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. 
The opening and closing of the stomata have been shown to be affected by light concentrations. When illuminated, the concentration of solutes in the guard cell vacuoles increases because starch is converted to malic acid, and a proton pump in the plasma membrane is stimulated. The proton pump removes hydrogen ions (H+) from the guard cells, and in response, potassium ions (K+) flow into the cell. Chloride ions (Cl-) also flow into the cell via another pump in response to the H+ concentration difference. The accumulation of these ions and malate in the vacuole of the guards cells is enough to cause the water potential to drop within the guard cells. Water then flows in by osmosis, leading to the turgidity just described and opening the pore. As this opening process occurs in light, exactly the opposite happens at night. As light is lost, channels open to conduct Cl- and K+ out of the guard cells, water is lost, and the cells become flaccid and close.