AS Biology - practical
How do you measure pH?
- using an indicator - using a pH meter Universal indicator - produces a range of colours over the whole pH range, from 0-14
How do you calculate a percentage change? Making calculations from data
1. Find the difference between the first reading and the second reading, by subtracting one from the other. 2. Divide this value by the first reading, and multiply by 100 - this is the percentage change. State whether the change is an increase or a decrease.
What is a histogram/frequency diagram? Constructing bar charts and histograms
A graph where there is a continuous range of categories on the x-axis and the frequency with which each of these categories occurs is on the y-axis. The bars are drawn so that they touch.
What is a conclusion? Making conclusions
A simple, well-focused and clear statement describing what can be deduced from the results of the experiment. The conclusion should relate to the initial question that was investigated or the hypothesis that was being tested.
What is rennin (chymosin)?
An enzyme that clots milk. Found in the stomachs of young mammals which are fed on milk. Used commercially in cheese-making. Its substrate is a protein (casein).
How do you carry out a calculation from a set of results? Making calculations from data
Answers must have the same number of decimal points as the original measurements e.g. if the original measurements are 12.4 and 43.7 the answer must only have one decimal place.
How do you control pH?
Buffer solutions - solutions that have a particular pH even if the reaction taking place produces an acidic or alkaline substance that would otherwise cause pH to change. How to use - add a measured volume of the buffer to your reacting mixture.
How do you determine the gradient at a point on a curved graph? Making calculations from data
Draw a tangent to the curve and then draw a eight angle triangle. Find the values x1 and y1, and then divide y1 by x1 to find the rate.
What is a bar chart? Constructing bar charts and histograms
Drawn when there is a discontinuous variable on the x-axis and a continuous variable on the y-axis.
What is a grid and a haemocytometer?
Grid Used to count the number of cells in a given area or volume of a sample. It is a grid in the eyepiece of a microscope or marked on a special slide Haemocytometer - used to count the number of cells in a known liquid
What are the independent and dependent variables in this experiment? Variables and making measurements
Independent variable - the concentration of rennin. Definition of independent variable - the factor whose values you decide on, and which you change. Dependent variable - the rate at which the rennin causes the milk to clot. Dependent variable definition - the variable which is not under your control; you do not know what its values will be until you collect your results.
These notes are based on the experiment - investigating how enzyme concentration affects the rate of activity of rennin.
Investigating the effect of the concentration of rennin on the rate at which it causes milk to clot.
What is a colourimeter?
It provides quantitative measurements of colour intensity in a solution. It might be a sensible suggestion when asked how you could improve the reliability of the results collected in an experiment. It is an instrument that measures the amount of light that is absorbed by a tube containing a coloured liquid. The deeper the colour, the more light is absorbed.
How do you control windspeed?
May need to vary windspeed when investigating transpiration. Could be controlled by using a fan - can place at different distances from the plant Not able to measure the actual windspeed.
What is a continuous variable? Constructing bar charts and histograms
One where there is a smooth numerical relationship between variables
What is a discontinuous variable? Constructing bar charts and histograms
One where there is no continuous relationship between the items listed on the scale. Each category is discrete.
What are standardised variables or controlled variables? What are the standardised variables or controlled variables in this experiment? Variables and making measurements
Other variables that might affect the results that are kept constant. In this experiment, standardised variables would include: - temperature - the type of milk used - pH
How do you determine the gradient of a straight line graph? Making calculations from data
Select two points on the line and draw a right angle triangle using these points. Measure the values x1 and y1 and the calculate the gradient as y1/x1.
What is the definition of validity? More about measurements - accuracy, precision and reliability
Simple - A reading is valid if it really measures what it is intended to measure, this is why it is important to identify the dependant and independant variables carefully and to keep all other variables constant.
What are some suggestions of improvements to an experiment?
Suggestions should be focused on reducing sources of error. Improvements: Using measuring instruments that are likely to be more precise, accurate or reliable - e.g. measuring volumes with a graduated pipette rather than a syringe. Using techniques for measuring the dependent variable that are likely to be more reliable - e.g. using a colourimeter to measure colour changes, rather than the naked eye. Using techniques or apparatus that are better able to keep standardised variables constant - e.g. using a thermostatically controlled water bath rather than the naked eye. Controlling important variables that were not controlled in the original experiment (also have to say how you would control these variables). 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.
What are some examples of standardised variables or controlled variables? Variables and making measurements
Temperature pH Light intensity Windspeed Humidity Biological material Others: Time of day The kind of glassware
What is the definition of accuracy? More about measurements - accuracy, precision and reliability
The accuracy of a measurement of how 'true' it is. If you are measuring a temperature, then the accuracy of your measurement will depend on whether or not the thermometer is perfectly calibrated. If the thermometer is accurate, then when the meniscus is at 31C, the temperature really is exactly 31C. Simple - A measurement is accurate if it is close to its true value, so accuracy relates to the closeness of a reading and its true nature.
What is the definition of precision? More about measurements - accuracy, precision and reliability
The precision of a measurement depends on the ability of the measuring instrument to give you the same reading every time it measures the same thing. This doesn't have to be the 'true' value. So, if your thermometer always reads 32C when the temperature is really 31C, it obviously is not accurate but is it precise. Simple - This relates to the closeness of agreement between a series of repeated readings; there may be some variation between readings, if there is a lot of variation, your results may be less precise.
How do you decide what values of the independent variable to use in your experiment. How do you make decisions about the range and the intervals? Changing the independent variable
The range of the independent variable - the spread of values from lowest to highest. Range for this experiment - 0-1%. Tip - if you are given a solution with a concentration of 1% to work with, then that will be your highest concentration - you cannot make a more concentrated solution from it, only more dilute ones The interval is the 'gap' between the values that you choose within the range. Interval for this experiment: Simple dilution - 0. 0.2, 0.4, 0.6, 0.8, 1% (interval is 2.0). Serial dilution - 0.0001, 0.001, 0.01, 0.1, 1%. To produce this range of concentrations you dilute the original solution. When displaying results as a graph, there should be at least five values for the independent variable - if not you won't be able to see any trends or patterns.
What is the definition of reliability? More about measurements - accuracy, precision and reliability
The reliability of a measurement is the degree of trust that you have in it. If your measurements are reliable, then you would expect to get the same ones if you repeated them on other occasions. It is affected by both the accuracy and precision of your measuring instruments and by the kind of measurement you are making. One of the best ways of dealing with poor reliability is to repeat the readings several times. Simple - This indicates how much confidence you can place in your results; if results are reliable, you would expect to get the same readings each time you repeated the measurement.
What are the two different types of error?
There are two different types of errors: - Systematic - Random Systematic errors - Arise if a measuring instrument is incorrectly calibrated, for example. - The value read from the scale will always differ from the 'true' value by the same amount. Random errors - Arise as a result of difficulties in controlling variables or in measurement of the dependant variable. - For example, it is difficult to maintain the temperature of a water bath at an exactly constant value, or to determine the end-point of a reaction by eye, as there will always be a certain amount of variability. When making suggestions for improvement, think about the following possibilities: - Are there any variables that were not controlled? - How can these variables be effectively controlled? - Is there a better way of measuring the dependant variable? - Could I have used apparatus that is more accurate?
What are sources of error?
They are not mistakes that you might have made e.g: - not reading the thermometer correctly - not measuring out the right volume of a solution - taking a reading at the wrong time These are avoidable human mistakes that should not be made. Sources of error - unavoidable limitations of the: - apparatus - measuring instruments - experimental technique - experimental design They prevent the results from being reliable.
What do many experiments include? Experiments
They involve investigating how one thing affects another. For example: - investigating how enzyme concentration affects the rate of activity of rennin - investigating how temperature affects the rate of activity of 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 plasmolysed
How do you control light intensity?
Vary the distance of a light source. Light intensity is proportional to 1/distance squared
How do you control temperature?
Water bath Two types of water baths: - electrically controlled water bath - hand made water bath (using a large beaker of water)
How do you observe and measure colour changes and intensities?
When the dependant variable is colour - use simple words e.g. red, purple, green - to quantify these use simple terms e.g. pale or dark
What is a control? Controls
When the factor that we are investigating is absent. In this experiment: To make sure that it is the rennin that is making the milk clot, and not some other factor. Controls: 1. A tube that has no rennin in it (water). Everything else must be be the same - same volume of water is added to match volume of enzyme solution that is added to all the other tubes 2. A tube containing boiled rennin solution - boiling denatures the rennin enzyme so it is inactive
How do you estimate uncertainty in measurement? Estimating uncertainty in measurement
When using a measuring instrument with a scale (e.g. thermometer or syringe) the reading doesn't lie exactly on one of the lines on the scale. It is assumed that each reading could be inaccurate by 0.5C (for temperature using a thermometer) - this is the possible error in the measurement. The potential measurement is half of the value of the smallest division on the scale that is being read from. Example: If a temperature reading was 31.5C it would be shown as 31.5C +- 0.5C If two measurements are taken and the results are the difference between them it is assumed that both of the measurements could be out. Example: If the two measurements are 28.5C (error +-0.5C) and 39.0C (error +-0.5C) then the answer is 10.5C+-1C. Why? 39.0 - 28.5 = 10.5C and 0.5 + 0.5 = 1C How to express the size of the error as a percentage: - divide the error in the measurement by the measurement itself - then multiply by 100 Example: percentage error = (1/10.5)x100 = 9.5%
