How do you do the photosynthesis required practical?

What does the photosynthesis practical investigate?

It tests how light intensity affects the rate of photosynthesis. Pondweed is used because it lives underwater, so the oxygen it makes comes off as bubbles you can see and count. You change the light intensity by moving a lamp closer to or further from the pondweed.

It is one of AQA's required practicals in both GCSE Biology and Combined Science: Trilogy, in the bioenergetics topic. The pondweed sits in sodium hydrogencarbonate solution, which supplies carbon dioxide, so the plant has plenty for photosynthesis.

What equipment do you need?

The heart of it is pondweed in a tube of sodium hydrogencarbonate solution, with a lamp you can move along a metre ruler. A stopclock times each count.

• A piece of pondweed, and a paper clip to weigh it down
• Sodium hydrogencarbonate solution
• A boiling tube and a clamp stand
• A lamp, ideally an LED one, which gives off less heat
• A metre ruler
• A stopclock
• A thermometer
• A tank or beaker of water to act as a heat shield, if the lamp gets hot
• A gas syringe or an upturned measuring cylinder, if you collect the gas rather than count bubbles

What is the method?

Put the lamp at one distance, let the pondweed adjust, count the bubbles it gives off in a minute, then move the lamp and do it again.

• Fill a boiling tube with sodium hydrogencarbonate solution and clamp it upright.
• Attach a paper clip to a piece of pondweed so it stays under the water, and put it in the tube with the cut end at the top.
• Darken the room, so the lamp is the only light.
• Put the lamp 10 cm from the pondweed, measured with the metre ruler.
• Wait a few minutes so the pondweed can adjust to the light.
• Count the bubbles given off in one minute. Do this three times and record each count.
• Move the lamp to 20 cm and repeat, then to 30, 40 and 50 cm.
• Check the temperature of the water at the start and the end.

What are the variables?

You change the light intensity by moving the lamp, and measure the rate by counting bubbles. The lamp also warms the water, so temperature is the control variable that needs the most care.

How do you work out and present the results?

At each distance, work out the mean number of bubbles per minute from your three counts. That mean is your rate.

Worked example: at 10 cm you count 38, 42 and 40 bubbles in a minute. The mean is 120 ÷ 3 = 40 bubbles per minute, with a range from 38 to 42. At 20 cm the counts are 22, 19 and 22, so the mean is 63 ÷ 3 = 21 bubbles per minute.

Plot the rate on the y-axis against the distance on the x-axis: the rate should fall as the lamp moves away. Higher tier students also need the inverse square law, which says light intensity is proportional to 1 ÷ distance². Doubling the distance from 10 cm to 20 cm cuts the light intensity to a quarter. If you plot rate against light intensity instead, the curve rises and then levels off, because another factor, such as carbon dioxide or temperature, starts to limit the rate.

How do you make it accurate and safe?

Bubbles come in different sizes, so counting them is only a rough measure. Collecting the gas in a gas syringe, or in an upturned measuring cylinder full of water, and measuring its volume is more accurate. Keep the temperature steady: an LED lamp gives off little heat, or a tank of water between the lamp and the tube can soak up heat. Give the pondweed a few minutes after each move before you start counting.

Lamps get hot, so don't touch the bulb, and hold the lamp by its base when you move it. Keep water well away from the lamp and its plug, and dry your hands before you touch the switch. Wash your hands after handling pondweed.

What do exam questions ask about it?

Expect to explain the set-up: why sodium hydrogencarbonate is added, and why a tank of water might sit between the lamp and the tube. You may be asked what the gas is, or how to measure it more accurately than by counting. Graph questions ask you to describe the pattern and say which factor limits the rate on each part of the curve, and Higher tier papers add the inverse square law. An anomaly might come from counting too soon after moving the lamp. The 6-mark version asks you to describe the method, including how you keep it a fair test.

An exam-style question we wrote: "Suggest why counting bubbles might not give an accurate measure of the rate of photosynthesis." A good answer: the bubbles are not all the same size, so the same count can hold different volumes of oxygen.

Common mistakes

• Forgetting that the lamp warms the water. Say how you kept the temperature the same.
• Calling the bubbles carbon dioxide. They are mostly oxygen, made by photosynthesis.
• Counting straight after moving the lamp. Give the pondweed a few minutes first.
• Mixing up the direction. A bigger distance means a lower light intensity and a slower rate.
• Halving the light intensity when the distance doubles. By the inverse square law it drops to a quarter.