The Scientific Method

The fundamental cornerstone of ALL

SCIENCE

Words we will understand by the end of today…

• Observation• Inference

• Hypothesis…and the Null Hypothesis• Data – quantitative and qualitative• Variables• Controlled experiment• Control group

• Based on our observations, we formulate a research question

• Our research question is presented as a hypothesis, based on our biological reasoning

• We typically present the Null hypothesis

So why do all experiments require a hypothesis?

‘If... then…’We must have a principle that we can

objectively test• We generally state the Null Hypothesis for

cause-and-effect relationships

Identify the experimental VARIABLES

• Dependent – what you measure (Y-Axis)

• Independent – what you change (X- axis)

• Controlled – what remains constant

• (Uncontrolled – variables which may be hard to control)

Ideally you should have all variables controlled, excepting one dependent and one independent variable

When does a hypothesis become a theory?

• When a hypothesis is repeatedly confirmed by experiment and observation (USING THE SCIENTIFIC METHOD), it may become generally considered as a THEORY

• THEORIES are the most reliable, rigorous, and comprehensive form of scientific knowledge

Some well known scientific theories:• Cell theory• Evolutionary theory• Theory of relativity• Climate change theory

Designing a controlled experiment

Designing a controlled experiment

1. Formulate a focused research question2. Identify ALL of the relevant variables

(independent, dependent, controlled, uncontrolled)

3. Decide how to manipulate the independent variable – range

4. Decide how many repeats of the experiment you need to do

5. Decide if you are including a control group

How much data is enough?

Osmosis Experiment (Monday)

Your Research Question…

How does sucrose concentration affect the rate of osmosis across potato cells?

Or a more sophisticated question:Using sucrose solution as a reference point of

known water potential, what is the water potential of white potatoes (Solanum

tuberosum]?

Osmosis

‘Facilitated diffusion of water molecules from a region of their higher concentration (high water

potential, dilute solution) to a region of their lower concentration (concentrated solution, low water potential, through a partially permeable

membrane ’Let’s remind ourselves…McGraw Hill animation

What are the factors which can affect osmosis?

1. Temperature: The higher the temperature, the higher the rate of osmosis

2. Concentration gradient across the semi-permeable membrane (Difference in water potential*)

3. Surface Area for osmosis4. (Pressure difference across the semi-permeable

membrane)

Potato cells

Check out the amyloplasts, storing

starch granules…

Your Research Question…

How does sucrose concentration affect the rate of osmosis across potato cells?

Or a more sophisticated question:Using sucrose solution as a reference point of

known water potential, what is the water potential of white potatoes (Solanum

tuberosum]?

…So how could we ‘measure’ the effect of

sucrose concentration on the rate of osmosis in

potatoes?(i.e. what will be your dependent

variable?)

Other folks ideas…

• Mr Bozeman's tips• Rate of osmosis could be measured as mass change

per unit time (gmin-1 or gh-1)• In a closed chamber, [where temperature was

controlled, and not manipulated], rate of osmosis could be measured as pressure change per unit time (kPamin-1)

Identification of Independent, Dependent, controlled and

measured variables

Make a table of accurately defined variables

Variable Definition, Units

Independent

Dependent Rate of osmosis

Controlled

Measured

Factors which affect osmosis

• Sucrose concentration will be your independent variable (what you manipulate)

• You must ensure that all other factors are controlled

Results tableSucrose

concentration1% 10% 25% 50%

Initial mass of potato (g

±0.01)Final mass of

potato (g ±0.01)

Change in mass (g ±0.01)

% change in initial mass

Is one set of results enough?

How could we improve the accuracy of our results?

Your Research Question…

How does sucrose concentration affect the rate of osmosis across potato cells?

Or a more sophisticated question:Using sucrose solution as a reference point of

known water potential, what is the water potential of white potatoes (Solanum

tuberosum]?

Reminder: Water potential (y)

• A term used to describe the ‘water concentration’of a cell or tissue

• Pure water in a free standing solution has a water potential of zero, while most plant cells have a negative water potential.

• All solutions have water potential < 0

• A dilute solution has a high water potential

• A concentrated solution has a low water potential

Water potential

Water potential = solute potential + pressure potential

Do we know the water potential of sucrose solution?

How can that help us define the water potential of our potatoes?

Your challenge for next Monday/ Tuesday

1. Set up experiment2. Collect and collate CLASS data ( Dr P will email you the

class data)3. Calculate the mean and standard deviation of the %

change in mass of the potato strips for each sucrose solution

4. GRAPH the mean and standard deviation of the % change in mass of the potato strips for each sucrose solution

5. Use your knowledge of osmosis to explain your results6. Think of ways in which you could improve your experient

to make your results more reliable