Practical Techniques - Need To Know!

Question 1

How could scientists obtain data to produce a calibration curve and how they would use it to find the conc of x in a sample

Answer 1

1. Use a known stock conc of x to produce a dilution series 2. Measure the absorbance of each conc or measure each conc with a colorimeter 3. Plot a graph of absorbance on the y axis against conc on the x axis and draw a calibration curve 4. Measure the absorbance of the x sample and find the x conc from the curve by interpolation

Question 2

How would you use an optical microscope and a slide of stained x cells to find the mean diameter of one x cell

Answer 2

1. Measure with an eye piece graticule 2. Calibrate the eyepiece graticule using a stage micrometer at the same magnification/ calibrate against something with a known size 3. Take a number of measurements to calc a mean

Question 3

A student found the mean diameter for the x cells in a section, give 2 precautions that they should take when sampling the x cells and give reason for each

Answer 3

1. Should sample at random to avoid bias 2. Take a large number to be representative and minimise the effect of anomalies

Question 4

Apart from temp and PH what are variables that need to be controlled when preparing a liquid medium culture of nitrogen fixing bacteria

Answer 4

Same volume of bacterial same conc of glucose, same conc/vol of oxygen, same time for bacteria to divide

Question 5

How to answer evaluate conclusion questions

Answer 5

Give findings to support but then points for counter: only used one species, may be due to another factor in the experiment

Question 6

How could the volume of co2 produced be measured by changing the apparatus used to find vol of 02 (tube with seeds, potassium hydroxide to absorb carbon dioxide, graduated scale with coloured liquid

Answer 6

Repeat the experiment but get rid of potassium hydroxide, calculate vol of co2 by measuring distance the liquid has moved with and without KOH and the difference in drop moved is the co2 produced

Question 7

How to calculate the percentage uncertainty for a specific value

Answer 7

1. The uncertainty is 1/2 of the smallest increment used e.g if using a ruler, smallest is 1mm and half is 0.5mm 2. You have two values means two uncertainties so add together both measurements e.g. 0.5 + 0.5=1mm 3. Look up the specific value which may be trail 1:79mm. Do your value (1mm) over 79mm x 100

Question 8

The scientists used an optical microscope to measure the number of capillaries in thin sections cut from samples of heart muscle.
Describe the method they would have used to find the mean number of capillaries per mm2

Answer 8

Measure diameter of field of view and calculate area; Using micrometer slide and eyepiece graticule; Count number of capillaries in large number of fields of view and calculate mean; Select fields of view randomly

Question 9

Practical investigating pigment loss from beets in ethanol, hcl and temp - give a way the student could insure the beet cylinders were kept at 25 degrees through the experiment

Answer 9

Measure temperature/ use a thermometer in each tube at regular inter pad and use appropriate corrective measures if temp has fluctuated

Question 10

How to work out uncertainty and how too reduce it

Answer 10

Half of the smallest interval e.g. if interval is 2cm3 then the uncertainty is 1. Use an instrument with smaller intervals/scale

Question 11

How to work out % uncertainty and the % uncertainty of areas

Answer 11

The uncertainty (half smallest increment) divided by the measurement given x 100. For area: find %uncertainty of the width (uncertainty/length x 100) find %uncertainty for length (uncertainty/length x 100) then add them together and the answer is plus minus the %

Question 12

Why is a layer of oil added onto of water in experiments

Answer 12

To prevent evaporation of water or to stop o2/gasses from entering

Question 13

When would you use the median for a set of measurements

Answer 13

When there is the presence of extreme values/outliers in a small sample size

Question 14

How can you obtain a quantitative measurement of a samples cloudiness

Answer 14

Place in colorimeter and measure % transmission of light through samples. Controls: same vol of water, 0 it with water, use the same wave length each times shake the sample

Question 15

What is a serial dilation (overview) and how to make a 1 in 10 dilution to make a 1 in 1000 dilution of bacterial liquid culture

Answer 15

In a serial dilution, the solution made becomes the stock conc for the next. Add one part bacterial culture to 9 parts sterile liquid (water) to make 10 to the minus 1 dilution. Mix well. Repeat using 9 parts water and 1 part of previous dilution 1 and dilution 2 to make dilution 3

Question 16

Why would a student use a more diluted mixture in a dilution series

Answer 16

Count unlikely to be accurate/repeatable because too many cells are cells would be overlapping

Question 17

Equation for making concentrations

Answer 17

C1V1=C2V2 c1-conc of starting stock solution, v1- initial volume c2- conc of final dilute solution v2-final volume

Question 18

Controls used in the beet experiment

Answer 18

Length and diameter of beet pieces, time beets spent in solution

Question 19

How to set up and use a potometer

Answer 19

1. Immerse in water to fill (no bubbles) 2. Cut shoot at and angle underwater (no air in xylem, increases SA for water uptake) and add to bung 3. Take out and close tap 4. Put beaker full on water at the end 5. Add Vaseline around shoot /seal joints(ensure water/airtight)6. Dry leaves 7. Wait for 5 mins to equilibrate 8. Add air bubble by lifting tube out of beaker then back in 9. Record initial position, wait then record final 10. Open tap/valve to add water from reservoir to reset bubble 11. Repeat 12. Rate of movement to get rate of respiration

Question 20

Give two reasons why the potometer doesn’t truly measure the rate of respiration

Answer 20

Water used in support/turgidity, water used in photosynthesis, apparatus not sealed/leaks

Question 21

How to find the rate of water loss per surface area of leaves

Answer 21

Draw around each leaf on graph paper and count squares of both sides of each leaf. Divide rate of water loss/uptake from potometer by total SA of leaves

Question 22

When counting using a haemocytometer/daisy sqaure, why are only top and left counted not bottom or right

Answer 22

To avoid dealing with parts of cells, to avoid counting the same cells twice, to be consistent (get comparable results and be standardised)

Question 23

Why do you dilute solution less when counting white blood cells than red

Answer 23

There are fewer white cells so no need to dilute further to see enough to count

Question 24

How to make a temporary mount of a piece of plant tissue for an optical microscope

Answer 24

Cut a thin layer of tissue with a scalpel onto a drop of water on a slide. Add iodine in potassium iodide stain. Gently lower a coverslip onto at an angle using forceps/mounted needle so no air bubbles

Question 25

Difference between t test, chi squared and correlation co efficients and their null hyps

Answer 25

T text compares the diff between two means (processed data)- there is no stat sign diff between the means of a and b. Chi is used on raw data null- there is.no stat sig diff between O and E data/ no. orgs… correlation null- no stat sig correlation between x and y (all diffs due to chance)

Question 26

How to answer a t test question

Answer 26

1. Talk about the t value or the p value 2. Reject the null is the calc is greater than the crit. 3. Talk about less than 5% prob diff between means is due to chance 4, state there is a statistically significant diff between the two means E.g. if the calc value is less than the critical value, the null is accepted, there is not a sig diff between the two means, more than 5% prob the diffs are due to chance

Question 27

How to answer a correlation Q

Answer 27

Uses Pearson’s or spearman’s rank. If the calculated value is greater than the critical value, reject null so there is a stat sig diff between variables,prob chanceis less than 5%. Describe the correlation based on calc (evidence of a correlation, strong?, +/-), d.f is n-2

Question 28

How to answer chi squared question

Answer 28

O, E, O-E, O-E squared, O-E squared over E. to work out E it’s what there would be if the conditions had no effect (split equally among conditions) if the calc value is bigger than critical value, reject null

Question 29

Suggest ways to improve a scientific drawing

Answer 29

Only use single lines/no sketching . Add labels/title. Add a scale bar, no shading or hatching, draw all parts to same scale

Question 30

List some aseptic techniques

Answer 30

Wash hands to kill x or wear gloves to prevent contamination. Burning Bunsen close by to create upward current of air. Disinfect bench with disinfected cloth to prevent contamination. Flame equipment to sterilise. Lift lid slightly to prevent entry of microbes

Question 31

Suggest why the number of bacteria would have been lower after incubating if no aseptic techniques were used

Answer 31

Different microorganism could have been introduced, which limit the growth of other bacteria or use the food source/space

Question 32

Precautions when carrying out a dissection

Answer 32

Carry sharp instruments by holding handle, disinfect surfaces and hands with soap and water. Cut away from body against non slip surface