Osmosis Experiment (Monday)

Your Research Question… How does sucrose concentration affect the water content (rate of osmosis) in root vegetables (potatoes, sweet potatoes) ? Or a more ‘correct’ scientific question: Using sucrose solution as a reference point of known water potential, what is the water potential of white potatoes (Solanum tuberosum]/ Sweet potatoes (Ipomea batatas)?

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 factors which affect osmosis? Temperature: The higher the temperature, the higher the rate of osmosis Concentration gradient across the semi-permeable membrane (Difference in water potential*) Surface Area for osmosis Pressure difference across the semi-permeable membrane (Difference in water potential*)

Check out the amyloplasts, storing starch granules… Potato cells Check out the amyloplasts, storing starch granules…

Designing an experiment

The fundamental cornerstone of ALL SCIENCE 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

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

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

Identify the experimental VARIABLES Independent – what you change (X- axis) Dependent – what you measure (Y-Axis) Controlled – what remains constant (potential to influence experimental data) (Uncontrolled – variables which may be hard to control) (potential to influence experimental data) 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 Formulate a focused research question Identify ALL of the relevant variables (independent, dependent, controlled, uncontrolled)that could influence outcome Decide how to manipulate the independent variable – range Decide how many repeats of the experiment you need to do Decide if you are including a control (‘untreated’) group

How much data is enough?

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]?

(i.e. what will be your dependent variable?) …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 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

Sucrose concentration Results table Sucrose concentration 1% 10% 25% 50% Distilled water 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 relatively high water potential A concentrated solution has a relatively low 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 Set up experiment Collect and collate CLASS data on Google form Calculate the mean and standard deviation of the % change in mass of the potato strips for each sucrose solution GRAPH the mean and standard deviation of the % change in mass of the potato strips for each sucrose solution Use your knowledge of osmosis to explain your results Think of ways in which you could improve your experient to make your results more reliable