7.1 Practical Skills

A student investigates how light intensity affects the rate of photosynthesis in pondweed.

The diagram shows how he sets up his investigation.

• He places the lamp at distances of 10, 30, 50, 70 and 90 cm from the beaker.
• At each distance, he measures how much gas is given off from the pondweed in 1 minute.

(ii)

Sketch a line on the axes below to show the results you would expect.

[2]

(i)

Describe how and where oxygen is produced in photosynthesis.

(ii)

Explain why the amount of oxygen gas given off is not a true measure of the rate of photosynthesis.

A student wants to investigate the effect of air movement on transpiration.

The diagram shows how she sets up her experiment.

These are her results.

Explain the difference in her results.

The student kept environmental conditions like light intensity and temperature the same.

(i)

(ii)

A student investigates how different concentrations of sucrose solutions affect potatoes.

• Three chips are cut from a potato.
• Each chip is 5.0 cm long.
• Each chip is left in a different concentration of sucrose solution for two hours.

These are the results.

Explain why the length of the chip increases in the 0.0 M solution.

Explain why the length of the chip stays the same in the 0.5 M solution.

(ii)

Answer = ........................................ % [2]

(ii)

Some students investigate how the rate of diffusion in animal cells is affected by the surface area : volume ratio.

The table shows their results.

answer = ........................................ [1]

• Write your answers in the table below.
• Show your answers in standard form.
  
  

  Length of one side of cube (cm)	Rate of colour change (s–1)
  1	........................................
  2	........................................
  3	........................................

[2]

Students investigate how to extract DNA from peas.

Stage 1:

• Chill 10cm³ of ethanol. Keep it on ice throughout the method for use in stage 2.
• Make a thick ‘soup’ by blending 100cm3 of peas with salt and cold water. Blend for 15 seconds in an electric blender.
• Strain the ‘soup’ through a mesh strainer and collect the liquid part in a beaker.
• Add 30cm³ of washing-up liquid and swirl to mix.
• Let the mixture settle for 5–10 minutes in a water bath at 60°C.

One group of students made a water bath using a beaker of water, thermometer and Bunsen burner. Another group used an electric water bath.

Write down two advantages of using an electric water bath.

1 ........................................
2 .........................................

Low temperatures protect DNA by slowing down the activity of enzymes that destroy DNA. High temperatures break down membranes in the cell.

To extract DNA, some methods use a water bath at 60°C but other methods do not use an increased temperature.

Suggest two reasons for the different methods.

Stage 2 isolates the DNA.

• Pour the mixture collected from stage 1 into a test tube until a third full. Add protease enzymes to the test tube.
• Slowly pour cold ethanol at an angle of 45° into the tube. Ethanol will float on top.
• DNA is soluble in water, but salted DNA does not dissolve in ethanol and will form white clumps where the water and ethanol layers meet.
• Twirl a glass rod and the DNA will collect on the rod.
• Dry the sample on a pre-weighed filter paper and measure the mass of product.

Suggest two safety precautions which should be taken at stage 2.

Explain why each safety precaution is needed.

1 Safety precaution: ......................................
Explanation: ......................................................

2 Safety precaution: ........................................
Explanation: ........................................................

Look at the table. It shows the results from the two groups of students in the investigation.

(i)

Answer = ..................................... mg [2]

(ii)

[2]

A student prepares onion cell slides to view under a microscope.

Put the stages in the correct order by writing the numbers 1 to 5 in the boxes.

Explain why the iodine solution is used.

Look at the image of some onion cells.

[2]

The diagram shows a layer of onion cells.

The actual length of the layer is 1.5 mm.

Calculate the average length of one onion cell.

Answer = ........................ mm 

A student thinks that using the highest magnification of a microscope is always best.

Explain why this may not be true.

A student wants to compare the transpiration rates of two plants.

The plants have different sized leaves.

Fig. 18.1 shows how she sets up her experiment.

Fig 18.1

Suggest why the student put a layer of oil on top of the water.

The student makes sure that each plant has the same number of leaves.

Which other experimental conditions should she keep the same?

The table shows the results of the experiment shown in Fig. 18.1.

Write a conclusion with an explanation about this experiment.

Use the results and calculations in your answer.

A student investigates how light intensity affects the rate of photosynthesis in pondweed.

The diagram shows how he sets up his investigation.

He plans to place the lamp at distances of 10 cm, 15 cm and 20 cm from the beaker.

He plans to measure how much gas is given off from the pondweed in 10 seconds.

His teacher says he could improve his plan.

Write down two improvements he could make to his plan.

The student counts the number of bubbles to get a measure of the amount of gas given off in photosynthesis.

Give two reasons why counting bubbles is not an accurate way of measuring the amount of gas given off.

What is the gas given off in photosynthesis?

Explain why the amount of this gas given off is not a true measure of the rate of photosynthesis.

(i)

Sketch a line on the axes below to show the results you would expect.

[2]

(ii)

[2]

A student investigates the plants growing underneath a tree.

He lays out a tape measure on the ground, starting at the tree. He then places a quadrat on the ground.

He measures the percentage of the ground in the quadrat that is covered by plants. He repeats this every metre away from the tree.

The table shows his results.

Plot a graph of the student’s results and draw a line of best fit.

The student thinks that shade from the tree is affecting the plants.

Explain how the student’s results show this.

Erythromycin is an antibiotic drug.

What is an antibiotic?

It is important to get the dose of erythromycin right.

Too much erythromycin can be harmful.
However, recently some strains of bacteria have developed resistance to low concentrations of erythromycin.
To see how effective erythromycin is, it is tested using bacteria grown on agar plates.

This method is used:

• A petri dish is used that has the bacteria growing evenly over the surface.
• A disc of filter paper is soaked in erythromycin.
• The disc is placed on the agar in the centre of the petri dish using sterile forceps.
• The dish is incubated at 37°C.

(i)

The diagram shows the actual size of the dish after incubation.

This table is used to analyse the results of the experiment.

Use the results of the experiment and the table to judge the level of resistance in this strain of bacteria. A ruler has been added to assist with accurate measurements (The area of a circle = π r² and π = 3.14.)

answer= ....................................... mm²[3]

(ii)

Student A and student B study the plants growing on a salt marsh.

They both sample the plants present by laying out two tape measures at right angles across the salt marsh.

They then place a square frame on the ground in different places and count the number of plants in the square, as shown below.

What is the name of the square frame that they use to sample the plants?

The diagrams show the position of each student’s samples across the salt marsh.
Each small square in the diagrams represents one sample.

(i)

The whole salt marsh has an area of 2500m².

Each square frame has an area of 0.25m².

Calculate the percentage of the whole salt marsh that was sampled by student A.

(ii)

Look at the two students’ sampling shown in the diagrams.

Explain which student is likely to get the most accurate estimate for the number of plants in the salt marsh.

Their teacher said that they should take care as there may be harmful bacteria in the salt marsh.

State two things that the students could do to reduce the risk of infection from the harmful bacteria.

1 ....................................
2 ....................................

A student investigates the plants growing on a salt marsh. He uses a quadrat to sample the plants.

He puts down two long tape measures at right angles to each other across the salt marsh. He then picks numbers at random and uses them to decide where to place a quadrat.

The diagram shows the position of all his quadrats across the salt marsh.

(i)

The salt marsh measured 50m × 50m.

Each quadrat measured 0.5m × 0.5m.

Calculate the percentage of the whole salt marsh that was sampled by the student.

Percentage = ............................... % [3]

(ii)

A second student sampled by placing five quadrats close together in the centre of the salt marsh.

Evaluate the sampling method of the second student compared to the method of the first student.

(iii)

Suggest one factor that the students should consider in a risk assessment for their experiment.

Scientists investigate antibiotic pollution in two different lakes.

They collect samples of water from the two lakes. The scientists then use aseptic techniques to investigate how resistant the bacteria in the water are to antibiotics.
Fig. 16.2 shows the apparatus they use.

Describe how the scientists could use this apparatus to measure how resistant the bacteria are to antibiotics.

The scientists also counted how many species of bacteria were resistant to antibiotics and how many species of bacteria were killed by antibiotics.

The scientists found these results.

Which lake has the highest levels of antibiotic pollution?

Tick (✓) one box.







Explain your answer.

Some students decide to use lichens to try and work out how polluted their school grounds are.

They read about a scale called the Lichen Diversity Value (LDV).

It is worked out in this way:

• choose four trees in the area
• hold a quadrat on the north side of the trunk of one tree
• count the total number of all the lichens in the quadrat
• then do this on the east, south and west side of the tree
• repeat this for each tree.

The LDV is found by adding together the four mean values.

The students calculate the mean number of lichens on the north, east and south sides of the trees.

Calculate the mean for the west side and use this to calculate the LDV.

LDV = .............................. [2]

This scale shows the diversity of the lichens shown by the LDV.

What does the LDV show about the diversity of lichens in the school grounds?

What else about the lichens could the students look for to make a better assessment of pollution?

The diagram shows a simple potometer.

The apparatus can be used to investigate the effect of light intensity on transpiration rates.

[2]

[3]

The heat sink is a transparent tube of cold water.

Explain why a heat sink is used in this experiment.

[2]

The table shows the results from using the potometer.

The mean distance the gas bubble moved along the tube at 10cm from the light was 72mm.

The diameter of the narrow tube was 1mm.

Calculate the volume of water taken up by the plant.

Use the equation: volume = πr²l
where r is the radius of the tube and l is the distance the bubble moves
π = 3.14

Give your answer to 2 significant figures.

Volume of water = ................. mm³ /minute [3]

(ii)

(iii)

How should the scientists deal with this anomalous reading when they process the data?

(iv)

The scientists described their results for 20cm as 73 ± 2.
Explain why they did this.

An experiment is set up to investigate how substances move into and out of cells.

Look at the results.

Explain the results of this experiment.
Use ideas about molecules in your answer.

Plant cells are also affected by osmotic conditions.

Look at the graph. It shows the percentage change in mass of potato chips in different concentrations of sucrose.

Concentration = .......................................... mol/dm³ [1]

The carrot chip shows a 10% decrease in mass compared with its original mass for every 0.2mol/dm³ increase in sucrose concentration.

Calculate the x-axis intercept for the carrot chip.

x-axis intercept = ......................... mol/dm³ of sucrose [1]

Stomata are found on the surface of leaves.

The diagram shows some of the surface cells of a leaf.

Look at the diagram showing a plant cell.

Answer = ....................................... × [2]

Look at the picture of part of a plant cell.



Use the arrow (measuring 75 mm) on the picture and the scale (measuring 15 mm) to estimate the length of the chloroplast.

Answer = ....................................... μm [1]

Photosynthesis takes place inside chloroplasts.

The graph is from an experiment to show the effect of temperature on the rate of photosynthesis.

(i)

(ii)

The rate of photosynthesis was recorded in 5°C intervals.

The experiment could be improved to get a more precise value for the optimum temperature.

Explain how.