, Pasteur broke the neck on the flask
Now that the flask was open, microorganisms grew within a few days.
This supported Redi’s hypothesis, not the theory of abiogenesis.
200 years had passed since Redi’s original experiment and his hypothesis that abiogenesis was
wrong had finally received enough support to be accepted.
It was now widely believed that living things came from other living things.
During this 200 years, the microscope had been invented.
The invention of the microscope allowed scientists to see cells for the first time.
Many scientists were observing cells and their work combined with the work of Redi, Pasteur
and others lead to the development of the modern cell theory.
The modern cell theory states:
1. all living things are made of cells
2. all cells come from other cells
3. cells are the basic units of structure and function of all living things