20 April 2015

#76 Summary of Practical Skills

1 In an experiment investigating the effect of one variable on another, the independent variable is the one that you change and the dependent variable is the one that you measure. All other variables should be controlled (kept constant).

 2 The range of the independent variable is the spread from lowest to highest value. The interval is the distance between each value in the range.

 3 Temperature can be kept constant or varied using a water bath. pH can be kept constant or varied using buffer solutions.

 4 The accuracy of a measurement is how true it is. For example, an accurate measuring cylinder reads exactly 50 cm3  when it contains 50 cm3 of liquid.c

 5 The precision of a measuring instrument is how consistent it is in giving exactly the same reading for the same value.

 6 The reliability of a set of measurements is the degree of trust that you can have in them. A reliable set of measurements are likely to be very similar if you are able to do the same experiment again. If you are concerned about reliability, then do at least three repeat measurements for each value of your
independent variable, and calculate a mean.

 7 In general, the error in any measurement is half the value of the smallest division on the scale. For
example, on a measuring cylinder marked in 2 cm3 divisions, the error in any reading will be ± 1cm3. If you are taking two readings and calculating the diff erence between them, then the error is ± 1 cm3 for each reading, making a total error of ± 2 cm3.

8 Results tables should be constructed with the independent variable in the first column and the
readings for the dependent variable(s) in the next column(s). Units go in the headings, not in the body
of the table. Each value should be recorded to the same number of decimal places. This is also the case for any calculated values.

 9 In a line graph, the independent variable goes on the x-axis and the dependent variable on the y-axis. Headings must include units. Scales must go up in even and sensible steps. Points should be plotted as small crosses or as encircled dots. Lines should be best fit or ruled between successive points. Do not extrapolate.

10 Bar charts are drawn when there is a discontinuous variable on the x-axis. Bars do not touch.

11 Frequency diagrams or histograms are drawn when there is a continuous variable on the x-axis. Bars touch.

12 Conclusions should be short and to the point. They should use the results to answer the question posed by the investigation. They should not go beyond what is shown by the results. Do not confuse
conclusion with discussion.

13 When describing data displayed on a graph, begin by stating the general trend and then describe any points at which the gradient of the curve changes. Quote figures from the x-axis and y-axis coordinates for these points. Do not use language suggesting time (e.g. ‘faster’) if time is not shown on the x-axis or y-axis.

14 Show every small step whenever you are asked to do a calculation.

15 Do not confuse mistakes with experimental errors. Mistakes should not happen. Experimental
errors are often unavoidable, unless you have the opportunity to use a better technique or better
apparatus. Systematic errors are those which have the same magnitude and direction throughout the
experiment, and are usually caused by limitations in the measuring instruments. Random errors are
those which vary in magnitude and direction during the experiment, and may be caused by difficulty
in controlling variables or in making judgements. When asked to suggest improvements in an experiment, concentrate on the main sources of error and suggest ways of reducing them.

16 When making drawings from a microscope, a low-power plan should show only the outlines of
tissues and no individual cells. Be prepared to go up to high power to get more information about where one tissue ends and another begins. High-power drawings should show as much detail as possible, including details of individual cells.

End-of-chapter questions

An investigation   is carried   out  into  the  effect  of substrate   concentration    on  the  activity   of catalase.  What   could  be the dependent    variable?
A   the  concentration    of catalase
B   the  pH  of the  enzyme   solution
C   the  rate  of production     of oxygen
D   the  temperature     of the  substrate

2  An investigation   is carried   out  into  the  effect  of temperature     on  the  activity   of lipase.  Separate   tubes  of substrate solution   and  enzyme   solution    are left  in  temperature-controlled      water  baths  for  ten  minutes   before  mixing.   Why  is this done?
A   to activate   the  enzyme
B   to allow  time  for  the  enzyme   and  substrate    to react
C   to control   the  independent     variable
D   to keep  a standardised    variable   constant

4   For this  question   you  need  two  sheets  of graph   paper.
The light  micrographs    below  are cross  sections   of a young   root  and  a representative   part  of a young   stem  of Ranunculus (buttercup).

a    Name   the  tissues  A, B, C and  D.        [4]                                                                                                                                             
 i On  one  of the  sheets  of graph   paper,  draw  the  outline   of the  root.   Use  at  least  half  the  width   of the  graph   paper  when   making   your  drawing.
Now  draw  inside  your  outline   a low-power    plan  of the  xylem  only.   Be as accurate   as you  can  in  drawingthe  correct   proportions     compared    with   the  overall  size of the  root  -  you  may  find  it useful  to make some  measurements     with  a ruler.       [4]                                                                                                                                        
   ii Now  take  the  second   sheet  of graph  paper   and  draw  the  outline   of the  stem.   It does  not  have  to  be exactly  the  same  size as your  drawing   of the  root.
Carefully   make  a low-power   plan  to  show  the  vascular   bundles   only.   Draw   in  outline   the  lignified tissues  sclerenchyma    and  xylem,  and  the  tissue  labelled   C  berween   them.    [2]                                                                
   iii Sclerenchyma    and  xylem  are tissues  which   contain   dead  cells whose  walls  are  thickened    with   a mechanically    strong   substance   called  lignin.   Lignin   is used  for  strength   and  support.    Count   the number    of squares   of graph   paper  covered   by lignified   tissue  (xylem)  in  the  root.   Count   the  squares   that  are more  than  half  included    in  the  drawing   as whole   squares,   and  do  not  count   squares   that  are less than  half included.  [1]                                                                                                                                                                                           
  iv     Count   the  number    of squares  covered   by the  whole   root  section   (including    the  lignified   tissue).                         [1]
 v  Calculate    the  percentage    of squares  occupied    by lignified   tissue  in  the  root  as follows:

 vi     Repeat   steps  iii to v for  the  stem  (remember    lignified   tissue  in  the  stem  is sclerenchyma    plus  xylem).               [3]

vii    Assuming    the  results  you  have  obtained    are  typical  of the  whole   stem,  suggest   an  explanation for  the difference in percentage of lignified  tissue  in  the  root  and  the  stem.  [2]                                                                            
viii   If you  try  to imagine   these  structures    in  three  dimensions,     the  lignified   tissue  in  the  root  is a central   rod, but  in  the  stem  it is a circle  of separate   rods.  Suggest   the  reasons   for  the  different   distribution   of lignified tissues  in  the  root  and  the  stem.   [2]   

[Total:  20]

5  A student   decided   to investigate   the  effect  of temperature  on  the  activity   of enzymes   in yeast.  The  student   measured the  activity   of the  enzymes   by counting    the  number    of bubbles   of carbon   dioxide   which   were  released   in  three minutes.

The  results  of the  student's   investigation    are shown   in the  table.

a i  Plot  a graph  of the  data  shown   in  the  table.         [4]                                                                       ii From  the  graph,   estimate   the  enzyme   activity   at 25°C.                                          [1] 
        iii Suggest  how  the  student   should   make  sure  that  the  results  of this  investigation    are as accurate   as possible and  as reliable   as possible.                                                                 [3]

b   In  carrying   out  this  investigation,   the  student    made  the  hypothesis    that  'The  activity   of the  enzymes   in yeast increases  as temperature      increases.'    State  whether   you  think   this  hypothesis    is supported    by the  student's   results. Explain   your  answer.                          [2]
[Total:   10]

[Cambridge  International   AS  and A Level Biology 9100  Paper 31,  Question  1c and d, June 2009]

6  A student   investigated    the  time  taken  for  the  complete    digestion   of starch  by amylase  found   in  the  saliva of 25 individuals   of a species  of mammal.

A sample  of saliva  was collected   from  each  individual    and  mixed  with   5 cm3 of starch  suspension.    Samples   of the mixture   were  tested   for  the  presence   of starch.

The student   recorded   the  time  taken   for  the  complete    digestion   of starch.

The investigation   was  repeated   with   the  same  individuals    on  the  following   day. The results  of the  student's    investigation   are shown   in  the  table.

a    Plot  a graph   to  display   these  data.   [4]
b   Describe   the  patterns    in  the  results.  [3]
c    Suggest  a reason   for  the  differences   between   the  results  for day  1 and  day  2.  [1]
d    Suggest  how  you  might   control   the  variables   in  this  investigation   to compare   a different   species  of mammal  with  the  mammal    studied.     [3]                                                                                                                                                             [Total:   11]
[Cambridge  International   AS  and A Level Biology 9100  Paper 33,  Question  1b, November  2009]

3. End-of-chapter answers
1 C
2 C

Exam-style questions

4  a 
     A epidermis;
     B cortex/parenchyma; 
     C phloem;
     D endodermis; [4]

   b i LP plan draw with no cell detail;
        xylem only draw inside circle;
         correct proportions;
       lines continuous, not sketchy and sharp pencil used; [4]

     ii LP plan drawn showing vascular bundles only and no cell detail;
        sclerenchyma, xylem and phloem drawn in outline; [2]

     iii no. of squares of graph paper covered by lignifi ed tissue in root counted; [1]
     iv no. of squares of graph paper covered by whole root section counted; [1]

     v % squares occupied by lignifi ed tissue in root calculated correctly from student’s
answers to iii and iv; (answer should be around 1%) [1]

   vi no. of squares covered by lignifi ed tissue in stem counted;
        no. of squares covered by whole stem counted;
         % squares occupied by lignifi ed tissue in stem calculated correctly; (answer should
be around 1%) [3]

   vii stem needs more support than root;
         because upright in air and needs support to prevent it falling over / collapsing; AW [2]

   viii roots subjected to tugging/pulling pressure from parts above ground;
        roots spread out, so like a series of guy ropes;
        stem a single column;
        greater strength from a ring of rods than from one central rod;
       ring of rods provides greater resistance to compression from above than a single central rod;
       accept any reasonable suggestion(s) which are based on diff erent stresses to which roots and
      stems are subjected. [max.2]

5 a i ‘Temperature / °C’ on x-axis and ‘Enzyme activity / mean number of carbon dioxide
bubbles released per minute’ on y-axis;
      suitable scales on both axes – range from 10 or 15 to 40 on x-axis and 0 or 5 to 20 on
y-axis, in intervals of 2 or 5;
      all points plotted accurately, using crosses or encircled dots;
      thin, clear, best-fi t line drawn or points joined with ruled lines – no extrapolation; [4]

       ii correct reading from graph, including unit (mean number of bubbles per minute); [1]

      iii accuracy: use water bath to change independent variable;
          control of signifi cant named variable plus method of control (e.g. use same type of yeast);
          use named apparatus (e.g. gas syringe) to collect gas (for measurement of dependent
        reliability: increase number/range of temperatures;
        repeat each temperature three times and calculate mean; [max. 3]

b hypothesis is supported;
   quote figures for change in mean number of bubbles between any two temperatures between
15 °C and 40 °C;
    reference to no data below 15 °C or above 40 °C;
    so cannot tell if hypothesis is also supported outside this range; [max. 2]
 [Total: 10]

6 a x-axis is ‘Time / minutes’, y-axis is ‘Number of individuals’;
       scales on both axes with suitable range and interval;
       all bars plotted accurately or points plotted accurately (using a cross or an encircled dot);
       all lines neat and thin, plus key; [4]
 b on both days, minimum time taken is 35 min and maximum time taken is 55 min;
     on both days, number of individuals is greatest near the centre of the range;
     on day 1, greatest number of individuals take 45 minutes to digest starch, but on day 2 greatest number of individuals take 10 minutes to digest starch;
      mean time is greater on day 1 than on day 2; [max. 3]

c temperature may have been higher on day 2;
   animals on day 2 may have eaten recently and so had more saliva/amylase in their mouths; [max. 1]

d use individuals of same age/mass/body weight;
   ensure pre-treatment is the same (e.g. food given, environment);
   use same volume of saliva;
    use same volume and concentration of starch;
     keep temperature the same by using a water bath; [max. 3]
 [Total: 11]

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