You are going to investigate the effect of temperature on the rate of respiration in yeast cells. When yeast cells respire they release carbon dioxide gas. Read all the instructions but DO NOT DO THEM until you have drawn a table for your results in the space provided in 1(a)(i). You should use the safety equipment provided while you are doing the practical work. Step 1 Stir the yeast suspension with the glass rod and fill the syringe with $$\(10 \mathrm{~cm}^{3}\)$$ of yeast suspension. Ensure you place the syringe nozzle under any foam that is present on top of the yeast suspension so that the foam is not drawn up into the syringe. Step 2 You have been provided with a small piece of modelling clay. Shape the modelling clay around the end of the syringe plunger as shown in Fig. 1.1. It is important that the modelling clay is firmly attached to the plunger so that it does not fall off. Step 3 Gently lower the syringe into the measuring cylinder, as shown in Fig. 1.2. If necessary, reshape your modelling clay so that it fits inside the cylinder but do not remove any modelling clay. Step 4 Fill the measuring cylinder with cold water. Do not let the water in the measuring cylinder overflow. The nozzle of the syringe should be below the surface of the water in the measuring cylinder. Fig. 1.2 Step 5 Measure the temperature of the water in the measuring cylinder. Record this measurement in your table in 1(a)(i). Step 6 Start the stop-clock and wait for two minutes. Step 7 Reset the stop-clock to zero. Step 8 Start the stop-clock again and count the number of bubbles produced by the yeast in three minutes. Record this number in your table in $$\(1(a)(i)\)$$. Step 9 Carefully pour the cold water in the measuring cylinder into the waste container. The syringe containing the yeast suspension should remain in the measuring cylinder. Step 10 Raise your hand when you are ready for hot water to be poured into your measuring cylinder. Ensure that the water level is above the nozzle of the syringe but not overflowing. Step 11 Repeat steps 5 to 8. Bread is made from flour, water and yeast which are mixed to form a dough. Fig. 1.4 shows a person making bread. The carbon dioxide gas produced by yeast causes the volume of the dough to increase. Sodium chloride (salt) is often added to dough when making bread. The sodium chloride affects the rate at which the yeast respire. Plan an investigation to determine the effect of the mass of sodium chloride on the volume of dough.

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