Lesson 5 (Week 6) 11 March 2014

Diffusion and osmosis experiments

Examples of diffusion • Gas exchange in lungs/leaves (CO2 and O2 enter/exit the organim) • Gas exchange between blood and cells • Glucose within a cell (moving from edge of cell to mitochondria • Osmosis (involves movement of water across a membrane)

Diffusion ‘rules’

• Occurs down the concentration gradient

• Occurs in gases and liquids, may or may not involve a membrane

• High diffusion rate=short distance, large surface area, big concentration difference

• Increase in temperature in creases diffusion

• Large molecules slow diffusion

• Semi permeable membrane affects diffusion

Examples of osmosis

• In the kidney (this is why dialysis works)

• leaf turgor (cell membrane pushing against cell wall)

• root system of a plant (sucking up water)

Osmosis ‘rules’ • In osmosis we are looking specifically at how water will diffuse

across the membrane

• When a cell swells up or shrinks due to water moving in and out of a membrane this can result in build up or loss of pressure against the cell membrane

• Pressure can influence diffusion rate (ie high pressure will slow diffusion, as in plant turgor)

• Pressure and concentration both contribute to the water potential (ie similar concept to concentration gradient)

Water Potential (ψ) = Pressure Potential (ψp) + Solute Potential (ψs)

The scientific method

Question: Does smoking cause emphysema?

Previous research suggests there may be a link. There may be factors that influence this, eg length of time smoking, how many cigarettes a day, exposure to other substances (eg asbestos), etc.

What variables are we testing? (Design experiment)

What is the hypothesis? (prediction of likely finding)

Do the experiment/study..the fun bit!

Report the results…the paperwork!

Ask a question & do background research-

Diffusion:

• How does viscosity affect diffusion?

• How does temperature affect diffusion?

• How does molecule size affect diffusion?

Viscosity: The resistance of a substance to flow. For example, water has a lower viscosity than molasses and flows more easily.

Design an experiment

• Design a good experiment

– Two variables (factors), can apply various ‘treatments’

– Control all other variables that may influence the result

– Do repetitions

• Make sure you consider ‘lab’ safety

Construct a hypothesis to test

• A hypothesis is a prediction of what you will find if you do an experiment

• It is phrased as an if (I do this)…..then (this will happen) statement

• Eg- If a cohort of smokers and non-smokers are tested for emphysema, then it the smokers cohort will show a higher incidence of emphysema.

Plan how you will record ‘results’- ie raw data

What will you record?

How will you record it?

Make sure that everyone does the same thing!!

Example of results table

Treatment Diff rate (1/2min)

Diffusion rate (1min)

Diffusion rate (1 ½ min)

Add more columns as long as you continue to measure results…….

Our osmosis experiment

Question: Does osmosis occur into a substance (a ploymer), which will demonstrate how osmosis works? Do different types of solutions (isotonic/hypotonic/hypertonic) influence the direction of water movement?

Experiment: Gummi bears are soaked in pure water, then soaked in a salty solution.

What would you expect to happen?

Reporting the results

An experimental report includes the following: • Aim • Background (this is your homework, worksheet for week 5,

Q3) • Hypothesis • Risk assessment • Materials & methods • Results • Discussion & conclusions (or recommendations) • References • Appendices (log book- homework is to answer the

reflection Qs)

Logbook • Use a diary entry format (see proforma on wiki) • Record any points you think are relevant/raw

data • Reflections for 11 March 2014 (include in

logbook under this date): – Do you think the scientific method is useful?

Explain. – How did you solve any difficulties that you may have

to do your experiment in class, but still get useful data?

– Did you learn something new today? How can you use this in other areas of your life?

Risk assessment

Risk/hazard: Addressed by: Eg use of hot water

Limit water temperature to 60deg C, all persons wear enclosed

shoes, beakers wrapped with serviette, beakers placed on tile and

moved by handling tile, work on bench close to source of hot water.

Use the table below to make a list of risks and hazards that may have been present during your investigation and how these could be addressed. (this proforma is on the tpc bio wiki)