Photosynthesis

Experiment 2. Testing a leaf for starch (a) Half fill a 250 cm3 beaker or tin can with water (hot water if available) and place it on a tripod over a Bunsen burner. Heat the water till it boils and then turn down the Bunsen flame sufficiently to keep the water at boiling point. (b) Hold the leaf in forceps and plunge it into the boiling water for 5 seconds. This will kill the cells, arrest all chemical reactions and make the leaf permeable to alcohol and iodine solution later on. (c) With the forceps, push the leaf carefully to the bottom of a test-tube and cover it with methylated spirit. (d) TURN OUT THE BUNSEN BURNER. (e) Place the test-tube in the hot water and leave it for 5 minutes. The alcohol will boil and dissolve out the chlorophyll in the leaf (See Figure on p.2.02). (f) Use a test-tube holder to remove the test-tube from the water bath and tip the green alcoholic solution into the receptacle for waste alcohol but take care not to tip the leaf out as well. If the leaf is white or very pale green, go on to (g). If there is still a good deal of chlorophyll left in the leaf, boil it for a further 5 minutes with a fresh supply of alcohol, using the hot water bath. If it is necessary to relight the Bunsen to heat the water to boiling point, remove the test-tube and do not replace it until the Bunsen flame is extinguished. (g) Fill the test-tube with cold water and the leaf will probably float to the top. I. Tough leaves (e.g. Tradescantia). Hold the leaf stalk with forceps and dip it into the hot water in the water bath for 2-3 seconds. Spread it flat on a tile or Petri dish lid with the aid of a little cold water. 2. Soft leaves (e.g. Busy Lizzie). Use forceps to place the leaf on a tile or back of a Petri dish lid and, holding the leaf stalk firmly against the tile or lid, let a fine trickle of water from the cold tap run over it to wash away the alcohol. (h) If necessary, use the forceps to spread the leaf quite flat on the tile or lid. Using a dropping pipette cover the leaf with iodine solution for one minute. (i) Take the leaf to the sink and holding it on the tile or lid, wash away the iodine solution with a fine trickle of cold water .