BPrac A - General

Errors describe = explain explicitly 

• Drops of different size

• Stacking of potato discs

• Difficulty in making precise cuts using scapel

• Drops counted were of different sizes/volume -> number of drops released per minute may not proportional to the rate of H2O2 decomposition → Use a gas syringe to record the volume of gas evolved per unit time

• Stacking of the potato discs reduces the surface area exposed to solution -> reduce the catalase concentration exposed to H2O2 -> lower number of drops released per minute

• Difficulty in making precise cuts using the scalpel -> Imprecise cutting leads to different surface area exposed -> different concentration of catalase being in contact with H2O2 in each syringe -> A higher catalase concentration will result in a higher number of drops released per minute

If all significant sources of error were eliminated from this investigation, improvement should be (if narrow conc used)

Prepare 5 copper sulfate concentrations between ______ to _______ for a better estimate of the concentration of U

1. Yeast 

• CANNOT SAY first few bubbles may be air and not O2 (idk why just don’t say it)

Describe visking tubing errors

1. Visking tubing 

• Squeeze out any water inside visking tubing before use: prevent leftover water from diluting solution -> affecting rate of osmosis 

• Ensure visking tubing does not lean against wall: reduce SATVR -> reduce rate of osmosis 

Osmosis errors

• Repeat the test using intermediate concentrations -> allows relationship between variables to be observed more accurately 

• Increase the number of concentrations used and use smaller intervals of sucrose concentrations such as… 

• Evaporation of salt solutions -> cover petri dish 

• Temp of water bath not maintained 

• Colour change is subjective -> use colour chart *NOT colorimeter 

Temp error

Temperature of water bath decreases, rate of respiration decreases -> monitor temp and top up hot water when needed to maintain a constant temperature

Time related error

• Stagger reactions such there is a 5-min interval between the set-up of each reaction

• Decrease time intervals to more accurately pinpoint the exact time….

Colour change error

• Only can use for difficult to see colours (Like light VS dark yellow) : Difficult to judge and is subjective -> 

  • Use a colorimeter to identify colour of solutions taken out at time intervals instead of relying on visualization as it is subjective (measure absorbance value of solution) 

  • Results should be compared to standard (DO NOT use for first colour change seen)

Colour change improvements (repeatability, accuracy)

• Repeatability : 

  • Colour in tubs is not the exact same as the colour standard -> take values as a range 

  • Difficult to compare colours of test and colour standard -> examine tubes against a white surface 

• Accuracy : 

  • Compare dry-weight of red precipitate produced 

Syringe error

• Use different syringes for each solution to avoid contamination

• Contamination using same stopper -> have each individual stopper for each test tube 

Compare accuracy, precision, reliability, repeatability

1. Accuracy is about being correct (close to the true value).

2. Precision is about being consistent (how close repeated measurements are to each other).

3. Reliability is about being repeatable (whether the process gives consistent results over time)

4. Repeatability is about closeness of agreement between independent test results (repeats), obtained with the same method

Improve reliability

• Repeat exp and calculate average to reduce variation in photosynthesis activity OR shorter intervals and increase number of temp or conc of solution -> more accurate best fit line 

• To see if measurements are comparable and if there are any outlier results

• Sample size too small may not be representative of cells -> increase the number of cells counted from 20 to 30-50 

Improve accuracy 

• Change data collection method: 

  • Use a more precise instrument to measure small volumes (Eg. burette) 

  • Add the yeast mixture to a boiling tube with rubber bung and collect gas with a gas syringe

• Include more temperatures in the pre-treatment of 10, 30, 50, 70, 100 degrees (MUST give values) 

• Increase duration of pre-treatment to 10min 

• Include a control experiment (Eg. Include a control tube with boiled and cooled amylase added instead to the starch solution so as to be sure that the reaction with I2/KI was due to reducing sugar formed from the hydrolysis of starch by amylase)

• Staggering the addition of amylase into tubes A to D to ensure equal reaction times (only say if experiment says put in all at once)

Improve repeatability (temp, pH, enzyme/substrate conc, measuring)

Temperature

• Conducted in a water bath set at 25C, using a thermometer to monitor the temperature and addition of hot and cold water to maintain the temperature

Be specific: Use water bath of tube C during pre-treatment at 30 degrees

OR

1. Use thermostatically-controlled water bath

pH: Add equal volume of pH buffer at a particular pH throughout the experiment 

Enzyme conc: Use same volume and concentration of enzyme 

Substrate conc: (same as enzyme)

Measuring: Measure size of beads using vernier calipers or micrometer screw gauge  to ensure that the same-sized beads of 0.3cm diameter are used

The results are valid only to a limited extent because…

• The results are valid only to a limited extent because

  • The identification of colour and pH is subjective, as the colours corresponding to different pH values are very similar and may be difficult to distinguish accurately

  • No repeats were carried out -> reliability of the two pH readings cannot be confirmed

  • Heat from the lamp and light from the surroundings may influence the results, affecting the measured pH

Replicate VS repeat

1. Replicates: Identical sets of experimental tubes and carried out simultaneously with the experiment -> ensure consistency of results 

2. Repeats: Duplicate experiments conducted after the first set of results have been obtained (experiment usually repeated 3 times)