Practical Skills

Question 1

Whta are the different types of experiments?

Answer 1

• Investigating how enzyme concentration affects the rate of activity of renin
• Investigating how temperature affects the rate of activity in catalase
• Investigating how surface area affects the rate of diffusion
• investigating how the concentration of a solution affects the percentage of onion cells that become plasmolyse
• Will use renin as example one

Question 2

What are the different types of variable and measurements made?

Answer 2

• In this experiment you would be investigating the effect of the contraption of rennin on the rate at which is causes milk to clot
  1. the concentration of renin wis the independent variable. This is the factor whose value you’d decide on and which you change
  2. The rate at which the rennin cause the milk to clot is the dependent variable. This is the variable,e which is not under your control; you do not know what its values will be until you collect your results

Question 3

What are other variable that you may change or need to keep the same?

Answer 3

• In an experiment such as this, it is important that all other variables are kept constant
• These are sometimes called standardised variables or controlled variables
• In this experiment important standardised variable would include:
  1. Temperature
  2. The type of milk used
  3. pH
• If you allowed any of these ti change during the experiments then they could affect the results and would make it difficult - if not impossible - to know he defect the enzyme concentration was having on the rate of reaction

Question 4

How might you change the independent variable?

Answer 4

• You may be asked to decide what values of the independent value to use in your experiment, You will need to make decisions about the range and the intervals
• AT LEAST 5

Question 5

What is the range of the independent variable?

Answer 5

1. The range of the independent variable is the spread of values from lowest to highest
2. In this case you might use concentrations of rennin ranging from 0 to 1%
   - If you are asked to do this in an examination, you will usually be given some clues that will help you to decide the range, for example if you are given solution with a concentration fo 1% to work with then that will be your highest concentration, because you cannot ,are a more concentrated solution from it, only more dilute ones

Question 6

What is the interval of the independent variable?

Answer 6

1. The interval is the ‘gap’ between the values that you choose within the range
   - In this case you could use concentrations of rennin of 0,0.2,0.4,0.6,0.8 and 1,0% and the interval would then be 0.2%
2. Another possibility would be to sue a series of values that a re each one tenth of each other, 0.0001, 0.001, 0.01 and 0.1 and 1.0 and in either case you can produce this range of concentrations by diluting the original solution

Question 7

How do you do a series dilution?

Answer 7

• Start: 1% standard solution
  1. A: 8 of ss and 2 of water (10cm of 0.8% solution)
  2. B: 6 of ss and 4 of water (10cm of 0.6% solution)
  3. C: 4 of ss and 6 of water (10cm of 0.4% solution)
  4. D: 2 of ss and 8 of water (10cm of 0.2 solution)

Question 8

How do you do the other dilution?

Answer 8

• Start: 1% standard solution
  1. A: 1cm of ss and 9cm of water (10cm of 0.1 solution)
  2. B: 1cm of A and 9cm of water (10cm of 0.001% solution)
  3. C: 1cm of B and 9cm of water (10cm of 0.0001% solution)

Question 9

What are some ways of measuring the dependent variable?

Answer 9

• in an enzyme experiment:
  1. You could determine the initial rate of reaction, by raking measurements very quickly to find out how much product has been formed, or how much substrate has disappeared, in the first minute or so of the reaction
  2. You could leave the reaction to take palace until it has completely finished, and that is all the substrate has been converted into product and record the time taken to do this
  3. You could time how long it takes to reach a clearly identifiable stage of the reaction, called an end-point

Question 10

What is important about graphs in biology?

Answer 10

• Join the points
• Independent on x
• Dependent on y
• axis with units
• Scale goes up in Kuala intervals

Question 11

What are classic variable you need to control?

Answer 11

• pH
• Temperature
• Light intensity
• Humidity
• Windspeed

Question 12

How could you control temperature?

Answer 12

1. A thermostatically controlled water bath
   - Don’t assume that use because electric water bath says a temperature that it is, use a thermometer, held int he water bath and not touching the sides or base of the container to measure the temperature
   - if possible read the thermometer whilst its bulb is still in the water and if you take it out to read it, then you will be measuring the temperature of the air
   - If you re standing tube of liquid in the water vary you should allow time for their temperatures to reach the same temperature was the water ad this can take a long time and it is a good idea to measure the temperature of the actual liquids in the tube, rather than assuming that they are at the same temperature as the liquid in the water bath
   - if you re doing an enzyme experiment, you may need to bring both he enzyme solution and the substrate solution tot he same temperature before you add one to the other, stand them, in two separate tubes, in the same water bath and leave them to reach the desired temperature. remember that as soon as you add the enzyme and substrate to one another, the reaction will start, so don’t do that until you are ready to begin taking measurements

Question 13

How do you control the pH of a solution?

Answer 13

1. Use a buffer solution
   - These are solutions that have a particular pH, and that keep the pH even if the reaction taking places produces an acidic or alkaline substance that would otherwise cause pH to change
   - You simply add a measured volume of the buffer to your reacting mixture

Question 14

How can you measure the pH?

Answer 14

• Using an indicator

- Using a pH meter

Question 15

What is a control?

Answer 15

-It is a tube with no renin in it, so where the factor we are investigating is absent, but everything else must be the same so the same volume of water is added to match the volume of the enzyme solution that is added to all the other tubes
-Another possible control could be a tube containing boiled rennin solution
0Boiling denatures the rennin enzyme so it is inactive

Question 16

What is the accuracy of a measurement?

Answer 16

• How ‘true’ it is
• If you are measuring a temperature than the accuracy of you measurement will depend on whether or not the thermometer is perfectly calibrated

Question 17

What is precision?

Answer 17

-The precision of a measurement depends on the ability of the measuring instrument to give you the same reading every time is measures date same thing and this does not always have to be the ‘true’ value

Question 18

What is the reliability?

Answer 18

• The reliability of a measurement is the degree of trust that you an have in it
• If your measurements are reliable then you would expect to get the same ones if you repeated them on other occasions
• Reliability is affected by both the accuracy and precision of your measuring instruments
• For example you have to make a decision about exactly when the end-point is reached and this us reeky difficult as there is no precise moment at which you can say the clots definitely from, so your measurement of time and which this happens will be very unreliable

Question 19

How do you improve reliability?

Answer 19

Repeat your readings and fin the man

Question 20

What tables do you need to draw?

Answer 20

1. Repeat one reading at least once and find the man
2. First column is independent, followed by readings of the dependent variable
3. Rule lines using a ruler
4. Heading with quantity / unit use SI
5. Result organised in sensible sequence, e.g. highest to lowest
6. Each measurement of the dependent variable is taken to the same number of decimal places
   - Exclude anomalies in mean! (measure again or ignore)
7. values calculated for mean are same number of dp

Question 21

How do you describe data?

Answer 21

1. Describe overall trend
2. Look for any changes in gradient and describe
3. Quote data stating both the x and y value
4. Don’t use time phrases unless time is on x axis

Question 22

How do you make a conclusion?

Answer 22

The greater concentration of rennin, the shorter the time taken to reach the end point. An increase in rennin concentration increases the rate of reaction

Question 23

How could there be uncertainty in you measurement?

Answer 23

1. Resulting from lack of accuracy or precision in the measuring instruments that you were using and limitation in reading the scale
2. These errors are likely to be the same through your experiment and they will be about the same size, and act in the same direction, on all of your reading and results
3. They are systematic errors

Question 24

What are some of the difficulties in controlling standardised variables?

Answer 24

1. For example, if you were using a water bath to maintain a constant temperature in the rennin experiment it may have been impossible to keep the temperature absolutely constant
2. Variations in temperature could have affected the rate of activity of the rennin, making it impossible to be sure that all changes in rate of activity were due to difference in your independent variable - the concentration of the rennin
3. These errors are likely to be different for different stages of your investigation and they are random errors

Question 25

What re some of the difficulties in measuring dependent variables?

Answer 25

1. Due to human limitations
2. For example, in the rennin experiment, you need to judge the end point and this is subjective and may have changed during experiment and so a random error

Question 26

How can you reduce the sources of error?

Answer 26

1. Using measuring instruments that are likely to be more precise, accurate or reliable, for example, measuring volumes with a graduated pipette rather than a syringe
2. Using techniques for measuring the dependent variable that are likely to be more reliable, for example using a colorimeter to measure colour changes rather than the naked eye
3. Using techniques or apparatus that a re better able to keep standardised variables constant such as using a thermostatically controlled water bath rather than a beaker of water
4. Controlling important variables that were not controlled in the original experiment (not that it is also important to say how you would control these variables)
5. Doing repeats so that you have several readings of your dependent variable for each value of your independent variable and then calculating a mean value of the dependent variable

Question 27

What is super important with drawings?

Answer 27

1. Be drawn with clear single lines (do not have several ‘goes’ at a line so that it ends up being fuzzy) and use a HB pencil and a good eraser so that when you do make a mistake you can rub it out completely
2. Show the overall shape, and the proportions of the different components of the structure you are drawing, accurately
3. DO NOT include shading or colouring
4. Be large using most of the space abatable but not going outside of the space (for example, it should not go over any of the words printed on the page) RTQ for what the q wants e.g. four whole cells!

Question 28

What do you draw in a plan diagram?

Answer 28

1. This is also called a low-power plan and only the outlines of different tissues are shown
   - Use low power lens to see the whole are that you will show in your drawing draw but high power to see where one tissue ends and another begins

Question 29

What is a high power detail?

Answer 29

1. A more detailed drawing using high power and it generally does show individual cells

Question 30

How do you label the diagrams?

Answer 30

1. You may be asked to label your drawings and the label lines should be drawn with a ruler and pencil and the end of the line should precisely touch the part of the diagram you are labelling
2. Do not use arrowheads
3. The label lends should not cross over one another
4. The labels themselves should be written horizontally (no matter what angle the label line is at), and should not be written on the drawing