Investigative Skills (AQA A Level Biology): Revision Note

I would mash the muscle tissue in a blender to release the organelles [1] . I must suspend the contents in a liquid that is cold to stop the enzymes working [1] and is buffered to maintain the pH. The liquid also needs to be isotonic to stop the organelles shrinking or bursting. Then, I would spin the sample in an ultracentrifuge, starting at a high speed to remove the biggest parts first, and eventually, the nuclei will settle in the pellet.

Mark: 2/6

Reason for mark:

The student gained 2/6 for correctly identifying and explaining two preparation conditions, gaining marks for MP3 and MP4.

No mark was given for homogenisation because the description did not match the required mark-scheme wording (MP1)

The phrase "maintain the pH" was too vague as an explanation for using a buffered solution, as it did not mention enzyme or protein denaturation (MP5)

Finally, the method for isolating nuclei was incorrect; nuclei should be pelleted at the lowest centrifuge speed in the first spin, not by starting at a very high speed (MP6)

I would first homogenise the muscle tissue to break open the cells [1] and release the nuclei. The resulting homogenate must be placed in a solution that is cold to prevent enzyme activity [1] , is isotonic to stop the organelles from bursting [1] by osmosis, and is buffered to maintain a constant pH and prevent protein denaturation [1] . After this I would filter the solution to remove any large cell debris [1] and then spin the solution at a low speed [1] in a centrifuge so that the nuclei form a pellet at the bottom of the tube.

Mark: 6/6

Reason for mark:

This answer clearly addresses all three required preparation conditions and links them to their necessary explanations

It also accurately outlines the three procedural steps: homogenisation, filtering, and low-speed centrifugation to obtain the nuclei pellet

I would wash my hands and briefly flame the bottle neck before pouring the culture onto the agar plate. I would make sure to keep the lid off the plate for the shortest possible time and make sure the working area is close to the heat so that the surrounding air is sterilised.

Mark: 0/3

Reason for mark: while the overall procedure here is correct, this answer is too vague to gain credit throughout:

• “wash my hands” is too vague for MP1 as it does not refer to soap

• Briefly flaming the bottle neck does not gain MP2 because the required step is “remove bottle lid and flame neck of bottle”

• "pouring the culture" does not earn MP3 because it fails to refer to a pipette or syringe

• “keep the lid off the plate for the shortest possible time” does not describe lifting the lid slightly or at an angle for MP4

• working near the heat “so that the surrounding air is sterilised” is explicitly rejected in MP5: the Bunsen flame is used to create upward air movement, not to sterilise the air

Before starting I would wash my hands with soap [1] and disinfect the bench surface. I would then remove the lid of the culture bottle, briefly flame the neck of the bottle [1] and replace the lid as soon as possible.
To transfer the bacteria, I would use a sterile pipette [1] and would only lift the lid of the Petri dish slightly [1] while making the transfer.

Mark: 3/3

Reason for mark: this answer matches the mark scheme with washing hands with soap and disinfecting the bench (MP1), removing the lid and flaming the neck of the culture bottle (MP2), using a sterile pipette to transfer the bacteria (MP3), and lifting the Petri dish lid only slightly (MP4). Because the maximum for the question is 3 marks, any three of these valid techniques would be credited.

They used 380 volunteers, so this is a large sample size and the results can be applied to the whole population [1]. They were aged 25–60 and split into two equal groups, with one group doing the weight-loss programme and the other not, and the scientists measured their mass and diabetes remission after two years.

Mark: 1/3

Reason for mark: this answer earns 1 mark for correctly identifying that using 380 volunteers from many health centres gives a large, more representative sample (MP2). The rest of the answer just repeats details from the stem (age range, splitting into two groups, measuring mass and remission) without explaining how these features reduce bias (MP1), provide a control comparison (MP4), or support long-term validity (MP3)

The volunteers were selected using a computer-generated list, so the selection was random and this reduced bias [1]. There were 380 volunteers from many health centres, so the sample size is large and could be representative of the whole population [1]. The scientists also included a control group that did not do the weight-loss programme, so the remission rate in group P can be compared directly with group Q [1].

Mark: 3/3

Reason for mark: this answer gains full marks because it both identifies valid design elements and explains how they contribute to validity; the computer-generated list makes selection random and reduces bias (MP1); 380 volunteers give a large, representative sample (MP2), and having a control group allows a direct comparison with the weight-loss group (MP4)