Describe an experiment by which you could test to see whether alcohol concentration affected membrane

permeability (5).• Same volume discs of beetroot• Same volume of alcohol• Same temperature• Same time in alcohol• Range of alcohol concentrations• Use colourimeter to read amount of pigment in solution• Graph of colour intensity (% absorbance etc.) over alcohol concentration

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Passive Transport

Passive Transport

Learning Objectives• Explain what is meant by

passive transport • Explain what is meant by

facilitated diffusion• Identify the role of

membrane proteins in transport

Success Criteria• Recall the definitions of

diffusion, facilitated diffusion and osmosis (Grade D-E)

• Compare diffusion and facilitated diffusion (Grade C)

• Construct models/diagrams to show how the processes differ (Grade A-B)

Transport across the cell membraneAll cells are surrounded by a partially-permeable membrane that controls what substances can enter and exit the cell.

A cell needs to be able to import the substances it needs to survive, and to export waste materials and substances that are needed outside the cell.

There are several methods by which substances (molecules and ions) can cross the cell membrane:

diffusion osmosis active transport.

On your whiteboards write down what you understand from KS4 about each methods

What is diffusion? - DEMODiffusion is the net movement of particles down a concentration gradient: from a region of high concentration to a region of low concentration.

No metabolic energy is expended during diffusion so it is an example of passive transport.

One example of diffusion is gas exchange across respiratory surfaces, such as the lungs of mammals and birds, and the gills of fish.Mind map – what factors affect diffusion!! Give examples where possible

net movement

of particles

What factors affect diffusion?

The rate of diffusionThe rate of diffusion in a given direction across an exchange surface can be summarized by Fick’s law, which states that:

rate of diffusion is proportional to:

surface area × difference in conc.

length of diffusion path(membrane thickness)

Increasing the surface area across which the particles diffuse, or increasing the size of the concentration gradient will increase the rate of diffusion.

Increasing the distance (or thickness of the membrane) over which diffusion takes place will decrease the rate.

Cell membranes, polarity and diffusion

Generally, the smaller and less polar a molecule, the easier and faster it will diffuse across a cell membrane.

Small, non-polar molecules such as oxygen and carbon dioxide rapidly diffuse across a membrane.

The non-polar, hydrophobic tails of phospholipid molecules in a cell membrane act as a barrier to most substances.

Small, molecules such as water (even though it’s polar can also diffuse across, but much more slowly.

Charged particles (ions) cannot diffuse across a membrane, even if they are very small.

Facilitated diffusion

Facilitated diffusion

• Write down a definition of facilitated diffusion• What is the difference between carrier proteins

and channel proteins?

• Construct models/diagrams to show how the all the passive processes of transport occur

- Diffusion- Facilitated diffusion using channel proteins- Facilitated diffusion using carrier proteins

Beetroot practicalQuantitative Task – Practical skills in Biology 1 • Record all your raw data in the most suitable form in the space provided on page 2

(5 marks)Qualitative Task – Practical skills in Biology 1• Looking at the depth of colour from each of the temperatures, which temperature

do you think affects membrane permeability the most? (2 marks)

Evaluative task – Practical skills in Biology 1• Plot a suitable graph on the graph paper provided (3 marks)• Use your raw data and graph to describe the trends (2 marks)• Outline the conclusions that can be made from your results

(2 marks)• Assess the validity (accuracy, precision and reliability) of the conclusions that you

have drawn using the raw data you have been presented with. (3 marks)

Beetroot practical (SA)• Quantitative Task – Practical skills in Biology 1 Record all your raw data in the most suitable form (5 marks) Columns correctly labelled Units/correct place Means calculated All accuracy 1 dp Recognising anomalous results, left out if necessary

• Evaluative task – Practical skills in Biology 1 Plot a suitable graph on the graph paper provided deduct one mark for each of the following done incorrectly axes correct (independent variable on x axis) ; axes have appropriate scales and labels ; points accurately plotted ; points joined by straight, ruled lines ;

(3 marks)

• Qualitative Task – Practical skills in Biology 1 Looking at the depth of colour from each of the temperatures, which temperature do you think affects

membrane permeability the most? Using your data describe which temperature affects permeability the most, refer to depth of colour (2 marks) Use your raw data and graph to describe the trends • trend described correctly • If more points – use data from graph Describe – state overall pattern or trend (explain is – give

reasons for trend) (2 marks)

Outline the conclusions that can be made from your results (2 marks) Temp below 0 – phospholipids don’t have much energy so can’t move much. Channel/carrier proteins may

denature increasing permeability.• Ice crystals may form and pierce membrane, making it highly permeable when it thaws• As temp increases phospholipids move more due to more energy – increases permeability• Above 45 – phospholipids bilayer starts to melt, water inside cell expands putting pressure on membrane,

Proteins denature – increase permeability

Assess the validity - Were you truly measuring the correct thing, ensuring that the outcome is not distorted by extraneous factors

• Reliability – more repeats at each temperature, or more temperature/Insufficient raw data collected/more variables to control/

• Accuracy - use a timer to ensure the cubes were left in the water bath for the same amount of time, digital thermometer to ensure the temperature was accurate

(2 marks)

Evaluative QQE• Reliability – results can be consistently reproduced in independent

experiments• COULD BE ASKED EVALUATE RELIABILITY OR SUGGEST WAYS HOW TO

IMPROVE RELIABILITY1) size of data2) More variables you control the more reliable3) Data collection – Are there any problems with the method, has biased

slipped in. Less bias = more reliable4) Controls are needed to draw valid conclusions5) Repetition by other scientists6)  7) Accuracy – The correctness of the measurement - results that are close to

the true answer. Usually dependent on the calibration of instrument used for measuring.

8)  9) Precise – those taken with sensitive instruments that measure small

increments eg. Mm rather than cm / to 1 dp

Plenary - whiteboards