One of the important differences between previous educational qualifications (and the earlier grading of A-levels) and the later GCSE qualifications was supposed to be a move from norm-referenced marking to criterion-referenced marking.  On a norm-referenced grading system, fixed percentages of candidates achieve each grade. With criterion-referenced grades, in theory, all candidates who achieve the criteria can achieve the grade. A comparison of a clearly norm-referenced assessment, such as the NFER Cognitive Ability Test or CAT, with GCSE grading seems to show an unexpected correlation, which challenges the idea that the GCSE is a properly criterion-based assessment. 
A sound investigation which could be improved with some simple scientific experimental procedure alterations.
1. Be clear of the aim of the investigation. It is clear that water will move in and out of the potato cell by osmosis, the aim of the experiment was to determine the solute concentration of the potato cell, which
was achieved. The conclusion should have focussed on this.
2. Try to avoid talking in first person throughout the paper instead of saying "I did this, then I did this..." write in third person "Firstly the potato cylinders were added to the sodium chloride solution, they were then left for 24 hours..."
3. Be consistent with terminology. If you refer to the potato cylinders as cylinders at the start of the paper call it this throughout the paper.