2. Explain the purpose for having controls in light of the goals of experimental

Question

2. Explain the purpose for having controls in light of the goals of experimental research. In other words, what is the ultimate goal of experimental research and how does having controls allow the research to achieve those goals. 3. How did Spallanzani initially disprove the idea of spontaneous generation of microorganisms? Do you think Needham's criticism of Spallanzani's results were valid? Explain. 4. List and describe the important variables that Pasteur controlled in his experiment to disprove spontaneous generation. s. An experiment by Ernest Wynder was used as evidence in support of cigarette smoke as a causative agent for lung cancer. What criticism(s) can you make about his experimental design and therefore the weakness in any conclusions drawn from it? 6. What is the one major flaw or weakness of all controlled experimental designs'?

Explanation / Answer

Q2)

In an scientific experiment, a scientist uses the scientific method, to ask questions and come to a conclusion about the nature of the world. After observing a phenomemon, a scientist investigates it, he seeks answer to the cause of that phenomenon, so he creates a hypothesis. A Hypotheis is a proposed explanation or answer he is seeking for the phenomenon. A hypothesis doesn’t need to be correct, but it should be testable.

So a hypothesis is a prediction that, what will happen during the experiment, and if the hypothesis is correct then the results of the experiment will match with the scientist’s prediction. If the results do not match than a new hypotheis is formulated.

There are certain things in an experiment they are i) Dependent variables ii) independent variables.iii) control group iv) experimental groups.

The independent variable is a variable which the experimenter either manipulates or controls in an experiment to test the effects of this manipulation on the dependent variable. A dependent variable is a variable which is measured to see if the manipulation puts any effect.

Example of dependent and independent variables, are suppose we are interested to know how stress affects the heart rate in humans. Here independent variable is stress and the dependent variable is the heart rate. we can manipulate stress levels on the human subjects and measure how these stress levels changes their heart rate.

Also two groups are under observation that is i) Experimental group ii) Control group.

control group is used as a baseline to which the experimental group can be compared to. Example is - suppose two groups of people are receiving an experimental treatment for a medical condition, one group will be given the actual treatment ( it is called the experimental group) and another group will be given a placebo or sugar pill (it is called the control group).

Without a control group, the effects of the independent variable on the dependent variable in an experiment can not be determined, because outside factors always try to influence the behavior of the experimental group. so control group acts as a point of comparison,it ensures the variable under examination (that is the impact of the medicine) is responsible for creating the results of an experiment.

Control group allows the scientist to eliminate the varying amounts of uncertainty in his experiment.

Controls gives a baseline to which the results of an experiment can be compared, it also allows the researcher to correct if there is any error in the experiment. If some error happened in the experiment, a scientist can check with the controls of the experiment, to see if the error also affected the controls. If so,then they can correct the error next time they do the experiment.

Q3) Spallanzani an  Italian scientist was the first to disprove spontaneous generation of microorganisms, he proved microorganisms can be killed by boiling.

The Theory of spontaneous generation says that living creatures develop from nonliving matter,

Spallanzani researched in 1768 the theory of the spontaneous generation of microbes. According to Buffon and Needham, there is a life-generating force present in certain kinds of inorganic matter which creates living micro organism, if given sufficient time. but spallanzani's experiment showed that, it is not an inherent feature of matter, it can be destroyed by an hour of boiling. As the microbes do not re-appear as long as the material was properly sealed, he proposed that microbes move by air and they can be killed by boiling.

But Needham argued that experiments destroyed the "vegetative force" necessery for the spontaneous generation.

we do not agree with Needham's criticism because in louis pasteure experiment when the flask was heated and left there was no growth of microorganisms, but when the necks of the swan neck flask broken and microorganisms allowed to enter , they start growing, this disproves the theory of sponatneous generation.

Q4)

In Psteure's experiment ,he prepared a nutrient broth like a soup. He placed equal amounts of the broth into two long-necked flasks. , Then he left one flask with a straight neck. The other flsk he bent the neck to form an "S" shape.he boiled the broth in each flask to kill any living matter to be present in the liquid. The sterile broths were then allowed to sit, at room temperature and exposed to the air, in their open-mouthed flasks.After several weeks, Pasteur observed that the broth in the straight-neck flask is discolored and cloudy, but the broth in the curved-neck flask had not changed.

He concluded that germs in the air were able to fall unobstructed into the straight-necked flask and contaminated the broth. In the other flask, whose neck is bent ,however trapped the germs in its curved neck,­ and prevented them from reaching the broth, as a result the color of the flask neither changed nor it became cloudy.

He argued that ,if spontaneous generation is the reason than, the broth in the curved-neck flask would also have become reinfected, because the germs would have spontaneously generated. But the curved-neck flask do not got infected, This indicates that the germs only came from other germs.

Pasteur performed various experiments to disprove spontaneous generation.

He placed boiled liquid in a flask and let hot air enter the flask. Then he closed the flask, and no organisms grew in it.

The number of flasks in which organisms grew was lower at higher altitudes, this shows that air at high altitudes contains less dust and fewer organisms.

Pasteur also used swan neck flasks having a fermentable liquid, and air is allowed to enter the flask by a long curving tube that made dust particles to stick to it, In this broath nothing grew, till the flasks were tilted, making the liquid touch to the contaminated walls of the neck. This showed that the living organisms which grow in such broths come from outside, with dust, rather than spontaneously generating within the liquid or from the action of pure air.

The important variables pasteure controlled in this experiment were the broath which he used, was same for all experiments, Time given was also same, also he used different flsks to see the effect of the experiment.

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