practical

Question 1

How to calculate total magnification?

Answer 1

total magnification = eyepiece lens magnification x objective lens magnification
An eyepiece lens, which often has a magnification of x10
A series of objective lenses, each with a different magnification, e.g. x4, x10, x40 and x100

Question 2

magnification formula

Answer 2

Formula: Magnification = Image size/actual size

Question 3

Preparing a microscope slide (liquid)

Answer 3

1. Add a few drops of the liquid to a clean slide using a pipette
2. at a 45 degree angle Lower a coverslip onto the specimen and press down to remove air bubbles

Question 4

Preparing microscope slide: solid

Answer 4

1. Use scissors or a scalpel to cut out a sample of tissue/peel away a thin later of cells
2. Place the sample onto the slide (sometimes a drop of water is added before)
3. Apply a stain (normally iodine or methylene blue)
4. Gently lower a coverslip over the specimen and press down to remove any air bubbles

Question 5

Why is excess benedicts reagent used in benedicts test

Answer 5

to ensure a complete reaction with all the sugar present in the solution

Question 6

Reducing sugar test

Answer 6

Benedict’s reagent is copper(II) sulfate in an alkaline solution. It has a distinctive blue colour.
Add excess Benedict’s reagent to solution
Heat in a water bath at 80-90 for about 5 minutes
If reducing sugar is present the colour will change from a range of blue to red depending on the concentration of sugar
Brick-red: High concentration of sugar
Orange: Moderate-high concentration of sugar
Yellow: Medium concentration of sugar
Green: Low concentration of sugar
Blue: No sugar present

Reducing sugars reduce the soluble blue copper sulfate to insoluble brick-red copper oxide, containing copper(I). The copper oxide is seen as a brick-red precipitate.
reducing sugar + Cu2+ → oxidised sugar + Cu+

Question 7

what level of risk is harmful irritant

Answer 7

medium

Question 8

Protein test

Answer 8

All proteins have peptide bonds, containing nitrogen atoms. The nitrogen forms a purple complex with copper(II) ions and this forms the basis procedure of the biuret test.
The reagent used for this test is called biuret reagent. It is made up of a dilute solution of potassium hydroxide or sodium hydroxide, and a dilute solution of copper(II) sulfate.
Biuret test
Grind up sample
Add Biuret reagent
If colour changes from blue to lilac/purple protein is present, if no colour change no protein is present
(if the sample contains amino acids or dipeptides, the result will be negative because there must be at least 2 peptide bonds)

Question 9

Non-reducing sugar test

Answer 9

1. Add dilute hydrochloric acid to solution
2. Heat in a water bath that has been boiled (still at boiling temp)
3. Neutralise the solution with sodium hydrogen carbonate or sodium hydroxide
4. Repeat Benedict’s test
5. If there is change in colour non-reducing sugar is present, if no change in colour no sugar present

Question 10

Lipid test

Answer 10

Lipids are insoluble in water, but soluble in ethanol (alcohol). This fact is made use of in the emulsion test for lipids.
Emulsion test
1. Grind up the sample if solid
2. Add ethanol to test tube and shake
3. Add this solution into a test tube with water
4. If a milky emulsion forms there are lipids present, if it remains clear there are no lipids present

Question 11

Starch test

Answer 11

To test for starch, you use iodine solution (since iodine doesn’t dissolve in water the iodine solution is actually iodine in potassium iodide solution). The starch–iodine complex that forms has a strong blue-black colour.
Iodine test
1. Put the food sample on a spotting tile
2. Add a few drops of iodine in the form of potassium iodide solution
3. If colour changes from orange-brown to blue-black then starch present, if no colour change no starch present

Question 12

Examples of reducing sugars and non-reducing sugars

Answer 12

Non-reducing
Sucrose

Reducing
Maltose
Glucose
Fructose
Galactose

Question 13

Explain how enzyme is affected by change in temperature

Answer 13

Less enzyme substrate complexes are formed

Question 14

Qualitative test errors

Answer 14

Qualitative food test errors
1) difficult to judge colours, especially if concentrations
are low resulting in light colours or small colour
changes
2) temperature of the water bath may not remain
constant throughout heating

Question 15

Improvement for qualitative test errors

Answer 15

Improvements for qualitative tests
1) use a thermostatically controlled water bath
2) use a colourimeter
3) place a white card or tile behind the test tubes

Question 16

Quantitative food test errors

Answer 16

Quantitative food test errors
1) difficulty in comparing colours
2) when determining an unknown concentration, it may
be between 2 concentrations
3) 4) temperature not being constant for all samples
DCPIP test for vitamin C – drops fall on the sides of
the test tubes

Question 17

Improvements for quantitative tests

Answer 17

1) carry out more experiments with a narrower or wider
range of concentrations
2) 3) plot a graph with results to estimate the unknown
use a colourimeter to help compare colour changes
better
4) use a white card/tile to observe the colour changes
better
5) 6) use a thermostatically controlled water bath
in the DCPIP vitamin C test, use a wider test tube, or
a wide-mouthed one
7) repeat and take the average

Question 18

formula for dilutions

Answer 18

c1v1 = c2v2

Question 19

Errors in enzyme experiments

Answer 19

1) counting bubbles is inaccurate due to different bubble sizes, bubbles being too fast to count, bubbles too small so they’re missed
2) temperature or pH not constant (if they aren’t the
factors being investigated)
3) difficulty in judging endpoint e.g., with renin the
coagulation may not be clear

Question 20

serial dilution of 1% solution by half each time

Answer 20

20cm^3 of 1.0% solution
transfer 10cm^3 of that into new beaker and add 10cm^3 of water to get 0.5% solution
transfer 10cm^3 of that into new beaker and add 10cm^3 of water to get 0.25% solution
etc

Question 21

results for timing in tables

Answer 21

to the nearest second

Question 22

Improvements in enzyme experiments

Answer 22

1) use a gas syringe to measure volume of gas instead
of counting bubbles
2) control temperature using a thermostatically
controlled water bath
3) control pH by using buffers
4) use a colourimeter/put a white tile or card to judge

Question 23

Effect of heavy metals such as copper sulphate
and lead nitrate

Answer 23

• can act as inhibitors
• may cause protein to clot or coagulate
• may denature proteins
• may breakdown bonds so that can alter tertiary and
  quaternary structures

Question 24

Immobilising enzymes error

Answer 24

1) beads are not of equal sizes
2) beads stuck to the sides of the tube and to each
3) other
4) forceps may cause damage to the beads
5) difficulty to introduce drops using syringe
6) test tube is not vertical and test tubes are not of
equal sizes
7) temperature/pH is not controlled (if they are not the
factors under investigation)

Question 25

Immobilising enzymes improvements

Answer 25

Improvements 1) use sieve with equal size holes to produce beads with equal diameter 2) use wider test tubes 3) 4) 5) 6) 7) stain the beads for clearer movement use retort stand to make the test tube vertica use test tubes with equal sizes use spatula or spoon instead of forceps control temperature using thermostatic waterbath 8) control PH using buffer

Question 26

Effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes errors

Answer 26

1) difficulty in cutting the agar squares into equal dimensions 2) difficulty in judging the colour change 3) agar is not of equal depth 4) pigmentation of the agar is not even 5) agar may be damaged during cutting

Question 27

Effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes improvements

Answer 27

Improvements 1) use moulds for preparing agar squares with equal dimension 2) use a different indicator with clearer endpoint 3) use wider or narrower range of concentrations (in case of determining unknown) 4) place a white card below the beaker for better judgment of colors

Question 28

Deciding which variable to standardise

Answer 28

volume (must be suitable for apparatus) * concentration * temperature * pH * biological material (e.g., same species, age, storage conditions, time of year, mass) * humidity (plastic bag/Ca(OH)2) * apparatus

Question 29

Decide how the independent variable should be changed within a suitable range to provide accurate results.

Answer 29

concentration – simple/serial dilution * temperature – water bath/freezer * pH – use buffers * moving air – fan * humidity – plastic bag/calcium hydroxide

Question 30

modifications to a procedure

Answer 30

1) change a different independent variable standardise all other variables including a previously used independent variable use an improved method to obtain accurate and precise results for dependent variable

Question 31

How to keep fair comparison in osmosis experiments?

Answer 31

when using petri dishes, use petri dishes of the sme size * when using sucrose or any other solution, put the same volume in each dish * when using onion epidermis or potato, must be of the same size * leave the potato or onion epidermis in the solution for the same period of time

Question 32

General precautions in osmosis experiments

Answer 32

cover the petri dishes during the experiment to avoid evaporation of water which can affect concentration of the solution * when using droppers or syringes, use separate droppers for each solution or wash and dry after each step * while preparing a slide of onion epidermal cells, lower the coverslip gently to avoid any air bubbles

Question 33

Effects of immersing plant tissues in solutions of different water potentials errors

Answer 33

1) time tissues left in solutions is not enough to observe complete plasmolysis evaporation of solution may take place and therefore change the concentration difficulty in judging degree of plasmolysis difficulty in cutting the samples into correct dimensions parallex error may occur during reading the lengths

Question 34

Effects of immersing plant tissues in solutions of different water potentials improvements

Answer 34

1) leave for longer time in the solutions prepare more concentrations with narrower/wider range (give examples and method of dilution) 3) cover the petri dish to avoid evaporation of solution prepare more than one sample in each solution ensure that the tissues are completely immersed in the solutions

Question 35

Imrpovements to a procedure

Answer 35

Improvements 1) standardise relevant variables 2) use a measurement method for the dependent variable which is more accurate 3) collect more data by taking replicates to obtain mean

Question 36

Suggest modifications to this procedure that would allow you to obtain a more accurate estimate of the concentrations of reducing sugars in unknown solutions

Answer 36

- smaller intervals between standard concentrations ; - plot graph of standard concentrations against time and interpolate ; 3 AVP ;

Question 37

After soaking at 20°C for 24 hours, the concentrations of reducing sugars were different for the seeds from different species of plants. However, after soaking at the higher temperature for 24 hours, the concentrations of reducing sugars were the same for all the seeds tested. Suggest and explain why, at a higher temperature, the concentrations of reducing sugars were the same for all the seeds tested.

Answer 37

1 enzymes denatured so no reducing sugars released ; 2 the high temperature changes the shape of the enzyme's active site making it not complementary to the substrate

Question 38

features to mention when asked differences between two specimens

Answer 38

trichomes (outgrowths) presence of airspaces midrib (prominent vein) number of vascular bundles (one has more or less) endodermis size (thicker or thinner) size of vascular bundles cortex (cells in between root and endodermis) shape of stoma presence of nucleus in guard cells shape of stem position of vascular bundles (scattered or arranged in rings)

Question 39

feature of plants that live underwater

Answer 39

presence of air spaces, makes leaf buoyant

Question 40

plasmolysis

Answer 40

water moves (out of the cells) from a high water potential to low water potential ; 2 (water moves out) by osmosis ; 3 cell membrane pulls away from the cell wall ;

Question 41

serial dilution of 1.0% solution by a factor of 10

Answer 41

1cm^3 solution transferred to each beaker and 9cm^3 of water added

Question 42

identify which root extract was taken in the summer

Answer 42

the one with less starch

Question 43

As soon as a number of small clots appear, stop timing and record the value in (a)(ii). error with this

Answer 43

difficult to judge when clots first appear

Question 44

Suggest why the procedure was repeated.

Answer 44

to improve accuracy in results

Question 45

how to obtain a more accurate value for unknown solution

Answer 45

1 use more concentrations with narrower intervals ; 2 states concentrations both sides of the estimate ;

Question 46

explain the effect of pH on the activity of protease.

Answer 46

1 optimum pH around ___ 2 (away from optimum) shape of the active site is changed ; 3 (from pH1.8 to pH6.1) more enzyme substrate complexes/(from pH6.1 to pH8.5) less enzyme substrate complexes / (at optimum pH) maximum number of enzyme substrate complexes ; 4 low activity at extreme pH ;

Question 47

features of xerophytes

Answer 47

1 rolled leaf ; 2 thick epidermis ; 3 cuticle ; 4 sunken stomata ; 5 hinge cells ;

Question 48

table with 3 things

Answer 48

Ethanol concentration (%) Height of sediment / mm at time (min)

Question 49

Suggest why the percentage of ethanol produced by Schizosaccharomyces pombe is lower than the percentage of ethanol produced by Saccharomyces cerevisiae for all sources of carbohydrate.

Answer 49

1 the optimum pH for S. pombe not the same as for S. cerevisiae ; 2 the optimum temperature for S. pombe not the same as for S. cerevisiae ; 3 S. pombe was unable to ferment all the sources of carbohydrate ;

Question 50

Finding the mean diameter

Answer 50

take measurements at 2 different lengths

Question 51

what contains a higher quantity of salicylic acid urine or blood

Answer 51

urine bc it filters and excretes it from the blood

Question 52

Aspirin is also taken to reduce inflammation and blood clotting. Inflammation and blood clotting involve enzyme-controlled reactions. Suggest how aspirin reduces these enzyme-controlled reactions.

Answer 52

1 aspirin acts an inhibitor ; 2 aspirin changes the shape of the active site of the enzyme ; 3 (so) fewer enzyme substrate complexes form ;

Question 53

With reference to your results in (a)(ii), explain the effect of ethanol on cell membranes.

Answer 53

1 (ethanol) denatures the proteins in the cell surface membrane ; 2 (ethanol) dissolves the phospholipids (and cholesterol) ; 3 causing, increase in permeability / increase in fluidity ;

Question 54

Describe how you would modify the procedure to investigate the effect of temperature on the permeability of beetroot cell membranes.

Answer 54

uses one stated concentration of ethanol ; states using at least five different temperatures ;

Question 55

suggest why milk is used instead of water to dilute the starch-milk solution

Answer 55

water would dilute the colour

Question 56

explain the increase then plateau of an enzyme graph

Answer 56

Rate increases as more substrates become available Active sites become saturated

Question 57

A student suggested that the mean actual length of the vascular bundles calculated in (b)(i) was not accurate for the whole plant. Describe two modifications to the method used in (b)(i) that would allow a more accurate mean length of the vascular bundles for the whole plant to be calculated.

Answer 57

1 measure all the vascular bundles 2 larger magnification to make it easier to measure 3 take more sections

Question 58

With reference to your observations at 0, 5 and 10 minutes, describe the effect of concentration of hydrochloric acid on the rate of protein precipitation.

Answer 58

1 states that the higher the acid concentration, the higher the rate of precipitation ; 2 refers to observations of cloudiness at 0, 5 and 10 minutes

Question 59

Explain why the student left the test-tubes in the water-bath for 3 minutes before adding the starch solution.

Answer 59

reference to the contents of the test-tubes reaching the temperature of the water-bath ;

Question 60

Describe how the student could obtain a quantitative estimate of the concentration of reducing sugar in a solution.

Answer 60

1 at least five known concentrations of reducing sugar ; 2 Benedict’s test and time for first colour change for known concentrations of reducing sugar ; 3 test unknown sample of reducing sugar (with Benedict’s) ; 4 compare time for unknown sample with times for known samples of reducing sugar ; 5 draw a graph of known concentrations of reducing sugar against time (to first colour change) ; 6 read off unknown sample of reducing sugar ;

Question 61

Step 2 to step 4 improve the validity of the results by removing the pigment that was released when the beetroot tissue was cut. State how you could confirm that all the pigment released when the beetroot tissue was cut has been removed.

Answer 61

Water isn't red

Question 62

Use your knowledge of cell surface membranes to explain how temperature increase affects it

Answer 62

As temperature increases, the phospholipid bilayer becomes more fluid making it more permeable. At very high temperatures, membrane proteins denature, disrupting their structure and function. Together, these effects cause substances to leak more easily across the membrane, explaining the observed changes in the results.

Question 63

Describe how you would modify this procedure to investigate the effect of concentration of substrate on the time taken for the beads to rise.

Answer 63

1 same pH ; 2 5 different concentrations of hydrogen peroxide ; 3 serial or proportional dilution ;

Question 64

features of xerophyte

Answer 64

1 trichomes and traps water vapour inside the leaf ; 2 curled leaf and increases humidity to reduce the water potential gradient ; 3 sunken stomata and exposed to the humid atmosphere with a low water potential gradient ; 4 thick cuticle and reduces water loss by transpiration ;

Question 65

a possible explanation for the change in the concentration of reducing sugars in grapes as they age.

Answer 65

1 the concentration of amylase and reducing sugar increase as grapes age ; 2 amylase hydrolyses starch into reducing sugars ; 3 more amylase leads to an increase in reducing sugar ;

Question 66

sources of error in dropping milk into copper sulphate experiment

Answer 66

drops sometimes stick to the side of test-tube ; use a wider vessel ; drops of milk remain in the milk ; use fresh copper sulfate each time ;

Question 67

explain the shape of the graph between 0.1mmoldm–3 and 0.2mmoldm–3 of lactase.

Answer 67

1 more enzyme molecules so more active sites available ; 2 more successful collisions ; 3 more enzyme-substrate complexes form ;

Question 68

Describe two improvements to your procedure that would make the estimate of a salt concentration where there is no net movement of water into or out of the potato

Answer 68

1 using smaller intervals of salt concentrations around the value of the candidates’ estimate ; 2 use a more precise method of cutting discs ;

Question 69

Until the 1930s, most commercial fermenting equipment for growing yeast cells was made of copper. After this time, fermenting equipment was made using stainless steel instead of copper. Use your graph in Fig. 1.3 to suggest why stainless steel was used instead of copper.

Answer 69

copper will, reduce growth of yeast / reduce fermentation ;

Question 70

One way to improve the accuracy of the estimate of the total number of stomata on a leaf is to use a photomicrograph with a larger area. State one other way to improve the accuracy of the estimate of the total number of stomata on a leaf.

Answer 70

repeat / sample more areas ;

Question 71

A student set up a beaker as a control experiment. The result of the control experiment showed that the sucrose was hydrolysed by an enzyme. Suggest what substances the student put in the beaker for the control experiment.

Answer 71

boiled, yeast / enzyme ;

Question 72

Describe how you would modify the procedure to investigate the effect of changing the concentration of the sucrose solution on the rate of hydrolysis of sucrose.

Answer 72

1 use a set number of yeast beads ; 2 prepare at least five concentrations of sucrose ; 3 by, proportional / serial, dilution ;

Question 73

Suggest an explanation for why more fruit juice was obtained when using the free pectinase between the temperatures of 30°C and 50°C than when using immobilised pectinase.

Answer 73

1 free enzyme can more easily come into contact with apple pulp ; 2 less apple pulp can come into contact with yeast beads

Question 74

Suggest an explanation for why more fruit juice was obtained when using immobilised pectinase between the temperatures of 60°C and 70°C than when using the free pectinase.

Answer 74

1 free pectinase denatures at a lower temperature or yeast beads denature at a higher temperature ; 2 yeast beads have a higher tolerance of high temperatures

Question 75

starch solution risk

Answer 75

low

Question 76

A student observed that the rate of diffusion was not constant during the investigation. Suggest how the student could modify the procedure to investigate the change in the rate of diffusion.

Answer 76

1 use one concentration of hydrochloric acid ; 2 for five time intervals ; 3 calculate the rate of diffusion of the acid for each time interval ;

Question 77

Amino acids are transported into cells by facilitated diffusion. Explain the shape of your graph in Fig. 1.4.

Answer 77

1 states that the higher the concentration of alanine the higher the rate of absorption ; 2 correct reference to transport proteins ; 3 transport proteins saturated at 50 to 70 per cent concentration of alanine ;

Question 78

Suggest an explanation for the result at 0% concentration of catalase.

Answer 78

hydrogen peroxide, breaks down / forms oxygen, with no enzyme ;

Question 79

With reference to the total surface area, explain the trend in your results.

Answer 79

1 increases the (total) surface area in contact with the sodium chloride solution ; 2 increased surface area to volume ratio ; 3 (so) there is a shorter (diffusion) distance (to the cells in the middle of the potato cylinder) ;

Question 80

Explain why confidence in the results can be increased by repeating the procedure several times.

Answer 80

identify anomalous results / to exclude anomalous results / to allow a statistical test (not mean) ;

Question 81

Describe the movement of water molecules when the water potential of the sucrose solution surrounding the piece of potato is the same as the water potential of the potato cells.

Answer 81

1 ref. to molecules are moving / no overall movement / no net movement ; reject molecules are not moving 2 equal rate in both directions ;

Question 82

Explain why the percentage concentration of lactose in the milk after 5 minutes increases as the bead diameter increases.

Answer 82

1 larger beads have a smaller total surface area ; ORA plus any two explanations (ORA): 2 less enzymes in contact with lactose ; 3 less active sites for the substrate to bind to ; 4 fewer enzyme substrate complexes / ESCs ; 5 decreased rate of lactose hydrolysis ;