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Core Practical 3

Investigate membrane properties including the effect of alcohol and temperature on membrane permeability.
OBJECTIVES
● To know how the effect of temperature and alcohol on membranes can be determined
● To be able to recognise quantitative variables that should be controlled in an investigation
SAFETY
● Wear eye protection.
● Water baths at temperatures above 50 °C may scald. Take care when removing lids to allow steam to escape away from the face or body. If you are splashed by hot water, or scalded by steam, cool under cold running water immediately.

● Take care with sharp items such as the cork borer and knife. Always cut or push downwards onto the tile.
● Ethanol is highly flammable so keep away from naked flames and keep the stoppers on bottles.
● Do not handle electric plugs, sockets or switches with wet hands.
● Wash hands with soap and water when finished.
MATHS SKILLS
● Find arithmetic means.
● Understand the terms mean, median and mode.
● Plot two variables from experimental or other data.
EQUIPMENT
● eye protection
● water baths pre-set at required temperatures
● thermometer
● distilled water
● syringe
● beetroot
● cork borer
● ruler
● white tile
● knife
● 10 cm3 syringe
● pipette
● test tubes
● colorimeter
● cuvettes
● labels or pens for labelling
● forceps
● crushed ice
● ethanol (at 10%, 20%, 30%, 40% concentrations)

DIAGRAM

figure A Use a cork border to cut cylinders of beetroot.

PROCEDURE
Beetroots are root vegetables that appear red because the vacuoles in their cells contain a water soluble red pigment called betalain. These pigment molecules are too large to pass through cell membranes.
The effect of either temperature or alcohol on membranes can be investigated.
PART 1: EFFECT OF TEMPERATURE
1. Prepare eight water baths pre-set to a range of temperatures between 0 and 70 °C.
2. Take eight test tubes and label each one with the temperature of one of the water baths. Use a syringe to add 10 cm3 of distilled water to each test tube.
3. Place each tube in the water bath set to the corresponding temperature and leave for 5 minutes.
4. Check the temperature of each bath using a thermometer. The temperatures are unlikely to be exactly right, so record the actual temperatures.
5. Use a cork borer to cut eight beetroot cylinders (see figure A, page 13). Use a knife, ruler and white tile to trim them all to the same length (1 cm is sufficient). Wash the cylinders thoroughly with water until the water runs clear, then gently pat dry with a paper towel.
6. Add one beetroot cylinder to each test tube and leave in the water bath for 15 minutes.
7. Shake the tubes once. Then, working quickly and carefully, use forceps to remove the cylinder from each tube. Discard the cylinders but keep the supernatant liquid (the clear liquid above the solid). It may be easier to decant this liquid into clean test tubes.
8. Set the colorimeter to a blue/green filter and percentage transmission. Zero the colorimeter using a blank cuvette filled with distilled water.

9. Transfer liquid from each test tube in turn into a colorimeter cuvette, place in the colorimeter and take the percentage transmission reading. Record your results in a suitable table.
PART 2: EFFECT OF ALCOHOL
1. Take five test tubes and add 10 cm3 of ethanol to each one. Use a different concentration of ethanol in each tube (distilled water can be used for a 0% concentration).
2. Use a cork borer to cut five beetroot cylinders. Use a knife, ruler and white tile to trim them all to the same length (1 cm is sufficient). Wash the cylinders thoroughly with water until the water runs clear, then gently pat dry with a paper towel.
3. Add one beetroot cylinder to each of the five tubes and leave for 15 minutes.
4. Shake the tubes once. Then, working quickly and carefully, use forceps to remove the cylinder from each tube. Discard the cylinders but keep the supernatant liquid (the clear liquid above the solid). It may be easier to decant this liquid into clean test tubes.
5. Set the colorimeter to a blue/green filter and percentage transmission. Zero the colorimeter using a blank cuvette filled with distilled water.
6. Transfer liquid from each test tube in turn into a colorimeter cuvette, place in the colorimeter and take the percentage transmission reading. Record your results in a suitable table.
ANALYSIS OF RESULTS
1. Highlight the maximum and minimum values at each temperature and decide if any values are anomalies (i.e. results that show a substantial deviation from the general pattern of results). If possible, repeat the tests that produced these results and calculate a new mean.
2. Plot a graph of mean percentage transmission against temperature. Do not forget to use the actual temperature of each water bath. Add error bars to show the range of transmission values at each temperature.
LEARNING TIPS
● Make sure you know how to cite references from scientific journals correctly. For example:
● Butler, K.G. (2000) Pollen germination across the seasons. School Science Review, 82 (298), 93–94.
● This format should be followed for all references to scientific journals, even if articles are available online. Do not provide only the web address.
● Make sure you draw tables and graphs correctly. The independent variable should always go in the left-hand column of a results table and on the horizontal axis of a graph. Numbers in tables, including calculations, should only be reported to the limits of the least accurate measurement. Data on graphs should be scaled so that the graph fills more than half of the available space.