AS - skills Flashcards
Maths Skill: Drawing a Graph for Enzyme Rate Experiments
what are enzyme rate experiments?
experiments that are carried out to determine the effect of changing a particular factor on the rate of a reaction that is catalysed by an enzyme
which factors can be changed in an enzyme practical?
-temperature
-pH
-enzyme concentration
-substrate concentration
2 ways that ways of reaction can be measured…
-measuring how much of a product is made in a given time period
-measuring how much a substrate is broken down in a given time period
which graphs are used to present the results of enzyme rate experiments?
line graphs
where do variables stay on graphs?
independent variable on the x-axis
dependent variable on the y-axis
when are lines of best fit added to a graph?
if a trend can be identified
what can lines of best fits be used for?
-to interpolate (reading off values in between existing data points)
-to extrapolate (going beyond the range of existing data points to read off values)
multiple sets of data on a graph:
-it may be necessary to plot more than one set of data on the same graph (eg: temp)
-each line represents the data collected at a specific temperature
-when drawing a graph like this, make sure to clearly label each line
tips for plotting line graphs:
-choose scales that enable all data points to be plotted within the graph area
-label axes, with units included
-make graphs that fill the space the exam paper gives you
-draw a line (or curve) of best-fit to identify trends (smooth line with a balance of data points above and below the line)
-un some cases, the line of best fit should be drawn through the origin
Maths Skill: Using a Tangent to Find Initial Rate of Reaction
when are tangents needed for graphs?
-many enzyme rate experiments produce non-linear graphs, meaning they have an ever-changing gradient
↳ they are shaped this way because the reaction rate is changing over time
-in these cases, a tangent can be used to find the reaction rate at any one point on the graph
what is a tangent?
-a straight line that is drawn so it just touches the curve at a single point
-the slope of this tangent matches the slope of the curve at just that point
what is done with the tangent?
you find the gradient of the straight line (tangent) you have drawn
what is the initial rate of reaction?
the rate of reaction at the start of the reaction (time = 0)
steps of drawing a tangent:
1) make sure the line you draw is perfectly straight
2) choose the point where the tangent is to be taken and slowly line the ruler up to that point
3) draw a line that follows the general direction of the curve at that point
how to calculate a gradient:
y² - y^1 / x² - x^1
Maths Skill: Calculating pH
what can be used to calculate pH?
if the hydrogen ion (H+) concentration of a solution is known, the pH can be calculated using the equation
what formula is used to calculate pH?
pH = -log₁₀ [H⁺]
example:
the hydrogen ion concentration of a solution is 1.6 x 10^-4 mol dm-3. find the pH of this solution.
pH = -log₁₀ [H⁺]
pH = -log₁₀ x (1.6 x 10^-4) = 3.796
pH = 3.8
example:
the hydrogen ion concentration of a solution of sodium hydroxide is 3.5 x 10-11 mol dm-3. find the pH of this solution.
pH = -log₁₀ [H⁺]
pH = -log₁₀ (3.5 x 10^-11) = 10.456
pH = 10.5
exam tip:
-don’t forget the minus sign in the formula: pH = -log₁₀ [H⁺]
-pH must fall between 0 and 14 so if your answer is outside of this range, something has gone wrong!
-log’ is the same as log^10
Practical Skill: Control of Variables & Uncertainty
why are control variables important?
if these control variables are not kept constant, they could affect the results of the experiment
↳ this would make the results unreliable
what is uncertainty?
the amount of error your measurements might contain
how to calculate uncertainty?
uncertainty/measured value x 100
example:
in an enzyme-controlled reaction involving the breakdown of hydrogen peroxide by catalase, 50 cm³ of oxygen was produced, with an uncertainty value of 0.5 cm³ . calculate the percentage error of this measurement.
percentage error = (0.5 ÷ 50) x 100
percentage error = 0.01 x 100
percentage error = 1 %
in an enzyme rate experiment involving the breakdown of hydrogen peroxide by catalase, a student recorded that 10 cm³ of oxygen was produced in 5.245 seconds.
the student measured this using a stopwatch that counted in milliseconds. calculate the percentage error of the stopwatch measurements.
1) calculate the uncertainty value
-the stopwatch can measure to the nearest millisecond (0.001 second)
-this means the actual time taken could be up to 0.0005 seconds shorter or longer than this
-this means stopwatch measurements have an uncertainty of ± 0.0005s
2) calculate the percentage error
(0.0005 ÷ 5.245) x 100
0.000095 x 100
percentage error = 0.0095 % / 0.01 %
Practical Skill: Microscopy & Drawing Scientific Diagrams
the beginning of using an optical microscope:
start with the low power objective lens
↳ it’s easier to find what you’re looking for in the field of view
↳ this helps to prevent damage to the lens or coverslip, incase the stage has been raised too high
how to take measurements of cells?
with a graticule
where can graticules be used?
it can be placed into the eyepiece of a microscope to act as a ruler in the field of view
what must be done with a graticule?
-it must be calibrated for the objective lens that is in use
-this is done by using a stage micrometer
-by using the two scales together, you can find out how many micrometers each graticule unit is worth
why are biological drawings usually made?
to record the observations seen under the microscope
what are biological drawings?
line pictures which show specific features that have been observed when the specimen was viewed
rules of biological drawings:
-the drawing must have a title
-the magnification under which the observations were made must be recorded
-lines should be clear, single lines
-no shading
-label lines should not cross
-label lines should be kept to one side of the drawing
how many different lenses does a light microscope have?
-an eyepiece lens
-an objective lens
what is an objective lens?
a series of (usually 3) objective lenses, each with a different magnification
what is an eyepiece lens?
often has a magnification of x10
Magnification Calculations
magnification formula
I.A.M
image size/actual size = magnification
mm → µm
x1000
µm → nm
x1000
the actual thickness of the leaf below is 2000µm, but the image size of the leaf in the diagram is 50mm. what is the magnification of the diagram?
2000 / 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm
magnification = image size / actual size = 50 / 2 = 25
magnification = x25
Looking at the Gas Exchange under the Microscope
what to do with unclear or blurry images:
-switch to the lower power objective lens and try using the coarse focus to get a. clearer image
-consider whether the specimen sample is thin enough for light to pass through to see the structures clearly
what is the purpose of three way taps?
they can be used in conjunction with potometers and respirometers
what do three way taps allow?
for repeat readings to be taken easily, eliminating the need for the apparatus to be reassembled each time
three way taps & potometers
when used with a water reservoir they allow for the air bubble to be returned to the start of the tube by allowing water to enter the system
three way taps & respirometers
the three-way tap can be used with a syringe to push the bead of coloured liquid back to the start
** Maths Skill: Using Logarithms When Investigating Bacteria**
how do bacterial colonies grow?
at rapid rates when in culture with very large numbers of bacteria produced within hours
which scales are useful when investigating bacteria?
logarithmic scales
what do logarithmic scales allow for?
a wide range of values to be displayed on a single graph
Quantitative Investigations of Variation
what is a mean?
the average value of a data set
what is a standard deviation?
a measure of the spread or dispersion of data around the mean
what does a small/low standard deviation indicate?
results lie close to the mean
what does a large/high standard deviation indicate?
the results are more spread out
limitations of measures of central tendency:
its best to use them with means as comparisons are easier
what does an overlap between standard deviations suggest?
the results are not significantly different
what does no overlap between standard deviations suggest?
the results are significantly different
a group of scientists wanted to investigate the effects of a specific diet on the risk of coronary heart disease. one group was given a specific diet for 8 weeks, while the other group acted as a control. after the 8 weeks scientists measured the diameter of the lumen of the main artery in the arm of the volunteers. the results of the experiment are shown in table 1 below:
before experiment
experimental - 0.69 (±0.02)
control - 0.71 (±0.02)
after 8 weeks
experimental - 0.74 (±0.03)
control - 0.72 (±0.05)
use the standard deviations above to evaluate whether the diet had a significant effect?
experimental group before:
0.67 to 0.71mm
experimental group after:
0.71 to 0.77mm
control group before:
0.69 to 0.73mm
control group after:
0.67 to 0.77mm
-there is an overlap of standard deviations in the experimental group before and after the experiment (0.67~0.71mm and 0.71~0.77mm) so it can be said that the difference before and after the experiment is not significant
-there is also an overlap of standard deviations between the experimental and control groups after the eight weeks (0.71~0.77mm and 0.67~0.77mm) so it can be said that the difference between groups is not significant
Serial Dilutions
how to produce a serial dilution:
1) take one part of the stock solution (100% conc - bacteria) and add it to 9 parts of water (10% concentration)
2) take one part of this new solution and add it to 9 parts of water (1% concentration)
3) take one part of this new solution and add it to 9 parts of water (0.1% concentration)
4) mix thoroughly between each step
what are the benefits of serial dilutions?
it enables you to make solutions with very low concentrations, without having to measure out very small volumes, this improves accuracy
a student needed to estimate the number of bacterial cells present in a solution by counting them under the microscope. they need to use a dilution series to investigate the number of cells present, otherwise there would be too many to accurately count.
describe a method for how they could make a 1 in 10 dilution and then go on to use this to make a 1 in 1000 dilution of the original liquid culture of bacteria. (3 marks)
1) add 1 part bacteria culture to 9 parts water (to make 10-^1 dilution);
2) mix
3) repeat using 9 parts water and 1 part of 10-^1 dilution to make 10-^2 dilution
4) repeat using 9 parts water and 1 part of 10-² dilution to make 10-³ dilution
dilution series calculation formula:
C1 x V1 = C2 x V2
the meanings of the values of the dilution series calculation:
Cl = concentration of the stock solution
VI = colume of stock solution used to make new concentration
C2 = concentration of the solution you are making
V2 = volume of new solution you are making
V2 = V1 + …
volume of distilled water to dilute with
V2 = V1 + …
volume of distilled water to dilute with
what statistical test is used to see if a factor has a significant effect?
chi-squared
