AQA A Level Biology Topic 3 Exam Questions

AQA A Level Biology Topic 3 Exam Questions A different student used coloured water to investigate the movement of water in leaf stalks of celery. During the procedure she: • cut equal lengths of stalk from each plant • put the cut end of each stalk into coloured water • left these stalks to take up the coloured water for 20 minutes • used a sharp scalpel to cut slices from the stalks at 1 mm intervals until she reached a slice with no coloured water. Figure 2 shows a slice of leaf stalk with coloured water inside groups of xylem vessels. Figure 2 Explain why coloured water moved up the stalks. [3] - 1. Water evaporates/is transpired (from leaves/ stalk/celery/plant); 2. Water potential gradient/lower water potential creates tension/pulls up water OR Osmosis creates tension/pulls up water; Accept negative pressure for tension 3. Hydrogen bonds/cohesion/adhesion maintains column; Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind. Explain why. [2] - 1. Binding of first oxygen changes tertiary / quaternary (structure) of haemoglobin; Ignore ref. to 'positive cooperativity' unqualified