Practicals

• Describe how you would test a liquid sample for the presence of lipid and how you would recognise a positive result.

  1. (Mix / shake sample) with ethanol, then water;

  Sequence is important

  2. White / milky (emulsion);
• Explain why the student:

  1. used only the first 5 mm from the tip of an onion root.

  2. pressed down firmly on the cover slip.


  1. Where dividing cells are found / mitosis occurs

  2. Single / thin layer of cells / spread out cells so light passes through (making cells / nuclei visible);
• Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate.


  1. Examine large number of fields of view / many cells;

  Mark as pairs only

  Accept large number / 20 or more for many

  2. To ensure representative sample;

  Accept typical / reliable

  OR

  3. Repeat count;

  4. To ensure figures are correct;

  OR

  5. Method to deal with part cells shown at edge /count only whole cells;

  6. To standardise counting;
• Describe how he made a 1 in 10 dilution and then used this to make a 1 in 1000 dilution of the original liquid culture of bacteria.

  1. Add 1 part (bacteria) culture to 9 parts (sterile) liquid (to make 10-1 dilution);

  Accept water / nutrient / broth for liquid

  2. Mix (well);

  Accept stir

  3. Repeat using 9 parts fresh (sterile) liquid and 1 part of 10-1 and 10-2 dilutions to make 10-3 dilution;

  OR

  Add 1 part 10-1 (suspension) to 99 parts (sterile) liquid (to make 10-3 dilution);
• Describe the method the student would have used to obtain the results in the graph. Start after all of the cubes of potato have been cut. Also consider variables he should have controlled. (osmosis)

  1. Method to ensure all cut surfaces of the eight cubes are exposed to the sucrose solution;

  Credit valid method descriptions to fulfil mp1, 2 and 3 (no explanation is required).

  2. Method of controlling temperature;

  Accept 'at room temperature' for method

  3. Method of drying cubes before measuring;

  4. Measure mass of cubes at stated time intervals;
• The scientists collected bees using a method that was ethical and allowed them to identify accurately the species to which each belonged.

  In each case, suggest one consideration the scientists had taken into account to make sure their method

  1. was ethical.

  2. allowed them to identify accurately the species to which each belonged.


  1. Must not harm the bees

  2. use photographs/specimens (to identify species)
• Suggest and explain two ways in which the scientists could have improved the method used for data collection in this investigation.

  1. Collect at more times of the year so more points on graph/better line (of best fit) on graph;

  Both suggestion and explanation is required for each mark point.

  The explanation must relate to the graph.

  2. Counted number of individuals in each species so that they could calculate index of diversity;

  3. Collected from more sites/more years to increase accuracy of (mean) data;
• Suggest one reason why it was important that the student left the apparatus for one hour after the yeast culture reached a constant temperature.

  (So the) oxygen is used/absorbed/respired;
• Explain why a log scale is used to record the number of cells.

  Large range/difference/increase in numbers;
• Explain the purpose of:

  1. boiling the agar

  2. transferring the same volume of liquid culture onto each agar plate.


  1. So no contamination / other bacteria;

  2. So same number of bacteria transferred to allow comparison;
• Describe how you would use cell fractionation techniques to obtain a sample of chloroplasts from leaf tissue. Do not include in your answer information about any solutions

  1. Macerate / homogenise / blend / break tissues / cells (in solution);

  2. Centrifuge;

  3. At different / increasing speeds until chloroplast fraction obtained;
• The student put the same volume of water in each tube.

  Explain why it was important that he controlled this experimental variable.

  1. (If) too much water the concentration of pigment (in solution) will be lower / solution will appear lighter / more light passes through (than expected);

  OR

  (If) too little water the concentration of pigment (in solution) will be greater / solution will appear darker / less light passes through (than expected);

  2. So results (from different temperatures) are comparable;
• Describe a method the student could have used to monitor the temperature of the water in each tube.

  (Take) readings (during the experiment) using a (digital) thermometer / temperature sensor;
• They used a calibration curve to find the concentration of protein in samples of blood plasma.

  Describe how the scientists could obtain data to produce a calibration curve and how they would use the calibration curve to find the concentration of protein in a sample of blood plasma


  1. Produce known concentrations of protein;

  2. Measure absorbance of each concentration

  3. Plot a graph of absorbance on y-axis against concentration (on x-axis) and draw curve;

  4. Use absorbance of sample to find protein concentration from curve;
• Describe how the student could use an eyepiece graticule to determine the mean diameter of stomata.

  1. Measure (each stoma) using eyepiece graticule;

  2. Calibrate eyepiece graticule against stage micrometer / ruler / graph paper;

  3. Take a number of measurements (to calculate a mean);
• The scientists incubated the flasks containing the leaf discs at 26 °C and gently shook the flasks.

  Suggest one reason why the scientists ensured

  1. the temperature remained constant

  2. why the leaf discs were shaken.

  1. (Maintain temperature) so that the rate of diffusion (of ions out of cells) remains constant

  2. (Shaking) So all surfaces of the leaf discs are exposed (to water) / so all submerged;
• The solution that the student used to produce the chloroplast suspension had the same water potential as the chloroplasts.

  Explain why it was important that these water potentials were the same.

  1. Osmosis does not occur;

  2. Chloroplast / organelle does not burst / lyse / shrivel / shrink;
• Outline a method the ecologists could have used to determine the plant species richness at one site.


  1. A method of selecting sampling sites at random;

  2. Use of quadrat;

  3. Identify (plant) species (at site / in each quadrat)

  OR

  Count number of (different plant) species (at site / in each quadrat);
• Outline a method she could have used to find this rate. You should assume that all water loss from the shoot is from the leaves.

  1. Method for measuring area;

  e.g. draw round (each) leaf on graph paper and count squares;

  2. Of both sides of (each) leaf;

  3. Divide rate (of water loss / uptake from potometer) by (total) surface area (of leaves);
• Suggest two variables the biochemist controlled when investigating the effect of temperature on the rate of breakdown of a protein by the protease.

  1. Initial / starting substrate concentration

  2. Enzyme concentration

  3. pH.
• Describe two aseptic techniques she would have used when transferring a sample of broth culture on to an agar plate.

  Explain why each was important.

  1. Keep lid on Petri dish

  2. To prevent unwanted bacteria contaminating the dish.

  OR

  3. Wear gloves / Wash hands;

  4. To prevent contamination from bacteria on hands / mouth

  OR

  Prevent spread of bacteria outside the lab;
• 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.

  1. Measure diameter of field of view and calculate area;

  2. Using micrometer slide and eyepiece graticule;

  Accept descriptions

  3. Count number of capillaries in large number of fields of view and calculate mean;

  4. Select fields of view randomly
• Explain why the data in the table above are described as processed results.

  Calculations made (from raw data)
• Describe how you would use the student's results in the table above to find the water potential of the potato tissue.

  1. Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis;

  2. Find concentration where curve crosses the x-axis / where percentage change is zero;

  3. Use (another) resource to find water potential of sucrose concentration (where curve crosses x-axis).
• Give two reasons why it was important that the student counted the number of stomata in several parts of each piece of leaf tissue.

  1. Distribution may not be uniform

  OR

  So it is a representative sample;

  Accept: more/fewer stomata in different areas.

  Ignore: anomalies/random/bias.

  2. To obtain a (reliable) mean;
• Suggest two variables the student should control in using the filter paper discs in this investigation.

  1. (Same) size;

  Ignore: 'same shape' as shape shown on the diagram.

  2. (Same) material/absorbency;
• Suggest one advantage of using a pH meter rather than a pH indicator in this experiment.

  1. Numerical readings / not subjective / colour change subjective / gives quantitative data / not qualitative / gives continuous data;

  2. Greater accuracy;
• Describe how temporary mounts are made.

  1. Thin slice/section;

  2. Put on slide in water / solution / stain;

  3. Add cover slip;
• What measurements should the student have taken to determine the rate of photosynthesis?

  Oxygen production / concentration and time.
• Describe how the students would have returned the air bubble to the start of the capillary tube in this investigation.

  Open / use tap / add water from reservoir;
• Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.

  Dry off leaves;

  Ensure no air bubbles are present

  ensure airtight
• A potometer measures the rate of water uptake rather than the rate of transpiration. Give two reasons why the potometer does not truly measure the rate of transpiration.

  1. Water used for support / turgidity;

  2. Water used in photosynthesis;

  3. Water produced in respiration;

  4. Apparatus not sealed / 'leaks';
• Describe how the student would show that reducing sugars were present in a solution.

  1. Add Benedict's;

  2. Heat;

  3. Red / orange / yellow is a positive result
• The level of coloured liquid in the right-hand side of the manometer tube went down during Experiment 1. Explain why.

  1. Oxygen taken up / used (by seeds);

  2. Carbon dioxide (given out) is absorbed by solution / potassium hydroxide;

  3. Decrease in volume / pressure (inside flask);
• The pea seedlings were kept in the dark after each treatment. Explain why this was necessary.

  1. (Seedlings) respond to light / are phototropic;
• The scientists gave their results as ratios. Explain the advantage of giving the results of this investigation as a ratio.

  1. Allows comparison;

  2. Different amounts of fungus added / fungus is different size at start;
• The ecologists could have used the mark-release-recapture method to estimate the number of one species of fish in the lake. Describe how.

  1. Capture sample, mark and release;

  2. Appropriate method of marking suggested / method of marking does not harm fish;

  E.g. Cutting a fin / attaching a tag / paint / marker.

  3. Take second sample and count marked organisms;

  4. Population = No. in sample 1 X no. in sample 2

  -------------------------------------

  No. marked in 2 sample
• This species of fish breeds at a certain time of the year. During this fish-breeding season, the mark-release-recapture technique might not give a reliable estimate.Suggest one reason why.

  population increases
• Explain the purpose of the strip of filter paper in the potassium hydroxide solution.

  to increase surface area (for carbon dioxide absorption);
• Describe how you could measure the dry biomass of the parts of the herbaceous plants above ground in a sample area of 1 m2.

  (collect and) dry all above ground plant material;(reject collect one / small sample / whole plants)in an oven at or just below 100 oC;weigh and repeat until constant mass;other precaution, e.g. cooling in desiccator;