Teachers’ Conference for AL Biology TAS (2004) Formulation & Testing of Hypothesis in Scientific Investigations

Posing a question  Proposing a hypothesis A tentative explanation What is a hypothesis? Posing a question  Proposing a hypothesis A tentative explanation

If the hypothesis is correct, what predictions can be made? Why need a hypothesis? If the hypothesis is correct, what predictions can be made?  Design experiments or plan observations.

This guides us to plan observations or design experiments If the hypothesis is correct, we should observe xxx when a certain test is conducted [predictions]  This guides us to plan observations or design experiments

A question that asks “why” usually requires a hypothesis. A question that asks “what” usually requires making objective observations.  No hypothesis is needed.

降落傘的大小如何影響其下降速度？ 降落傘的形狀如何影響其下降速度？ What hypothesis should be put forward for the following investigations? 降落傘的大小如何影響其下降速度？ 降落傘的形狀如何影響其下降速度？

水的溫度與受熱時間的關係？

水的溫度與受熱時間的關係？

用玻璃瓶蓋著燃燒的蠟燭，你會觀察到什麼？ 為什麼火焰會熄滅？

Do we need to suggest a hypothesis for the following questions? What is the W.P. of potato cells? Why does a potato become more soft after storing for a long time?

Do we need to suggest a hypothesis for the following questions? What is the optimum temperature for salivary amylase? Why is salivary amylase still very active at 70oC?

Do we need to suggest a hypothesis for the following questions? What is the effect of light intensity on the rate of photosynthesis? Why does the rate of photosynthesis becomes constant with increase in light intensity when the light intensity is high?

What food constituents are present in peanut, banana, grape, potato and biscuit? Hypothesis: I think that peanut, banana, grape, potato and biscuit all have proteins and lipids.

What is the respiration rate of mealworms? Hypothesis: I think that the respiration rate (oxygen uptake) is constant.

What is the transpiration rate of a plant outside and inside the laboratory? Hypothesis: The plant outside the lab should have a higher transpiration rate than the plant inside the lab.

What is the effect of light intensity on the rate of photosynthesis? Hypothesis: The photosynthetic rate rises with increase in light intensity.

What is the osmotic potential of the cell sap in red onion epidermal cells? Hypothesis: When 50% of the observed cells is plasmolysed, the O.P. of the cells is the same as that of the surrounding sucrose solution.

In the Coleus leaf, more stomata are present on the lower epidermis than on the upper epidermis. What is the significance of this uneven distribution of stomata? Hypothesis: To reduce the rate of water loss due to transpiration when sunlight shines directly from above. To allow a faster rate of photosynthesis as more carbon dioxide can be taken in.

If a potted plant is placed outside the laboratory for two days without any water supply, it becomes wilted. A similar plant inside the laboratory will not show wilting. How would you explain these observations? Hypothesis: The plant outside the laboratory becomes wilted because it has a higher transpiration rate due to the windy conditions outside. The plant outside the laboratory becomes wilted because it is hotter outside.

Potatoes stored for a long time becomes soft to the touch Potatoes stored for a long time becomes soft to the touch. Why does this occur? Hypothesis: The longer the time of storage, the more water is lost through transpiration. The cell membrane is damaged during storage. So the cells lose their turgidity.

Posing a question (Why …?)  Proposing a hypothesis A tentative explanation A tentative idea that generates predictions – for testing the idea.

A tentative idea that generates predictions – for testing the idea. Examples What factors affect photosynthesis? What conditions are necessary for seed germination?

Posing an investigatable question  Is it necessary to put forward a hypothesis ?  Designing experiment/observation

Designing experiment/observation Biological principle  Designing method of investigation  What is the independent variable? How to change it?  What is the dependent variable? How to measure it? What are the assumptions made?

Designing experiment/observation What are the controlled variables? Any assumptions made? Is it necessary to set up a control experiment? If yes, how to do it?

What are the assumptions that you need to make in order to draw a valid conclusion from your results? To measure transpiration rate using a bubble potometer: rate of water absorption = rate of transpiration To find the W.P. using the % change in mass method: change in mass of potato cylinder  amount of water uptake/loss by osmosis To find the growth rate of a plant by measuring the change in dry mass: change in dry mass  amount of cytoplasm formed To compare the sugar content of green, red and dark grapes using Benedict’s test: amount of reducing sugar  total amount of sugar

Assumptions made in the controlled variables? Effect of light intensity on photosynthesis: rate of respiration remains constant in different light intensity; gas bubbles are identical in size To compare the vitamin C content of different food samples using DCPIP: Each drop of solution has the same volume; the end points are the same To compare the rate of anaerobic respiration of yeast using different substrates: air temperature & pressure unchanged; the yeast samples are identical

Bacteria A and B are different strains of the same species Bacteria A and B are different strains of the same species. They can be distinguished by their colony morphology. A scientist conducted an experiment to investigate what molecules inside the bacteria controlled colony morphology. The diagram below shows the method of investigation and the results obtained.

Which tube(s) is the control?

Which region is the control?

Which set-up is the control?

Which tube(s) was the control? What was the aim of the investigation? What hypotheses were the scientist trying to test? What was the purpose of setting up tubes II to IV? According to the hypotheses under test, which tube(s) should be the control?